Symposium ST02—In Situ Mechanical Testing of Materials at Small Length Scales, Modeling and Data Analysis

Polymeric coatings having nano-to-microscale thickness show immense promise for enhancing thermal transport, catalysis, energy conversion, and water collection. Characterizing the work of adhesion (G) between these coatings and their substrates is key to understanding transport physics as well as mechanical reliability. Due to the rapid development of nanoscale devices and coatings, there is an increasing need to develop methods to quantitatively measure the work of adhesion at ever smaller length scales, both in terms of lateral spatial resolution (< 100 µm) and film thickness (< 100 nm). Experimentally quantifying the work of adhesion of nanoscale films with high spatial resolution is difficult. The most common method for measuring work of adhesion involves characterizing the contact or ‘pull-off’ force between a probe coated with the thin film of interest and the surface of interest using atomic force microscopy. However, surface force measurement is highly sensitive to adsorption of airborne hydrocarbons or water vapor on the test surface. To obtain accurate measurements of adhesion, it is necessary to separate the two surfaces of interest prior to interaction with the surrounding atmosphere. Conventional mechanical approaches for adhesion energy measurement, such as the peel test and four-point bending test delaminate the film from its substrate directly, thus preserving the interface. However, these methods are only appropriate at larger scales (> 1 mm lateral length for adhesion measurement) due to the difficulty of applying the delamination force without damaging the film via stress concentration. Here, we demonstrate that water vapor condensation blistering is capable of in situ measurement of work of adhesion at the interface of polymer thin films with micrometer spatial resolution. We use our method to characterize adhesion of interfaces with controlled chemistry such as fluorocarbon/fluorocarbon (CF_n/CF_m, n, m = 0 ~ 3), fluorocarbon/hydrocarbon (CF_n/CH_m), fluorocarbon/silica (CF_n/SiO₂) and hydrocarbon/silica (CH_n/SiO₂) interfaces showing excellent agreement with adhesion energy measured by the contact angle approach. We demonstrate the capability of our condensation blister test to achieve measurement spatial resolutions as low as 10 µm with uncertainties of ~10%. The minimization of energy dissipation enables us to probe extremely weak adhesion, which can be as low as 10^-6 J/m² at Sapphire/Teflon interface. The outcomes of this work establish a simple tool to study interfacial adhesion.

In the past decades, high-entropy alloys (HEAs) have been intensively investigated not only because of fundamental scientific interests, but also their outstanding mechanical properties, for example, high ductility and fracture toughness. Among hundreds of different combinations of principal elements, the equiatomic FeCrCoMnNi alloy, the so-called Cantor alloy, has been studied as a model system, which is a single-phase material with face-centered cubic (FCC) structure at room temperature and shows outstanding ductility and strain hardening especially at cryogenic temperatures.

However, dislocation-based deformation mechanisms of HEAs remain elusive and require a fundamental understanding to tailor their mechanical properties. Several models have been suggested possible strengthening mechanisms of HEAs, for instance, the high entropy effect and the lattice distortion effect. In the case of the Cantor alloy, the main strengthening mechanism was identified as deformation twinning with critical twinning stress of 720 MPa. At room temperature, dislocation slip by full dislocations is dominant, however, at strains exceeding 20 % and high flow stresses, deformation twinning was also observed. To reveal the strengthening mechanism in more detail, direct observation of dislocation plasticity and deformation dynamics is required.

Here, we present a study correlating the microstructure and dislocation plasticity of FeCrCoMnNi FCC phase HEA by employing an in-situ transmission electron microscope (TEM) compression and tensile deformation. The threshold shear stress for dislocation glide in a thin foil is measured from dislocation curvature as exceeding 400 MPa. Interestingly, dislocations are prevented from straightening upon unloading due to high frictional stresses. Moreover, an atomic-scale chemical analysis is conducted by aberration-corrected scanning TEM energy-dispersive X-ray spectroscopy (STEM-EDS) and atom probe tomography to investigate chemical inhomogeneity. Lastly, the origin of the jerky glide motion of dislocations observed during the in-situ TEM straining tests is further analyzed by using atomistic simulations.

Despite possessing similar glide systems as an fcc metal, the equiatomic CoCrFeMnNi high entropy alloy (HEA) (Cantor alloy) continues to intrigue metallurgists around the word. Many key questions such as the origin of its strength, the preferential activation of twinning at low temperature or the nature of obstacles to dislocation movement remain to be understood. In particular, the possibility of a friction stress that would arise from the distorted structure of the HEA instead of an extended core as in bcc metals or covalent materials is still debated. In this study, we have carried out tens of in situ straining tests in a TEM at room and liquid nitrogen temperature to assess these questions. Dislocations are used as probes, whose dynamic behavior under stress serve as local markers of the average of punctual obstacles strength they have to overcome. Quantitative stress measurements are made and confronted to those provided by bulk tests, including the size effects arising from thin foil testing.

The development and application of metallic glasses can benefit from the in-depth understanding of the structure-properties relationship during plastic deformation. The limited tensile plasticity of metallic glasses hindered the in-situ study of structural response under tension. Here, we report an in-situ synchrotron study of a metallic glass after canning compression under tension at a micro-scale. Our experimental results show that the canning-compressed metallic glass has much better tensile ductility and much higher ultimate strength than its initial state. The real-space analysis of synchrotron diffraction data indicates that the cluster connectivity and ordering at medium-range length scale play an essential role in plastic deformation. Our results may shed light on developing ductile metallic glasses and their devices at micro-scales.

We attempt here an in-situ experiment to reconcile the recorded mechanical activity of dislocation avalanches during small-scale plasticity of a high-entropy alloy with the observed formation and evolution of slip line morphology on the sample. Correlating the intermittent microplastic events with their corresponding slip-line pattern allows us to define two main event types: events that lead to the formation of new slip lines, and those that involve reactivation of already existing slip lines. The formation of new slip lines reveals statistically larger and faster avalanches that lower the stress-integrated scaling exponent of the global distribution of all occurring events. The opposite tendency is seen for avalanches involving reactivation of already existing slip lines. The combination of both these types of events represents the highest degree of spatial avalanche delocalization, forming a group of events that determine the truncation length-scale of the truncated power-law scaling. These observations link stress-strain fluctuations in a typical small-scale deformation experiment l to the spatial localization of dislocation avalanches, underlining their non-trivial scaling behavior.

Dynamic properties of materials at high strain rates are vital to assess their suitability and reliability in applications ranging from common drops to demanding impact-protection applications. Macroscale mechanical testing at high strain rates, though challenging, is already a well-established field of research. But recently, given the push towards miniaturization, small scale mechanics has also been a topic of intense research over the last two decades. However, till date the micro and nanomechanical experiments have been largely limited to testing samples made using focused ion beam (FIB) milling at quasi-static speeds. Small scale sample preparation using FIB-based milling is a serial and time-consuming process that typically limits the number of samples tested in micromechanical studies, while allowing the fabrication of only simple geometries. On the other hand, the limitation in testing speed is primarily attributed to the lack of instrumentation capable of high speed actuation and simultaneous high speed capture of loads and displacements with micronewton and nanometer resolution.

This presentation will introduce a newly developed piezo-based micromechanical tester capable of conducting high strain rate micromechanical testing at speeds up to 10mm/s. The relevant hardware requirements and protocols specific to high speed testing at the small scale will be elaborated. Further, a localized electrodeposition based metal additive micromanufacturing (μAM) technique will be introduced as a viable method for fabricating full-metal microarchitectures. A large array of ideal copper micropillar test-beds built using this technique will be presented along with electron backscatter diffraction (EBSD) based microstructural characterization showing two distinct microstructures: microcrystalline and ultrafine grain (UFG). The mechanical properties of these copper micropillars, explored as a function of strain rate from 0.001/s to 500/s, will be shown as a function of initial pillar microstructure. Relevant, stress-strain signatures, thermal activation parameters and post-deformation microstructural analysis using EBSD and transmission kikuchi diffraction (TKD) for the copper micropillars will also be presented.

