Symposium MS01 : Extreme Mechanics

Slender beams undergo large deformations when subjected to moderate mechanical forces: they are therefore promising building blocks for the design of multistable, reconfigurable metamaterials. Combining these structures with active materials through advanced additive manufacturing techniques paves the way towards engineering truss metamaterials with properties tunable in time (so-called 4d-printing).
In this presentation, I use a discrete, geometrically exact beam formulation that can efficiently and accurately simulate the nonlinear deformation of slender beams featuring complex material behaviors, such as those found in the design of such metamaterials. I implement a numerical method that fully decouples the kinematics from the material behavior, and can handle finite rotations as well as a wide class of constitutive laws depending on the stretching, flexural and torsional strain and strain rates.
After presenting the underlying geometric description of framed curves, and the treament of inelastic material models using variational constitutive updates, I will show examples covering different constitutive laws, including rate-dependent bevhaviors. These benchmark problems feature instabilities, nonlinear geometric couplings between bending and twist, as well as natural curvature. I will show that the convergence of the numerical method competes with classical nonlinear beam formulations.
Finally, I present an extension to beam assemblies and trusses with rigid joints, which I demonstrate on a set of examples including shape transforming structures, triggered by thermal and mechanical stimuli.

Architected materials have long been explored for their ability to achieve extremal combinations of mechanical and functional properties. Optimal design of the topology has allowed numerous demonstrations of mechanical metamaterials with unique combinations of high stiffness, strength, energy dissipation, low density, and many other properties. More recently, progress in additive manufacturing has enabled fabrication of nano-architected materials, which can achieve far superior properties via combined optimization of their unit cell topology and base material micro/nano-structure. This presentation will cover recent progress in the design, manufacturing and characterization of ultralight nano-architected ceramic materials with unprecedented combinations of strength, energy absorption and high temperature stability. Lattice, plate and shell-based architected materials with nanoscale topological features are fabricated by two-photon polymerization Direct Laser Writing (DLW). The polymeric preforms are subsequently pyrolyzed in vacuum, resulting in a ceramic metamaterial (carbon or silicon oxycarbide, depending on the choice of resin). The effect of the processing parameters on the microstructure and mechanical properties (ultimate strength and strain to failure) of the resulting constituent materials is discussed first. Subsequently, we present ceramic mechanical metamaterials with various topologies and relative densities, and demonstrate extremal combinations of mechanical properties. In particular we show that with proper choice of topology, ceramic nano-architected materials can exhibit progressive failure, dissipating large amounts of energy. Finally, opportunities for scalability are discussed.

Smart materials have enabled self-directed deformations of initially flat geometries into 3D shapes. However, most self-morphing structures are not capable of time-dependent actuation paths. To achieve more complex target geometries and avoid self-collisions, it is critical to locally embed target curvatures and morphing rate information in shells prior to their deployment. While some structures with intrinsic actuation capabilities have incorporated temporal programming through the sequential folding of discrete hinges, these examples do not allow for changes in Gaussian curvature. Here, we demonstrate a method for encoding temporal morphing behaviors in architected bilayer shells that assume complex shapes and doubly curved geometries. Each shell layer is a tessellation of polymeric unit cells that soften in water at rates locally prescribed by a non-periodic mesostructure. The shells contract due to the effect of a pre-stretched elastic membrane, and stiffness differentials across each of their shell layers, inducing curvature changes. Morphing is locally finalized by rigid contacts between adjacent elements, locking shells in their target geometries. Mesoscales are encoded to perform specified behaviors through an inverse design tool that is based on a data-driven model of unit cells’ temporal responses.

Mechanical metamaterials are materials with micro-architectures, which give rise to unusual mechanical properties that are difficult or impossible to achieve in homogeneous materials. In this work, new types of phase-transforming metamaterials are developed to demonstrate domain switching, shape-memory effects, and energy absorption. In the first design, a metamaterial composed of an elastomer with periodic holes can undergo a phase transformation when subjected to a compressive load, yielding a large transformation strain. We show that such phase transformations can be broadly tuned by changing the geometric parameters of the metamaterial. Examples of domain switching and shape memory effects in such a metamaterial are demonstrated. In the second design, a metamaterial composed of a compliant elastomer and rigid granular particles is proposed as a reusable energy absorber. When the metamaterial is subjected to an external load, it undergoes phase transformations between the multistable states. Correspondingly, the input energy is partially trapped as elastic energy in the elastomer, and partially dissipated by friction between the granular particles, forming hysteresis between the loading and unloading force–displacement curves. Through tuning the structural design of the metamaterial, the pretension and stiffness of the elastomer, and the size of and friction between the particles, a vast design space is achieved to program the mechanical behavior of the metamaterial. Impact tests confirm the energy-absorbing capability of the proposed metamaterial.

We present thin, undulating sheets that switch rapidly and reversibly between flat, rolled, helixing and crumpled shapes under loading. While these complex states are stable when the undulations are big and the sheet is slim, switching back to the rest state is easy under external forcing: an ideal starting point for multi-shape materials.

We show that the sheets' reversible shape-switching is caused by the occurrence of many local, interacting, and hysteretic pop-through defects. We classify the various stable shapes into distinct shape families, and find a transition from rolled to helixing states depending on sheet aspect ratio. We also study how mechanically trained memory arises under repeated compression of the sheets. Our work provides a novel platform for designing multi-shape metamaterials with memory.

This talk will discuss the use of core-shell structures to create tough mechanical “metamaterial-like” fibers that stretch to large tensile strains. Tough materials have large areas under a stress-strain curve. Thus, materials that require high stress to extend to large strains have high toughness. Metals require large stress in extension but fail at low strains. Elastomers require low stress yet can extend to high strains. Here, we combined these two properties using a metal (core) and elastomer (shell) fiber structure to achieve a tough material that requires high stress to extend to large strains. The core consists of gallium, which can be injected into the fibers and then solidified at room temperature. Extending these fibers causes the metal to fracture at high stress. Rather than experiencing catastrophic failure upon fracture of the metal, the elastomer shell holds the fibers together and additional strain, counterintuitively, causes additional fracturing of the metal. Polymer “bridges” (i.e. elastomeric shell with no metal core) form between the fractured metal segments, as the metal core segments continue to fracture into smaller pieces and dissipate energy. Stress is transferred from the elastomeric shell to the metal core segments via the core-shell interface. Although there are a variety of ways to create tough materials (including chemical and physical approaches), this approach is interesting because of its simplicity and the fact that the fibers are conductive and can self-heal. These types of tough materials have applications in stretchable electronics and soft robotics because of their ability to conduct electricity, sense touch, extend to large strains (500%), and provide toughness in a manner that is reminiscent of human skin. This talk will discuss this new approach to creating tough stretchable materials for future extreme materials design.

A structural parameter proposed almost two centuries ago, the so-called Poisson’s ratio (PR), is still a very effective way to characterize the mechanical behaviors of materials and structural models. PR is defined as the ratio of transverse strain to longitudinal extension strain along the direction of a stretching force. The vast majority of materials present positive PR, i. e. they laterally expand (shrink) when subjected to compressive (tensile) strains. Materials with negative (the so-called auxetic materials [1]) or zero PR can be found in nature and man-made and have many technological applications, such as sensors and actuators. In particular, null PR materials, i. e. the ones that neither contract nor expand laterally due to axial strains, have received much attention towards applications in different fields as medicine and aeronautics. The most common examples of zero PR materials are gases and cork [2]. Cork has inspired the development of semi-reentrant honeycomb-like structures [3], which have became the widely used paradigm for constructing zero PR structures. In this work, we have designed and experimentally realized a new type of topological model that presents null PR under compression. Our model is not based on either reentrant honeycomb or hexagonal structures. It is composed of arbitrary planar junctions of a given structural unit cell containing two main parts: two parallel rigid bars of smooth surfaces and a soft and elastic membrane or spring connecting the rigid bars. These unit cells or blocks were printed using a homemade 3D printer and randomly distributed on a surface to form sample arrays of m by n cells. Using a homemade vise, the arrays are compressed and the strains on both transversal and longitudinal measured, the obtained PR values were almost zero. These new and very simple proposed topological models present great potential for a new paradigm in creating zero or near-zero PR structures.
[1] R. H. Baughman and D. S. Galvao, Nature v365, 735 (1993).
[2] G. N. Greaves et al., Nat. Materials v10, 823 (2011).
[3] J. N. Grima et al., Adv. Eng. Mater. v12 855 (2010).

Materials that can efficiently dissipate impact energy are attractive for various lightweight applications including aerospace, automotive, and personal protection. Liquid crystal elastomers (LCEs) can exhibit remarkable energy dissipation behaviors originating from the coupling of dynamics of LC molecules (Mesogens) and polymeric networks. Here, we investigated the extreme energy dissipative behavior of LCEs over a wide range of strain rates for a fundamental understanding of their strain-rate dependent energy dissipation behaviors. We synthesized main-chain LCEs based on a two-stage thiol–acrylate reaction to examine the effect of the ordering of mesogens and chain alignment on the rate-dependent dissipation behaviors. The impact energy dissipation capability was characterized through bistable structures consisting of LCE beams to have enough time for measuring the energy dissipation by reducing the peak values of acceleration even under high impact. We found that LCEs exhibited 1 to 3 orders of magnitudes higher energy dissipation compared to conventional elastomers (Polydimethylsiloxane (PDMS)) with an increase of the energy dissipation as a function of strain rate in the tested range of 10^-3 to 10³ (1/s). Moreover, the strain-rate dependency was able to be controlled by the degree of alignment of mesogens within LCEs through simple control of the applied tensile strain during crosslinking. Then, we performed finite element analysis (FEA) to find the relationship between the arrangement of mesogens and strain-rate dependent energy dissipation based on Prony series parameters obtained from dynamic mechanical analysis. The results from FEA analysis showed good agreement with data from experiments at different strain rates. We envision that our findings can contribute to rationally designing and synthesizing LCE materials and structures with desired strain rate-adaptive energy dissipating capability, which will be useful for various applications including impact mitigation for aerospace, automotive, and personal protection materials and structures.

Structural reciprocity (SR) is a concept of self-supporting of load-bearing bars that together form larger mechanical resistant structures [1]. Dating back to the Neolithic, SR was found from native tepees and tents, to old bridges like the one over the Rhine that was built in the Roman Empire by Julius Caesar, and even in the drawings of Leonardo Da Vinci [2]. Commonly seen in floors or roofs, SR is also present in art, religious symbols and decorative objects. Although SR involves a mutual exchange of action and reaction between parts of the whole structure, it is also known to rely on a perfect symmetric relationship between them [1]. A structure having SR is called reciprocal structure (RS). The main characteristics of a RS are, first, the role of supporting and being supported should not occur in the same part of the structure, i. e., they must be separated, not overlapping like in truss bars. Second, each element of a RS must, at the same time, support the others and being supported by the others. These two properties make beams and two-dimensional materials very much appropriated to build RSs. Here, we propose a simple nano version of a Da Vinci’s RS based on graphene nanoribbons. The stability and resistance against mechanical impacts (ballistic projectile) were investigated through fully atomistic molecular dynamics (MD) simulations. We considered different structures with three and four joins with and without RS topologies. Our MD results showed that structures with RS topologies are more impact resistant than the ones without SR, despite the fact that the used graphene nanoribbons are highly pliable. We discuss these results in terms of building self-sustained and resistant nano-domes and nanocages and possible applications in nanoengineering.

[1] A. Pugnale and M. Sassone, Nexus Netw. J. 16, 9 (2014).
[2] L. da Vinci, Il Codice Atlantico della Biblioteca Ambrosiana di Milano. Florence: Giunti (2000).

The ability to engineer micro-granular crystals so that their constituent microspheres can be positioned within lattices as desired would enable the control of extreme stress-wave propagation behavior due to the nonlinear stiffness interactions between their microspheres. Whereas engineered granular media that consists of macro-sized spheres (>1mm) support Hz-kHz stress waves for audible applications, engineered micro-granular crystals support MHz-GHz stress-waves for more extreme high-energy applications. Such applications include armor that controls or redirections the propagation of shock waves caused by explosions or high-speed impacts or acoustic lenses that focus high-frequency phonons for underwater imaging, noninvasive surgery, and material damage detection technologies.

Here we propose an approach that utilizes automated optical tweezers to simultaneously assemble many 1-10μm-sized spheres at their intended locations within 3D micro-granular crystals. Unlike existing self-assembly, directed self-assembly, or electrophoretic deposition approaches that can fabricate only a limited selection of ordered micro-granular crystal lattices, the proposed optical tweezers approach possesses the flexibility to arrange microspheres of different materials and sizes anywhere within a lattice to achieve high control over extreme stress-wave propagation behavior. Moreover, the approach is automated to increase the placement accuracy and precision of the microspheres and to dramatically increase the speed of assembly so that large test samples can be fabricated within reasonable build-times. Once fabricated these samples, which can’t be made any other way, are tested to collected data that is used to learn about the extreme mechanics that govern the behavior of these crystals.

We aim to present these results for both quasi-static compression tests and for dynamic laser-induced impulse force tests conducted on differently packed micro-granular crystal lattices. The data collected is used to inform course-grain molecular dynamic simulation models that when matched to accurately mimic the experimental results are used to simulate numerous ‘what-if’ scenarios aimed to uncover what effects (e.g. sphere location, size, sphericity, surface roughness, friction, electrostatic forces, adhesive forces, asperities, surface charges, and hydrodynamic effects) govern the extreme mechanics of such crystals. This work sheds light on what mechanisms give rise to the nonlinear stress-wave propagation behaviors of micro-granular crystals as well as those that explain how they deform and ultimately fail.

The design and fabrication of double hollow tube lattice architectures is pursued to enhance both the stiffness and recoverability of lattice materials. In essence, these structures correspond to the superposition of two truss lattices made from hollow tubes. Both trusses feature the same lattice parameters. As a result, the smaller diameter structure can be built inside a second structure of larger tube diameter. Design maps are developed through theoretical and computational analysis that express the stiffness of the proposed double hollow tube lattice as a function of the relative density and the outer-to-inner tube radius. Nanolattices with wall thicknesses ranging from 27 to 110 nm are fabricated from alumina through direct laser writing of a polymer scaffold, atomic layer deposition of alumina followed by removal of the polymer phase. In-situ compression experiments are performed on both single- and double tube configurations. While both configurations provide about the same stiffness of the same weight, a higher recoverability is observed for the new double hollow tube configurations.

Nonwoven fibrous mats are found in many applications such as cellulose nanofiber paper, electrospun polymeric fibrous scaffolds used in tissue engineering, filtration and insulation materials etc. In these materials, fibers are held together by adhesion and friction acting at inter-fiber contacts. We present a study of the mechanical behavior of random, non-crosslinked fibrous mats of nanofibers stabilized by inter-fiber adhesion. Fibers of various degrees of tortuosity, and of infinite and finite length are considered. The variation of structural parameters such as the mat thickness and the mean segment length between contacts with the strength of adhesion is determined. Mechanics in such systems has an energetic component, associated with stored strain energy in fibers, and a dissipative component, associated with friction. The dissipative component is dominant. The response to tensile loading of the mats has three regimes: a short elastic regime in which no sliding at contacts is observed, a well-defined sliding regime characterized by strain hardening, and a rapid stiffening regime at larger strains. Networks of finite length fibers loose stability during the second regime of deformation and do not exhibit stiffening. We relate the parameters that describe this behavior and the network strength to the structural network parameters. These results are relevant for the design of electrospun mats and other planar fibrous non-cross-linked networks.

Phase transforming cellular materials are architected materials whose unit cells have multiple stable configurations. If designed correctly, these materials can absorb energy by allowing non-equilibrium release of stored energy through controlled elastic limit point transitions as the cells transform between different stable configurations. Most previous works on similar architected materials with elastic limit point transitions have focused on material behavior under mechanical loading. The stress-induced phase transformation in these materials takes place under an applied mechanical load, while the reverse transformation can be driven either by elastic energy stored in the material during the forward transformation or by an external force acting in the direction opposite to that of the force applied during the forward transformation. In this work, we present and discuss a new family of PXCM designs that can undergo both stress-induced and temperature-induced phase transformation, mimicking in most part shape memory alloys.

Beams, shells and plates are used as the basic building blocks of lattice materials. It is shown through combined theoretical and numerical analysis that the choice of the elementary building block strongly influences the mechanical performance of mechanical metamaterials. Increases in stiffness of up to 200% are observed when building isotropic lattices from plates instead of beams (while keeping the mass unchanged). Furthermore, it is shown that shell-lattices are ranked between truss- and plate-lattices as far as stiffness and yield strength is concerned. However, from the point of view of fatigue strength and impact energy absorption, shell-lattices might have a substantial advantage. In addition to deriving the effective yield surfaces through homogenization, the plastic anisotropy of elastically-isotropic metamaterials is analyzed in detail. All theoretical results are also validated through detailed finite element simulations. Selected metamaterials are additively fabricated from polymers using two-photon lithography and from stainless steel using selective laser melting. Static experiments are performed to confirm the theoretical modulus and yield stress estimates for uniaxial tension, compression and shear. Furthermore, dynamic experiments are performed on a Split Hopkinson Pressure Bar (SHPB) system to determine the effect of strain rate on the specific energy absorption response of mechanical metamaterials of 20% relative density.

We present the ballistic energy absorption characteristics of thin (100-250 nm) mats comprised of an interconnected network of multiwall carbon nanotubes (MWCNT) investigated using a micro-projectile impact test with incident velocities from 300-900 m/s. The quasi-static properties of the MWCNT mats are quite modest but at the extreme strain rates and large strains of ballistic impact, a distinctive set of morphology dependent deformation mechanisms leads to extraordinary energy absorption. As the spherical projectile engages the film, the unoriented bundles of MWCNT tubes rotate, straighten, align and translate into the impact region, dissipating the kinetic energy of the projectile via frictional forces and elastic stretching energy of the principal tubes until axial fracture occurs. The specific energy absorption depends on impact velocity and film thickness and can range from 9-12 MJ/kg, much greater than any other material.

Self-assembly is becoming increasingly important as a bottom-up method for building novel structures and controlling functions at length scales ranging from the nanometer to millimeter. Various driving forces for self-assembly have been studied at these length scales, including gravity, magnetic force, electrostatic interaction, entropy, and capillary interaction. Among these, capillary effects have received particular attention at the microscale because of their relatively significant magnitude, tunability, and simplicity, and a number of studies have demonstrated the formation of complex two- and three- dimensional structures by this method. However, such studies have been limited to isolated structures like pillars and posts where the neighboring units interact with each other only through capillary force, limiting the complexity of the assembly architectures can be achieved. In this project, we study the response of connected elastic lattices with the capillary force introduced via rapid evaporation of volatile organic solvents. Such micro lattices, different from micro-pillars, possess strong interaction between unit cells through capillary forces as well as the elastic coupling from the geometric compatibility of the underlying lattices, giving rise to programmable lattice transformations from the coalescence of the adjacent edges. Furthermore, as compared to micro-pillar systems, lattice structures are known to exhibit stronger mechanical strength and have a broader control space over the surface properties accompanied by lattice transformations.

Soft robots have been shown to be able to perform a variety of tasks, including walking, grasping, crawling and swimming. However, to achieve all these functions their constituent actuators need to be inflated sequentially. In this work, we demonstrate a new generation of flexible machines that can be realized by harnessing propagation of non-linear elastic waves. To demonstrate this concept we focus on an iconic stretchable toy that has captivated children and adults all over the world: a Slinky. We show that by exploiting the propagation of nonlinear pulses this popular toy can be transformed into a robot capable of moving around without the need of legs.

From earthworms, snakes and caterpillars, to slugs and snails, many creatures successfully combine the flexibility of their body with the ability to locally manipulate frictional forces to move in complex environments without the need of legs. These limbless organisms have recently inspired the design of compliant robotic systems that take advantage of elastic deformation to crawl and maneuver through confined spaces. For example, snake-inspired robots with locomotive skills that surpass more conventional designs have been designed by incorporating anisotropic friction into soft elastomeric inflatable structures. Moreover, soft robots capable of worm-like motion have been realized by sequentially actuating contracting segments to induce peristalsis. However, these machines require multiple inputs to achieve the desired peristaltic motion, as each segment must be addressed independently according to a specific preprogrammed sequence.

Here, inspired by the retrograde peristaltic waves observed in earthworms, we show that the propagation of non-linear elastic waves in flexible structures provides opportunities for locomotion. To demonstrate the concept, we focus on a Slinky -- an iconic stretchable toy that has captivated children and adults all over the world -- and use it to realize a pulse-driven robot capable of propelling itself. Our simple machine is built by connecting the Slinky to a linear actuator and using an electromagnet and a plate embedded between the loops to initiate nonlinear pulses that propagate from the front to the back. Notably, we find that the directionality of these pulses enables our simple robot to move forward. Moreover, our results indicate that the efficiency of such pulse-driven locomotion is optimal when the initiated waves are solitons - large amplitude (non-linear) pulses with stable shape and constant velocity along propagation. As such, our study expands the range of possible applications of solitary waves and demonstrate that they can also be exploited as simple underlying engines to make flexible machines move.

