Symposium F.SF08 : Defect-Dominated Plasticity and Chemistry in Metals and Alloys

Ti-6Al-4V is one of the most commonly used alloys for additive manufacturing (AM) due to its applications as orthopaedic implants and aerospace components which can uniquely benefit from the advantages of AM. However, the inherent inhomogeneity and cyclic thermal loading in additive manufacturing processes significantly complicate the microstructural evolution of this alloy. The current understanding of the microstructural evolution in Ti-6Al-4V is based on traditional processing and cannot adequately predict its behaviour during AM.

Analysis of different interfaces (prior beta grain boundaries, alpha inter-variant boundaries) formed during AM of Ti-6Al-4V can clarify the competition between different modes of alpha variant selection in determining the final microstructure. Here, traditional 2D characterization methods are often insufficient for uncovering complex, interconnected microstructural features that arise from solid-solid phase transformations during AM.

In this work, Ti-6Al-4V blocks were produced using different scanning strategies during electron beam powder bed fusion and characterized using 2D and 3D electron backscatter diffraction. This revealed that the microstructure is more complex, and that alpha laths are more interconnected than previously believed. For example, inter-variant boundary character distributions showed significant evidence of different variant selection mechanisms as a function of the position in the build and the scanning strategy. Some of these variant selection mechanisms are more desirable than others. Understanding the complexity of the microstructure in 3D provides new insights into the fundamental phenomena behind the microstructural evolution of titanium alloys in AM, and will help to give recommendations for optimising process design.

Acknowledgements: Funding by the AUSMURI program, Department of Industry, Innovation and Science, Australia is acknowledged. Samples were provided by Prof. Suresh Babu and Miss Sabina Kumar, The University of Tennessee, Knoxville.

With recent advances in computational modeling and in situ experimental technologies, there have been increased efforts to combine these approaches to understand microscopic deformation mechanisms in metals and alloys. In this talk, I will present our recent studies that integrate in situ electron microscopy and diffraction experiments with crystal plasticity and atomistic simulations for gaining a deep understanding of microscopic deformation processes. For example, we have combined in situ synchrotron X-ray diffraction experiments with crystal plasticity simulations to investigate the microscale residual stresses in additively manufactured stainless steel. We have also combined in situ transmission electron microscopy experiments and atomistic simulations to study the effects of atomic structures and elemental distributions on the exceptional mechanical properties of high-entropy alloys. In addition, we have used this strategy to reveal the grain boundary deformation atom by atom, step by step, thus uncovering the unexpected grain boundary sliding mechanisms in real time.

The stacking fault energy (SFE) changes with an addition of solute atoms for FCC metals. Tendency to form deformation twins and stacking faults increases with decreasing SFEs and affects the mechanical properties. In severe plastic deformation (SPD), alloying elements generally enhance grain refinement by inhibiting dynamic recovery and accumulating dislocations leading to smaller grain size than that in pure FCC metals. The role of alloying elements on grain refinement has two possibilities. The first is the formation of extended dislocations on decreasing SFE with the addition of solute atoms. The second is the segregation of solute atoms at dislocations and stacking faults. Both the two effects can suppress the dynamic recovery and retard the formation of grain boundaries until the significant dislocation densities is accumulated. Previous studies have focused only on the effect of SFEs on grain refinement and do not take the effect of segregation of solute atoms into consideration. In these cases, it is not obvious which one is the major effect. The purpose of this study is to investigate the relative importance of the effect of SFEs and solute atoms on microstructural evolutions for ultrafine grained (UFG) materials. Pure copper, pure silver, Cu-4.6at%Al and Cu-6.8at%Al were selected (SFEs are about 41, 22, 37 and 23 mJ/m², respectively). UFG structures were formed by ECAP at the same homologous temperature so that the effect of temperature on thermal activation can be eliminated. Samples processed from one to eight passes through Route Bc at 5mm/min for pure metals and 1mm/min for alloys. Microstructural evolutions were observed by EBSD, TEM and XRD. The microstructural investigation is focused on variation of the subgrain or grain size, the distribution of grain boundary misorientations and the dislocation density. The results show that grain size significantly decreases at initial stage of pressing while it saturates at the final stage of pressing. The final grain size after eight passes for Cu, Ag, Cu-4.6at%Al and Cu-6.8at%Al were 325, 268, and 251 and 76 nm. It is found by comparing Ag and Cu-6.8at%Al, that grain size of the Cu-6.8at%Al was smaller than that of Ag despite the same SFE. It seems that the final grain size is determined by the ultimate cell size because cell boundaries lose mobility once they transformed to sharper grain boundaries by dynamic recovery. It is assumed the solute effect is greater to inhibit the transformation of cell boundaries than the effect of stacking faults, which result in smaller effect of SFE than solute atoms om grain refinement.