The last part of the presentation will highlight the capabilities of μAM to fabricate complex full-metal 3D microarchitectures such as microlattices and microsprings. Copper microlattices with different geometries, chosen from an energy absorption perspective, including honeycomb, octet and kelvin foam will be presented along with their dynamic compressive properties upto a strain rate of ~150/s. Further, a detailed structural and microstructural characterization conducted using FIB-based 3D slice-and-view and EBSD/transmission electron microscopy (TEM) respectively will be compared between the undeformed and deformed lattices. Finally, a finite element methods (FEM) based simulation aimed at understanding the structural evolution of the metal lattices under high speed deformation will be presented. It will be explained that the FEM simulation model takes into account the defect structures determined using the reconstructed 3D slice-and-view lattices and the appropriate constitutive laws identified from the high strain rate compression of copper micropillars.

Plastic deformation in crystalline materials consists of an ensemble of collective dislocation glide processes, which lead to strain burst emissions in micro-scale samples. Prior investigations of plastic intermittencies involved the large strain bursts triggered with dynamically-reactive testing systems. Under these driving conditions, the microcrystal is rapidly compressed as a function of the system’s dynamic response, which ultimately governs the termination of the instability and its overall size. We argue that a dynamically-induced plastic instability or strain burst is comprised of many of the currently measured individual dislocation avalanche events, propagating over minuscule time scales, which are ubiquitous to crystal plasticity.
To investigate the onset of the ultrafast individual dislocation avalanches, we performed a comprehensive set of strict displacement-controlled micropillar compression experiments in conjunction with large-scale molecular dynamics and physics-based discrete dislocation dynamics simulations. It is shown that the size distributions of these dislocation events exhibit a gradual transition from an incipient power-law slip regime (spanning ≈2.5 decades of slip sizes) to a large avalanche domain (spanning ≈4 decades of emission probability) at a cut-off slip magnitude s_c. This cut-off slip provides a statistical measure to the characteristic mean dislocation swept distance, which allows for the scaling of the avalanche distributions vis-à-vis the archetypal dislocation mechanisms in face-centered cubic (FCC) and body-centered cubic (BCC) metals. Our statistical findings provide a new pathway to characterizing metal plasticity and towards comprehension of the sample size effects that limit the mechanical reliability in small-scale structures.

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 platinum-aluminide bond coatings are an example of compositionally and microstructurally graded coatings with significant variation in engineered mechanical properties across the cross-section. 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 1000^oC. With combined analysis of chemistry and microstructural changes, the results were used to understand local mechanical properties variation as a function of temperature.

Relatively little is known about freestanding thin film creep at elevated temperature. In MEMS, polycrystalline silicon (polysilicon) thermal actuators are frequently used as on chip actuators, but they typically operate at temperatures 300-500 °C higher than the substrate to generate useable displacement and force output. This is because polysilicon’s coefficient of thermal expansion (CTE, 2.7 µε/°C) is the same as the silicon substrate. When actuated by Joule heating, heat flows to the specimen, which can induce experimental artifacts. Instead of using polysilicon, we demonstrate refractory metal tantalum (Ta) thin films as a new structural material for MEMS nanomechanical test platforms. The CTE of Ta films is double that of polysilicon ( 5.9 µε/°C). Therefore, self-actuation simply by raising the temperature is possible. A stress relaxation test can be performed under an isothermal condition, and strains up to 30 % are possible. We have fabricated Ta thermal actuators using conventional surface micromachining after resolving key fabrication issues. Self-actuation has been successfully demonstrated. Using a platinum specimen for demonstration, we have validated a process flow to co-fabricate the specimen with Ta thermal actuators. High-temperature, stress relaxation experiments on structural Pt films at temperatures up to 500 °C will be performed and presented. This work demonstrates for the first time a powerful drift-free in-situ stress relaxation test of structural thin films at elevated temperatures.<!--![endif]----><!--![endif]---->

A wide variety of materials, including thin film microelectronics and lignocellulosic materials, have moisture-dependent mechanical properties that can vary substantially depending on the ambient relative humidity (RH). Under high RH conditions, moisture sorption can weaken interfaces or plasticize materials. The effects of moisture can be especially challenging to study at interfaces and in composite materials with small micron- and nanoscale components. To overcome this challenge, protocols were developed to perform nanoindentation experiments over a wide range of humidity. Using an external RH generator, the environment inside of a Hysitron TriboIndenter enclosure was controlled and varied from dry air up to 95% RH. Analysis of calibration nanoindentations in the air and in fused silica supported that the nanoindenter functioned properly over the entire humidity range provided that enough time was given to equilibrate at the RH. However, some corrosion was observed in metal fasteners on the instrument, which suggests that keeping the nanoindenter under high RH conditions is not advisable over the long term. Nanoindentation results studying the moisture-dependent properties of porous organosilicate glass (SiCOH) thin films and wood will be presented. In SiCOH, elastic modulus was found to not depend on humidity, whereas the hardness and fracture behavior were intimately linked to the nature and concentration of the absorbed water. Wood cell walls are plasticized by the absorbed water and both the elastic modulus and hardness decrease with increasing RH. Additionally, using dynamic nanoindentation the mechanical damping of the wood cell walls was assessed over three decades of frequency and a moisture-dependent glass transition of the amorphous polysaccharides in the wood cell wall could also be observed as a peak in mechanical damping.

The chemo-mechanical effect in solids refers to dimensional change due to change in stoichiometry. Dimensional change due to electrochemically-induced compositional change has been termed the electro-chemo-mechanical (ECM) effect. The mechanical instability inherent in this effect is clearly deleterious for batteries or fuel cells, but, as recently suggested, has potential for use in actuation[1]. The structure of an actuator device that operates on the ECM principle comprises a micrometer thick solid electrolyte (SE) sandwiched between two ECM-active, working body (WB) layers. An electrochemical reaction must occur in these layers, causing them to alternately expand or contract. In order to facilitate the ECM response, the WB layers should have mixed ionic and electronic conductivity and a large chemical expansion coefficient.
We have constructed a 2mm diameter thin film membrane ECM actuator device comprising 20mol% Gd doped CeO₂ (20GDC) as the SE and [TiO_2-δ\20GDC] or [V₂O_5-δ\20GDC] composites as the WBs[2]. Selected area electron diffraction measurements showed the composite to be nanocrystalline, a morphology that promotes interfacial oxygen ion diffusion. Synchrotron X-ray absorption (XAS) measurements detected a mixture of Ce³⁺/Ce⁴⁺ (~[0.4]/[0.6]) and Ti³⁺/Ti⁴⁺ (~[0.1]/[0.9]) oxidation states in the WB. XAS measurements under bias showed changes in the short-range order of Ti and V oxides supporting the presence of a redox reaction. The deformation of the ECM actuator was observed to be in the bending regime producing large vertical displacements (~3 μm) and ~4 MPa stress. The stress/voltage ratio yields a pseudo piezoelectric stress coefficient of e₃₁=1.26 C/m², comparable to common lead-free piezoelectrics such as lithium niobate, lithium tantalate and alkali niobates.

[1] J. G. Swallow et al., Nat. Mater. (2017) 16, 749
[2] E. Makagon et al. Adv. Funct. Mater. (2020), 2006712.