Architected materials represent an area of active research because they exhibit exotic properties such as negative Poisson’s ratios and negative refractive indices, and decouple historically correlated material properties such as strength vs. density and thermal conductivity vs. stiffness. Most architected materials reported to date are passive in the sense that they have a prescribed geometry fulfilling a single functionality. It was recently shown that architected materials can be reconfigured by mechanical deformation and instabilities, hydration-induced swelling, and magnetic actuation. Such smart, stimulus-responsive materials can make a long-lasting impact on adaptive, deployable, and dynamically tunable devices if they could overcome existing challenges of (1) requiring bulky external control, (2) toggling between “on” and “off” states, and (3) reverting to original configuration once external stimulus is removed. Most of these reconfigurable systems are small and idealized; scaling them up requires substantially increasing the number of repeating units that would inevitably introduce inhomogeneities similar to defects, gradients or grain boundaries that govern properties of classical materials.

Here we demonstrate a new mechanism to dynamically reconfigure architected materials by exploiting electrochemically driven alloying/dealloying reactions to induce continuous, non-volatile and reversible structural transformations. We used the alloying couple of silicon and lithium as a prototype system because Si is a high-capacity battery electrode material notorious for its ~300 % volumetric expansion after full lithiation. Buckling instabilities were observed in Si nanowires and etched honeycomb patterns during lithiation but they have not been systematically investigated as a design tool to achieve structural reconfiguration. Leveraging on Si’s mechanical resilience at small scales, we designed and fabricated Si-coated microlattices purposely structured to promote lateral in-plane buckling. In situ lithiation/delithiation observations revealed cooperative buckling among neighboring beams that reconfigures the tetragonal unit cells into curved ones with pairwise opposite concavity. Through experiments and finite element modeling, we discovered that the bistability of in-plane buckling leads to formation of multiple lateral domains separated by distorted domain boundaries, with domain sizes and distribution governed by defects and lithiation rate. We analyzed this phenomenon using a statistical mechanics approach analogous to the Ising model, and designed artificial defects to effectively program domain boundaries to emerge in prescribed patterns upon lithiation. Understanding and controlling defects provides a pathway to drive the dynamic response of architected materials along a particular trajectory. This new class of electrochemically reconfigurable architected materials has significant implications as it creates new vistas in designing dynamic mechanical metamaterials with novel stress-relief mechanisms and tunable phononic band gaps.

Reference:
Xia, X. et al. Electrochemically Reconfigurable Architected Materials. Nature 572, in press (2019)

Complex deformation of a polymer jet appears in many manufacturing processes, such as 3D printing, electrospinning, blow spinning, and rotary jet spinning. In these applications, a polymer melt or solution is first extruded through an orifice and forms a jet. The polymer jet is then dynamically deformed until the polymer solidifies. The final product is strongly affected by the deformation of the polymer jet. And the deformation is strongly affected by the viscoelasticity of the polymer. Here we develop a beam theory to incorporate both the nonlinear viscoelasticity and the bending/twisting stiffness of a polymer jet. As a demonstration, we study the formation of a polymer fiber under strong centrifugal force, a fiber manufacturing process known as rotary jet spinning.

Epithelial monolayers are very simple yet very important animal tissues with crucial mechanical functions. They consist of cohesive and highly organized cells that adhere to each other and often to an underlying matrix to form a barrier that lines internal and external organ surfaces, controls transport of gas or nutrients, protects us from pathogens or desiccation, and is involved in morphogenesis. Seen as materials, epithelial monolayers are truly remarkable. They can maintain barrier integrity while increasing their lateral area by 10-fold or while deforming into highly curved 3D shapes. In apparent contradiction, they can dynamically remodel by selectively disengaging cell-cell adhesions to heal wounds or to transport cellular materials through organized active flows during morphogenesis. This tunable mechanical behavior relies on a hierarchical organization. At a sub-cellular scale, structural proteins dynamically self-organize into cytoskeletal networks and adhesion complexes, which transduce chemical energy into mechanical work and determine cellular architecture and mechanics. In turn, groups of cells self-organize to form tissues and homeostatically maintain these tissues in a dynamical steady-state. A major question in mechanobiology is to understand the multi-scale self-organization of adaptable functional units at different scales and their integration to perform mechanical functions including morphogenesis, homeostasis, or motility. I will discuss our theoretical efforts to understand various aspects of cell and epithelial tissue mechanics and morphogenesis starting from sub-cellular models of the cytoskeleton and cellular adhesion. I will illustrate how subcellular dynamics, including molecular turnover and active force generation, determine mechanical properties at a tissue scale including solid-like or fluid-like rheology, active superelasticity, or pulsatile dynamics and ratcheting remodelling.

We investigate instability-induced pattern transformations in soft particulate composites under large deformations [1]. The onset of instabilities are detected by numerical Bloch-Floquet techniques and experiments on 3D-printed specimens. We find that the system of stiff inclusions periodically distributed in a soft elastomeric matrix experiences dramatic microstructure changes upon the development of elastic instabilities. We observe experimentally the formation of microstructures with antisymmetric domains and their geometrically tailored evolution into a variety of patterns of cooperative particle rearrangements. In combination of numerical instability analyses and experiments, we find that the formation of domain occurs in macroscopic instability regime, while microscopic instability induces the cooperative new pattern of particles rearranged in a wavy chain. Our observation shows that these patterns can be tailored by tuning the initial microstructural periodicity and concentration of the inclusions. Thus, these fully determined new patterns can be achieved by fine tuning of the initial microstructure. Our findings open new ways for developing reconfigurable mechanical metamaterials that can find applications in a large variety of fields from acoustic metamaterials, actuators, and soft robotics to morphing devices remotely controlled by external fields for biomedical applications.

References
[1] Li J, Tarkes DP, Slesarenko V, Goshkoderia A, Rudykh S. Domain formations and pattern transitions via instabilities in soft heterogeneous materials. Adv. Mater. 2019, 31(14): 1807309.

The problem of a beam on an elastic foundation has a long history in engineering mechanics. Its deformation and onset of buckling behavior have been employed as a surrogate for understanding an extensive list of technologically important applications, including sun-kinking of railroads, pipeline transportation engineering, and thin films on hard or soft substrates in microelectronic and biological applications. Despite this history, the global bifurcation behavior of the problem has not been fully explored. Indeed, the complexity of the equilibrium solution set will come as a surprise to many practicing and research engineers.

With an eye toward the study of creasing and deformation-localization problems in soft materials, this work studies the behavior of an inextensible infinite Euler-Bernoulli beam that is subjected to a compressive axial force and connected to a nonlinear (polynomial) elastic foundation. We seek global post-bifurcation equilibrium paths and their stability as a function of the applied force. All bifurcating paths (stable and unstable) are of interest due to the possibility of stable segments occurring on any path in the deep post-bifurcation regime of the global solution set. However, standard solution techniques (incremental Newton-Raphson coupled with strategically chosen imperfections) are foiled by the complexity of the problem's bifurcation behavior. Thus, it becomes necessary to employ sophisticated equivariant bifurcation theory and numerical methods, based on group theory, to tame the complexity and systematically obtain the desired results. These methods will be reviewed and their application to the problem at hand will be demonstrated as a typical example of their power to solve and bring order to an otherwise intractable set of equations. Finally, the results will be interpreted physically to explain the existence of localized-deformation and crease-like behavior. Parallels with phase transformation problems will also be drawn.

Although the study of the effect of shape and geometry on the mechanical response of solid objects has a long history, the surge of modern techniques to fabricate structures of complex form paired with our ability to simulate and better understand their response has created new opportunities for the design of architected materials with novel functionalities (also referred to as metamaterials). Since the properties of architected materials are primarily governed by the geometry of the structure (as opposed to constitutive ingredients at the material level), I’ll show that their deformation and instabilities can be harnessed to achieve new modes of functionality, including motion, energy trapping and releasing, wave guiding and shape changes.

Origami is not rigid; facets and hinges bend and stretch. This compliance enriches the physics of origami, leading to mechanical multistability, and opens up a rich design space to control shape-morphing in thin materials.
We explore this new design space starting from an undulating "origami" sheet. This simple material switches reversibly between many stable shapes via snap-through instabilities. Its reshaping results from a hierarchical, geometry-mediated competition between different sources of compliance, which we demonstrate via experiments and minimal computational models.

Origami structures are finding numerous technological applications including lightweight composites, soft robotics, metamaterial design, and deployable space structures, where their nonlinear deformations and instabilities are leveraged to form novel material behavior. The contrast in material (stiffness mismatch among deformation modes) and geometric (slender elements) properties in origami structures can lead to highly nonlinear mechanical behavior with unique macroscopic properties, such as multi-stability. We have recently integrated our nonlinear origami truss model with a robust continuation solver for bifurcation detection and branch tracking to map multistable energy landscape in order to characterize the role of discrete fold stiffness distributions. Towards the goal of developing more robust design tools to navigate this non-convex design space, involving energy landscapes with many bifurcations and stable equilibrium points, we investigate how changing stiffness distributions (stretching, folding, and facet bending stiffness) and non-zero fold reference angles shift the bifurcation and equilibrium points. These insights are leading to the development of a generalized design tool for origami structures exhibiting many stable configurations for deployability and lightweight material design.

Locomotion strategies employed by unicellular organism are a rich source of inspiration for studying mechanisms for shape control. The ones used by unicellular organisms are particularly interesting because they are invisible to the naked eye, and offer surprising new solutions to the question of how shape can be controlled, and for which function.
In recent years, we have studied locomotion and shape control in several biological systems using a broad range of tools ranging from theoretical and computational mechanics, to experiment and observations at the microscope, to manufacturing of prototypes.
A particularly interesting case study is provided by Euglena gracilis. This unicellular protist is particularly intriguing because it can adopt different motility strategies: swimming by flagellar propulsion, or crawling thanks to large amplitude shape changes of the whole body (a behavior known as “metaboly”, or “amoeboid motion”). The shape changes required for the two strategies are completely different and consist of bending waves along the flagellum in swimming motility, peristaltic expansion/contraction waves of the body in crawling motility. Interestingly, however, both the general morphing principle and its embodiment in the microscopic architecture of the active structural elements enforcing the shape changes are the same. Shape changes are achieved by Gaussian morphing, the paradigm by which curvature can be produced by differential in-plane stretches of the mid-surface (the tubular shell of the flagellum in one case, the body envelope in another). These stretches are in turn produced by molecular motors walking along microtubules, and causing the bending of microtubule bundles (flagellum) and the twisting of pellicle strips (cell envelope).
The behavior displayed by Euglena has been observed under the microscope, and reproduced quantitatively thanks to mathematical and computational models. The detailed understanding of the mechanics of the shape-shifting mechanism in terms of the body architecture provides the basis to design new engineering devices, based on bio-inspired, morphable structures.
We will survey our most recent findings obtained also in collaboration with M. Arroyo, G. Cicconofri, G. Noselli, within this stream of research which has been supported by ERC Advanced Grant 340685-MicroMotility.

Stretchable materials—elastomers, hydrogels, ionogels, and organogels, along with their hybrids—are under intense development to enable numerous and far-reaching applications. In developing a material for a load-bearing application, attention inevitably falls on the resistance of the material to the growth of a crack, characterized by toughness under monotonic load, and by threshold under cyclic load. Many methods have been discovered to enhance toughness, but they do not enhance threshold. Here we describe a principle of stretchable and fatigue-resistant materials. To illustrate the principle, we embed unidirectional fibers of a soft and stretchable material in a matrix of a much softer and much more stretchable material, and adhere the fibers and the matrix by sparse and covalent interlinks. When the composite is cut with a crack and subject to a load, the soft matrix shears readily and delocalizes the high stretch of a fiber over a long segment. The material achieves a fatigue threshold more than one order of magnitude higher than natural rubber. The principle of stretchable and fatigue-resistant materials is applicable to various materials, layouts, and methods of fabrication, opening an enormous design space for general applications.

Aero-engine titanium components endure extremes of loading which may be both mechanical and thermal, and which are known to give rise to the phenomenon of cold dwell fatigue [1]. This is argued to develop through combined effects of crystallography, localised slip due to plasticity and creep [2], and the redistribution of stress, often termed load shedding [3], which occurs during the stress hold stage of flight cycles. Evidence is emerging that a potentially important aspect of the deformation occurring is that due to negative creep. This is the activation of localised reversed time-dependent slip resulting from back stress development in titanium alloy (IMI834) and occurs interestingly on partial stress unloads (which are representative of in-flight loading cycles). The negative creep is potentially helpful in contributing to the thermal alleviation of the dwell fatigue phenomenon, and this is addressed in this paper.

A new thermo-mechanically coupled discrete dislocation plasticity (DDP) formulation is established, incorporating thermally activated dislocation escape and creep, which is integrated with experiments on polycrystalline IMI834. The experiments consider dwell fatigue cycling without and with a partial unload, as well as the inclusion of a thermal cycle [4], which are investigated with the DDP model.

The mechanistic basis of negative creep is shown to be the establishment of dislocation pileups and back stress development such that on partial unloading, reversed creep occurs by thermally-activated dislocation escape and reverse glide during secondary stress hold. Anomalous cyclic strain accumulation in both isothermal and anisothermal stress-loaded alloy IMI834 is thus explained by negative creep.

The magnitude of predicted macroscopic negative cyclic creep strain in dwell fatigue loading is found to be consistent with independent experimental literature. The inclusion of the thermal cycle including elevated temperature facilitates further negative creep by promoting dislocation escape via thermal activation. The experimentally observed ‘anomalous’ macroscopic cyclic strain accumulation in both isothermal and anisothermal loadings has been explained by negative creep at the dislocation scale. The latter also provides the underpinning explanation for the beneficial effect of elevated temperature excursions in diminishing cyclic creep accumulation and hence reducing dwell fatigue sensitivity in titanium alloys by thermal alleviation.

Keywords: Negative creep; Thermal alleviation; Discrete dislocation plasticity; Titanium alloys; Dwell fatigue


Reference
[1] W.J. Evans, M.R. Bache, Dwell-Sensitive Fatigue under Biaxial Loads in the near-Alpha Titanium-Alloy Imi685, Int J Fatigue 16(7) (1994) 443-452.
[2] J.K. Qiu, Y.J. Ma, J.F. Lei, Y.Y. Liu, A.J. Huang, D. Rugg, R. Yang, A Comparative Study on Dwell Fatigue of Ti-6Al-2Sn-4Zr-xMo (x=2 to 6) Alloys on a Microstructure-Normalized Basis, Metall Mater Trans A 45a(13) (2014) 6075-6087.
[3] Z.B. Zheng, D.S. Balint, F.P.E. Dunne, Discrete dislocation and crystal plasticity analyses of load shedding in polycrystalline titanium alloys, Int J Plasticity 87 (2016) 15-31.
[4] Rolls-Royce, Private Communication, 2018.

The development of novel fabrication techniques for amorphous materials, including metallic glasses and high entropy alloys, is important for various structural and biomedical applications. The processing routes often involve mechanical and thermal treatments of disordered alloys that lead to changes in potential energy, microstructure as well as mechanical and physical properties. In this presentation, we discuss recent results of molecular dynamics simulations of three loading protocols: oscillatory deformation, flash annealing, and elastostatic loading of metallic glasses. In each case, the simulations are performed in a wide range of processing conditions and the optimum control parameters for the most relaxed and rejuvenated states are determined. The structural analysis is performed by identifying clusters of mobile atoms with relatively large nonaffine displacements. It is generally found that the most rejuvenated glasses are characterized by reduced elastic moduli and lower yield stress, thus leading to improved plasticity. These results are useful for the thermomechanical processing of metallic glasses with optimized mechanical properties.

Materials can fail when subjected to cyclic loading at stress levels much lower than the ultimate tensile strength or yielding limit, which is known as mechanical fatigue. Understanding the fatigue behavior is critical for any emerging material in order to evaluate its long-term dynamic reliability. Two-dimensional (2D) materials have been widely applied to mechanical and electronic applications, where they are commonly subjected to cyclic stress. However, the fatigue life and underlying damage mechanisms of these atomically thin, nearly defect-free, materials are unknown. Here we show the first fatigue study of freestanding 2D materials, in particular graphene and graphene oxide (GO). Monolayer and few layer graphene were found to exhibit ultrahigh fatigue life of more than one billion (10⁹) cycles at large mean stress of 71 GPa. Such a remarkable fatigue life is higher than that of any material reported to date at similar stress levels. Monolayer graphene exhibits global and catastrophic fatigue failure without progressive damage, and the failure is attributed to bond reconfiguration near the defective site. The presence of functional groups on GO imparts a local and progressive fatigue damage mechanism. The fatigue life of GO was found to diminish significantly when the material is scaled up in thickness (10s of layers). This work not only provides new fundamental insights into the widely observed fatigue enhancement behavior of graphene-embedded nanocomposites, but also serves as a starting point for the mechanical dynamic reliability evaluation of other 2D materials.

Measuring thermodynamics and kinetics for single grain boundaries has long been experimentally laborious and challenging. Developments in small scale experimentation present oppurtunities to develop new more efficient approaches to obtain this important data. This presentation will discuss ultahigh temperature in situ transmission electron microscopy based mechanical testing experiments designed to measure grain boundary diffusivity, surface diffusivity, surface energy, and grain boundary energy. The general approach, based on laser heating, is also demonstrated for more conventional stress-strain measurements. The approach is applied to zirconium oxide at temperatures between ≈1700 ^oC and ≈2500 ^oC.

Two continuum frameworks will be presented to simulate the deformation of polycrystals under different scenarios using coupled crystal plasticity finite element models.
The first one is a model to study polycrystals at elevated homologous temperatures under relatively small stresses, where deformation is controlled by the diffusion of vacancies to-and-from the crystal bulk to the grain boundaries as well as the diffusion within the grain boundary. This creep model is formulated as a coupled problem of diffusion and mechanical equilibrium. It is assumed that vacancy emission and absorption within grain boundaries is due to the climb of grain boundary dislocations, and therefore the creep kinetics is derived from physically-based mechanisms. The effect of grain size, applied stress, and grain boundary dislocations mobility will be presented.
The second framework is a fully coupled thermo-mechanical framework including thermal strains, temperature dependency of the crystal behavior and heat generation by dissipation due to plastic slip. The framework allows to bridge the plastic deformation and its rate with heat diffusion at the microscale. The model is used to simulate the machining process accounting for the effect of the microstructure heterogeneity, in this case the presence of soft and stiff grains.

The past decade has seen enormous development of buckled materials, materials in which one or more elements is in the form of a slender structure that buckles under compression. Such buckled materials are being considered for flexible electronics, as surfaces with tunable adhesion and friction, or materials with unusual mechanical properties such as a negative Possion ratio. In theoretical and experimental analyses, such situations are almost always treated within an elasticity framework, i.e. all materials are generally regarded as elastic, often linearly elastic. Here we examine plasticity effects in such buckled materials.

The situation considered is of a thin film of stiff plastic material (polyethylene) attached bonded to a softer elastomeric film. Upon stretching this bilayer composite, the plastic thin film stretches irreversibly, whereas the elastomer stretches reversibly, thus creating a “strain mismatch”. Upon releasing, the elastomer imposes compressive stress on the polyethylene film which then buckles. We explore the effects of film plasticity in this situation by experiment and numerical simulations. We show that plasticity has numerous effects beyond simply causing the “strain mismatch”: the film can yield in-plane during release; the film can buckle during release to form a strongly wrinkled surface; the film can yield and form plastic hinges during wrinkling. We show that plasticity can be exploited to achieve wrinkled surfaces with aspect ratios much higher than possible with analogous elastic systems. We also point out that the tendency of the plastic film to undergo necking sets a lower limit on the rubber yield stress necessary to achieve uniform wrinkling. Although these results are obtained for the specific case of a layered thin film geometry, the insights apply in general to all buckled materials in which a plastic material is combined with an elastic one.