Hydrogen in steel is associated with a catastrophic failure mode, known as hydrogen embrittlement. This phenomenon is abrupt and is initiated at the atomic level. Although the exact mechanisms are still subject to debate [1], some mitigation strategies are available, including minimizing hydrogen ingress with surface coatings, or controlling hydrogen diffusion within via the introduction of microstructural ‘traps’, e.g. second phase precipitates such as niobium carbide [2]. It is believed that the incorporation of fine, distributed traps can reduce the mobility of detrimental hydrogen atoms and mitigate the macroscale embrittlement. However, as hydrogen is difficult to examine at fine scale, the experimental evidence is lacking for further optimization of the microstructural design.

Atom probe tomography is a powerful technique that can provide accurate 3D maps showing the position and identity of atoms. H is easily detected but, because it is so mobile, researchers are never sure whether it arises from the sample or the chamber itself. It has recently been demonstrated [3] that this issue can be tackled by charging samples with deuterium (D), the less common stable isotope of H, and cooling the specimen to cryogenic temperatures immediately after charging to slow diffusion. This approach allows the D to serve as a marker for H, so that the location of the H atoms can be determined unambiguously.

The University of Sydney has recently installed a suite of tools that allow the preparation and transfer of samples into and between instruments via ultra-high vacuum cryogenic transfer. These facilities include a purpose-built controlled-atmosphere glovebox (Microscopy Solutions), a Zeiss Auriga scanning electron microscope-focused ion beam (SEM-FIB) equipped with a custom-designed cryogenic stage (also Microscopy Solutions), and a CAMECA laser-assisted local electrode atom probe (LEAP) equipped with a Vacuum and Cryo Transfer Module (VCTM) on the load lock, all which are connected by a Ferrovac UHV suitcase. This new specimen treatment chamber allows specimens to be charged with D in a gaseous environment, at various temperatures, quenched to cryogenic conditions and then transferred to the atom probe for analysis. This talk will include an overview of this system as well as atom probe data from deuterated steel samples, including direct observation of deuterium at trapping sites, such as dislocations, grain boundaries and precipitates in martensitic steels [4].

References:
[1] I.M. Robertson et al., Metall Mater Trans A, 46a, 2323-2341 (2015).
[2] H. Bhadeshia, ISIJ International, 56, 24-36 (2016).
[3] Y.S. Chen et al., Science 355 (2017) 1196-1199.
[4] Y.S. Chen et al., Science 367 (2020) 171-175.

Using two-temperature thermodynamics, we derive an important constraint on the Taylor-Quinney coefficient, which quantifies the fraction of plastic work that is converted into heat during plastic deformation. We show that the Taylor-Quinney coefficient is a function of a thermodynamically defined effective temperature that measures the configurational disorder in the material, and increases as deformation progresses. Finite-element analysis of a recent experiment on the AA6016-T4 aluminum alloy, using the thermodynamic dislocation theory (TDT) of Langer et al., shows good agreement between theory and experiment for both stress-strain behavior and temporal evolution of the temperature.