The past several years has witnessed a surging popularity of two techniques for defect characterization in crystalline materials: (i) scanning transmission electron microscopy (STEM) using diffraction contrast imaging, and (ii) electron back-scattered diffraction (EBSD) mapping. Advantages of diffraction-contrast STEM methods over conventional TEM include the suppression of dynamical effects and spurious contrast, as well as the ability to image relatively thick specimens. In parallel, the use of transmission modalities in the scanning electron microscope (SEM) has attracted recent attention. Here, we link these capabilities by employing an field emission SEM equipped with a transmission detector for defect characterization – termed transmission SEM (TSEM). Imaging modes that are similar to conventional CTEM bright field (BF) and dark field (DF) and STEM are explored, and some of the differences due to the varying accelerating voltages highlighted.
As an alternative approach more amenable to bulk materials, EBSD has evolved to be a widespread and powerful characterization technique for the mapping and analysis of phases in crystalline materials, providing key information about crystal orientation, morphologies, lattice strain, topology, and crystallographic texture. Whereas the advent of direct electron detection (DED) that circumvents inefficient conversion between electrons and photons has revolutionized the field of TEM, owing to single electron sensitivity for beam sensitive samples and ultrafast detection for time-resolved studies, the use of DED in the vastly more accessible SEM environment is in its infancy. Here, we demonstrate how the richness of information encoded in EBSD patterns is amplified by a new generation of direct electron detectors that enable high speed mapping and acquisition of high-fidelity patterns that can be used for statistically-meaningful defect analyses.
We further show that these SEM-based approaches provide significant advantages for dynamic in situ characterization. We employ location-specific in situ tensile experiments to study the nature of dislocations dynamics in several structural alloys. By selecting specific crystallographic orientations relative to the tensile axis, we observe the underpinnings of several plasticity mechanisms including shear localization, cross-slip, and dislocation junction formation and evolution. To illustrate the power of these new methods for defect-contrast studies, we show examples on several emerging structural materials such as superalloys and refractory multi-principal element alloys.

In situ transmission electron microscopy (TEM) experiments are typically recorded either in real space or diffraction space. However, it would be ideal to have both real and diffraction space for when transient events occur that cannot be repeated exactly (ie- defect generation or irreversible phase transformations). Real space imaging provides context for these transient events by spatially-resolving microstructural features to one another while diffraction space provides better structural clarity about phase identification and lattice parameters. Four-dimensional scanning transmission electron microscopy (4D-STEM), can come close to providing both simultaneous real-space imaging and diffraction analysis during in situ testing. With the advent of fast direct electron detectors it is possible to perform strain mapping via diffraction pattern analysis during in-situ deformation in a TEM with the precision of a few nanometers without stopping the experiment. Images of the same overall experiment can then be formed using virtual apertures applied to the accumulated diffraction patterns. This talk will highlight recent in situ 4DSTEM experiments that explore transient events where both information from diffraction space and real space are used. The diffraction patterns are used to identify different phases, orientations and relative strain, while the images formed by using virtual apertures provide microstructural context for the analysis. Example experiments include defect generation and interactions in metals, local strain evolution in bulk metallic glasses, and structural transformations in functional oxides under deformation.

This study assesses the applicability of advanced atomic resolution displacement measurement techniques to characterize dislocation character in metallic materials using simulated images derived from anisotropic elasticity and actual measurements in high entropy alloys. We draw attention to two techniques: the real space method of template matching (TeMA) and the reciprocal space method of geometric phase analysis (GPA) and provide a critical assessment. These techniques have limitations for direct evaluation of full dislocations Burgers vector or when local displacements exceed certain value. This value depends on the displacement direction in relation to the crystal structure. A cut-off for reliable displacement measurement is proposed as the boundaries of the Wigner-Seitz cell drawn on the 2D HRTEM image of the crystal structure. This effect is illustrated with simulated arctangent displacement profiles reminiscent of dislocation cores. An approach for circumventing this limitation is suggested in the form of a nearest neighbor correction. Additionally, a methodology for determination of the Burgers vector is introduced on the basis of a vectorial rendering of the displacement field upon consideration of two zone axis measurements and applied to TeMA and GPA. The experimental results conform to the Burgers vector of a full lattice dislocation in the FCC crystal structure of the High-Entropy Alloy (HEA). The comparison of simulated and experimental images proves the efficacy of the HR-TEM (High Resolution Transmission Electron Microscopy) displacement mapping techniques while pointing to the need for caution in case of large displacements.

The mechanical characterization at small length scales is of fundamental importance to obtain insights of deformation mechanisms. In particular, stress mapping is of great interest since it allows the study of fracture mechanics, e.g. visualizing the stress distribution around a crack tip. Conventional lab scale stress mapping techniques are based on electron microscopy and Raman spectroscopy. Common drawbacks are the required high vacuum for electron microscopy and the need of polarizable materials for Raman spectroscopy, hindering the range of materials that can be analyzed. X-ray diffraction is also an established technique for mechanical studies, however its need for a large interaction volume poses problems for ultra thin film studies while still being limited in lateral resolution. Synchrotron X-ray diffraction bypasses these drawbacks and achieves high spatial resolution and strain sensitivity for thin crystalline materials but requires significant resource investment.
In contrast, reflectance anisotropy spectroscopy (RAS) can be employed for a wide range of materials, environments and sample thicknesses to become an attractive technique for non-destructive mechanical studies. RAS is an ellipsometric technique that measures the difference in reflectance between linearly polarized light along two orthogonal directions of the sample. However, usual resolution of conventional RAS setups is in the order of millimeters. Here, we present an advanced reflectance anisotropy spectroscopy (RAS) microscope based on a super continuum laser source as a non-destructive stress mapping technique. Our microscope enables insight into the electronic band structure, phase and crystal orientation.
We sputter metallic thin films on flexible substrates and employ focused ion beam (FIB) milling to modify surface topology. We then demonstrate the capabilities of the technique by externally applying uniaxial strain on the fabricated FIB structures and mapping of the strain distribution on the surface of the sample. Furthermore, mechanical properties are highly influenced by microstructure. Since RAS signal is dependent on crystal orientation, we also prove grain mapping in polycrystalline copper and compare the RAS signal of different grains to outline the possibility of grain orientation indexing in a similar fashion to electron backscattered diffraction. The potential simultaneous stress and grain orientation mapping will enable future studies on the fracture mechanics of thin films and novel materials with different microstructure and environmental conditions.

In the past decades, Coherent X-ray Diffractive Imaging in Bragg geometry (BCDI) has become one of major nanoscale imaging techniques. Unique sensitivity to lattice [1] enables BCDI to image three-dimensional morphology as well as internal deformation field distribution of nano-scaled and/or micro-scaled materials including metal, metal oxide, minerals, and so on. Nowadays, in-situ and operando BCDI is a main driver to address scientific questions on materials.
In this talk, I will introduce current state-of-the-art of BCDI and the 34-ID-C beamline in the Advanced Photon Source where BCDI experiments can be performed. This talk will also cover recent experimental results on in-situ and operando BCDI, e.g. unique lattice distortion in ZSM-5 zeolites [2], annealing effect on gold grains on gold thin films [3], dislocation imaging under tensile loading [4], and current development on combining Laue diffraction to BCDI [5]. In addition, some estimates of BCDI in the future will be discussed.

[1] M. A. Pfeifer, et al., Nature 442, 63 (2006).
[2] W. Cha, et al., Nat. Mater. 12, 729 (2013).
[3] A. Yau, et al., Science 356, 739 (2017).
[4] M. Cherukara, et al., Nature Communications 9, 3776 (2018).
[5] A. Pateras, et al., Journal of Synchrotron Radiation 27, 1430 (2020).

Sub-10 nm metals have fascinating physical, optical and chemical properties. By applying stress to these nanomaterials, it is possible to study atomistic motion near surfaces, as well as investigate structure-property relationships at the smallest length scales.

Here, we use diamond anvil cell techniques to compress nanocrystals and nanoclusters under hydrostatic and non-hydrostatic pressures. Synchrotron X-ray diffraction is used to detect structural changes within 4 and 6 nm Au nanocrystals. It is found that crystalline defects (e.g. stacking faults) form in the 4 nm nanocrystals under pressure, and remain in the nanocrystals after pressure is removed. In contrast, twins within 6 nm Au nanocrystals are removed during high-pressure compression, such that single crystalline nanocrystals are formed. These single crystalline nanocrystals were found to be metastable. The nanocrystals were recovered after compression, and placed in the TEM, where the nanocrystals reverted to the twinned state under the electron beam.