Thermal environment has been considered as one of the most important extreme conditions that affect mechanical properties of materials. Particularly, the mechanical properties at a low temperature has been extensively studied due to their importance in space exploration or marine engineering. The decrease in temperature usually causes the ductile-to-brittle transition (DBT) in body-centered-cubic (bcc) metals, which must be prevented to avoid the catastrophic failure of engineering devices working at a low temperature. Recently, extreme mechanics at the micrometer scale began to receive a great attention due to the strong interests in the development of micro-electro-mechanical system (MEMS) working under extreme conditions. If these devices operate at a low temperature, it is critical to prevent the DBT in micron-sized components. At the micrometer-scale, mechanical behaviors of single crystalline metals are remarkably differently from those at bulk scale due to the size effects. Source-controlled plasticity causes the size-affected strength as well as the intermittent plastic flow. In order to prevent the DBT at the micrometer scale, it is critical to understand how the source-controlled plasticity is influenced by the decrease in temperature. For bcc metals, the lattice resistance, cross-slip of screw dislocation, the evolution of dislocation structure, and the presence of free surface should be critical factors to affect the DBT at the micrometer scale.
In this work, therefore, we present our recent results on the DBT in a [0 0 1] bcc niobium (Nb) single crystal at the micrometer scale. The dog-bone shaped tensile samples with 2μm in width were fabricated via focused-ion beam milling and were tested at the nominal strain rate of 10^-3 s^-1 at 298, 100 and 56K using custom-built in-situ cryogenic micromechanical testing system. Stress-strain data showed the extensive ductility (~25%) at 298K but the limited ductility (~6%) at both 56 and 100K. Post-mortem scanning electron microscopy revealed that samples tested at 298K underwent uniform and homogeneous plastic deformation while samples tested at 56 and 100K underwent highly localized plastic deformation followed by brittle fracture that was initiated at slipped regions. Thus, micron-sized niobium single crystals indeed show the ductile-to-brittle transition. Post-mortem transmission electron microscopy showed the significantly high dislocation density in samples tested at 298K, but almost no dislocations in samples tested at 56K. These results imply the strong suppression of dislocation multiplication at a low temperature. The reduction of mobile dislocation, i.e., dislocation starvation, would induce the DBT in Nb at the micrometer scale. This DBT mechanism differs from that at bulk scale, which is caused by the decrease in mobility of screw dislocation. At the micrometer scale, all mobile dislocations seem to be annihilated at the free surface. Brittle fracture would occur due to the stress concentration at slipped region before dislocation nucleation occurs. Note that this presentation reports the first observation of DBT in bcc metals at the micrometer scale and in cryogenic environments. Our results will not only enable a deeper understanding of the combined effects of sample dimension and temperature on plasticity and fracture processes in bcc metals, but also guide a design of mechanically-robust materials used at the micrometer scale and at a low temperature.

One of the great concerns in fusion reactor operation is disruption events that deliver 10s to 100s of GW/m² of heat over microseconds to first wall materials, severely damaging plasma facing components, and therefore hamper their long-term durability. Specifically, the diverter region is one the most sensitive parts of the plasma facing portion of the reactor. In addition to causing damage to the vessel wall, disruption events can produce debris or lead to the diffusion of high Z material into the fusion plasma region.
Tungsten based materials are promising as plasma for use in plasma facing components because of its high melting point, high thermal conductivity, and low tritium retention. However, it also has disadvantages such as radiation induced embrittlement and thermally driven high ductile to brittle transition. Most importantly, erosion by particle bombardment is great issues in off normal, i.e. disruption, events which gradually weaken the function of the vacuum vessel. To date, different types of tungsten, alloys, and ceramics have been proposed to minimize these challenges in first wall materials selection.
In this study, tungsten carbide was used because of having comparable characteristics that of tungsten thus could perform as a plasma facing material. At the same time, tungsten metal was used as a reference material to compare the emerging features from the microscale to the macroscale. HELIOS, the electrothermal plasma experiment was used to replicate the small angle, high heat and particle flux like conditions that these materials will experience in off normal events. As prepared tungsten (W) and tungsten carbide (WC) samples were subject to HELIOS device with the controlled variables such as heat flux, particle flux, sample temperature, types of ions and their energy, and duration. Scanning Electron Microscope was used to visualize the microstructure of sample both at the surface and depth. Similarly, Energy Dispersive Spectroscopy (EDS) was carried out to see the compositional profiles both at the surface and along the depth direction. To see the cross-sectional view, surface treatment including cut, mount and polishing is necessary to take advantage of Focused Ion Beam (FIB) technique. Atomic Forces Microscope (AFM) was conducted on the surfaces of the samples to see the emerged surface morphology due to plasma interaction. X-ray Diffraction (XRD) patterns were recorded to reveal the possible alteration of lattice structure after exposure. Nanoindentation was performed to see any deviations in microhardness to obtain the microscopic mechanical strength information. Collection of data were then connected to draw the plasma material interaction at the surface and bulk materials. Based on the concentration profiles along the depth dimension, it is available to quantify the penetrative behavior of tungsten carbide and tungsten in these environments. The overall damage on tungsten and tungsten carbide are analyzed and correlated to materials characteristics.

Indentation cracking of 6H-SiC up to 700 °C were introduced via a high-temperature nanoindenter equipped with a Berkovich tip. Fracture toughness (K_c) for indents with radial cracks (<400 °C) were calculated based on the Ouchterlony-modified Laugier equation. However, this equation does not apply for indents above 400 °C, due to the more complicated crack morphology revealed by FIB tomography. Here, an alternative method for calculating fracture toughness based on nanoindentation and FIB tomography was proposed. Good agreement with literatures was found for RT but slightly overestimates for 700 °C, suggesting the dissipation of energy through plastic deformation. High-resolution electron diffraction (HR-EBSD) was also conducted to measure the residual stress/strain distribution and geometrically necessarily dislocations (GNDs) density around the nanoindents. The combined effort of FIB tomography and HR-EBSD will provide better understanding of the thermally-controlled deformation process.

Multicomponent alloys (MCAs) represent a new class of alloy that are composed of multiple principal elements, contrasted to conventional alloys that rely on a single principal element with additional alloying elements in relatively small abundance. Despite their nominal chemical disorder, several studies have reported short range order (SRO) in MCAs – i.e. preferential bonding, local elemental enrichment and/or clustering – and such SRO may have broad implications for MCA performance. To tackle this problem, a suite of spatially resolved, electron imaging, diffraction, and spectroscopy techniques is used to correlate coordination chemistry and local order with microstructural evolution and related dislocation phenomena. Specifically, microstructures of so-called medium entropy alloys and high entropy alloys subjected to a variety of deformation regimes are analyzed. The resulting microstructures are quantified using diffraction-based techniques, and ultimately compared to simulations of similar alloy families in order to determine the extent to which localized dislocation-based phenomena play a role in microstructural evolution, and how alloy chemistry plays a role in these determining factors. The techniques presented allow for the direct observation of the interplay between chemistry and microstructure, and thus, provides us with key tuning knobs for future MCA development, especially for use in extreme environments such as high strain rate deformation, corrosion, and irradiation conditions.

The hexagonal close packed (HCP) family of metals is of growing interest as it provides a potential solution to the ever-increasing variety of structural applications that require advanced materials with unprecedented combinations of mechanical properties. As an example, Mg and its alloys are promising candidates for light-weighted structural applications, e.g., aircraft, automobile, electronic, etc. Successful incorporation of Mg and Mg alloys into engineering designs is however hindered by their limited plasticity compared to that of current structural metals, which can be attributed to the anisotropic dislocation slip and twinning behavior that is associated with the HCP structure. Higher critical resolved shear stresses (CRSS) are required to activate the non-basal slip systems on the prismatic and pyramidal planes to produce an arbitrary shape change.

Over the years, the scientific community has proposed various approaches to enhance the performance of HCP metals, including introducing pre-existing nano-scale growth or deformation twins and nano-particles/precipitates in a nanostructured matrix. While these strategies and many others have led to several 100% enhancements in strength, to simultaneously attain strength and strains-to-failure values that are comparable to those of other advanced metals requires important breakthroughs in fundamental science and innovative design and processing strategies. Recently, in-situ experimental techniques in TEM and HRTEM and multi-scale simulation tools have emerged as successful strategies to enhance our fundamental understanding of the deformation mechanisms in materials. In this presentation, results from our group on Mg and Mg alloys are discussed with particular emphasis on the following topics: recent findings on twin nucleation and its relationship to dislocation slip; the influence of slip-twin and twin-twin interactions on mechanical behavior; formation of twin-twin junctions in single crystal Mg at the nanoscale and synthesis of high densities of twin-twin junctions in polycrystalline Mg at the macroscopic scale; and rare earth element alloying effects on dislocation glide and twinning activity in Mg alloys. The fundamental deformation behavior and the underlying mechanisms in Mg and Mg alloys are discussed in an effort to facilitate the development of synthesis pathways to produce low-density, high strength, high toughness HCP materials that are suitable for advanced industrial applications.

It is well understood that strength of crystals goes up as specimen size goes down with defects in the crystals decrease. When the size is down to below 30~50 nanometer, stress for defects nucleation becomes extremely high for crystals with large lattice friction resistance such as W. In such extreme condition, high stress plasticity occurs with unique deformation mechanism. I will cover experimental molecular dynamics with the in-situ high resolution transmission electron microscope for observation on the ultra-high stress induced lattice disturbance, dislocation dipole nucleation, competition between slip and twinning, and twinning nucleation of nano-sized body center cubic metals pillar (30-50 nm diameter) with defects free condition under ultra high stress above 10 ~20 GPa compressive loading

When external loads are applied to a material containing a crack, a characteristic stress field s_crack builds up around the crack. If the material does not undergo any microstructure evolution during loading, the deflection of the crack faces depends on s_crack alone. However, in polycrystalline metals, applied loads may cause the microstructure to evolve, e.g. through grain boundary migration, martensitic transformations, or differential distortion. This microstructural evolution generates complex stress fields s_mstr of its own. The displacement of the crack faces then depends on the superposition of s_crack and s_mstr, rather than on s_crack alone. This coupling between mechanically-driven microstructure evolution and internal stresses gives rise to counterintuitive crack behavior, such as closure under applied mode I (tensile) loading. This talk will present atomistic and continuum-level simulations of stresses induced at a triple junction during mechanically-driven grain boundary migration and their effect on the behavior of a nearby nano-scale crack. The simulations will be compared with corresponding experiments on fatigue-induced nano-cracks in polycrystalline Pt.

Boron carbide is superhard, but its extended engineering applications are prevented by the abnormal brittle failure arising from the high-density amorphous shear bands. To improve the mechanical properties of B₄C, we investigated how grain boundaries (GBs) determine the deformation and failure mechanism of B₄C. The deformation and failure mechanism of polycrystalline B₄C were studied using the reactive force field (ReaxFF) simulations. We found that the main deformation mechanism of nanocrystalline B₄C is grain boundary sliding, leading to a reverse Hall-Petch relationship. This GB sliding triggers the amorphous shear band formation at predistorted icosahedral GB regions with initiation of cavitation within the amorphous bands. Our simulation results are validated by the nanoindentation experiments in which an intergranular amorphous GB phase was observed due to GB sliding. Although most GB sliding events in our ReaxFF simulations lead to the intergranular amorphization, we do observe one intragranular amorphization initiates from grain boundaries (GBs) and propagates along the rhombohedral (011)[2-1-1] slip system. Combining density functional theory (DFT) and ReaxFF simulations, we found that the dislocation nucleation occurs along this particular slip system, accompanied with bond breaking of icosahedral-icosahedral bonds within B₁₁C cage. This leads to the amorphous shear band formation and intragranular amorphization. Our studies provide an atomistic explanation for the influence of GBs on the deformation behaviour of nanocrystalline ceramics, helping design strong and ductile superhard ceramics.

An overview of the mechanical behavior of metallic nanocomposites in terms of the interface-dominated microstructure will be presented. The interface microstructure will be characterized using parameters such as interface spacing, morphology, crystallographic orientation relationship, texture with respect to processing direction, interface defect structure and chemistry. A series of model Cu-X systems where X is BCC element such as Mo, Ta, Nb, V, Cr, Fe, etc will be used to highlight the novel mechanical behavior enabled by interface microstructures. The presentation will highlight the integration of theory and computational simulations across relevant length scales and in situ characterization of defect phenomena and mechanical behavior. In particular, examples from high strain rate testing and failure by shear band formation in ultra-high yield strength metallic nanocomposites will be highlighted.

Shear deformation of metallic alloys can induce deformation and microstructural changes which can be used to process materials in solid state, helping avoid melt processing and in achieving highly refined microstructures not achievable by conventional processing. In order to develop such solid phase processing methods for metallic alloys, we aim to better understand the fundamental atomic scale mechanisms of mass and energy transfer in materials under shear deformation. To achieve this aim, we employed synchrotron based in situ and ex situ high energy x-ray diffraction capabilities under high pressure with and without shear deformation, to enable real time investigation of microstructural evolution under such high deformation conditions. Such synchrotron based XRD results were correlated with detailed microstructural characterization before and after shear deformation using transmission electron microscopy and atom probe tomography, to develop a comprehensive understanding of the structural and compositional changes in the microstructure due to the shear deformation. Our results on structural and chemical modifications of several metallic alloys such as Al-Si, Cu-Nb and Cu-Ni provide new insights on the unique role of shear deformation in formation of metastable states as well as modifying the phase transformation pathways of these alloy systems. These new insights will be presented and compared with what is currently understood about role of shear deformation on generating super saturated solid solution states or self-organized nanolayers in metallic alloys.

Understanding mechanistic pathways of far-from-equilibrium processes can help design materials with desired properties and control their behavior. Here we investigate how shear stress applied to nano-scale heterostructures of immiscible metals leads to the formation of metastable intermixed regions and establish the conditions for their stability. Heterostructures of Cr and Cu were grown using molecular beam epitaxy and subjected to linear shear by performing atomic force microscopy (AFM) scratch test. Controlling the force applied to the AFM tip and the ordering of the Cr and Cu layers allows us to control the tip penetration depth, the width of the wear track, and the amount of debris formed by the displaced material. Subsequent analysis performed using transmission electron microscopy and atom probe tomography reveals the character of the deformation field and the extent of shear induced intermixing under the wear track. Complementary molecular dynamics simulations provide additional insight into the mechanistic pathways of these off-equilibrium processes leading us to establish how the coupling between nano-structuring in the as-grown material and the shear loading determine the formation of mechanically-driven Cu-Cr intermixed regions.

2-dimensional (2-D) sharp interfaces with distinct boundaries demarcating an abrupt discontinuity in material properties in nanolayered composites have been studied for almost twenty years and are responsible for enhanced behaviors such as strength, radiation damage tolerance, and deformability. However, 2-D interfaces have their limitations with respect to deformability and toughness. 3-D interfaces are defined as heterophase interfaces that extend out of plane into the two crystals on either side and are chemically, crystallographically, and/or topologically divergent, in three dimensions, from both crystals they join. Here, we present the synthesis, structure, thermal stability, and mechanical behavior of nanolayered Cu/Nb containing interfaces with 3-D character. By co-sputtering the bimaterial interfaces between the constituent pure phases, the resulting compositional gradient gives rise to new interphase boundary structures. Micropillar compression results show that the strength of Cu/Nb nanocomposites containing 3D interfaces is significantly greater than those containing 2-D interfaces. Mechanical anisotropy, as well as shear banding is observed during pillar compression with retention of continuous layers across the shear band. We will present our recent results on deformation of such 3-D interfaces and structures, and describe their structural evolution mechanistically through the use of atomistic simulations.

Polymer bonded explosives consist of high energy particles in a polymeric binder. When these composites are subjected to heat, impact, or other stimulus they may undergo a rapid chemical change. This process is controlled by the formation of high temperature localized regions known as “hot spots”. The mechanisms of hot spot nucleation are controlled by the microstructure, for example in the same sample some particles ignite while others do not.

The sensitivity of the microstructure to initiation is studied with finite element simulations. The results help to identify the mechanisms of hot spot formation under a range of mechanical stimulus. The finite element model incorporates anisotropic plasticity and fracture and heat transport using a phase field approach. Microstructures with different initial defects, including cracks, debonding and voids are analyzed. Furthermore, we analyze the relative importance of plastic dissipation and friction for different crystal orientations and grain sizes.

We measured the hot spot and combustion temperatures of shocked insensitive high explosives in plastic-bonded explosive (PBX) formulations with nanosecond time resolution using optical pyrometry. The experiments used our shock compression microscope, which generates short-duration (4 ns) shocks using laser-launched flyer plates with velocities of 1-4.5 km/s. The microscope has, in addition to the optical pyrometer, a Photon Doppler Velocimeter (PDV) and high-speed imaging cameras.
The explosives were fabricated in the form of an array with 187 charges 1 mm in diameter and various thicknesses. The tiny charges were 80% explosive and 20% binder. The binder was polydimethylsiloxane (PDMS) and the explosives were triaminotrinitrobenzene (TATB), 1,1-diamino-2,2-dinitroethene (FOX-7), and 2,6-Diamino-3,5-Dinitropyrazine-1-Oxide (LLM-105). These explosives are particularly interesting because they are powerful but insensitive to accidental initiation.
The explosive temperatures were determined by measuring the spectral radiance in the visible region and fitting the radiance to a graybody model. However, since all these explosives are yellow, their absorption in the blue region gives apparent temperatures that are too cold. We measured the absorption spectrum in the PBX using diffuse reflectance spectroscopy. Knowing the absorption spectra, we have developed a modified graybody model that accounts for this absorption which allows us to measure the temperature more accurately.

The ability to probe the time-dependent microstructural response in explosives under shock compression is critical to understanding the initiation of explosives used in munitions and nuclear weapons. We have developed a tabletop apparatus that allows us to send initiating/detonation shocks to a sample and study the effects of microstructure on initiation. By using laser-driven impactors, we impart shocks lasting several nanoseconds and compress sample targets up to 10s of GPa. Using multi-frame fast photography with nanosecond temporal resolution we visually track thermal emission and its evolution over time. Additionally, we measure the time-dependent temperatures using a multichannel optical pyrometer coupled to a microscope objective that can resolve features down to 2 μm. The sample targets in this study consist of miniature arrays of explosive crystals embedded in various polymers, but sample targets can range from miniature, cylindrical explosive charges, thin films, and liquids. To date we have produced the first time-resolved images and simultaneous temperature measurements of shock-induced hot spots in a plastic-bonded explosive.

Metallic alloys with gradient microstructures have recently been shown to exhibit enhanced mechanical properties, namely increased strength with minimal loss in tensile ductility. One of the most common ways of engineering a gradient microstructure is via surface mechanical attrition treatment (SMAT) wherein hard balls are ultrasonically accelerated to impinge the surface and cause severe deformation and microstructural refinement. The improved properties are based on a number of proposed mechanisms, including dislocation cascading and mechanical incompatibility between layers, however less studied factors include the contributions of residual stress, texture, hardness and sample/process geometry. In this lecture, we will present research vignettes on each of these factors, and deliberate on their overall roles in property enhancement. The results will be used to explain seemingly incongruent phenomena observed in many reports, and further unravel and decouple deformation mechanisms in gradient materials.

Severe plastic deformation is a widespread method of making high-performance metallic materials. Single-phase polycrystalline metals undergoing severe plastic deformation develop steady-state textures that are characteristic of the mode of deformation. By contrast, we show that two-phase, Cu-Nb nano-laminate composites reach a variety of different steady-state textures under a single mode of deformation. Using molecular statics simulations and a novel algorithm for crystal rotation analysis, we observe that the final, steady state texture and interface character in these materials depends on the initial texture of the composite. This finding suggests that the range of bulk Cu-Nb nano-composite textures that may be made by severe plastic deformation is larger than previously demonstrated, with multiple plastically-driven steady states accessible, depending on initial texture. We propose a modification of accumulative roll bonding with highly textured seed layers as a means of accessing different driven steady states in layered composites.

Layered solids are prevalent in both natural and synthetic systems from geological formations and ice, to microelectromechanical devices and traditional composites. MAX phases are a particular type of layered ternary transition metal carbide or nitride that essentially bridge the gap between ceramics and metals. They possess a unique deformation mechanism of atomistic buckling, termed ripplocations, which upon further load can lead to mesoscale nonlinear kink band formation (NKB). This mechanism is closely related to, but distinct from intrinsic rippling of 2D layered materials. Unlike dislocation motion, bulk ripplocations have no polarity or Burgers vector, allowing for a potentially useful energy absorbing intrinsic toughening mechanisms. On the bulk scale, MAX phases catastrophically fail in a nominally brittle manner due to the fact that critical resolved shear stresses have drastically reduced pathways for dislocation motion. As such, understanding the competition of ductile, pseudo-ductile and brittle deformation mechanisms across stress states and strain rates is imperative to moving toward tailoring the layered anisotropy for specific strength or stiffness performance metrics. Consequently, this talk presents orientation and strain rate investigations of randomly and highly-oriented MAX phase titanium silicon carbide (Ti₃SiC₂), among others. Specifically quasi-static and dynamic compression experiments both parallel and perpendicular to the c-axis (on highly-oriented MAX) and fracture investigations utilizing 2D digital image correlation with ultra high-speed imaging map surface kinematics, damage behavior and by extension, crack tip energetics through analysis. Ripplocation nucleation, self-assembly and propagation to the point of permanent kink banding, which is fundamental to the deformation of all layered solids, as well as the potential benefits of highly-textured MAX phases will be discussed. A model bulk experiment replicating ripple formation with associated nonlinear buckling analysis will also be presented.