In specific conditions, grain boundary (GB) migration occurs in poly-crystalline materials as an alternative vector of plasticity compared to the usual dislocation activity. The shear-coupled GB migration, the expected most efficient GB based mechanism, couples the GB motion to an applied shear stress. For a given GB, several migration mechanisms referred as coupling modes can occur.
Both experimental and theoretical studies have evidenced that the migration of the GB occurs through the nucleation and motion of disconnections [1-3]. Disconnections are GB defects that have both a step and a dislocation (Burgers Vector) character. They can homogeneously or heterogeneously nucleates [4-6].
We use atomistic simulations (especially the Nudge Elastic Band method) on symmetric tilt grain boundary in FCC materials to evidence these disconnections and to characterize both their structural and energetic properties. In this study, we focus on homogeneous disconnections nucleation. Investigating the <100> and <110> coupling modes, we evidence the disconnections characteristics dependence on the grain boundary disorientation. Besides, we evidence the operation of specific disconnections involving coupling modes different from the usually observed <100> and <110> coupling modes.
[1] A. Rajabzadeh et al., Philosophical Magazine, 93, 1299 (2013)
[2] A. Rajabzadeh et al., Phys. Rev. Lett., 110,265507 (2013)
[3] A. Rajabzadeh et al, Acta Mat., 77, 223-235 (2014)
[4] N. Combe, et al., Phys. Rev. B, 93, 024109 (2016)
[5] N. Combe, et al., Phys. Rev. M, 1, 033605 (2017)
[6] N. Combe, et al. Phys. Rev. M,3, 060601 (2019)

We perform an in situ investigation of hydrogen-assisted crack initiation at grain boundaries in initially flaw-free samples of polycrystalline Inconel 725. We design specialized tensile specimens, introduce hydrogen into them using electrochemical charging, and perform in situ tensile tests in a scanning electron microscope (SEM). We used electron backscattered diffraction to correlate crack initiation processes with the underlying microstructure. Cracks are found to initiate at twin boundaries, including coherent twin boundaries. To elucidate the role of hydrogen and the effect of plasticity on the crack initiation process, we use digital image correlation (DIC) to characterize surface plastic strain distributions. Potential mechanisms for hydrogen-assisted crack initiation will be discussed.

Grain boundary-dislocation interactions play a crucial role in the deformation behavior of polycrystalline materials. While it is well known that grain boundaries can respond in a variety of ways to imposed deformation, the criteria for each response is not well known. We present work focused on determining the criteria for nucleation of dislocations from a grain boundary as well as efforts to determine the criteria for transmission of dislocations through a grain boundary. The nucleation simulations of a single grain boundary find unique criteria for every slip system, which include non-Schmid effects. The transmission simulations sample a larger population of grain boundaries and find more variation in the observed geometric criteria and resolved shear stresses. We also detail current efforts to examine the evolving criteria as repeated dislocations impact a grain boundary.

In plasticity, deformation mechanisms are controlled by dislocations. A considerable progress of research over time allowed the development of different techniques to observe and characterize experimentally and/or numerically these linear defects.
For example, the Transmission Electron Microscopy (TEM) was the most used technique that allowed experimental evidences and studies of the dislocation role in deformation mechanisms [1]. Accurate Electron Channeling Contrast Imaging (A-ECCI) is a non-destructive procedure that provides, also, TEM-like diffraction contrast imaging of defects in bulk specimens as well as their full characterization [2,3,4]. For both techniques, the presence of a free surface (two surfaces for a TEM thin foil about 100 nm thick, and one surface for bulk specimen characterized by A-ECCI) can affect the nearby dislocation configurations. Furthermore, in case of A-ECCI, the electron channeling intensity signal acquired comes from the top layer diffracting down to a depth of ≈ 150 nm inside the material. Therefore, an in-depth approach for understanding the influence of the free surface on dislocation configurations is required.
In order to address this crucial scientific challenge, we proposed to combine fundamental experience both in experiments and modeling. For the experimental part, several micro-volumes of different thickness, prepared by Focus Ion Beam (FIB), are characterized to be able to study the effect of the free surface on the dislocation configurations. Numerically, microstructures similar to the experimentally characterized micro-volumes is simulated by Discrete Dislocation Dynamics (DDD) [5]. In our approach, the microstructures are self-relaxed in the presence of numerically added free surfaces to simulate the progressive thinning of the sample. The resulting structures in a thin layer close to the surface will be analyzed and compared to the experimental observations. The model results are expected to better understand the effect of surfaces on the redistribution of internal stress and then on the dislocation dynamics, and further to explore how dislocation structures near the surface, the only ones accessible by these techniques (A-ECCI and TEM), can be different from those in the bulk, which have a considerable effect on the macroscopic behavior.
Key words: dislocations, free surface, A-ECCI, TEM, micro-volume, FIB, DDD.
References
[1] J. W. Edington. Interpretation of Transmission Electron Micrographs. London, 1976
[2] H. Mansour, M. A. Crimp, N. Gey, N. Maloufi. Scripta Materialia, 76-79, 2015
[3] H. Mansour, J. Guyon, M. Crimp, N. Gey, B. Beausir and N. Maloufi. Scripta Materialia, 84-85,11-14, 2014
[4]H. Kriaa, A Guitton, N. Maloufi. Scientific reports 7, 2017
[5]Aaron A. Kohnert, Hareesh Tummala, Ricardo A. Lebensohn, Carlos N. Toméa, Laurent Capolungo. Scripta Materialia 178,161–165, 2020.