At even smaller length scales, fcc cuboctahedral and icosahedral nanoclusters such as Au₂₁, Au ₂₅, and Ag₂₈Pt₁ were compressed while optical absorbance and photoluminescence spectroscopy were performed in-situ. In combination with density-functional theory, it was found that increased interaction between surface metal atoms and the surrounding organic ligands leads to the observed high pressure behavior.

Metallic nanowires have been widely used in a variety of nanoengineering applications, including nanoelectromechanical systems, nanosensors, transparent electrodes, optoelectronics, and flexible and stretchable electronics. Mechanical behaviors of metallic NWs play a crucial role in reliability of the nanowire-based devices. Here I will present the recent work in my group on in-situ nanomechanics of crystalline metallic nanowires inside transmission electron microscope (TEM). First, we report competition of deformation mechanisms - twinning and slip - in single-crystalline metallic nanowires. We found that the competition depends on the cross-sectional shape of the nanowire, which affects the change of surface energy associated with each deformation mechanism. Second, we found unexpectedly, through careful cross-sectional TEM study, that most of the synthesized single-crystalline nanowires include a central twin boundary along the entire length of the nanowire, so we call them bi-twinned nanowires. Here we also found two competing deformation mechanisms - localized dislocation slip and delocalized plasticity via an anomalous tensile detwinning mechanism - depending on the volume ratio between the two twin variants and the cross-sectional aspect ratio. The mechanism of the observed tensile detwinning was investigated. Furthermore, the twin boundary was found to cause interesting recoverable plasticity and Bauscinger effect, as well as strain hardening. Finally, we probe hydrogen embrittlement using metallic nanowires as a model system. We found increasing yield strength and brittle failure with the presence of hydrogen, which was attributed to the hydrogen-induced suppression of dislocation nucleation at the free surface of nanowires.

Molecular crystals consist of discrete molecules bonded together not by ionic, covalent, or metallic bonds, but by weaker cohesive forces. The almost endless variety of molecular shapes and bonding forces gives rise to a plethora of structures, and, accordingly, deformation and fracture processes that vary greatly among molecular crystals. Mechanical behavior in these materials generally remains less completely understood than in other materials. Small-scale and in-situ mechanical testing approaches have made great contributions to the understanding of deformation mechanisms in many other classes of materials; nanoindentation has already proven useful in examining molecular crystals, but to date in-situ methods have seen little application. This talk present results from single particle compression studies conducted in-situ inside of a scanning electron microscope. Individual micron-sized particles of hexanitrohexaazaisowurtzitane, an organic molecular crystal and energetic material, were compressed to the point of fracture. Challenges and successes in sample preparation, experiment execution, and interpretation of the resulting data will be discussed. The combination of microscopy and quantitative load-displacement data allowed for estimation of stresses and strains associated with yielding and fracture processes, and may suggest an extrinsic size-dependent strength phenomenon. The approach here is generalizable, and may contribute to the understanding of deformation and fracture in other molecular crystals.

This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. DOE or the United States Government.

While bulk material strength is determined by the size of microstructural features, such as the size and distribution of grains, precipitates, twin boundaries or dislocations, nanoscale deformation is strongly dependent on the size of the body being deformed. In the range of tens of nanometers of particle diameter, deformation is accommodated through a Coble-creep-like mechanism where the strength depends on strain rate and the size of the spherical particle. However, even this Coble-creep-like mechanism may break down for single-digit-diameter nanoparticles, when surface faceting becomes dominant.
In the present work, we used in situ compression tests of platinum nanoparticles to quantify the size-dependence of deformation mechanisms. Platinum nanoparticles were synthesized with sizes ranging from tens of nanometers down to single-digit nanometers. These particles were compressed inside of a transmission electron microscope (TEM), enabling the combination of high-resolution video with real-time measurement of force and deformation. The morphology and load response of the deforming nanoparticles were analyzed to demonstrate the dominant deformation mechanism in various size regimes. Finally, results were compared with matched atomistic simulations for atomic-scale insight into driving forces and particle behavior.

The indentation-induced phase transformation of crystalline silicon thin films was studied in-situ during the indentation with a Berkovich probe using Raman spectroscopy-enhanced instrumented indentation. The in-situ Raman spectroscopic analysis showed the formation of high-pressure phases during indentation loading, calling into question the common view that pressure release is a pre-condition for this transformation. The observations suggest strain and time as important but overlooked factors in nucleation and growth of high-pressure phases and provide new context to previous works on the pressure-induced phase transformations of silicon.

Single element metallic nanowires have been deeply studied for their wide range of applications. Alloyed nanowires are known to have distinct characteristics and deformation behavior compared with single element nanowires that comes from difference in stacking fault energies. They are known to have potential to be used in various plasmonic devices for their chemical stability and broad applicable range of wave lengths. However, more detailed analysis mechanical properties is needed for comprehensive understanding of such material systems. In this study, defect-free single crystalline Au₃₀Ag₇₀ alloy nanowires are synthesized by topotaxial growth method and tested in tension using an in-situ pico-indenter. Deformation twinning followed by superplastic deformation of the alloy nanowires is experimentally observed and the critical dimension of Au₃₀Ag₇₀ alloy nanowires at which transition from ordinary plasticity to deformation twinning is determined to be ~333 nm, which is about 2 time larger than that of Au nanowires. Stacking fault energy, which is the key element determining the deformation mode, of Au₃₀Ag₇₀ alloy nanowires is calculated to be 21 mJ/m², which is smaller than that of Au nanowire with stacking fault energy of 31 mJ/m². The decrease in the stacking fault energy in the alloy nanowire resulted in stabilization of deformation twinning to a larger critical dimension before transitioning to ordinary plasticity.

SU-8 is a cross-linked thermoset polymer which is extensively used for design micro-electrical mechanical system (MEMS) components. The effect of cross-linking amount on the mechanical properties i.e. modulus (E) and hardness (H) was studied using nanoindentation. Different cross-linked samples were fabricated by changing the curing steps (post-exposure baking, hard baking) of standard photolithography process. The accurate measurement of cross-linked segments is done via in-situ Fourier-transform infrared spectroscopy. Nanoindentation experiments were performed under constant strain rate loading conditions. Using the conventional method yielded high E and H values for less cross-linked sample and less values for high cross-linked samples. A careful analysis showed that the main reasons for these inverse values are adhesion between the tip surface and sample surface, viscoelastic effect on unloading and errors in calculation of contact area. We propose a new methodology to correct for these potential sources of error. Thereafter, less E and H values for less cross-linked sample and high values for high cross-linked sample were obtained. The values are 4.61 ± 0.13 GPa and 5.02 ± 0.18 GPa of E, and 256.97 ± 1.42 MPa and 285.48 ± 1.17 MPa of H for less (~ 82 %) and high (~ 95 %) cross-linked samples, respectively.
Keywords: SU-8; Photolithography process; FTIR; Nanoindentation; SPM

Ceramics have been widely used for structural applications because of their superior mechanical properties. It has been known that the behavior of GB fracture, deformation and migration is strongly dependent on the GB characters such as misorientation angle between two adjacent crystals and GB plane, however, such effect has not been clarified yet. In this study, in order to clarify the atomistic mechanisms of GB fracture and deformation, bicrystal studies have been performed to find the relationship between the atomic structures and GB behavior of SrTiO₃ and Al₂O₃ ceramics. Several kinds of bicrystals including GBs with specific geometrical configuration were fabricated, and some of them were doped by rare-earth elements to enhance the GB segregation.

It has been reported that single crystal of SrTiO₃ can be plastically deformed even at R.T. by dislocation slip like metals. So far, many experimental investigations have been tried for understanding the dislocation-grain boundary interaction, but these experiments were mostly carried out statically, and the fundamental processes are still not well understood yet. In this study, the nanoindentation experiments were conducted for SrTiO₃ crystals their bicrystals inside TEM. The SrTiO₃ single crystals were indented with the sharp diamond tip and successfully observed the dislocation dynamics. In the case of the GBs, the interaction between the introduced lattice dislocations and the GBs were directly observed. The dislocation-GB interaction and its dependence on the GB characters will be discussed in detail.