Three-dimensional (3D) metallic printing of macroscale engineering structures is a well-established field that promises precise spatial control and microstructure. The state-of-the-art metallic printer in the market that uses laser sintering has a minimum feature size of 20µm in thickness and 300µm in lateral direction. Thus the smallest structures that can be built are typically in the millimeter scale. On the other hand, micron scale metal architectures, with a simple extruded two dimensional shape such as micropillars and micro-cantilevers (with a length scale of ~2-30µm), can be fabricated only using focused ion beam (FIB) milling. Despite being widely used in the research communities, the use of FIB milling process in the industry is rather limited because of two key reasons: i) Defects are created in the milled structures due to gallium ion implantation and ii) it is an extremely time-consuming serial process, which severely limits the number of samples available for different studies. Thus, there is a critical need to develop a new method of 3D printing metals in the microscale (~1µm feature size) to create complex geometries (~10-100µm overall dimensions) such as microlattices, microsprings etc. Such a technology would enable us, for the first time, to design hierarchical microscale metallic structures with a unique combination of properties such as low density (<1000g/cc), high strength (>0.5GPa) and high ductility (>50% plastic strain). These full-metal microarchitectures can have profound applications in fabrication of freeform 3D metal microelectromechanical systems (MEMS) based devices and MEMS packaging for energy absorption/impact resistance.
In this presentation, for the first time, a new manufacturing technique for producing focused ion beam (FIB)-free templatable 3D metal microarchitectures such as microlattices, using a combination of two-photon lithography and electrodeposition will be described. The fabrication process which also involves a new electrodeposition overgrowth and inversion step, allowed the successful manufacture of full-metal 3D microlattices, with ~1µm feature size, to be made completely from nanocrystalline nickel, including the bottom substrate, the sample and a flat metal plate as the top-layer. Full-metal microlattices with such design are ideal for mechanical testing and for transfer to several applications/systems such as MEMS devices.
Subsequently, dynamic compressive properties of the full-metal nanocrystalline nickel microlattices, obtained using a state-of-the art piezo-based in situ micromechanical tester in a scanning electron microscope (SEM), will be presented as a function strain rate from 0.001/s to 200/s. Remarkably, the nickel microlattices with a density of ~1200g/cc (1/7^th density of solid nickel) exhibit high yield strength of ~0.5GPa, due to the nanocrystalline nature of the electrodeposited metal. Also, the metal microlattices deform in an almost ideal plastic manner with a constant plateau-like crushing stress without initial stress peaks and are able to sustain significant strains upto 50% before densification. Further, given that the live monitoring of the high strain rate compression tests was not possible due to the low frame rates of the SEM (~35fps) finite element analysis (FEA) was used instead to understand the structural evolution of the nickel microlattices at high speeds. Thus the presentation will also report the results from FEA, including the determination of the constitutive laws using high strain rate nickel micropillar compression.

Atomistic level modeling is a highly effective and has been recently used tool to study complex multiphase cement composite systems. It can be used to predict the mechanical characteristics and associated constitutive material constants of the composite system, as well as an atomistic level understanding of how individual material phases affects the overall properties of the composite structure. Cement paste is a complex composite system that consists of both hydrated and unhydrated minerals and voids. Being the most popular construction material, cement materials are regularly subjected to high pressure under different loading conditions including those from underwater applications, impact, and shock loading. The overall mechanical behavior of cement composite paste under hydrostatic compression is a result of the mechanical deformation of each constituent phase of this composite material system. In this study, predictive molecular dynamics was used to model the atomistic deformation of the unhydrated phases under increasing pressure. A reactive forcefield was employed to capture the appropriate material chemistry changes in contrast to the classical forcefield that fails to capture the molecular reactions. Hydrostatic compression behavior of major clinker minerals were determined by the pressure-specific volume Birch-Murnaghan equation of state (EoS) for Tricalcium aluminate (C₃A), Dicalcium silicate (C₂S), Tri-calcium silicate (C₃S) phases. Current modeling results indicate that calculated bulk moduli of the phases are in good agreement with previous experimental work reported for C3A and C2A. Among these three phases, C3S was found to exhibit larger bulk modulus as well as isotropic compressibility, which can influence the behavior of cement composite of hydrated and unhydrated components.

Computationally efficient methods for bridging length scales, from highly resolved micro/meso-scale models that can explicitly model crack growth, to macro-scale continuum models that are more suitable for modeling large machine components, have been of interest to researchers for decades. In this work, a brittle damage model that includes the effects of material plasticity is presented for the simulation of dynamic fracture in continuum scale quasi-brittle metal components. Crack evolution statistics, including the number, length, and orientation of individual cracks, are extracted from high-fidelity, finite discrete element method (FDEM) simulations and are used to generate effective material moduli that reflect a material's damaged state over time. This strategy allows for the retention of small-scale physical behaviors such as crack growth and coalescence in continuum scale hydrodynamic simulations. A stress-based degradation criterion is introduced for the degradation of individual material zones. This allows for the development of a heterogeneous damage distribution within the bulk material. The effective moduli constitutive model is used to simulate beryllium flyer plate experiments. The results of these simulations are found to be in good agreement with numerical and experimental velocity interferometer data. Extrapolation of the effective moduli model to a higher rate flyer plate case shows promise for further reducing computational costs associated with crack statistics generation from high-fidelity simulations.

The objective of this work is to determine the utility of three architected lattice types for energy dissipation when a sandwich panel is subjected to buried blast loading. The load is imparted on a panel by detonating a scaled explosive charge underneath a layer of engineered soil and monitoring the top surface deflection. Each panel was additively manufactured out of Ti-6Al-4V using powder bed fusion and the density of each panel was varied by changing the characteristic thickness of the lattice struts within a unit cell. The three classes of lattices studied in this work are octet, auxetic, and continuous shell lattices. Large finite element simulations of the structure-blast interaction, employing direct numerical simulation, are used to determine how the structure dissipates energy and ultimately collapses when subjected to the blast loading event. Finite element predictions of performance, which is characterized as maximum dynamic deflection of the top surface of the panel, are compared with in-situ measurements taken via Digital Image Correlation to rank order the structures. The key dissipation mechanisms of each lattice type are discussed in detail based on the finite element simulations, in-situ measurements, and post-mortem X-Ray CT scans of the tested structures.

Continuum dislocation dynamics (CDD) is becoming an important approach for modelling metal plasticity at the mesoscale. CDD is a density-based approach for modeling dislocation evolution under the effect of the applied load and the dislocation short and long range interactions. The method is based on transport equations for dislocations for which the driving force comes from the stress in the crystal. Most CDD models were developed for small deformation. In order to capture lattice rotation effects, a finite deformation kinematics must be accounted for in CDD model development. We present a finite deformation formalism of CDD that distinguishes between the dislocation measures in various configurations of the deforming crystal, thus yielding both Eulerian and Lagrangian forms of the transport equations governing those measures. A strain driven homogenization scheme is used to solve the coupled dislocation transport and crystal mechanics equations at finite deformation under periodic boundary conditions. A numerical scheme based on a staggered solution of the transport and stress equilibrium problems has been implemented within a finite element framework. We present simulations of several test problems such as large angle grain boundary formation, known geometrically necessary boundaries, as well as twist boundaries forming under special loading modes. We also present preliminary predictions of the dislocation microstructural patterns and compare them with TEM data of the same. The lattice rotation effects of the forming patterns will be discussed.

Continuum dislocation dynamics is becoming a popular framework for investigating the collective dislocation dynamics in single crystals undergoing plastic deformation. Continuum dislocation dynamics describes the spatiotemporal evolution of dislocations by transport-reaction equations that are coupled and solved concurrently with crystal mechanics, which is often formulated as eigenstrain problem. We present an approach to that facilitates an accurate implementation of dislocation reactions and cross slip in continuum dislocation dynamics. This approach seeks to rigorously enforce the dislocation line continuity from single slip system perspective in describing the reactions and cross slip. This is accomplished by adding virtual dislocation densities on each slip system to provide closure for all dislocation loops involved in cross slip or junction reactions, thus rigorously enabling the dislocation population on each slip system to satisfy the divergence free condition individually. We discuss the implementation of this approach and present results for the following: the stress-strain behavior of single crystals and the dislocation density evolution, the dislocation patterns, and the orientation dependence of the stress-strain response and the dislocation pattern. The later will be compared with experimental data for [001], [011] and [111] loading for FCC crystals.

The mechanical loading of metallic materials to high pressures and over short time durations remains a technically challenging arena of research. These loading conditions typically preclude the availability of detailed experimental results for physics involving finite elasticity, plasticity mechanisms including slip and twinning processes, and structural phase transformation. Although wave dynamics study via free-surface velocity diagnostics is a long-standing discipline, much of what is believed about the response of materials to these loading conditions remains inferred from comparison to computational results. The pursuit of questions involving coupled physics and the relationship between phase transformation and elastic and plastic deformation mechanisms is then best executed by integration of theory and experiment. We will present a study of structural phase transformation of high-purity titanium single crystals loaded by split-Hopkinson pressure bar (SHPB) and plate-impact conditions. Some sample material from these experiments was also soft-recovered and metallographically examined for structural evolution information. A thermodynamically consistent theory describing the finite elastic, dislocation slip, deformation twin, and phase transformation response of single crystals is presented. This theory presents physics coupling within both the energetic and structural components of the model for general high deformation rate loading conditions. The model performance is compared against the experimental results through numerical simulations of the experimental conditions. The SHPB and shock loading along the [0 0 01] and [ 10 -1 1 ] directions of single crystal high purity Ti is investigated computationally. Resonably good correspondence between simulation and experiment is obtained, which includes pole figures, volume fractions of components, free surface velocity, peak pressure, and phase transformation pressure. Multiple experimental phenomena are interpreted based upon the progression of dislocation slip, deformation twinning, and phase transformation. In compression with the [ 10 -1 1 ] crystal, a higher volume fraction in the primary twins but a lower secondary twin volume fraction in the [0 0 01] crystal in experiment was observed. The higher propensity for phase transformation occurs in the [0 0 01] crystal is reproduced. In addition to material texture, distributions of temperature, stresses, and plastic strains dependent on the impact loading directions are revealed.

In recent years two-phase nanolayered composites with individual layer thicknesses varying from 200-300nm down to 1-2 nm have been the subject of intensive study because of their unusual physical, chemical and mechanical properties. For example, with decreasing layer thicknesses (down to nanometer length scales) the mechanical response of these nanocomposites becomes increasingly interface dominated, and they exhibit ultrahigh strengths approaching the theoretical limit for ideal crystals. Moreover if the constituent phases present large differences in strength, elastic modulus and ductility, these multilayers give rise to new possibilities for the deformation mechanisms and properties of the composite as a whole. In this work we explore the possibility of synthesizing multilayered composites where one constituent phase has a low ductility, with a final goal of enhancing both the strength and ductility of the system.

Using physical vapor deposition (PVD) techniques we synthesized a hexagonal close-packed (HCP) – body-centered cubic (BCC) Mg-Nb system (where twinning in Mg leads to its lack of ductility), over a range of layer thicknesses ranging from 5 nm to 200 nm. We investigated the structure of the hitherto-unknown bcc Mg phase in the Mg/Nb multilayer nanocomposite under high pressures in a diamond anvil cell experiment using synchrotron radiation x-ray diffraction (XRD). We utilize a suite of small scale testing techniques to evaluate the deformation mechanisms in these nanocomposites that involve both compressive loading scenarios such as (a) high throughput nanoindentation testing, to more-specialized FIB-fabricated (b) micro-pillar compression, and (c) micro-tensile tests. The mechanical data obtained from these tests are correlated with the structure information at complementary length scales using techniques such as XRD, SEM, and TEM. The use of such diverse testing technologies allows a detailed characterization of the structure-property correlations at each length scale of interest; ranging from the response of each individual interface within the laminate to properties of the ensemble.

The specialized testing techniques such as FIB-fabricated micro-pillar compression and micro-tensile experiments are used to characterize the anisotropic response of the nanolayered composite, with interfaces oriented either normal, parallel and oblique (45^o) to the compression axis. Each of these configurations generate unique mechanical information: thus while compressions and tensions normal and parallel to the interface are important for a complete understanding of the anisotropy of properties and size-related effects, the tests with the interfaces loaded obliquely provide a measure of interfacial shear strength. Ductility and fracture toughness at the micrometer level are investigated using in-situ observations inside an SEM coupled with a micro-tensile geometry. Tests repeated as a function of layer thickness and interface orientations provide us with valuable information regarding the Mg-X interface character.

We have also extended our techniques to measuring the strain rate sensitivity (SRS) of the Mg-X nanocomposites using in situ strain rate jump experiments conducted under both indentation and micro-compression/micro-tensile loading scenarios. Elevated temperature measurements are especially important, since the SRS for nano-(bcc)Mg is expected to differ from its nano-(hcp)Mg counterpart as a function of temperature from which activation energies/volumes for deformation mechanisms can be extracted. Results from these tests were analyzed in terms of the measured activation energies and activation volumes from sub-micrometer sized Mg/Nb multilayer nanocomposites. These findings reveal an alternative solution to obtaining lightweight metals critical needed for future energy efficiency and fuel savings.

The residual mechanical properties acquired from shock-compressed solids are often times dramatically different from those received under quasi-static conditions. This suggests that the deformation mechanisms present during shock compression may be significantly different than those seen under quasi-static conditions.

In this presentation, we will discuss shock-induced softening in single crystal magnesium. Single crystal magnesium samples were shock compressed to approximately 0.8 GPa and 1.7 GPa respectively along the <a> and <c> axes then released back to ambient conditions. Nanoindentation was performed on the shock recovered samples at regions free of deformation twins. We compared the nanoindentation hardness of these samples to those received from samples compressed under quasi-static conditions as well as those of pristine samples that did not undergo any plastic deformation. Our results show that quasi-static deformation increases the hardness due to conventional Taylor hardening. However, if only the deformed specimens are compared, the hardness decreased with increasing strain rate. The microstructure of the shocked samples showed a large number of deformation twins but there seems to be negligible storage of dislocations. Even pre-existing dislocations seem to be swept out. We hypothesize that due to the nature of the shock-compression experiments, although dislocations will be generated during deformation, they are able to glide through the sample unimpeded and emerge at the free-surfaces, whereas they would normally be constrained in conventional quasi-static compression. The stress-driven removal of dislocations could lead to abnormal softening in shock-compressed magnesium. Our observation counters the conventional belief that large numbers of dislocations are stored in samples subject to severe plastic deformation as usually seen in shock-compressed face-centered cubic metals. These results give an insight into the fundamental understanding of the residual mechanical response in shock-compressed materials.

The heterogeneous structure of granular materials makes their response to dynamic deformation quite complex. The size, shape, and orientation of the grains as well as the pore structure influence the distribution of forces and thus the constitutive behavior of the material. These structure-properties relationships have been actively investigated for quasi-static loading, where imaging (such as x-ray computed tomography) can provide detailed information about the evolving 3D structure of the material. But relationships determined for low loading rates may not carry over to dynamic loading, where inertial effects become important. Several groups have used high-speed x-ray imaging to study dynamic deformation of granular materials but the insights obtained have mostly been qualitative, because the information available is limited to 2D projections of complex 3D structures. This has limited the usefulness of such studies for the development and validation of models of dynamic behavior of granular materials.

In this talk, we present a robust algorithm with which we can measure the heterogeneous pore size distribution of a granular material from single projected x-ray phase contrast images. As a demonstration we apply our algorithm to sandstones undergoing dynamic wedge impact, using a sequence of images to track from their measured pore size distribution the evolution of porosity, pore number density, and median pore size with micron-scale spatial resolution and sub-microsecond temporal resolution. We observe initial pore compaction close to the wedge tip to form an extended wedge that induces pore dilation downfield, leading to the development of intergranular cracks and failure. The insights available from these studies can provide a better understanding of how granular material response on the mesoscale influences macroscopic properties such as shear strength. This improved understanding may be beneficial for understanding complex dynamic loading problems, including planetary impacts and mine blasting.

In-situ neutron and synchrotron X-ray diffraction deliver unique and complementary insight into the material’s response to high temperature, deformation and extreme conditions. Neutrons illuminate a larger bulk volume and reveal quantitative phase abundance, bulk texture, lattice parameter changes and other ensemble averaged quantities. In contrast, fine-bundled high-energy X-rays deliver reflections from a number of individual grains. For each constituting phase, their statistics and behavior in time reveal information about grain growth or refinement, subgrain formation, static and dynamic recovery and recrystallization, slip systems, twinning, etc. Examples will be presented on selected in-situ examinations on materials undergoing thermo-mechanical processing, high-pressure and response to shock.

Understanding and predicting the response of materials under dynamic loading is a challenging problem due to complexities involved with the loading state and its interaction with various features in the microstructure. Previous experiments to study dynamic fracture in Tantalum (Ta) manufactured via Additive manufacturing (AM) has shown differences not only in the elastic plastic transition but also its spall properties. The goal of this work is to understand this difference in the dynamic response of AM vs. wrought Ta through the use of non-equilibrium molecular dynamics (MD) simulation. Both experiments and simulation data showed that altering the processing conditions also changed the number fraction of specific grain boundary types in the wrought and AM materials. To investigate if this change in boundary type distribution is the main cause of differences in the dynamic response of these materials, bi-crystal simulations were performed to quantify the effect of boundary type and structure on spall strength.

Magnesium is a potentially useful lightweight structural material for applications such as aerospace and defense, where materials can be subjected to extreme environments such as hypervelocity impact. In this study, we examine the effect of microstructure on spall void initiation during shock loading of pure magnesium. Using equal channel angular extrusion (ECAE), we processed pure Mg to produce an average grain size of 10 microns. Subsequent annealing increases the average grain size and influences the crystallographic texture of the material.

To study void initiation in these materials, we carried out spall experiments with a laser-driven micro-flyer apparatus capable strain rates exceeding 10⁶ s^-1, using photon Doppler velocimetry (PDV) of the back-surface velocity to determine the spall strength. We use x-ray computed tomography (CT) to characterize the incipient spall voids, and correlate this with the microstructure (texture and grain size) measured with backscattered electron diffraction (EBSD).

Ultrafast laser-induced shock waves in metals can produce strain rates as high as 10¹¹ (1/sec). The mechanical response to these extreme strain rates is important for ultrafast micro-machining, the study of extreme strain rate phenomena, de-orbiting space junk with ultrafast lasers, the response to biological systems as in eye surgery, and many emerging ultrafast materials applications. What is unique about our approach to studying these extreme conditions, which are on the order of ~10⁴ K and pressures up to 100 GPa, is that we are able to recover the material afterwards to study as well as use pump-probe methods to observe the dynamics on picosecond timescales. We deposit thin films (1 nm to 10 microns) on sapphire or glass substrates and irradiate the material through the transparent substrate. The fluence we use dictates which path the material will take through a temperature-density phase field. We are especially interested in studying materials in the vapor dome, the two-phase region where the liquid and gas phases are in equilibrium. Our work has demonstrated that we can force a material into the vapor dome 10-12 ps after the laser pulse ends [1]. Depending on the material and the irradiating fluence, we are able to keep the material in the vapor dome for up to 25 ps while maintaining its temperature to be, at most, 90% of that at the critical point[1]. We have demonstrated mixing of W and Ni to thermally stabilize the quenched nanograin material (~2 nm grain size). We have also shown that we can mix Ni and Ag layers at the higher temperatures in the vapor dome. These studies exploit the extreme quenching that can be observed—we are capable of quenching at 10¹⁰ to 10¹² degrees per second.


This talk will focus on the dynamics of high strain rate shock loading and rapid heating. We will study Ni films of 200 to 300 nm thickness that are calculated to be capable of supporting a strong shock wave. Multiple deformation mechanisms are observed upon irradiation including delamination and deformation without fracture, fracture, spall, and melt. Results from 1-D plasma hydrodynamics codes (HYADES) with DOE equations of state show that internal mechanical spall from the large tensile component of the shock wave is possible. Our modeling results suggest that, with a 150 fs, 780 nm laser pulse, we are able to generate up to 23 GPa of tensile stress at the trailing edge of shocks using a fluence (0.9 J/cm²⁾ that is just below the melt threshold for Ni. The spall strength of Ni in static conditions is about 3 GPa. We will present results of ultrafast pump-probe imaging and interferometry of the back side to extract the dynamics and relative competition between heat flow and mechanical stress. This experimental and computational platform will be used to explore the limits of the phonon drag regime and the ultimate strength of materials. We will present the in-situ observations of the dynamics along with ex-situ microscopy of the recovered material. These will be discussed in the context of our computational results to show that material heating and loading at these extreme conditions lead to unique final states. Finally, we will show that this experimental approach is ideal for studying a large variety of other high-temperature and high-pressure phenomena.

References
[1] Schrider, K. J. Femtosecond Laser Interaction with Ultrathin Metal Films: Modifying
Structure, Composition, and Morphology. Ph.D. Dissertation, University of Michigan, Ann Arbor,
MI, 2017.

Pulsed lasers with power on the order of terawatts, once deposited on a target surface, will launch a strong stress pulse that propagates into the material. Owing to the ultrashort duration of the laser pulses, unprecedented extreme conditions which combine high pressures (and/or shear stresses), strain rates and temperatures can be generated in materials, yielding a yet unexplored regime of study: extreme mechanical metallurgy. During the talk, I will first summarize our efforts on laser shock compression of four covalently bonded materials, namely, silicon, germanium, boron carbide and silicon carbide (SiC). These materials are known to have high Peierls-Nabarro stress and negative Clapeyron slope. The profile of the shock waves was measured by a velocity interferometer system for any reflectors (VISAR). The shock deformation microstructure has been revealed by high-resolution scanning/transmission electron microscopy and all the materials exhibit shock-induced amorphization. This discovery indicates shock-induced amorphization as a generalized deformation mechanism under strong shock condition. Laser shock experiments can also be extended to the tensile regime when the compressive wave was reflected by the free surface. The tensile wave, once exceeds the ultimate tensile strength of the materials, will lead to a catastrophic failure. Our recent efforts on dynamic tensile fracture of tantalum with different grain size will be discussed. In addition to laser shock experiments, I will also talk about the dynamic behavior of CoNiCr-based high entropy alloy (HEA). These materials show strong resistance to shear localization under impact loading, which is mainly due to HEA's low stacking fault energy and therefore easy propensity to undergo twinning. Our results suggest that CoNiCr-based HEA may have good ballistic protection application.

We have investigated the performance and applicability of a single-crystal and core-shell Cu@Ni nanoporous (NP) structure under shock-loading using atomistic simulations. Core-shell structure exhibits lesser volume compressibility than a single-crystal NP. Core-shell NP also demonstrates increased plastic deformation i.e., enhanced shock-energy absorption efficiency of around 10.5% larger than a single-crystal NP, albeit shock Hugoniots reflect a weak dependency on core-shelling of an NP structure. Shock-induced deformation behavior of NP-Cu with and without nano-coating of Ni has been studied. At u_p < 0.15 km/s, partial collapse of the pores are observed via direct crushing in both NP structures. At 0.15 km/s < u_p < 0.55 km/s, single-crystal NP Cu shows complete collapse of pores through ordinary dislocation-mediated softening in the ligaments, while the core-shell Cu@Ni NP demonstrates increased strain-hardening in Cu/Ni interface. At u_p = 0.75 km/s, shock-induced deformed microstructure of both structures has been observed to be recovered through recrystallization.

A systematic examination of the cyclic compression response in three different orientations (r,θ and z) of micropillars obtained from a polycrystalline thin-walled NiTi tube (that is a precursor to laser machining of medical stents) was undertaken and correlated with texture measurements. The tube was textured with a strong {112}-{111} duplex structure along the drawing direction (z), a strong {111}-{123} along the r-direction and a near {011}-{012} texture in the θ-direction. Strain amplitudes examined included the entire transformation cycle as well as reduced amplitudes about targeted mean strains; the latter enables a comparison of the cyclic response of a predominantly austenitic microstructure with that of a predominantly martensitic microstructure. Further, single crystal micropillars were milled from an annealed NiTi sheet with 25-30 µm grain size with loading axis aligned in orientations corresponding to the tube texture and their cyclic response was compared to those obtained directly from the tube. Results from these studies will be presented and discussed.

Shear bands in metallic systems are narrow regions of intense strained materials which could act as premature failure locations. A comprehensive understanding of shear band microstructure is the key to control the ductility of materials. However, large localized deformation usually develops within this thin region and nanoscale recrystallized grains are formed. Conventional characterization techniques such as EBSD and regular TEM imaging are inadequate to visualize the detailed microstructure inside these shear bands. For example, EBSD cannot provide high enough spatial resolution. TEM has the ability to determine the nanoscale grain size, but to elaborate the orientations of all grains is labor-intensive. Moreover, detailed orientation information cannot be obtained. In this work, we demonstrated that ASTAR (a technique that acquires the diffraction pattern of each pixel) could offer direct orientation information of microstructure feature down to 1 nm scale. Using the ASTAR results, we revealed the hierarchical structure including high-angle grain boundaries, low-angle grain boundaries, defect bundles in the shear bands.

The mechanical properties and fracture mechanisms of taper-free InP twinning superlattice (TSL) nanowires were ascertained by means of in situ uniaxial tensile tests in a transmission electron microscope. The nanowires were grown along the zinc-blende close-packed [111] direction and presented an an average twin spacing of ~ 13 nm. The elastic modulus along the [111] orientation was 87 ± 17 GPa, while the fracture strain was 2.9 ± 0.3% close to the one reported in ZnO nanowires (~5%) but smaller than that of GaAs nanowires (10–11%). Fracture was brittle in all cases and occurred by the propagation of a crack along the twin boundary interface. No evidence of inelastic deformation mechanism was observed neither in the experimental stress-strain curve nor in the TEM images before fracture. MD simulations of the tensile deformation of untwinned and twinned InP nanowires of different diameters (12 to 30 nm) at 300 K were carried out to assess the experimental data. The elastic modulus obtained from the simulations (84 GPa) was fairly independent of the diameter and close to the experimental values. The failure strain and the tensile strength of the twinned nanowires in the MD simulations (~ 10% and ~ 6.7 GPa) were much higher than those observed experimentally but smaller than those simulated for untwinned nanowires, and these differences were attributed to the stress concentrations associated with the zig-zag waviness at the nanowire surface. Moreover, the simulations did not show any evidence of non-linear deformation mechanisms prior to fracture, that was triggered by the nucleation and propagation of a crack at the twin boundaries, in agreement with the experimental observations.

High-strength structural materials such as Ni-based superalloys and diffusion bond coats are widely used in challenging environments and with exposure to mechanical fatigue, particle impact, and erosion at elevated temperatures. Diffusion aluminide bond coats are an example of compositionally and microstructurally graded coatings with significant variation in engineered mechanical properties across the cross-section. Nanoindentation and pillar compression, particularly in situ, can be considered as a well-suited technique for measuring the properties of such complex microstructural materials as the deformation volume can be carefully controlled to probe different precipitates and microstructural zones. In this study, an SEM nanomechanical instrument with an integrated high-temperature stage and an active tip heating was used to conduct pillar compression of aluminide bond coating and substrate at room temperature to well above 800degC. This is the first study of an in situ nanomechanical testing of any sample at such higher temperature with capturing deformation events in detail at that temperature inside an SEM. With combined analysis of chemistry and microstructural changes, the results were used to understand local mechanical properties variation as a function of temperature.

Nanomechanical tests are moving beyond the basic measurement of hardness and elastic modulus to encompass a host of different mechanical properties such as strain rate sensitivity, stress relaxation, creep, and fracture toughness by taking advantage of focused ion beam milled geometries. New developments, such as high cycle fatigue, are extending the range of properties which can be studied at the micro and nanoscale. However, such techniques are challenging due to low oscillation frequencies, long duration of tests and large thermal drift when attempted with standard indentation instruments. Novel piezo-based nanoindentation methods are now allowing access to extremely high strain rates (>10⁴ s^-1) and high oscillation frequencies (up to 10 kHz).

Until only recently, high strain rate testing of materials at strain rates from 100/s – 10000/s has only been possible using macroscale techniques, such as split Hopkinson bar, Kolsky bars and plate impact testers. At the microscale, strain rates have typically been limited to 0.1/s or less, owing to limitations in instrumentation and insufficient data acquisition rates.

This talk will focus on the most recent developments in instrumentation for in-situ extreme mechanics testing at the micro and nanoscales, with specific focus on a testing platform capable of strain rate testing over the range 0.0001/s up to 10’000/s (8 orders of magnitude) with simultaneous high speed actuation and sensing capabilities, with nanometer and micronewton resolution respectively.

Other recent innovations include cryogenic and high temperature tests covering the temperature envelope from -150 to 800 °C. The challenges in variable temperature tests and the associated technological and protocol advances will be discussed along with select case studies. The inherent advantages of using small volumes of sample material, e.g., small ion beam milled pillars, will be discussed together with the associated instrumentation, technique development, data analysis methodology and experimental protocols. Some examples of test data will be presented where a wide range of strain rate has been combined with variable temperature in order to investigate rate effects as a function of temperature. Finally, future research directions in this sub-field of micromechanics will be discussed.

We present a dynamically deformed material with a unique gradient-nano-grained (GNG) structure and martensitic phase transformation. GNG structures have been created through techniques such as surface mechanical attrition treatment [1], electroplating [2], surface rolling [3], and laser shock processing in germanium [4]. Studies show that nanograins provide increased strength while coarse grains retain ductility [1–3], simultaneously improving two mechanical properties that are often found to be mutually exclusive. However, a detailed structure-property relationship enabling predictions of material properties based on the gradient grain structure has not been developed yet. We investigate the process-structure-property relations in GNG metals through an experimental approach that allows us to tailor the grain structure and probe mechanical properties while observing microstructural changes. Using a laser induced projectile impact testing (LIPIT) apparatus, we impact single-crystal silver microcubes [5] onto a stiff target at ~400 m/s. The high-strain-rate deformation creates a gradient nano-grained structure with small ~10 nm grains near the impact plane and progressively larger grains farther from the impact region [6]. By varying impact parameters such as sample orientation and impact velocity, we produce different GNG structures with varying ranges of grain size and degree of gradient.
We show that a [100] impact orientation of the crystal initiates a phase transformation from a face-centered-cubic (fcc) to a hexagonal-close-packed (hcp) structure. Very rare naturally occurring hcp silver was found in north-eastern Russia in the 1970’s [7], and synthetic pathways have been developed recently [8]. A single-step method to dynamically induce such a phase transformation will allow for further tailoring of favorable material properties. We also find that this phase transformation is orientation dependent as cubes impacted along the [100] direction undergo phase transformation while impact along the [110] direction does not yield a phase transformation. This is due to the varying stress states present in the sample during deformation and the favorable [100] loading geometry to create stacking faults on the {111} planes. We also show that over time small grains coalesce into larger grains due to recrystallization and results in a subsequent hcp-to-fcc phase transformation.
With an understanding of the process-structure relations, we investigate how these structures lead to improved mechanical properties of the material. Using a focused ion beam (FIB), we mill micropillars out of the impacted samples and perform micro-compression tests using an in situ SEM nanoindenter to measure strength and toughness. We also fabricate planar cross-sections of the sample using FIB and measure the hardness as a function of the gradient grain structure using the SEM nanoindentator. Developing detailed process-structure-property relations will enable the design of metals with optimal strength and toughness for a given engineering application.
[1] X. Wu, P. Jiang, L. Chen, F. Yuan, Y.T. Zhu, Proc. Natl. Acad. Sci. 111 (2014) 7197–7201.
[2] Y. Lin, J. Pan, H.F. Zhou, H.J. Gao, Y. Li, Acta Mater. 153 (2018) 279–289.
[3] C. Yuan, R. Fu, D. Sang, Y. Yao, X. Zhang, Mater. Lett. 107 (2013) 134–137.
[4] S. Zhao, B. Kad, C.E. Wehrenberg, B.A. Remington, E.N. Hahn, K.L. More, M.A. Meyers, Proc. Natl. Acad. Sci. 114 (2017) 9791–9796.
[5] S. Jeon, S. Yazdi, R. Thevamaran, E.L. Thomas, (2017).
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[8] I. Chakraborty, D. Carvalho, S.N. Shirodkar, S. Lahiri, S. Bhattacharyya, R. Banerjee, U. Waghmare, P. Ayyub, J. Phys. Condens. Matter 23 (2011).

Two-photon polymerization (TPP) is a recently developed technique that was used in these experiments to “write” millimeter-size cellular structures (~0.2g/cm³) with micrometer-size resolution.¹ This process is used to make auxetic structures, which are characterized by having a negative Poisson’s ratio.² The purpose of this study is to develop a material that can dissipate energy from impacts or vibrations over a wide temperature range—from cryogenic to room-temperature conditions. Different auxetic structures were designed using finite element analysis methods to maximize the structural and plastic deformation that can occur at cryogenic temperatures without exceeding the strength properties of the material. These structures were then printed using a commercial laser writer (Nanoscribe GmbH) and tested at 23°C and 140 K using a thermal mechanical analyzer (Perkin Elmer) at loads ranging from 10⁴ to 10⁶ Pa and multiple cycles. The calculated energy (~32 J/kg) that is dissipated from a brief impulse (<17 ms) is compared to the measured energy dissipation. The robustness and performance of the structures is compared with the laser “writing” parameters used to make the structure, which requires full polymerization of the photoresist material while avoiding embrittlement.
This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856, the University of Rochester, and the New York State Energy Research and Development Authority.

1. K.-S. Lee et al., Polym. Adv. Technol. 17, 72-82 (2006).
2. M. Sanami et al., Procedia Engineer. 72, 453-458 (2014).

In this talk, we propose a design concept of architected mechanical metamaterials based on the crystalline quartz structure. Crystalline quartz is a repeating double helix structure of the SiO₄ tetrahedra with plenty of empty space that shows a zero-Poisson’s ratio. We mimic this crystalline alpha quartz structure to design a truss-based mechanical metamaterial. Both experiments and numerical analysis demonstrated that the inter-penetration of the SiO₄ tetrahedra results in the zero-Poisson’s ratio similar to the crystalline quartz. Through dynamic analysis at various levels, it was possible to figure out several important features of this mechanical metamaterial.

Selective laser melting (SLM) refers to an additive manufacturing (AM) technique for metallic alloys, which uses a scanning laser to melt and fuse metallic powder bed layer-by-layer. It has advantages over traditional manufacturing methods in the design freedom and manufacturing flexibility. However, the SLM process with repeated melting and solidification leads to the unique microstructure that can include large columnar grains and periodic cracks. Moreover, the rapid solidification intrinsic to SLM can produce nanoscale structure different from traditional manufacturing processes. While post-heat-treatments are widely employed to improve the mechanical property of SLM-produced alloys, the unique, as-built microstructure can lead to varying response to traditionally defined heat treatment parameters. Therefore, the detailed understanding of the dynamic microstructure evolution as a function of temperature and its effect on the mechanical behavior is essential for optimizing the SLM and post-heat-treatment processes. Here, in-situ transmission electron microscopy (TEM) is a useful tool to observe nano-scale phenomena in real time. It provides direct observations of the deformation processes in nano-scale to study the deformation mechanism/procedures, and their relationships with the microstructural evolutions during heating. In this study, we designed a unique sample shape that enables tensile experiment of thin TEM lamellae without undesired off-plane deformations or bending.
We selected IN718 as representative SLM-produced alloy to discover the relationship between the microstructural evolution during heat treatment and the associated change in the mechanical behavior. The in-situ TEM captured a microstructural evolution of IN718 during exposure to a high temperature, with an emphasis on precipitation of spherical γ'/ γ"phases. Subsequent tensile experiment demonstrated brittle fracture in contrast to a ductile fracture of as-printed IN718 indicating a proper heat treatment (solutionizing and ageing) is necessary to improve the mechanical property. This study provides correlation among heat-treatment, microstructural evolution, and the mechanical behavior, which can be used to optimize the post-heat-treatment for SLM-produced alloy through microstructure control.

Phase-change materials (PCM) owing to their extreme electro-optical contrast between crystalline and amorphous states are used in optical data storage, flexible devices and neuromorphic computing. However, atomistic mechanisms governing photo-excitation induced non-thermal amorphization processes are still unknown. We perform excited-state dynamics within the framework of density functional theory to investigate the mechanism behind such crystalline to amorphous transition for Germanium Telluride (GeTe). Our study shows the GeTe non-thermal amorphization process is the key. Amorphous phase is characterized by computation of diffraction pattern in real time. Further analysis of bond-overlap analysis suggests a change in bonding behavior between Ge-Ge from anti-bonding to bonding state that leads to increase in Ge-Ge bonds and decrease in Ge-Te bonds. A rapid heat extraction during excited-state simulation does not lead to amorphous phase. Overall, our excited-state dynamics study demonstrates that the non-thermal amorphization in GeTe occurs due to antibonding character of excited electronic states. Since structural transformation limits the lifetime of PCM-based devices, a deeper understanding behind this mechanism may allow us to a new avenue in the design of future devices.
This work was supported as part of the Computational Materials Sciences Program funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award Number DE-SC0014607

Metal 3D printing is a versatile additive manufacturing (AM) process that produces parts from powders layer-by-layer directly from a digital model. Metal 3D printing allows for fabricating complex metallic components in an efficient and cost-effective manner compared to conventional manufacturing techniques. One significant roadblock for additively manufactured parts is that residual tensile stresses can be high in AM parts, which limits their applications, particularly for cyclic loading or at high temperatures. Post-processing methods to modify AM parts offer new opportunities for applications in extreme environments such as in turbines or nuclear reactors. Laser shock peening (LSP) is a technique that is used as a post-processing method to optimize the service life of critical components by inducing compressive residual stresses that increase the material’s resistance to surface-related failures such as creep, fatigue, stress corrosion cracking, etc. However, its application for ferrous materials has been limited to temperatures lower than ~400°C since the stresses are released at temperatures above that.
A novel technique was developed to stabilize the LSP-induced compressive residual stresses at high temperatures (above 400°C). Nickel-based superalloy specimens (Inconel 718) were printed using a direct metal laser sintering (DMLS) technique. Various tests including X-ray diffraction spectroscopy, transmission electron microscopy, and nanoindentation were performed. Complementary molecular dynamics and phase field simulations were utilized to understand the underlying physical mechanisms governing the dislocation motion and their interaction with the microstructures to provide further insight to the effect of postprocesses on the final performance of printed parts, e.g. wear and fatigue. Nanoindentation hardness mapping showed that LSP increases the hardness of the top ~1.2 mm of the specimens, with a maximum hardness of ~6.6 GPa on the laser shock peened surface that gradually decreases with an increase in the distance from the surface and reaches 5.2 GPa. Electron microscopy proved that dislocation generation is the mechanism responsible for plastic deformation and hence hardening in laser shock peening. Specimens that were treated by our new technique, reserved more than 50% of the maximum magnitude of their compressive residual stresses, even after exposure to 700°C for 48 hours. The results will lead to AM materials with better performance at high temperatures, not only attributed to their inherent properties, but more importantly by optimizing microstructure, surface properties, and residual stresses.

Shear banding is observed widely in a variety of materials and at different length scales. During shear banding, intense plastic shear is produced within a thin band, and shear localization strongly limits the material’s ductility, leading to catastrophic failure under stress. For these reasons, shear banding has attracted considerable research interest in uncovering the underlying mechanisms. However, connecting mechanical behaviors with microstructure evolution during shear banding is difficult, and the critical knowledge of structure-property relationship is still missing so far. In-situ observation during deformation is needed to uncover the origin of shear banding.
Here, we report a direct observation of shear banding in nanocrystalline ZrN nanopillars. ZrN belongs to a class of materials, called nanocrystalline ceramics (NCCs), which are popular as protective coating because of their outstanding mechanical properties. The high strength of NCCs derives from the effect of nanograin size, following the well-known Hall-Petch relationship. A major drawback of NCCs, or nanocrystalline materials in general, is their low toughness due to the limited dislocation activities. However, when the grain size is reduced into the nanoscale regime, the effect of grain size begins to soften materials via the increased grain boundary activities. We show that plastic deformation in ZrN is carried out by intermittent granular activities in the NCC nanopillars without brittle fracture. Cooperative granular activities are found along the regions where shear bands form. Complementary cumulative distribution function (CCDF) of the associated stress drops suggests dislocation avalanches are suppressed by nanograin size.
Nanocrystalline ZrN was prepared in the form of thin film with an average grain size of 18 nm using unbalanced magnetron sputtering deposition. Nanopillars were fabricated in the plane view direction of the thin film. Compression tests of separate nanopillars were conducted in either bright-field imaging mode or nanobeam diffraction mode using Hysitron PI95 picoindenter in JEOL 2010 LaB6 TEM. The experiments were simultaneously recorded as a video at the frame rate of 10 frames/s.
The study here demonstrates the secondary deformation mechanisms involving grain boundaries, in addition to the primary deformation mechanism operated by dislocations, and how together these mechanisms facilitate plastic deformation in nanocrystalline materials. The strong correlation of shear band formation and intermittent granular activities are demonstrated, from which a shear banding model of NCC materials is proposed.