GB fracture in Al₂O₃ is strongly dependent on the GB characters and the dopant segregated at GBs. In order to clarify the atomistic GB fracture mechanism and its dopant effect in Al₂O₃ ceramics, Al₂O₃ bicrystals including GBs with specific geometrical configuration were systematically fabricated, and some of them were doped by rare-earth elements. Then, the atomic structures and chemistry in thus fabricated GBs were characterized by atom-resolved STEM, and the dynamic behavior of GB fracture was observed by TEM nanoindentation experiment. The relationship between GB characters, segregated dopants and GB fracture behavior of Al₂O₃ will be discussed in detail.

It is known that Al₂O₃ shows twin formation at R.T. under high stress concentration. So far, many experimental investigations have been tried for understanding the twin formation mechanism, but these experiments were also carried out statically, and the fundamental atomistic processes are still not well understood yet. In this study, TEM in situ nanoindentation experiments were conducted also for Al₂O₃ single crystals and the bicrystals. Al₂O₃ single crystals were indented with the sharp diamond tip, and the twinning dynamics were successfully observed. In this case, the deformation twinning often occurs in the present experimental condition, and the mechanism can be explained by a shear process for each lattice layer, which is caused by twinning dislocations. It is suggested that the non-basal twinning systems, such as the rhombohedral twinning in Al₂O₃, can be completed by not only simple shear but also atomic shuffling. In this study, the dynamic behavior and atomic structures of the twinning dislocations were investigated for rhombohedral twinning in Al₂O₃, and the twin -GB interaction and its dependence on the GB characters will be discussed in detail.

References:
1) S. Kondo, N. Shibata, T. Mitsuma, E. Tochigi, Y. Ikuhara, Appl. Phys.Lett., 100(18), 181906(2012)
2) S. Kondo, T. Mitsuma, N. Shibata, Y. Ikuhara, Sci. A dv., 2[11], e1501926(2016).
3) J.Wei, B. Feng, R. Ishikawa, T.Yokoi, K.Matsunaga, N.Shibata, Y. Ikuhara, Nature Materials, DOI：10.1038/s41563-020-00879-z (2021)

Nitride ceramics have wide application as a protective coating but suffer extreme brittleness. In this work, the mechanical behavior of physical vapor deposited Ti/TiN multilayer has been studied. Effect of varying layer thickness and volume fraction of elastic-plastic Ti in Ti-TiN multilayer is observed by testing micro-cantilever bending. FEM simulations were carried out for various multilayer thickness and volume fraction to establish the theory behind it. R-curve behavior of multilayer is also discussed using simulation results. Finer spacing in multilayer enhances the fracture toughness of multilayer. Fracture properties of multilayer in comparison to monolithic TiN layer will be reported.

Barium Titanate (BTO) is a widely used lead-free piezoelectric ceramic used at micron length scales as thin films in MEMS applications. Here we study the mechanical behaviour BTO single crystals and thin film systems using different micromechanical experiments and finite element modelling (FEM). Microscale mechanical behaviour of single crystalline BTO was studied by uniaxial in situ micropillar compression for different sizes. It was observed that pillars below 1μm diameter reached their theoretical strength whereas larger pillars yielded at lower stress values with multiple stress drops confirming slip activity. The strain accommodation mechanism at smaller length scales is by plastic flow, with a size exponent close to 1 and enhances the elastic strain limit of the material, which is an important consequence that can be exploited in sensors/actuators. Microcantilever fracture measurements revealed that, single crystal BTO showed a 45% higher K_IC than the bulk, while the polycrystalline thin film showed a 60% lower K_IC due to the weak inter-columnar boundaries. Different geometrical aspects and loading parameters on the stress intensity factor of single cantilevers are investigated using FEM to propose testing standards that can be used by future users. The variation of stress intensity factor and mode mixity (K_II/K_I) with respect to the relative position of the notch, beam cross-section, notch tip radius, notch geometry and arm length of the cantilever and loading direction are studied. Effect of bilayers and elastic modulus mismatch between the layers on the crack driving force are also discussed.

Nanoscale metallic objects like thin films, nanoparticles, nanowires, or nanoporous metals receive sustained attention due to their size-dependent mechanical properties, which can include changes in deformation mechanisms, pseudoelastic behavior and increased yield strength compared to the bulk material. Many of these nano-objects have well-defined morphologies and are initially dislocation-free, which make them ideal model systems to study, e.g. dislocation nucleation and dislocation – interface interactions. These properties of such nano-objects also predestinate them for direct comparisons between experiments and simulations.

Here we present recent results on in situ experiments on single crystalline, nanotwinned and five-fold twinned nanowires as well as on nanoporous gold and compare them with atomistic simulations. The latter allow for controlled variation of the size and morphology, surface quality and loading conditions. Such carful studies allowed to show that twin boundaries not only act as obstacles to dislocation motion leading to hardening, but also fundamentally change the deformation mechanisms and failure mode. The loading mode (tension, compression, bending) and boundary conditions, the location of the twin boundary and most importantly the intersection of the twin boundaries with other interfaces are shown to critically influence the deformation behavior. These observations demonstrate the fundamental differences between dislocation processes in nanotwinned polycrystals and in nanotwinned nano-objects. The simulations furthermore highlight the need for realistic sample sizes and morphologies to capture and understand essential, experimentally-observed, deformation mechanisms.

In situ microscopy has been established itself as a key technique for studying the mechanical properties of materials at small length scales. In particular, the application of in situ microscopy to nanowires and nanowhiskers has been very successful, not only because such nanostructures can be produced with high quality but also because preparation artefacts like FIB damage can be avoided. As result the intrinsic mechanical properties of nanowires and nanowhiskers can be reliably studied down to very small dimensions. Such investigations consolidated the notion of ‘smaller is stronger’ which refers to an almost universal trend of increased strength with decreasing size of nanostructures subjected to mechanical loads. Reducing the size of nanostructures not only affect their plastic deformation behavior but also changes their elastic properties as result of the increased surface-to-volume ratio. However, the observations of size effects on elastic properties are less consistent and the underlying mechanisms are still controversially discussed.
In this contribution we present results of in situ microscopy studies on individual metal nanowires carried out over the past years at the Center for Nanoanalysis and Electron Microscopy (CENEM) in Erlangen. In the first part we focus on the elastic properties of gold nanowires and demonstrate a profound influence of both, shape and size of the nanowires. The elastic properties are determined using in situ mechanical testing in SEM and TEM by means of resonance excitation and uniaxial tension. The combination of bending and tensile load types, performed subsequently on the same wire, allows for an independent and correlative calculation of the Young’s modulus. For this, the cross-sectional area and resulting second moment of area are determined for each individual nanowire by performing high resolution TEM on FIB cross sections of undisturbed nanowire areas after mechanical testing. Based on a detailed comparison of nanowires with different size and shape we find both cases of softening as well as stiffening, depending critically on the interplay between size and shape of the wires. Our results clearly demonstrate that not only size but also shape matters for the elastic properties at small scales. Different models from the literature are discussed which can in part explain the behavior.
In the second part we focus on the plastic deformation behavior and failure modes of five-fold twinned silver nanowires which are used as conduction lines in flexible transparent electrodes. Upon uniaxial straining of such electrodes some nanowires experience tensile loading and fracture while others are set under compressive stress resulting in buckling and kink formation, depending on the orientation of the nanowires with respect to the external load axis. We employ different in situ microscopy techniques in TEM, SEM and light microscopy to study the mechanisms of kink formation for both freestanding nanowires and nanowires on flexible substrates, respectively. We determine the critical strain for kink formation under these conditions and study the fundamental defect mechanisms associated with this deformation mode. Moreover, we demonstrate the potential of in situ light microscopy to study the deformation behavior of single nanowires on flexible substrates under ambient conditions. Finally, we report on first in situ microscopy studies under conditions of cyclic loading and discuss the role of kink formation and nanowire fracture on the overall failure behavior of flexible transparent nanowire electrodes.