Carbon Schwarzites are 3D porous crystalline pure carbon structures [1]. They contain rings with more than six atoms, which create negative Gaussian curvatures and the ‘flatness’ of these curvatures is dependent on the ratio of hexagon to non-hexagon rings. There are different Schwarzite families, such as primitive (P), gyroid (G), diamond (D), etc [2]. Previous studies suggested [3] that under compressive loadings, the mechanical properties of schwarzites is directly related to the local ‘flatness’ of each structure. Impact tests were performed on macroscopic 3D printed structures (cm size) [4], which showed that some qualitatively trends of the impact resistance and energy absorption are preserved from nano to macroscale. In this work we have investigated the Schwarzite behavior under ballistic impact. We considered four structures from two families (primitive (P688 and P8bal) and gyroid (G688 and G8bal)), which differ mainly by their local ‘flatness’. We used a spherical projectile with a diameter of 15 Angstroms and velocity range from 1 up to 3 km/s and with the same mass value of the target. We carried out fully atomistic molecular dynamics (MD) simulations using reactive force fields (AIREBO). Our MD results show that Schwarzites are very effective to absorb kinetic energy but their performance decreases as their ‘flatness’ increases. In P8bal and G8bal the projectile penetration is about four times deeper than in P688 and G688, with more extensive structural fractures. Considering that it was already demonstrated that macromodels of these materials can be 3D printed [4], our results open new perspectives to create new or to improve Schwarzite-based functional engineered materials.

[1] A. L. Mackay, H. Terrones, Nature 1991, 352, 762.
[2] R. H. Terrones and M. Terrones. N. J. Phys. 2009, 5, 126
[3] C. F. Woellner, T. Botari, E. Perim, and D. S. Galvao, MRS Adv. 2018, 3, 448.
[4] S. M. Sajadi, P. S. Owuor, S. Schara, C. F. Woellner, V. Rodrigues, R. Vajtai, J. Lou, D. S. Galvao, C. S. Tiwary, and P. M. Ajayan, Adv. Mater. 2017, 1704820.

Due to their high storing capacities, silicon and germanium have emerged as a promising anode material for high-capacity Li-ion batteries. Initial lithiation of both materials results in a crystalline to amorphous phase transformation accompanied by a large (300%) volume expansion that drives plastic flow and fracture above a critical particle size, which is several fold larger in Ge than Si. This difference is believed to be due to the fact that the motion of the atomically sharp crystalline-amorphous phase boundary is strongly anisotropic in Si but nearly isotropic in Ge. However, how this difference affects elastoplastic deformation and fracture remains poorly understood quantitatively. Here we exploit the phase-field approach to describe both the crystalline to amorphous phase transformation and stress-driven plastic deformation and fracture within a self-consistent mathematical framework to shed light on the role of anisotropy. We model the phase transformation using a modified Allen-Cahn model with anisotropic and isotropic mobilities, respectively. We model the mechanical response due to lithiation-driven swelling of the anode using neo-Hookean elasticity and finite J-2 plasticity. Finally, we model fracture using a variational phase-field formulation. Our model can describe both nucleation and propagation of cracks without interpenetration of crack surfaces under compression.

We use this multi-physics phase-field framework to investigate the lithiation of Si and Ge nanopillars as a function of yield strength and fracture energy. Using simulations without fracture, we first show that, for isotropic swelling, the tensile stresses driving fracture in the nanopillar remain capped by the yield strength. In contrast, for anisotropic swelling, tensile stresses become amplified severalfold by localization of plastic deformation near the particle surface. We relate this amplification to the existence of a critical yield strength below which plastic deformation occurs successively under compression and tension and above which it only occurs under compression. Next, using simulations with fracture, we show that this amplification of tensile stresses causes nanopillar to fracture at a smaller radius in Si than Ge as experimentally observed. Finally, we use our simulation results to obtain consistent estimates of elastic properties, yield strength, and fracture energy, which are compared to existing experimental measurements.

Gradient nanostructured metallic materials have shown a combination of high strength and good ductility. A TWIP steel with large equiaxed grains was processed by surface mechanical grinding treatment, and the subsequent gradient structures formed near surface consist of surface nanolaminate layer, a mixed layer of deformation twins and shear bands, and a deformation twin layer. TEM studies reveal that detwinning occurs during microstructural evolution. Tensile studies show that the gradient structures increase the yield strength of TWIP steel prominently. The evolution of gradient microstructures and their influence on mechanical behaviors of the gradient TWIP steel are discussed.

Diamonds are used in many industrial applications due to their extreme hardness, particularly for polishing, cutting, and pressure application. There has been an increasing interest in hexagonal diamond in recent years, largely due to its potential to be harder than diamond [1]. However, experimental hardness measurements for this technologically interesting material have yet to be reported due to the extreme temperature/pressure conditions required to form hexagonal diamond [2, 3] as well as the inability to form a phase-pure sample without cubic diamond inclusions [4]. Our recent study reports formation of pure hexagonal diamond at a record low temperature of 400°C that is proposed to be due to a shear-induced plastic flow [5]. This region of pure hexagonal diamond was observed in an annular region around the central region containing a graphite-like structure. In this work, the proposed shear-induced transformation pathway from this graphite-like structure to the phase-pure hexagonal diamond is explored and a pathway to further lower the required temperature is presented.

Transmission electron microscopy of lamellae containing both the graphite-like and diamond structures was examined to provide microstructural evidence for the shear-induced transformation mechanism. Within the graphitic regions, striations associated with plastic flow of graphitic sheets can be observed. The graphitic sheets are aligned along the flow direction, with the amount of orientation within these sheets increasing with increasing shear. At regions of higher shear, these flowing graphitic sheets were observed to progressively “lock-into” hexagonal diamond within a mixed region where both structures were present. The distribution of the hexagonal diamond is in good agreement with modelled shear-strain distributions. Density consistent with turbostratic graphite and hexagonal diamond are measured within their respective regions, with the density increasing linearly in the transition region. Graphitic inclusions within the hexagonal diamond and transition region were also found to have decreased interlayer spacing with increasing shear. The interlayer spacing of the inclusions within the hexagonal diamond region having a lower interlayer spacing than uncompressed graphite, making the transition to hexagonal diamond more energetically favourable. This is evidence for a step-wise transformation from the graphitic precursor to the hexagonal diamond structure due to a shear-induced flow mechanism.

The authors acknowledge funding from the Australian Research Council (Discovery Project DP170102087)

[1] Pan, Z.C., et al., Harder than Diamond: Superior Indentation Strength of Wurtzite BN and Lonsdaleite. Physical Review Letters, 2009. 102(5).
[2] Kurdyumov, A.V., et al., The Influence of the Shock Compression Conditions on the Graphite Transformations into Lonsdaleite and Diamond. Journal of Superhard Materials, 2012. 34(1): p. 19-27.
[3] Kraus, D., et al., Nanosecond formation of diamond and lonsdaleite by shock compression of graphite. Nature Communications, 2016. 7.
[4] Salzmann, C.G., B.J. Murray, and J.J. Shephard, Extent of stacking disorder in diamond. Diamond and Related Materials, 2015. 59: p. 69-72.
[5] Shiell, T.B., et al., Nanocrystalline hexagonal diamond formed from glassy carbon. Scientific Reports, 2016. 6.

This contribution focuses on how impact modified polypropylene (iPP) is engineered to absorb energy and fail in a ductile fashion under extreme loading conditions. Herein, we report the results from a systematic investigation on three different polypropylene systems modified using soft block copolymers SEBS (Styrene-ethylene/butylene-styrene), POE (Polyolefinic elastomer) and a combination of SEBS and POE. Comparisons are made between extreme conditions (high strain rate at -15°C and -30°C) and quasi-static fracture tests at 25°C. The mechanisms of energy dissipation and fracture are then investigated in these systems with regard to soft particle size, spacing, shape, and other morphological features that govern their energy absorption.

At 25°C and quasi-static loading rates, the formulation containing lower modulus SEBS shows the highest fracture energy release rate when compared to the other two formulations. At extreme loading rates, the SEBS formulation still shows the highest energy absorption, but its relative performance is not as contrasting. Morphological investigation using a scanning electron microscopy (SEM) indicates the formation of phase-separated soft-particle domains with a larger particle size of 1-2 um for SEBS containing formulation as compared to a 200-300 nm domains for the other two formulations. Detailed investigation of failure mechanism shows the formation of interconnected crazes and a significant particle-particle interaction for polypropylene formulation containing SEBS, whereas, POE containing formulations show isolated craze formation. Failure mechanism in these formulations suggests that dilatational strain is relieved by a craze formation rather than cavitation and shear band formation. Results indicate that soft particle domains larger than the critical particle size (about 1um) governs the energy absorption for polypropylene under extreme conditions.

Herein, we also investigate the effect that an additional reinforcement in the form of talc has on these polypropylene formulations. It is observed that talc acts as a nucleating agent and improves the dispersion of soft particle domains. Morphology of talc containing formulations indicates a significant decrease in particle size, most predominantly for SEBS. These phase-separated domains have a particle size of 150 nm, smaller than the critical particle size. Room temperature fracture toughness evaluation for talc containing formulations show that SEBS containing formulation has 35% higher fracture energy when compared to the other two formulations. Interestingly, low-temperature testing at -15°C and -30°C with an impact velocity of 4 m/s shows that polypropylene formulation containing SEBS and POE in the presence of talc has 20% higher impact energy absorption as compared to the other two formulations. As seen for the formulations not containing talc, the mechanism of energy dissipation did not change; Craze formation provides release of the hydrostatic stress and acts as a major energy dissipating mechanism. Room temperature testing on polypropylene system shows that the lower modulus SEBS has a higher fracture energy release rate, with or without talc reinforcement.

Graphene aerogels (GAs) are a three-dimensional porous form of graphene with a high surface area. GAs have attracted a significant amount of interest in recent years due to their unique mechanical and electrical properties that show great potential for numerous applications in batteries, supercapacitors, electrochemical sensors and absorption of oil and organic pollutants. These unique properties are originating from a clever arrangement of two-dimensional graphene sheets in a three-dimensional porous monolith structure containing air-filled pores. Here, we investigate the mechanical and electrical properties of GA prepared via hydrothermal synthesis from graphene oxide without any stabilizing polymer. GAs prepared by this method are generally brittle materials because they contain a lot of defects and oxygen groups. However, the thermal annealing at temperatures over 1000 °C under inert gasses causes a significant improvement of the mechanical and electrical properties, resulting in highly flexible and electrically conductive GA materials. The high-temperature annealing causes healing or repair of defects, eliminating most of the oxygen from the structure, thereby enhancing the amount of π–π stacking between graphene sheets. The as-prepared GA exhibits superior compressive elasticity and structural integrity when subjected to the ultrahigh pressing (MPa).

Mechanical toughness of polymeric materials has long been understood to depend on the relaxations and interactions on the molecular level. Toughness arises as impact energy is dissipated through these molecular relaxations. However, the time and length scale of the molecular mechanisms are typically several orders of magnitude faster and more localized than the traditional experimental techniques used to characterize them.
Here, we employ an all optical laser-induced projectile impact test (LIPIT) to study the dynamic deformation response of polycarbonate thin films for impact mitigation applications. Micro-projectiles deposited on a polymeric launch pad are accelerated via laser ablation of an absorbing layer. The deformation response of PC films of different MWs and thicknesses are investigated at high strain rates (10⁵ to 10⁷ s^-1). LIPIT measurements can help to elucidate the rate-dependent ductile-brittle transition of polycarbonates and the molecular relaxation mechanisms that contribute to the toughening/dynamic stiffening properties that are desirable for impact mitigation applications.

With recent advances in dynamic scanning probe microscopy techniques, it is now a routine to image the sub-molecular structure of molecules with atomically-engineered tips which are prepared via controlled modification of the tip termination and are chemically well-defined. The enhanced spatial resolution is possible as atomically-engineered tips can preserve their integrity in the repulsive interaction regime. Although the mechanism of improved spatial resolution has been investigated both experimentally and theoretically, the ultimate temporal resolution while preserving picometer scale spatial resolution still remains an open question. Here, we computationally analyze the temporal resolution of atomic force microscopy imaging with atomically-engineered tips using previously developed computational models [1-3] . Our computational results reveal that non-metal terminated tips, e.g. oxygen-terminated copper, are well-suited for enhanced temporal resolution up to video rate imaging velocities while preserving picometer range spatial resolution. Contrarily, the highest-attainable spatial resolution of atomically-engineered tips with low-stiffness, e.g. CO-terminated, deteriorate with increasing imaging velocity. Our results reveal that when atomically-engineered tips terminated with molecules are in use, imaging velocities in the order of nanometers per second at most are inevitable even for atomically flat surfaces to retain the atomic resolution and avoid slip-stick motion. In addition to shedding light on the temporal resolution of atomic force microscopy imaging with atomically-engineered tips, our numerical results provide an outlook to the scalability of atom-by-atom fabrication using scanning probe microscopy techniques.

References:
[1] O. E. Dagdeviren, Physical Review Applied 11, 024068 (2019).
[2] O. E. Dagdeviren, Nanotechnology 29, 315704 (2018).
[3] O. E. Dagdeviren, Applied Surface Science 458, 344-349 (2018).

The helicoidal microstructures found in exoskeleton of mantis shrimp dactyl club, scarabaei, lobster provide them with remarkable toughness and high impact strength. During impact loading, these helicoidally arranged filaments tend to stretch, and twist along different planes and angles, absorbing energy, increasing the work of fracture and deflecting crack propagation. Such naturally occurring helicoidal architecture inspire us to develop bioinspired synthetic helicoidal structural materials to bolster toughness and impact resistance. Most existing studies employ conventional carbon epoxy or glass epoxy macroscopic prepregs to study the effect of helicoidal architecture in composites. We, however, use novel computer integrated near field solution electrospinning as an additive manufacturing method to fabricate 3D helicoidally arranged fiberous structures (micron/nano sized) using polyvinylidene fluoride. The aligned filament arrays are continuously collected one on top of the other with 45° angular offsets in polyvinyl alcohol bath to produce thin film composites (in range of 200 100 μm thick). This helicoidal arrangement of fibers in the matrix shows improvement in tensile behavior, crack propagation and toughness. These helicoidally oriented fiber composites show dramatical twisting and elongation of the fibers along the different directions, delaying the fracture events, as compared to its bulk counterparts. Further, surface treatment for interfiber and fibers-matrix adhesion shall be performed, and its influence on the mechancial characteristics of the helicoidal composite shall be studied.

Nanostructured systems under high-strain rate conditions, have been object of theoretical and experimental investigations in recent years. A recent joint theory-experiment study showed that carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs) under high-velocity impacts can be unzipped into nanoribbons [1]. However, the dynamics and fracture patterns of nanostructured systems under high-velocity impacts are not yet fully understood [2,3] and more studies are needed, especially involving hybrid systems, such as fullerenes encapsulated inside single wall carbon nanotubes (SWCNTs), the so-called CNT peapods [4]. In this work we have investigated C60 fullerenes encapsulated inside SWCNTs (CNT-C60) under high-strain rate conditions. Our theoretical study was carried out through fully atomistic molecular dynamics simulations with the reactive force field ReaxFF. We have considered CNT-C60 shoot at high-kinetic energy (ultrasonic velocities values varying from 1 km/s up to 60 km/s) against a rigid substrate and into two different orientation relative to the target (lateral and vertical configurations). Our results show that for specific velocities and orientations, CNT-60 experience large deformations and are fractured at different configurations: i) just fullerene ejections; ii) tube fracture; iii) tube fracture with fullerene coalescence; and iv) under extreme conditions the formation of amorphous carbon structures. These results provide helpful insights in understanding the structural changes and fracture dynamics of nanostructures under high-rate conditions.
[1] S. Ozden, P. A. S. Autreto, C. S. Tiwary S. Khatiwada, L. Machado, D. S. Galvao, R. Vajtai, E. V. Barrera, and P. M. Ajayan, Nano Letters v14, 4131 (2014).
[2] J. M. de Sousa, L. D. Machado, C. F. Woellner, P. A. S. Autreto, and D. S. Galvao, MRS Advances v1, 1423 (2016).
[3] C. F. Woellner, L. D. Machado, P. A. S. Autreto, J. M. de Sousa, and D. S. Galvao, Phys. Chem. Chem. Phys., v20, 4911 (2018).
[4] B. W. Smith, M. Monthioux and D. E. Luzzi, Nature v396, 323 (1998).

In this study, we aim to understand the connections between the negative Poisson's ratio and the pore structures of the auxetic polyurethane foam. The pore structure is visualized by X-ray tomography and key structure parameters were extracted. We found that the sphericity, tortuosity, bond angles are the most dominating structure parameters which determines the range and extense of the auxetic mechanical behaviors. In addition, the sensing performance of the CNT-coated polyurethane foams was also tested as a function of Poisson's ratio. The stronger the auxetic effect, the stronger the strain sensing capability.

Contact mechanics provides a method of characterizing how a material responds to local deformation under controlled loading conditions. The most widely used implementation of contact mechanics is indentation hardness, where a hard stylus of known geometry is pressed into a softer material to measure the resistance to permanent deformation. In this research the mechanical properties of engineering metals are assessed using the concept of frictional sliding where a stylus is indented into a material and then slides across the surface leaving a permanent scratch or groove.
Frictional sliding with a spherical stylus on metals results in a ductile response that can be related to the elastic-plastic response of a power-law hardening material through equations developed with finite element analysis (FEA) simulations. Experiments performed on more than 20 steel and aluminum materials has shown that this approach can predict the material yield strength and ultimate tensile strength with close agreement to traditional laboratory tensile tests on the same sample. When a wedge-shaped stylus is used during frictional sliding tests, a chip of material separates from the substrate through fracture processes. If a small gap is included on the upstream face of the stylus then material in this region is subjected to increasing elongation as the separated chip flows up the wedge until a tensile fracture occurs. FEA simulations of crack propagation in elastic-plastic materials and frictional sliding experiments on steel have shown that the geometry of the fracture surface can be correlated to the material ductile fracture toughness (J_C). This concept can be extended to monitor fatigue behavior by applying cyclic loading at varying proportions of the steady state cutting force to determine the number of cycles required to propagate an existing crack. Overall, these examples show how contact mechanics can be used to correlate how a metal microstructure responds during frictional sliding to traditional mechanical properties of the substrate, including hardness, yield and tensile strength, fracture toughness, and fatigue.

Magneto-active elastomers (MAEs) are soft polymeric materials embedded with magnetically responsive particles or carried with free currents, which respond to external magnetic fields by producing large deformations or varying mechanical properties. They have been increasingly used for many emerging applications such as soft robotics, printed untethered soft origami, magnetically anisotropic micro actuators, bistable switches and magnetoactive acoustic metamaterials. Thus, it is of interest and of paramount importance to develop a more rigorous continuum theory and numerical model to consider the strong field interaction in MAE analysis so as to have a deep understanding of the coupled mechano-magneto behavior of MAEs and facilitate the rational design of soft robotics with MAEs. In this paper, we present a continuum theory and finite element model to simulate the finite deformations of magneto-active elastomer (MAE) under applied magnetic fields or currents. The magnetic field is assumed to be measured in the background space with fixed spatial coordinates, rather than in the elastic MAE, leading to a concise and physically admissible continuum theory to describe coupled mechano-magnetic behaviors of MAE. Specifically, the magnetic field varies with prescribed free or localized currents and their spatial locations evolving with the motion of MAE, while the deformation of MAE is actuated by the surface or body forces applied by the external magnetic field or equivalent currents. A staggered finite element solution framework is developed to solve the strongly coupled governing equations in the two fields, and mesh distortion along the interfaces of MAE domain and free-space domain is resolved by considering concurrent deformation of the mesh in these two domains. Numerical examples are presented to demonstrate the validity and efficiency of the developed model for simulating the behaviors of MAE structures subjected to different sources of magnetic fields (e.g., free currents, magnetization or external magnetic field). This research offers a new method and mechano-magneto model for analyzing MAEs and will be useful for the rational design and analysis of MAE-based actuators and robotics in the future.

In this presentation, we will demonstrate an architected material for energy absorption based on the simple kirigami concept. We utilized our 2-dimensional hierarchical cut pattern named ‘fractal cut’ for an energy absorption structure. Both experimental and numerical analysis were carried out to systematically support the feasibility of our design concept. We found that the ‘Fractal cut’ structures can decrease the impulsive force of an incident object when implemented into soft materials because of high biaxial expandability. We were further able to optimize the effective design to dissipate the impulse energy through the dynamic analysis of fractal cut materials with various cut motifs and levels.

Tungsten (W) plasma facing components in magnetic fusion devices experience extreme dynamical power (>10 MW/m²) and particle (>10²⁴ m^-2s^-1) loads due to its interaction with the edge plasma [1]. The plasma-material interaction zone is a few nm deep, corresponding to the range of the implanted particles, leading to extreme concentration and thermal gradients. The large flux of implanted hydrogen or helium species, result in large near-surface strain fields due to their accumulation - driven by the low solubility of both hydrogen and helium in W. The resulting changes in the mechanical properties are critical to evaluate from the viewpoint of plasma operation and component lifetime issues. To date, the dynamical mechanical response of W under such non-equilibrium loading of hydrogen or helium far beyond its solubility limits is not well known due to: (1) the difficulty in probing the near surface properties, and (2) a lack of a suitable in-situ method that can be applied during plasma loading. This work focuses on overcoming the first problem, and we report on changes in the near-surface mechanical properties of W due to hydrogen or helium implantation using picosecond ultrasonics [2, 3]. W thin films (~40 nm) were implanted with deuterium (D) or helium using an ion or plasma beam, respectively. Ultrashort light pulses were used to excite and detect GHz-frequency elastic waves to determine the elastic modulus of the W thin films. It was found that the longitudinal elastic constant of the W thin film increased by up to 5 % following D implantation (1×10²³ m^-2), while helium had a significantly larger effect. The results suggest picosecond ultrasonic method is a sensitive probe for characterizing the near surface changes in mechanical properties of plasma facing materials, opening the way towards in-situ applications.

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[2] C. Thomsen et al., Phys. Rev. B 34 (1986) 4129.
[3] H. Ogi et al., Phys. Rev. Lett. 98 (2007) 195503.

Bulk tungsten is inherent room-temperature brittleness which limits its workability and performance in demanding applications. Pre-deformed tungsten microwires play a significant role in the strength, ductility, and damage tolerance of tungsten (W) fiber-reinforced metal matrix composites (MMCs) due to tungsten fiber’s ductile behavior and high tensile strength. Room-temperature fracture of extremely fibrous, pre-deformed tungsten wire does possess quite a different fracture in a ductile manner and substantial resistance to fracture at room temperature. It involves combinations of three fracture modes: cleavage, grain boundary delamination, and with individual grains necking to nearly 100% reduction in area. However, the difficulty of observing the action of individual tungsten fibers during plastic deformation has made it challenging to conclusively determine the mechanisms of a tungsten fiber-reinforced composite and presents a major bottleneck in the rational design of tungsten fiber-reinforced composites. Here, we conduct the in-situ tension of tungsten fibers as a promising platform to determine the precise role of individual tungsten fiber. It reveals that the continuous and massive dislocation nucleation and fast dislocation glide and escape by in situ TEM straining lead to the ultra-large elongation in nanoscale W fibers. This work could give insights for understanding pre-deformed tungsten wire at nanosacle and the design of pre-deformed tungsten wire based flexible devices and nanoscale tungsten fiber-reinforced composites.

It is well accepted that submicron- and nanometer-sized single-crystal metals demonstrate different mechanical properties from those of bulk counterparts; e.g. plastic resistance increases with decreasing specimen size. For the long time use of such nanomaterials, understanding of creep deformation and fracture is more important than plastic properties. It is significant to clarify whether small crystals can sustain high stress over a long period. The purpose of the study is to clarify the size effects on creep properties of submicron-sized single-crystal metals at room temperature and discuss the underlying mechanisms. We conducted long-term (up to 14 h) tensile creep experiments for submicron single crystal gold and aluminum under in situ field-emission scanning electron microscopy (FESEM) or transmission electron microscopy (TEM) observation. We directly measured the specimen elongation from the FESEM or TEM images to eliminate the measurement error in the displacement sensor owing to thermal drift and evaluate the creep strain accurately. The creep behavior transitioned from continuous to discrete manner as the applied stress decreased, which was different from that of bulk metals. In the small stress region, although the creep strain increased with the passage of time, an arresting region appeared in which the creep deformation temporarily stopped. The in-situ TEM observation suggested that the appearance of the arresting region was due to the depletion of dislocations by recovery or the reduction of dislocations and activated dislocation sources. The smaller specimens required higher stress to reach a similar strain rate, indicating that the resistance to creep deformation increased with the decrease of specimen size. We discussed the mechanisms of the size effects based on the experimental results.

Boron carbide materials possess attractive properties such as high hardness, lightweight and wear resistance, therefore it can apply various applications like cutting and grinding tools, ceramics bearing, etc. Boron carbide, so-called B₄C, has a relatively wide range solid solution; B_4.3C to B_10.6C. However, boron carbide has a problem in sintering especially in boron-rich region in boron carbide because of the low self-diffusion coefficient in boron and boron carbide caused by the strong covalent bonding of boron. For example, boron carbide bulk materials are mainly sintered over 2273 K^{1, 2)}. A few decades ago, Takagi et al proposed for new sintering method called “reaction boronizing sintering” for complex boride based hard materials³⁾. In the reaction boronizing sintering, boronizing reaction is occurred by solid-state diffusion between metal and boron in first and the metal boride is generated. After the boronizing reaction, the eutectic liquid phase is appeared by the pseudo-binary eutectic reaction between boride and metal. In this method, the eutectic liquid phase has important roles for chemical reactions and the densification of the boride sintering body. In this study, we applied “reaction boronizing sintering” method to the sintering for boron carbide materials. 3d transition metals such as Mn, Fe, Co, Ni and etc. were adding to boron carbide to generate eutectic liquid phase between boron and metal element. Metal added boron carbides were synthesized by powder metallurgy method from metal powders, amorphous B powder, and C powder. To prevent the remain of free carbon in the sintering body and investigate the effect of molar ratios of B and C on the sinterability and mechanical properties, chemical compositions of boron carbide were B_5.5C to B_8.5C. Starting materials were mixed and sintered by Spark Plasma Sintering at 1973K. To refinement for the crystal structure and phase ratios, Rietveld analyses were carried out using Rietan-FP software. To investigate the microstructures and dispersion of the elements for the sintering body, SEM-EPMA measurements were performed. With the increase of the Mn contents, Vickers hardness was increasing and it reached more than 3000 HV2 for Mn 10mol% added B5.5C, which value was almost equal to standard boron carbide sintered body. According to the SEM images of Mn added B5.5C, observed pores in the fracture surface were decreasing with the increase of the Mn content. We will discuss about the effect of metal doping and chemical compositions of boron carbides on the microstructure and the mechanical properties for the boron carbide.

References
1) D.O. Moskovskikh, et al., Ceramics International. 43, 8190-8194 (2017).
2) F. Neria, et al., Ceramics International. 44, 21842-21847 (2018).
3) K.Takagi, J. Jpn. Soc. Powder Powder Metall., 45, 507-514 (1998).

Ductile iron (DI) has attracted considerable interest in virtue of its good wear resistance, high strength together with good ductility, and high fatigue strength. A super DI with a multiphase structure was designed by a novel multi-step low-temperature austempering treatment. The influences of multiple microstructure on tensile strength and fatigue limit were examined applying tension and tensile-tensile fatigue tests. It showed that the tensile strength is over 1160MPa as well as an elongation of 3%, and fatigue life Nf exceeds 25.000 cycles in a high stress amplitude level of 600MPa, which were attributed to a synergistic strengthening and microcrack toughening from the super multiphase matrix comprising prior martensite (PM), bainitic ferrite (BF) and retained austenite (RA). It was found that the arrest and retardation of crack nuclei during either static stretching or dynamic deformation was controlled by a (BF+RA)_nano structure, which was formed around the PM.
The result can offer a great potential for super ductile iron in future critical application industry such as automotive gears under heavily impact loads combined with wear.

Small-scale metallic glasses have many applications in microelectromechanical systems (MEMS) and sensors which require good mechanical properties. Bending, tensile, compression properties of metallic glasses at micro/nano-scale have been investigated previously. Here, by developing a micro robotic system, we investigated the torsional behavior of Fe-Co based metallic glass microwires inside a scanning electron microscope (SEM). Benefiting from the in situ SEM imaging capability, the fracture behavior of metallic glass microwires has been uncovered clearly. Large plastic strain is found under torsional loading at room temperature, which could be evidenced by the large torsion angle. Through the postmortem fractographic analysis, it can be revealed that both spiral stripes and shear bands contributed to the fracture mechanism of the microscale metallic glass. Plastic deformation of the microwires included both homogenous and inhomogeneous plastic strain, which began with the liquidlike region, then a crack formed because of shear bands and propagated along the spiral direction. Moreover, three stages fracture mechanism of this metallic glass microwire was proposed. Although the metallic glass microwire broke in brittle mode, the shear strain was not lower than that of conventional metal wires. Moreover, we found an inverse relationship between the plastic strain and the loading rates.

Almost all natural and man-made surfaces are rough, and this roughness controls many interesting phenomena, such as adhesion or friction between bodies, or transport across interfaces. The scaling of surface heights is often self-affine. This has been observed over many length scales, from atoms to mountains. However, the origin of self-affine roughness is not entirely understood. Here, plastic deformation is investigated as a possible generative mechanism for self-affine roughness in perfect crystals, High Entropy Alloys and metallic glasses. Bi-axial compression of initially flat samples is simulated using molecular dynamics. It is shown that the subsurface deformation field exhibits a fractal, self-affine geometry, independent of the atomic structure of the material. During deformation, the fractal structure of the subsurface displacement field is imprinted onto the free surface. Self-affine roughness is therefore a recording of the non-affine deformation process occurring in the bulk.

Pore migration in solder joints driven by electric current and stress gradient is an important phenomenon that impacts the performance and reliability of electronic devices. It is a diffusional process at high homologous temperatures under large thermodynamic driving forces caused by applied electric fields and internal stress concentrations in complex microstructures. With the miniaturization of electronic devices, anisotropic properties of individual grains play increasingly important roles since a solder joint consists of only several grains. Tin-based solder alloys, the most common lead-free solder material, exhibit significant anisotropic characteristics in electrical conductivity, elastic modulus, and diffusivity. To investigate the effects of these anisotropic properties on pore migration, a phase field model is developed to simulate pore evolution in beta-tin polycrystals. In the model, mass diffusion describing pore migration is coupled with charge conduction and elastic deformation by solving microscopic Ohm’s law and microelasticity equations, respectively. The simulations reveal various anisotropy-caused pore migration behaviors including pore velocity and path, pore-pore interactions, and pore-grain boundary interactions. The results are analyzed in terms of internal electric and stress fields which relate the pore migration behaviors to the underlying grain structures.

Multiferroic composites have been an area of recently increased research interest due to their potential in magnetoelectric coupling applications to increase efficiency and reduce component size when compared to traditional electromagnetic devices. Multiphase magnetoelectric composites exhibit coupling coefficients an order of magnitude larger than their single-phase counterparts by way of strain mediation of magnetic and electric energies. Concentric, toroidal ring structures are of a particular interest as they exhibit greater magnetoelectric couplings compared to similarly sized stacked laminated structures. In this study, a multiferroic ring composite consisting of lead zirconate titanate (PZT) piezoelectric ceramic concentrically bonded to Terfenol-D magnetostrictive alloy was investigated. To achieve their magnetoelectric coupling potential, these structures are typically operated at or near their resonant frequencies, hence a pre-existing crack at the interface propagates and degrades the strain mediation between the piezoelectric and magnetostrictive phases. Thus, a pre-defined crack was introduced at the interface, where the composite structure was subjected to an A/C electric field with near-resonant frequencies concurrently with a magnetic field corresponding to saturation. The crack behavior is reported along with its effect on the overall magnetoelectric performance of multiferroic composite ring structures.

We present experimental results on collective dynamics and self-assembly in a suspension of Quincke electro-rotating granular particles (1). Depending of the value of the electric field or the concentration of the particles is it possible to find different active matter steady states such as gas or polar vortex structures. We perform Particle Image Velocimetry in order to compute the vorticity and velocity of the systems and also simple tracking procedure are performed as a complementary way of characterization (2,3). A striking new result is that the size of the vortices can be controlled with the electric field and also, that the shapes of the vortices are independent of the container' s shape. The results points to future applications in materials science to fabricate nano-/micro-structured surfaces with macroscopic functionalities including photonics, wetting, porosity and permeability.
(1) A. Bricard et al, Nature 503, 95 (2013)
(2) H. H. Wensink et al, PNAS 109,14308 (2012)
(3) A. Doostmohammadi et al,, Nature Communications 7, 10557 (2016)

The shock response of multiphase metallic microstructures comprising of immiscible fcc/bcc phases is determined by the shock wave propagation and reflection behavior that generates heterogeneities in the microstructure such as dislocation slip and deformation twinning, which can determine the dynamic strength, i.e. the spall failure, of the microstructure. For pure fcc-Cu and bcc-Ta microstructures, MD simulations indicate a contrasting role of twinning in the void nucleation stresses of both phases. An increment in the density of twin partials in Cu increases the dynamic strength, whereas in Ta it decreases as the twin volume fraction increases. However, the role of structure, the spacing of interfaces on the shock wave propagation, and the interplay between dislocation slip and deformation twinning have not been widely studied for these types of materials. Thus, large scale molecular dynamics simulations are carried out to investigate the role of deformation twinning in multiphase tri-layer microstructures comprising of Cu/Ta/Cu layers with interfaces perpendicular to the shock loading direction for layer thicknesses ranging from 100 nm to 500 nm. These MD simulations suggest that shock wave reflections at the Cu/Ta interfaces affect the twinning/de-twinning behavior in the bcc-Ta phase that can result in modifications in the spall strengths of the multiphase alloy. The links between twin volume fractions, dislocation densities and spall strengths in Cu-Ta multilayered structures will be presented.

Due to the incomparable strength of alloyed Iron among metals, an immense interest has been generated to characterize the deformation/phase transition behavior of single-crystal Iron under shock loading conditions for its applicability in extreme environments. Shock compression of Fe-based microstructures shows an interplay between dislocation slip, deformation twinning, and phase transformation behavior. Ductile failure behavior under the dynamic loading conditions (spallation) is characterized by nucleation, growth and coalescence of voids. Such failure transpires due to the interaction of reflected waves; forming triaxial tensile stresses in a microstructure deformed during the shock compression. For single crystal microstructures, the mechanical behavior of the system strongly depends on the crystal orientation and shock-loading conditions that render variations in dislocation plasticity, deformation twinning, and phase transformation behavior. This emphasizes the need for an in-depth understanding of the microstructure evolution of material under shock loading, and the interaction of voids, slips, twining, etc., mechanisms, which tailors the spall failure at the atomic scale. Molecular dynamics (MD) simulations are carried out to investigate the spallation behavior of single-crystal Fe systems-oriented along [110], [112], and [111] directions subjected to various impact velocities ranging from 750-2000 m/s. The MD snapshots are characterized to quantify the evolution of twin volume fractions, fractions of HCP phase, and damage (void sizes and fractions) for the range of pressures generated. The effect of crystal orientation and impact velocities on the deformation response and the spall strength will be presented.

The interaction between glissile dislocations and precipitates within a continuum is responsible for marked increases in material strength. Due to their desirable engineering features, dislocation-precipitate interactions have been the subject of study for decades. Towards enhancing our mechanistic understanding of the dislocation-precipitate bypass process, we present an analytic model of the Orowan bypass stress (τ_Orowan) required for a dislocation to bypass an array of precipitates. We initially consider spherical precipitates described by a diameter (D) and inner precipitate spacing (L). Our model suggests a τ_Orowan scaling logarithmically with the precipitate diameter, τ_Orowan ∼ ln De^-D/L, which we validate against a well established, yet empirical model. We also examine the influence of precipitate aspect ratio on τ_Orowan. Finally, we demonstrate the application of our model towards predicting the scaling of τ_Orowan for an array of plate-like θ'' precipitates within an Al-Cu molecular statics materials system. Our analyses provide insight into relationships between precipitate size, shape, density, orientation and metallic strengthening mechanisms.

A dislocation-density based multiple slip crystalline plasticity formulation and a new computational fracture approach have been used to investigate and predict thermo-mechanical fracture in hexagonal close packed (h.c.p.) materials with a focus on h.c.p. alloys with hydrides that have different crystalline structures than that of the matrix. This predictive framework has been used to understand and predict the interrelated effects of dislocation-density interactions, generation, and recovery on the competition between intergranular and transgranular crack nucleation and propagation The validated predictions indicate that transgranular fracture is dominated by dislocation-density interactions with hydrides and intergranular fracture is dominated by GB misorientations. The proposed modeling framework can provide guidelines for a fundamental understanding of materials subjected to thermo-mechanical loading conditions, such that failure resistant material systems can be attained for extreme loading conditions and environments.

Comprehensive knowledge of crack growth mechanisms is vital to monitoring and predicting failure in ductile materials such as aluminum. Mechanical testing, microscopy, and tomography of single-phase aluminum grades 1100 and 1050 were applied to illustrate Al failure mechanisms and how those mechanisms changed with alloy composition. Al1100 and 1050 have been previously overlooked in favor of high purity or more specialized grades. Through low strain rate uniaxial tension testing, Al dogbone samples were strained to separation to reveal their fracture surfaces. The fracture surfaces exhibited smooth, void-free edge regions and dimpled centers with a smooth transition and were oriented perpendicular to the loading direction. The texture of the fracture surfaces supports the assumption that Orowan alternating slip (OAS) contributes to the fracture mechanism for dog Al1050 and Al1100 but the perpendicular fracture angle indicates that intervoid necking is another contributing mechanism. Center hole rectangular Al samples were strained under the same conditions as the dogbone samples but paused at key points to monitor crack formation and growth using computed tomography. Diamond-shaped cracks extended from the center hole towards the sample edge surrounded vertically and laterally by small voids with larger voids just beyond the crack tip. As strain increased, voids at the crack tip coalesced with the growing crack. The crack shape is indicative of OAS while the existence and coalescence of voids indicate intervoid necking. These mechanisms work together and describe Al1100 and 1050’s complicated growth behavior more effectively than a single mechanism would allow.

Acknowledgement: Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

We present a thermodynamic description of crystal plasticity. Our formulation is based on the Langer-Bouchbinder-Lookman (LBL) theory of dislocation motion, which asserts the fundamental importance of an effective temperature that describes the state of configurational disorder and therefore the dislocation density of the crystalline material. We extend the LBL description from isotropic plasticity to crystal plasticity with many slip systems. Finite-element simulations show favourable comparison with experiments on polycrystal fcc copper under uniaxial compression. The thermodynamic theory of crystal plasticity thus provides a thermodynamically consistent and physically rigorous description of dislocation motion in crystals. We also discuss new insights about the interaction of dislocations belonging to different slip systems.

The performance of a crystal material depends on the evolution of underlying microstructures, e.g. dislocations. Continuum dislocation dynamics (CDD) is an effective tool to study the evolution of dislocations at microscale. In CDD, curved and connected dislocation lines are represented by density-like field variables. The evolution of dislocation systems are obtained by solving a set of transport equations. Here we discuss how the processes of dislocation reactions in which two interacting dislocations form junctions can be described within such a framework. The basic idea is that dislocation reactions are done by subtracting a dislocation density vector from each reacting slip system and adding to the corresponding junction density. This will result in an additional reaction term in dislocation transport equations. Criteria including both Burgers vectors and dislocation line directions are established to determine whether reactions should happen. The model is solved within a 3D vector-density-based dislocation dynamics. Simple dislocation structures are tested for junction formation, followed by a loaded plastic deformation simulation. The results show that the model is able to capture the change of dislocation network due to reactions and the mechanical response is affected by dislocation reactions.

Uniaxial micropillar compression tests on a CaFe₂As₂ single crystal have recently revealed the existence of extremely large elastic deformation, superelasticity, with an elastic limit up to 13%. The collapsed-tetragonal phase transition, which is the uni-axial process of making As-As bonding, is the main mechanism of superelasticity and differs entirely from the shear process of martensitic-austenitic phase transition in conventional shape memory alloys. Usually, superelasticity and its related structural transition are strongly affected by defect structures. In order to improve the superelastic performance of CaFe₂As₂, therefore, it is necessary to understand how defect structure influences the collapsed-tetragonal phase transition and the corresponding superelastic behavior.
In this study, therefore, we investigated the effects of microstructure on the superelasticity of [0 0 1]-oriented CaFe₂As₂ micropilllar using the state-of-the-art in-situ micromechanical testing and transmission electron microscopy. Three different samples (Sn-solution grown, quenched FeAs-solution grown, and annealed FeAs-solution grown crystals) were prepared to produce different microstructures. Transmission electron microscopy showed that while a Sn-solution grown sample is nearly defect-free, a quenched FeAs-solution grown sample contains the extremely dense rectangular network of screw dislocations on a (0 0 1) plane. An annealed FeAs-solution grown sample contains nano-scale FeAs precipitates with coherent phase boundaries. Rectangular network of screw dislocations in a quenched FeAs-solution grown sample resembles the low angle twist boundary, and the corresponding lattice distortion would make the formation of As-As bonds more difficult, leading to the suppression of collapsed tetragonal phase transition as well as superelasticity. An annealed FeAs-solution grown sample showed the similar mechanical behavior with a defect-free Sn-solution grown sample, but fracture often occurred at a lower stress. The presence of nanoscale FeAs precipitates would act as a stress concentrator, which cause the early brittle fracture. In sum, superelasticity of CaFe₂As₂ is significantly influenced by defect structures, and our research outcomes will be generally useful to design the enhanced superelastic performance in other ThCr₂Si₂-type intermetallic compounds with the same crystal structure.