Microcantilevers are already successfully implemented in mass sensing devices, offering an improved sensitivity down in the range of few picogram. The performance of such nanomechanical devices is given by a high resonance frequency combined with a high quality factor. While for mass sensing applications the frequency shift is used, the quality factor additionally depends on the ambient medium and scales inversely with the pressure in the molecular flow regime. However, the measureable pressure range of commercial micro-sized cantilevers is restricted, which limits the application ability. To expand the pressure range, the size of the cantilevers has to be further reduced: A beam with a thickness in nm range, would shift and additionally broaden the measurable regime to higher pressure values. Therefore, a precise characterisation of the vibrational properties of nanowires is required, which is the fundament for futher research of next-generation devices.
We present a correlative electron and light microscopic approach to characterize the vibrational properties of single nanowires in dependence of the gas atmospheres and pressure level. The resonance properties of nanowires and the assigned quality factor depends on the pressure of the ambient medium. The high vacuum state of 10^-6 – 10^-7 mbar in SEM and TEM enables the characterization of the intrinsic vibrational properties, which is directly related to the microstructure and surface quality. To analyze the damping effect caused by the interaction of the gas molecules with the nanobeam, the in situ resonance measurements have to be performed within the molecular flow regime ( ∼ 50 mbar -200 mbar). For this purpose, the single nanowire is mounted within a compact chamber for the light microscope, which allows to observe the changing resonant behavior in dependence of the applied gas atmosphere (He, N₂, Air, Ar) and pressure level. By using the resonance vibration, we demonstrate the pressure sensing capability of a single nanowire. Moreover, the damping effect is used to examine the molar mass of the surrounding atmosphere and therefore the nanowire can be seen as ultrathin nanoscale resonant gas sensor. The in situ measurements of the same nanowire in the electron and light microscope offers the opportunity to analyze the complex interplay of several damping factors and gives direct insights in the vibrational behavior. While the intrinsic damping factor can be tuned by the microstructure, the nanowire thickness determines the sensitive pressure regime. The extrinsic factors are given by the interaction of the vibrating nanobeam with the surrounding gas atmosphere and are used for sensing applications.

Thin film technologies have enabled the life we know today. There is barely any product released in today’s market that does not involve thin film technologies at any point during its fabrication. As less material usually means lower costs, these thin films have been downsized to the dimension that is just barely sufficient to fulfill their purpose. For applications like e.g. diffusion-barriers or reflective coatings these dimensions are reaching nowadays well below 20 nm.
It is of interest to study films of small dimensions, because the material exhibit different properties at small length scales, usually referred to as size effects. Mechanical size effects have been studied extensively during the last two decades. Amongst others, by the means of Nano-indentation, Synchrotron XRD, Wafer-curvature, micropillar-compression, bulge-testing, cantilever-bending, nanowire-tensile experiments, and lately also by Reflectance Anisotropy Spectroscopy (RAS).
When it comes to polycrystalline thin films however, all the above mentioned techniques (except RAS) are getting less reliable when it comes to ultra-thin films, with thicknesses below 50 nm. This is due to the nature that the probed interaction volume becomes too small and the signal-to-noise ratio decreases. In our setup, we use white light from a common Xe-lamp for excitation. For metals at these optical frequencies, the interaction volume is confined to a few tens of nanometers below the surface. This makes RAS an excellent candidate to investigate metallic ultra-thin films below 50 nm.
RAS is best described as phase-modulated near-normal incidence Ellipsometry. It probes the difference of reflectance in two orthogonal directions, which correspond to anisotropies in the dielectric function. The Piezo-optical tensor relates strain to the dielectric function, which makes this technique highly sensitive to anisotropic changes in lattice spacing (e.g. under uniaxial loading). The method is not limited to any material or materials class, as long as the films are continuous and the material has an electronic transition in our detection range (1.5 - 5.5 eV). Our metallic films are on a flexible substrate, which allows us to strain them considerably in uniaxial tension. By the subsequent unloading of the stress the polymeric substrate drives the metallic film into compression. This is required as the technique is blind to residual biaxial strains.
We will present a new in-situ mechanical size effect study on pure fcc metals (namely gold, copper, and silver) down to 20nm thin polycrystalline films on flexible substrates. Sample fabrication has been proven most critical, as the technique is highly sensitive to surface morphology (e.g. anisotropic pores, roughness) and texture. To reduce roughness of the metal surface that is probed a substrate transfer is performed after deposition, usually referred to as template stripping. This results in metallic thin films on polymeric substrates with the same roughness as a silicon wafer. We will clearly outlay the potential as well as the limitations of this technique.

The field of Graphene Twistronics has been gaining significant traction in recent times due to the excellent superconductive behavior of these materials and their potential use in various applications. However, while the electronic properties of twisted multi-layered graphene have been extensively studied in the literature, there is a lack of research concerning the mechanical properties of twisted tri-layer graphene (tTLG). Therefore, this paper aims to mechanically test tTLG under tensile loading and evaluate the mechanical properties by constructing an accurate model of the system using a twist angle of ± 1.53 ° which is known to exhibit highly superconductive behavior. To develop our system and assess the fracture mechanism, we opted to use the AIREBO-M potential to precisely mimic the behavior of multi-layer graphene with varying sizes of cracks. The results indicate that TTLG possesses exceptional mechanical properties with pristine tTLG exhibiting a Young’s modulus of 1.506 TPa and an ultimate strength of 146.36 GPa. Furthermore, the fracture characteristics are also excellent with a fracture toughness of 4.79 ± 0.085 MPa M^1/2 and as expected, the critical stress reduces with increasing crack size a0. it also interesting to note that critical strain equilibrates to approximately 0.072 after a crack length of 1.85 nm. Twisted Tri-layer graphene has been known to exhibit exceptional electrical properties and this study validates the excellent mechanical properties as well.

Mechanical behavior of crystalline materials strongly depends on the microstructure and its evolution at different length scales which is governed by, e.g., crystallography, the distribution of defects, grain morphology, or texture. Understanding and predicting macroscopic mechanical responses requires appropriate descriptions of the micro- and nanoscale response, i.e., the evolution of the underlying microstructure, or intra- and inter-granular stresses. Following the advancement of experimental techniques, it is becoming possible to link computational models with real-life observations of deformation and failure.

In this presentation, an overview of current approaches integrating advanced experimental methods with high-resolution characterization and modeling will be discussed in the context of dislocation plasticity. In situ microcompression experiments, electron backscatter diffraction, and high-resolution digital image correlation allow the determination of local strains and identification of activated slip bands. In combination with transmission electron microscopy and computational analysis, the formation of dislocation structures and the interaction of dislocations with grain boundaries is analyzed. Geometrically necessary dislocation densities can be quantified at the nanoscale by using electron diffraction methods and the Kröner-Nye dislocation density tensor, thereby giving insights into, e.g., the indentation size effect. With those methods, experiment and simulation move closer together facilitating an understanding of relevant mechanisms in plasticity and fracture.

High performance materials, from natural bone over ancient damascene steel to modern superalloys, typically possess a complex structure at the microscale. Their properties exceed those of the individual components and their knowledge-based improvement therefore requires understanding beyond that of the components’ individual behaviour. Electron microscopy has been instrumental in unravelling the most important mechanisms of co-deformation and in-situ deformation experiments have emerged as a popular and accessible technique. However, a challenge remains: to achieve high spatial resolution and statistical relevance in combination. Here, we overcome this limitation by using panoramic imaging and machine learning to study damage in a dual phase steel. This high-throughput approach now gives us strain and microstructure dependent insights into the prevalent damage mechanisms and allows the separation of inclusions and deformation–induced damage across a large area of this heterogeneous material. Aiming for the first time at automated classification of the majority of damage sites rather than only the most distinct, the new method also encourages us to expand current research past interpretation of exemplary cases of distinct damage sites towards the less clear-cut reality. Often, the collection of ground truth data may be the limiting step, for this reason we will also addressed using data augmentation to achieve transferability, for example between different stress states like uniaxial, bending and biaxial loading.