Interface morphology and other microstructural heterogeneities ultimately determine thin film performance under extreme conditions, especially high-strain rate deformation. Synthesizing novel and complex thin films with morphologies that go beyond single crystal and single-phase materials, is needed to understand the properties exhibited by multiphase materials under dynamic loading conditions. In this work we present a new PVD sputtering technique to process topologically complex and bicontinuous thin film microstructures that are dense with interfaces. Using the innovative High-power Impulse Magnetron Sputtering, or HiPIMS, technique to co-deposit immiscible Cu and Fe, the direction of phase separation with respect to a substrate can be manipulated via the deposition conditions and the introduction of metal ions during deposition that is unique to HiPIMS. This level of control -- which can be expanded to other FCC/BCC immiscible systems -- over phase separated morphologies allows for governable interface crystallography, chemistry, shape, and defect structure. All of these parameters will broaden understanding of the dynamic mechanical response of complex, multiphase materials.

The emerging concept of multiple-principle element alloys have received tremendous interest in the last decade due to the unprecedented mechanical properties [1]. The CrCoNi-based, fcc single-phase alloy exhibits exceptional tensile ductility, fracture toughness and impact resistance thanks to its continuous work hardening rate, high frictional forces on dislocations and the tendency of nano-twin nucleation. The ability of maintaining the mechanical properties at very large plastic strain is especially attractive for potential structural applications.

Traditionally, the CrCoNi MEA is treated as a random solid solution, where atoms form a random distribution in the lattice. Nevertheless, it is reported that chemical short-range order could form in the alloy to lower the energy, thus, changing the stacking fault energy of the alloy and affecting the mechanical properties [2]. Such formation, along with the impact on the mechanical properties has been experimental observed in our previous study. The formation of SRO significantly increases the stacking fault energy and induces a wavy-to-planar slip transition.

In binary alloy systems, the formation of SRO usually leads to deformation localization and jeopardized ductility due to the glide plane softening effect and the subsequent planar dislocation slip [3, 4]. However, in the CrCoNi MEA, the ductility remains unaffected with the SRO and planar slip present. Microscopically, the formation of a nano-twin/planar slip network was observed despite the larger stacking fault energy. We speculate that the planar slip could reduce the local stacking fault energy by interrupting the SRO structure and provide nucleation sites for deformation twinning. A new twinning mechanism was proposed based on the TEM observation and theoretical calculations.

[1] Li, Z., Zhao, S., Ritchie, R. O. & Meyers, M. A. Mechanical properties of high-entropy alloys with emphasis on face-centered cubic alloys. Prog. Mater. Sci. 102, 296–345 (2019).
[2] Ding, J., Yu, Q., Asta, M. & Ritchie, R. O. Tunable stacking fault energies by tailoring local chemical order in CrCoNi medium-entropy alloys. Proc. Natl. Acad. Sci. 115, 201808660 (2018).
[3] Gerold, V. & Karnthaler, H. P. On the origin of planar slip in f.c.c. alloys. Acta Metall. 37, 2177–2183 (1989).
[4] Van De Walle, A. & Asta, M. First-principles investigation of perfect and diffuse antiphase boundaries in HCP-based Ti-Al alloys. Metall. Mater. Trans. A Phys. Metall. Mater. Sci. 33, 735–741 (2002).

SAM2×5 is an amorphous steel with a chemical composition of Fe_49.7Cr_17.7Mn_1.9Mo_7.4W_1.6B_15.2C_3.8Si_2.4. In this study, we have determined the crystallinity of the dense specimens of our bulk metallic glass using a novel differential scanning calorimetry technique. The dense specimens were prepared using the spark plasma sintering (SPS) technique at a heating rate of 500°C/min, a pressure of 100 MPa, and temperatures ranging from 615 to 675°C. Room and high temperature X-ray diffraction results showed that increasing the SPS temperature resulted in the devitrification of mainly carbide-based phases from the amorphous matrix. TEM observations showed that crystalline domains of less than 20 nm in size are uniformly distributed within the amorphous matrix. The average macro and micro compressive strength values of the specimens was between 1.7 and 2.2 GPa. Split Hopkinson Pressure Bar tests were carried out to measure the mechanical response of the sintered specimens under dynamic deformation, showing a yield strength of 2200 MPa under a strain rate of 900 s^-1.

Cu-X alloys, where X is a BCC metal from group V or VI in the periodic table, are nearly completely immiscible through a wide range of compositions and processing temperatures under the melting temperature of Cu. Cu-X thin films produced via physical vapor deposition will self-assemble into phase separated regions during fabrication. These phase separated regions have atomically resolved interfaces that impede the movement of dislocations in the system, resulting in elevated mechanical performance compared to bulk samples. The extent of elevated mechanical performance will depend on the microstructure morphological geometries. Micron thick films of Cu-Ta were deposited via magnetron sputtering at four distinct temperatures: 25, 400, 600, and 800 °C. STEM and TEM characterization revealed the films deposited at room temperature to consist of nanocrystalline Cu-Ta matrix and films deposited at elevated temperatures as having interwoven bicontinuous structures with large Cu islands within a Cu-Ta matrix. Nanoindentation has demonstrated a direct correlation between the morphologies and the observed mechanical properties of hardness and Young’s modulus. The distinct morphologies arise due to the highly disparate temperature-dependent mobilities of the constituent elements. The morphologies can be affiliated with applications in extreme mechanics as a bicontinuous structure adds a third dimensional component to the dislocation-interface interaction and the exact thickness of phase separated regions in the film will control the dominant yielding mechanism during deformation.

We report theoretical and experimental investigations on the structure of strontium oxalate and Raman spectra at high-pressure. The system has shown progress in the generation of CO₂ and in the synthesis of high-energy polymeric carbon monoxide after X-ray irradiation facilitating reactions such as: SrC₂O₄ +hv -> SrCO₃ +CO₂ + poly-CO [1]. Density functional perturbation theory was used to calculate the zone center optical phonons and to identify the vibration modes in term of atomic displacements. The simulations were compared to experimental Raman spectra and previous IR spectroscopic studies [1] in an effort to elucidate the details of phase transition between monoclinic and triclinic phases. Additional calculations of the phonon dispersion and density of states, as well as the electronic band structure and elastic moduli were performed to gain better insight into the phase behavior in strontium oxalate under high pressure.
[1] E. Evlyukhin, E. Kim, P. Cifligu, D. Goldberger, S. Schyck, B. Harris, S. Torres, G. R. Rossman and M. Pravica, J. Mater. Chem. C 6, 12473-12478 (2018).

Permanent, plastic deformation occurs in crystalline metals that are subjected to large strains. This is due to the irreversible interactions between dislocations, and between dislocations and microstructural features such as grain and twin boundaries. Recently, reversible deformation from large strains has been observed in a number of metallic nanostructures^1,2. Rapid surface diffusion and reversible dislocation motion have been proposed as two possible mechanisms for this behavior,^1,3 but the correct mechanism has not been determined conclusively.

To resolve these issues, 3.9 nm Au nanocrystals were compressed under non-hydrostatic pressure in a diamond anvil cell (DAC) up to ~7 GPa. These nanocrystals were colloidally synthesized, and contained twins. Structural changes were observed using in-situ x-ray diffraction (XRD), optical absorbance spectroscopy and MD simulations. We observed that XRD peak position and LSPR peak position recovered completely with pressure cycle, which indicates that the elastic strain in the lattice and the shape of the nanocrystal were recoverable. The XRD peak width increased with pressure and remained at larger values even after unloading, which indicates that the defects like stacking faults and dislocations were nucleated in the nanocrystals and remained in the nanocrystal even after unloading. LSPR peak height also reduced with pressure cycle, which corroborates the XRD results and indicates that defects form in the nanocrystals.

MD simulations were used to compress nanocrystals of a similar size. Partial and full dislocations were nucleated at the surface of the nanocrystals. Some of the full dislocations interacted strongly with twin boundaries within the nanocrystals and were present in the nanocrystal even after unloading. XRD patterns were generated from the MD simulations, and found to be similar to the experimental XRD patterns. Hence, we conclude that the interaction of dislocations and twin boundaries is responsible for the permanent deformation observed in the nanocrystals. This dramatically changes our current understanding of dislocation nucleation and recovery in such small nanocrystals.

References
1 Sun, J. et al. Nat. Mater. 13, 1007–1012 (2014)
2 Gu, X. W. et al. Phys. Rev. Lett. 121, 056102 (2018)
3 Bernal, R. A. et al. Nano Lett. 15, 139–146 (2015)

Separate disks of Al and Mg were mechanically bonded through high-pressure torsion (HPT) processing for 100 turns under a compressive pressure of 6.0 GPa. The material after processing was followed by natural aging at room temperature for 60 days in order to evaluate the microstructural stability. The materials characterization revealed that such high straining through the HPT processing synthesized a bulk nanostructured metastable Al with grain sizes of 35-40 nm in a state of supersaturated solid solution with the measured maximum Mg solubility of ~38.5 at.%. A superhard Al solid solution with the maximum Vickers microhardness value of ~370 was observed consistently across the disk diameter and the high hardness was considered mainly due to grain refinement and solid solution strengthening. X-ray diffraction analysis was applied to quantitatively evaluate the supersaturated solubility of Mg in Al and the stability of metastability with computing microstrain and crystallite size of the aged material. Significantly high microstrain of 0.0202 was observed for the metastable Al and it was even enhanced to 0.0274 after natural aging. It was attributed solely to the high supersaturation of Mg solute in Al. Moreover, advanced analysis using high-energy synchrotron radiation was conducted to visualize the uncovered phase and texture heterogeneity in the metastable Al alloy. This study demonstrates one of the ultimate states of metals when they are diffusion bonded mechanically under severe plastic straining at ambient temperatures.

The synthesis pathway to diamond from graphitic precursors using high pressures and temperatures has been studied and applied for many decade [1]. Industrial manufacturing methods require temperatures up to 1500 K, pressures between 5-6 GPa, and the use metallic catalysts [1]. Without these catalysts the pressure and temperature conditions required are far more extreme to overcome kinetic energy barriers. Recently, there has been considerable interest in the synthesis of pure, catalyst-free diamond and other novel forms of carbon from non-crystalline precursors, with the hope that the energy barrier impeding transformation will be reduced. For example, nanocrystalline diamond has been formed from carbon nanotubes following compression to 17 GPa at 2500 K [2]. The size of the nanocrystals were found to be similar to the diameter of the nanotubes, demonstrating that the type of precursor can influence the microstructure of the resulting diamond. Sumiya et al. observed the formation of nanocrystalline diamond (<10 nm in diameter) from a glassy carbon (GC) precursor when compressed for 10s at pressures of ~20 GPa and temperatures of ~3000 K [3]. When compressed for longer times, the nanocrystalline diamond evolved into larger crystals then exhibited a lamellar structure. Recently, it has been reported that it is possible to create an amorphous form of diamond has been synthesisd from GC following compression to 50 GPa and laser heating to ~1800 K [4]. However, this previous work has only investigated the compression of non-crystalline graphitic precursors within a limited temperature range. There is a need for a more thorough experimental study at temperatures above 3000 K, in the region of the PT diagram encompassing the liquid-diamond phase boundary.
In this study, GC samples were loaded into diamond anvil cells with an Ar pressure medium and compressed to 16 GPa. A 1070 nm pulse laser was then used to heat the samples to temperatures ranging from 1900-4500 K. A total of 15 samples were made for ex-situ analysis using raman spectroscopy, scanning electron microscopy and transmission electron microscopy. At low temperatures (1900-2200 K), the GC was found to have transformed into an oriented graphitic material in which its graphene layers are preferentially aligned perpendicular to the compression axis. Nanodiamonds (~10-200 nm) begin to form near the surface of the GC at temperatures of ~2200 K. These nanodiamonds increase in size and density as the temperature increases up to 4500 K. Interestingly, above ~3500 K voids were observed in the microstructure, some of which contained Ar, which appears to have an epitaxial relationship with the surrounding diamond. This observation supports the proposition that at these high temperatures, the GC may have entered a liquid state prior to the formation of diamond crystallites.

The authors acknowledge the Australian Research Council for financial support (Discovery Project #DP170102087).

References:
[1] F. P. Bundy, W. A. Bassett, M. S. Weathers, R. J. Hemley, H. U. Mao, and A. F. Goncharov, “The pressure-temperature phase and transformation diagram for carbon; updated through 1994,” Carbon N. Y., vol. 34, no. 2, pp. 141–153, 1996.
[2] H. Yusa, “Nanocrystalline diamond directly transformed from carbon nanotubes under high pressure,” Diam. Relat. Mater., vol. 11, no. 1, pp. 87–91, 2002.
[3] H. Sumiya, H. Yusa, T. Inoue, H. Ofuji, and T. Irifune, “Conditions and mechanism of formation of nano-polycrystalline diamonds on direct transformation from graphite and non-graphitic carbon at high pressure and temperature,” High Press. Res., vol. 26, no. 2, pp. 63–69, Jun. 2006.
[4] Z. Zeng et al., “Synthesis of quenchable amorphous diamond,” Nat. Commun., vol. 8, no. 1, p. 322, 2017.

Small scale metal forming, with characteristic forming dimensions ranging from millimeters down to micrometers, is of current interest in manufacturing of miniaturized components and devices. As compared to subtractive manufacturing techniques such as micro mechanical milling and micro electrical discharge machining, small scale metal forming offers the potential of high throughput, low cost, and dimensional repeatability. Metal forming operations typically involve deformation to large plastic strains [1]. Changes in the mechanical response of materials during forming as the characteristic forming dimension decreases to the meso/micro scale prevents straight forward adoption of macroscale forming practices to the small scales, such changes thus need to be better understood.

We report our recent results on characterizing the mechanical response and deformation characteristics of Cu in microscale axisymmetric reversion extrusion [2]. Specifically, the characteristic plastic strain for the extrusion process and the influence of the initial grain size of extruded material on the extrusion mechanical response and shape of extruded parts were examined through scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). We show that the mechanical response of extrusion exhibits deviations from continuum scaling behavior as the characteristic forming dimension becomes small as compared to the initial grain size. Materials based justification to the observations will be discussed.

References:
[1] B. Zhang, Y. Song, G.Z. Voyiadjis, W.J. Meng, Assessing texture development and mechanical response in microscale reverse extrusion of copper, J. Mater. Res. 33, 978-988 (2018).
[2] B. Zhang, M. Dodaran, S. Ahmed, S. Shao, W.J. Meng, K.J. Juul, K.L. Nielsen, Grain size affected mechanical response and deformation behavior in microscale reverse extrusion, Materialia 6, 100272/1-13 (2019).

Boron carbide (B₄C) is a hard and lightweight material, which has many engineering applications. However, B₄C loses its strength and toughness when subjected to high shear stresses. To improve its mechanical properties, the pervious computation work has suggested micro-alloying B₄C with Si. Very limited understanding of the failure mechanism of boron carbide, both Si-free and Si-doped, under high shear stress conditions is due to the lack of direct experimental observation at the relevant length scale for damage zone. Here we investigate the local deformation microstructure of regular and Si-doped boron carbide under indents, using a novel precession electron diffraction technique and high-resolution transmission electron microscopy. We observed that Si-doped boron carbide displays dispensed micro-cracks, while Si-free boron carbide exhibits major local cracks and low interfaces.

Developing revolutionary lightweight protective materials with superior specific energy dissipation and specific strength is critical for many impact mitigation applications—from ballistic protection of soldiers to foreign object damage prevention in air and spacecraft. Recent advancements in high strain rate experimental characterization techniques have revealed the excellent properties of nanoscale polymer thin-films over conventional bulk polymers. For glassy polymer thin-films, decreasing the film thickness effectively modify the polymer chain morphology and mobility, therefore bringing additional energy dissipation mechanisms, such as crazing, yielding, and adiabatic heating, which increases the projectile’s kinetic energy required to perforate the film. Comparing to glassy amorphous polymer thin-films, semi-crystalline polymers exhibit more complex deformation mechanisms, being strongly related to their microstructure, including crystallinity and molecular conformation. In this work, we study the dynamic mechanical behavior of PVDF-TrFE polymer thin-films with thicknesses varied from 75 nm to 400 nm using a micro-ballistic testing apparatus with projectile speeds ranging from 200 m/s to 1000 m/s. We examine the size- and strain-rate-dependency of the specific penetration energy and the dynamic deformation mechanisms of the thin-films. The polymer films that are thinner than 100 nm exhibit superior protective properties, with specific penetration energy as high as 3.7 MJ/Kg for 1000 m/s impact, about 5 times larger than bulk steel plate from macroscopic experimental measurements. It is demonstrated that as the film thickness is decreased or impact projectile speed is increased, there is increase in the specific penetration energy and evident transition in failure mechanisms of the film. The microstructure of PVDF-TrFE thin-films is further modified by applying appropriate heat-treatment. We additionally study the relationship between intrinsic crystalline microstructure and dynamic mechanical properties in semi-crystalline polymer thin-films. Understanding those key structure-property relations in nanoscale semi-crystalline polymer thin-films will provide promising approaches for developing novel flexible and lightweight polymer-based protective materials.

Electroceramics are often subject to high temperature, reducing/oxidizing environment and/or applied electric field. Such is the case for stabilized zirconia during field-assisted processing and many applications in solid oxide fuel/electrolysis cells, thermal barrier coatings and others. Against the conventional wisdom that stoichiometry and microstructure of zirconia is insensitive to reduction because of its large bandgap of ~5 eV, we found hugely enhanced grain growth under severe electrochemical reduction, creating (i) a graded microstructure with 100 times grain size difference at the cathode and anode side and (ii) a sharp grain size transition halfway across the electrically loaded sample. Continuum-level transport theory and first-principles calculations were conducted to gain better mechanistic understandings, which bring out the hidden yet critical role of electronic defects (localized electrons on Zr⁴⁺, i.e. reduced cations; and localized holes on O²⁻, i.e. oxidized anions) on microstructural evolution in nominally ionic compounds under extreme conditions.

References
[1] Y. Dong, H. Wang, I.W. Chen, Electrical and Hydrogen Reduction Enhances Kinetics in Doped Zirconia and Ceria: I. Grain Growth Study, J. Am. Ceram. Soc. 100 (2017) 876-886.
[2] Y. Dong, I.W. Chen, Electrical and Hydrogen Reduction Enhances Kinetics in Doped Zirconia and Ceria: II. Mapping Electrode Polarization and Vacancy Condensation, J. Am. Ceram. Soc. 101 (2018) 1058-1073.
[3] Y. Dong, L. Qi, J. Li, I.W. Chen, Electron Localization Enhances Cation Diffusion in Transition Metal Oxides: An Electronic Trebuchet Effect, arXiv preprint arXiv:1808.05196 (2018).

Fracture toughness of a material depends on the crack path, which in turn depends on the material microstructure. In a variety of technologically important structural materials, fracture involves preferential crack nucleation and growth along the grain boundaries particularly under extreme environmental loading conditions. For example, stress corrosion cracking, hydrogen embrittlement and low damage tolerance are all dominated by the grain boundaries. Over the past decades, our understanding of the relation between specific grain boundaries and their resistance to the aforementioned processes has increased along with the ability to choose processing routes leading to specific grain boundary character distributions, geometrical properties of the grains and chemical parameters. Recently, we have shown that the crack growth resistance of a crack propagating through a grain boundary network can be described by discrete unit events, and the final crack path in a specified grain boundary network is not necessarily the path of least resistance. This raises an important question; if the crack path is not dictated by the path of least resistance then can we change the crack path through a grain boundary network by introducing heterogeneities such as weak grain boundaries. The extent to which an admixture of weak grain boundaries can enhance the fracture toughness of a grain boundary network and the corresponding toughening mechanism will be discussed.

Thermal transport properties of minerals and melts at high pressures and temperatures is of central importance to the evolution and dynamics of planets. The pressure of the Earth’s interior continuously increases with the depth from the surface of the Earth: several hundred MPa at the region of the crust, ∼20 GPa at the upper mantle and ∼130 GPa at the lower mantle. Precise data of thermal conductivity and/or thermal diffusivity for minerals at elevated pressure make it possible to estimate the heat budget in the Earth.
In the Earth’s core and mantle, thermal conductivity of minerals containing dominantly iron-bearing silicates and iron alloys defines the heat and energy flow as well as the geodynamo over the Earth’s history. Direct measurements of thermal conductivity of Earth minerals at extreme pressures and temperatures are very challenging; the available data are limited, inconclusive, and not corresponding to the theoretical calculations. Therefore, there is a lack of experimental data, which is currently substituted by extrapolations and estimations.
In the present work, we have studied the direct measurements of thermal conductivity of iron-bearing and non-iron-bearing minerals as well as iron alloys at different high pressure and temperature conditions up to 130 GPa using diamond anvil cells with continuous and pulse laser heating. Fast measurements allowed us to resolve the heating response through the sample and understand the tendency of minerals behavior within the mantle and core. The analysis of the obtained experimental data is currently in progress and the results will be reported at the conference.