Small metallic nanoparticles are promising in a variety of fields due to their chemical, optical and electronic properties. Understanding the deformation mechanisms and mechanical response of these small nanoparticles to compression is necessary to determine their stability and suitability for applications. Most prior descriptions of nanoparticle deformation, such as Cobble-creep-like deformation, are defined assuming spherical nanoparticles. However, at very small sizes (<10 nm), metallic nanoparticles tend to form different shapes (such as icosahedra, dodecahedra, and cubes). The shape/size dependency of nanoparticles is a result of the anisotropy of the surface energies of the different facets. These facets determine the mechanical deformation of the small nanoparticles because different facets facilitate or impede processes such as atomic diffusion and defect nucleation.
Here, molecular dynamics simulations were performed to understand the relationship between faceting and the mechanical deformation of platinum nanoparticles. Simulated compression tests were performed, with matched conditions to in situ experimental testing inside of a transmission electron microscope. Dynamic facet rearrangement was observed during compression, and qualitatively linked as a result of crystal deformation, dislocation mobility, and residual mechanical strain. Taken together, the simulated and experimental results demonstrate the size- and facet-dependence of deformation in small platinum nanoparticles.

High throughput nanoindentation is powerful technique for evaluating highly localized mechanical properties in materials with complex microstructures. This allows for study of spatial and statistical variations in properties for materials with different phases or features of interest. Prominent examples include composite materials, multi-phase metallic alloys, gradient microstructures, or heat affected zones from processing. This makes it useful for speeding up development of new structural materials, especially since a lot more data can be generated versus traditional bulk mechanical testing for the same sample preparation time. This data can furthermore be easily correlated to the microstructure through use of complementary techniques, such as scanning electron microscopy and electron backscatter diffraction. In order to extract useful information from these techniques, automated analyses also need to be developed in tandem since such datasets are composed of many thousands of points, typically.
We have been exploring clustering as such an accompanying analysis technique. This encompasses several algorithms which are used to group similar data together. The primary current application is getting statistics from individual phases in the microstructure. This can also identify the size and position of these phases. In the future, more advanced analyses are expected to be developed as well. For now, determining the best clustering methods is still a relevant question. Many algorithms exist to choose from, along with choosing the correct number of clusters, how to do outlier removal, and the number of measurements needed to capture the property distributions. To that end, we have been developing an analytical framework based upon bootstrapping to compare the bias and uncertainty in different clustering techniques, as applied to nanoindentation. This will be demonstrated on complex structural materials, including Damascus steel, dual-phase iron high entropy alloys, and additively manufactured T91.

The thermodynamic equilibrium states of materials under shock loading (i.e. the shock Hugoniot) can be used to characterize their high strain-rate behaviors. The shock Hugoniot for a given material is generally obtained by conducting shock wave experiments. The current experimental techniques are, however, unable to elucidate all the complex atomistic mechanisms associated with the shock wave propagation, which can only be realized through comprehensive atomistic simulations. For example, first-principle density functional theory (DFT) calculations can be used to compute the shock Hugoniot of materials with very high accuracy. However, the system size that can be simulated in DFT is quite small (a few hundred atoms) due to the extremely high computational cost associated with DFT, which could be problematic if the dynamic response of the systems is size-dependent. Moreover, the high computational cost of DFT only allows us to model the equilibrium thermodynamic state behind the shock front (i.e. the Hugoniot states), rather than the dynamic shock wave propagation itself. Alternatively, classical molecular dynamics (MD) simulations can be used to explicitly model the shock wave propagation in larger systems containing millions of atoms. In addition to providing the shock Hugoniot, the explicit modeling of the shock wave propagation provides more useful information like spatiotemporal evolution of temperature and pressure.

In this work, we first conducted both DFT-based-quantum and classical MD simulations to obtain the shock Hugoniot of polymers and compared them with the existing experimental data. This exercise revealed the upper limits of the shock wave speed that can be accurately modeled using classical MD. Thereafter, we employed classical MD to explicitly model the dynamic shock wave propagation and spallation of polymers as well as polymer/ceramic multilayers. We conducted DFT calculations to obtain accurate MD force field parameters to model the adhesive interactions between the polymer and the ceramic. In order to investigate the atomistic mechanisms associated with the penetration process, we conducted full-scale classical MD simulations of ballistic impact tests of the polymer/ceramic multilayers. Our study demonstrates that the atomistic simulations provide a great insight into the nanoscale mechanisms associated with the dynamic behavior of materials under extreme conditions.

Acknowledgement: This work was supported by the Natural Sciences and Engineering Research Council of Canada.

Molecular dynamics (MD) simulations and first-principles calculations are carried out on first order pyramidal plane (FOPP) of magnesium (Mg) to study both compression twinning (CTW) and dislocation slip. To this end, a generalized stacking fault energy (GSFE) analysis is employed on dense and loose spaced planes of Mg FOPP. The crystal shearing resistance is extracted by using a minimum-energy path (MEP) finder called the nudged elastic band (NEB) method. The MEP regarding slip system on loose plane of FOPP shows that the unfaulted crystal structure is recovered in the middle of the path with non-straight and pronounced curved slip trajectories. Besides, it will be revealed that metastable configurations on the half of the MEP for the slip system are indeed related to a dissociated extended dislocation of loose pyramidal-I slip. Also, after extracting the dissociation mechanism related to this dislocation, it will be shown that loose pyramidal-I slip can involve shuffling. Moreover, the MEP for dense pyramidal-I slip shows transmutation of this slip into CTW in the middle of the path. This transmutation process will be further examined on CTW growth, and it will be demonstrated that this CTW mechanism is energetically more favorable compared to other twinning mechanisms.

Four dimensional scanning transmission electron microscopy (4DSTEM) is a versatile and powerful electron microscopy technique that has been used to measure strain, polarization and orientation, among other properties, at the scale of traditional TEM analysis. This technique, enabled by a new generation of electron detectors, is based on the computational analysis of thousands to hundreds of thousands of diffraction patterns rapidly acquired while the electron beam is scanned across a sample. It can further be combined with in situ TEM to provide novel insight into bulk metallic glasses (BMG). This talk will highlight how the evolution of strain and structural order in bulk metallic glasses prior to shear band formation can be spatially correlated during in situ deformation. The development of in situ heating and crystallization 4DSTEM techniques for BMGs will also be discussed. Additionally, if time allows, this talk will cover some of the more versatile aspects of 4DSTEM, such as in situ strain mapping in crystalline materials, orientation mapping nanoparticle grains, and the development of the software required to perform the analysis shown.

Understanding dislocation generation mechanisms and interactions with obstacles such as grain boundaries and other dislocations is central to understanding the mechanical behavior of metals and alloys. This has motivated decades of research into the unit processes governing dislocation interactions by in situ transmission electron microscopy (TEM) mechanical testing, resulting in the establishment of basic rules that govern how these interactions occur. However, much of this research has focused on the deformation of coarse-grained polycrystals where the grain boundaries are largely isolated from each other and dislocation glide is primarily transgranular. In ultrafine grained and nanocrystalline materials, the high volume fraction of grain boundaries can lead to the activation of additional deformation mechanisms, such as grain growth, grain boundary sliding, and intergranular dislocation propagation. This paper presents results on quantitative in situ TEM experiments of deformation of ultrafine grained materials with a focus on deformation-induced grain boundary migration. A unique aspect of this study is the ability to measure the apparent and true activation volumes associated with the material deformation using a micro-electromechanical system (MEMS)-based deformation platform. This study also takes advantage of progress in electron detection technology, with simultaneous high and low resolution recording now possible via digital zoom.

In this paper, I will discuss our results from two different systems: nanocrystalline Al and ultrafine grained Au thin films. In both cases, grain growth accompanies the deformation, but the rates and migration behavior differ significantly. The reasons for these differences, including the activity of disconnections and the role of grain boundary character, will be discussed and explored using atomistic simulations. I will also discuss the effects of the electron beam on deformation mechanisms, which are significant even when operating well below the knock-on damage threshold. This influence can be seen in the deformed microstructure and quantified via activation volume measurements.

Grain boundary (GB) stability may be defined as the ability of a GB to continue to absorb point defects without becoming saturated and without changing its macroscopic degrees of freedom (misorientation, inclination), or degrees of freedom. Much debate revolves around local GB stresses and how they evolve with radiation. Through the use of quantitative in situ microscopy, atomic and mesoscale simulations, and machine learning, we explore how GBs evolve as a function of point defect absorption, and specifically, how local stresses evolve throughout the dynamic radiation process. The results have profound implications in developing thermally stable, bulk nanocrystalline materials, and damage tolerant materials.

Stress-driven grain boundary (GB) migration can lead to grain growth and GB network evolution during the deformation of polycrystalline materials. The theoretical models with disconnection, dislocation, and shuffling mechanisms have been proposed to describe the GB migration. However, GB phase transformation during migration, the process of which cannot be described by these models, has never been explored experimentally at the atomic scale, especially for asymmetrical tilt GBs (ATGBs), one of the most common GBs in polycrystalline materials. Here, by performing in-situ high-resolution transmission electron microscopy, GB phase transformation was discovered at two faceted ATGBs in Au. Both of them were found to form ∑11(113) symmetrical tilt GBs, either via facet phase transformation or GB dissociation, during the shear-mediated migration process, which was further confirmed by molecular dynamics simulations. This work provides the first insights into the atomistic mechanisms of GB dynamic phase transformation during the migration of ATGBs and the impact of it on the microstructural control during the thermal-mechanical processing in polycrystalline materials.

Nanomaterials, such as carbon nanotubes, hexagonal boron nitride (h-BN) and graphene platelets, are promising candidates for reinforcing ceramic matrix composites. They are known for their high strength and have shown potential for improving the fracture toughness of monolithic ceramics. Much of the current understanding of these nano-reinforcements is based on extrapolation from the existing literature on much larger reinforcements such as microfibers. According to these studies on micro-reinforcements, the interaction between a crack and the interface is typically an energy governed process. However, in the case of nano-reinforcements, especially 2D reinforcements, this mechanism has yet to be fully explored due to the lack of systematic experimental studies on the interfacial properties of composites reinforced with nanomaterials.
In this work, the interfacial behaviors between multi-layer h-BN nanosheets and a polymer-derived ceramic (PDC) were investigated to promote the understanding of interfacial failure at small scales with in-situ pull-out experiments. By using nanoindentation-assisted micro-mechanical devices integrated with scanning electron microscopy (SEM), the detailed interfacial sliding and debonding behaviors between 2D materials and ceramic matrix were quantitatively measured for the first time. Interfacial mechanical properties, i.e., interfacial modulus and strength, were measured to be 5.65 ± 1 GPa*µm^-1 and 66.4 ± 16.8 MPa, respectively. Furthermore, based on the experimental observations, an analytical cohesive shear-lag model was established to understand the failure mode of the interface in h-BN/PDC composite. It is found that the strength of the interface, rather than interfacial fracture energy, governs the interfacial failure at nanoscales, which provides valuable insight that is expected to motivate future work on the mechanical properties of nanomaterial-reinforced composites.

Upon sustained loading, gels tend to feature delayed viscoplastic creep deformations. However, the relationship, if any, between the structure and creep dynamics of gels remains elusive. Here, based on accelerated molecular dynamics simulations and classification-based machine learning, we reveal that the propensity of a gel to exhibit long-time creep is encoded in its static, unloaded structure. By taking the example of a calcium–silicate–hydrate gel (the binding phase of concrete), we extract a local, non-intuitive structural descriptor (“softness”) that is strongly correlated with the dynamics of the particles—wherein the macroscopic creep rate exhibits an exponential dependence on the average softness. We find that creep results in a decrease in softness in the gel structure, which, in turn, explains the gradual decay of the creep rate over time.

This talk describes the design and integration of a custom-built optical instrument for in-situ Raman microscopy suitable for collecting high-quality spectroscopic data during the indentation of glass materials. It will further show that the reported experimental setup and optimized experimental parameters enable meaningful in-situ spectroscopic observations during indentation of fused silica at forces in the millinewton range. In this context, the talk will also touch upon possible pitfalls in in-situ Raman measurements on indented glasses and describe the misinterpretations that may result.

The quest for in-situ characterization has placed an emphasis on spectroscopic methods with penetrative, noninvasive, nondestructive attributes, thus bringing terahertz-based techniques to the forefront. Terahertz time-domain spectroscopy operating in the transmission mode is most suitable for investigation of polymers in-situ or ex-situ. Polymers are nearly transparent to terahertz waves, indicating inheritance of the molecular information by the propagating electromagnetic wave as it interacts with the polymer continuum, regardless of the sample form. The research leading to the presentation is focused on elucidating the thermal transition, e.g., glass transition, of polyurea. Polyurea is a thermoset elastomer prepared by mixing oligomeric diamine and diphenylmethane diisocyanate at a stoichiometric ratio of 96%, yielding a viscoelastic polymer with significant strain-rate sensitivity. Polyurea has been integrated in a plethora of applications due to its superior physical properties, the prime of which is the impact mitigation to guard structures and human from hypervelocity projectiles and biomechanical dynamic loading, respectively. However, prolonged exposure to environmental conditions was reported to deteriorate the material and adversely affect its mechanical properties. Therefore, this research strives to elucidate the structure-property interrelationship of polyurea using the noninvasive THz-TDS technique as a function of temperature spanning from -103°C to -3°C since the glass transition temperature of polyurea was previously reported to be -49°C. The approach is to control the temperature of the sample in a cold finger by isothermally soaking at each temperature step for 2 min, before collecting the terahertz signal with the sample in the propagation path with a spectral bandwidth of 1.7 THz extending from 0.3 - 2.0 THz and peak width of 0.18 mm. The time-domain data was transformed to the frequency domain using discrete Fourier transform. The optical and dielectric properties were deduced from the spectral data of each sample, where a shift in the sensitivity to change of the optical properties with respect to temperature was noticed around the thermal transition of polyurea. The polyurea samples, with ~16 mm diameter, were extracted from a 1 mm sheet that was prepared using slab molding technique, where a subset of the samples were continually exposed to ultraviolet radiation of 15 weeks at an exposure level of 5122 mJ/cm²/hr and 8553 mJ/cm²/hr of UV-A and UV-B, respectively. This prolonged exposure resulted in discoloration, minor changes in the elastic, hyperelastic, and viscoelastic behavior, and materials degradation in the form of crazing and microcracks. However, the extent of degradation was found to be limited to 45 μm below the surface, which is thought to attribute to the insignificance of the property changes. Investigation with THz-TDS in cryogenic temperature was able to accurately identify the glass transition temperature and radiation-related changes, since changes in molecular vibrations associated with conformational changes are encoded in the terahertz signal as it propagates through the sample. Future research seeks to couple the experimental results with molecular dynamic simulations to accurately elucidate the type and source of conformational changes.

We propose and demonstrate a rheological yielding mechanism in metallic matrix nanotube composites, which may aid in improving the mechanical properties of such composites. Machine learning-based analysis of in situ transmission electron microscopy (TEM) tensile test videos is used to identify microstructural defects in an aluminum-carbon nanotube (CNT) nanocomposite and thereby uncover nanoscale yielding processes during mechanical deformation. Observed defects near CNTs include notch evolution, new surface creation, material sliding, dislocation glide/cross-slip, and localized shear strain at <100nm length scale. Non-shearable CNTs provide strong pinning points against dislocation glide or cross-slip, triggering high local shear strains, which result in deformation via shear transformation. CNTs parallel to the load axis bridge evolving notches, hindering further penetration and inducing substantial nanoscale load transfer, which may contribute to the exceptional strength observed in these Al-CNT composites.