The design of new metallic materials is essential in fulfilling the promise of emerging and improving key technologies from efficient energy conversion over lightweight transport to safe medical devices. Over the last decades, two approaches in materials physics have proven immensely successful in the design of new metallic materials: Firstly, thermodynamic descriptions of crystalline phases have enabled materials scientists and engineers to tailor and process alloys to obtain a desired internal structure at the microscale. Secondly, better understanding and manipulation of the crystal defects, which govern the material’s strength, formability and corrosion resistance, has led to the development of new alloying and processing concepts that provide some of the most advanced high-performance alloys in operation today. However, to date, these two concepts remain essentially decoupled.
Our vision is to bridge the gap between these existing and powerful approaches by bringing them together in one new conceptual framework, which will consider defects and their thermodynamic stability in a holistic manner. Consequently, a new material design concept originating from the atomic scale will be accessible to materials physicists and engineers which jointly considers the local crystalline structure of defects (structural complexity), the atomic distribution of each element among the different types of phases and defects (chemical complexity) and the stability of defects under the given conditions, such as bulk composition, temperature and applied stress or electrode potential. We will show here, how these can be linked by defect phase diagrams, which will describe the transition between and the coexistence of defect phases. This structural and chemical complexity at the atomic scale naturally exists in all materials. On this basis, we believe that a new physical description of metallic materials will possible and will provide a powerful toolbox for future design of engineering materials with tailored properties regarding concurrent mechanisms and properties, such as mechanical and corrosion performance of engineering materials.

The interactions between hydrogen and various lattice defects play an important role in the damage of metal structural materials. The vacancy is one the of most general defects in metals, which could be introduced by the interactions of moving dislocations, irradiation, or ion implantation. The interaction between vacancy and hydrogen attracted huge interests in the past decades. The concentration of Hydrogen in α-iron is low due to the positive solution energy in bulk, however, it could be high around defects, such as grain boundaries or vacancies. The normal sense suggests that vacancies in metals could be greatly stabilized by forming hydrogen-vacancy complex. Our previous work shows that the diffusivity of vacancy could be enhanced when the hydrogen concentration is high in face-centered-crystal metals. In this paper, with the help of the neural network interatomic potential, an inflection point of vacancy diffusivity in α-iron, relates to the concentration of Hydrogen, is presented.
[1] Predictive model of hydrogen trapping and bubbling in nanovoids in bcc metals, J. Hou, X-S. Kong, X. Wu, J. Song, C.S. Liu, Nat. Mater., 2019, 18:833-839
[2] Hydrogen bubble nucleation in α-iron, W.T. Geng, L. Wan, J-P. Du, A. Ishii, N. Ishikawa, H. Kimizuka, S. Ogata, Scri. Mater., 2017:105-109
[3] Interplay between hydrogen and vacancies in α -Fe. E. Hayward, C-C. Fu, Phys. Rev. B, 2013, 87:174103
[4] Hydrogen embrittlement controlled by reaction of dislocation with grain boundary in alpha-iron, L. Wan, W.T. Geng, A. Ishii, J.-P. Du, Q. Mei, N. Ishikawa, H. Kimizuka, S. Ogata, Int. J. Plas., 2019, 112:206-219
[5] Interatomic potentials for hydrogen in α–iron based on density functional theory, A. Ramasubramaniam, M. Itakura, E. A. Carter, Phys. Rev. B, 2009, 79:174101
[6] Hydrogen-enhanced vacancy diffusion by an activation excess, J.-P. Du, W.T. Geng, Kazuto Arakawa, Ju Li, Shigenobu Ogata, submitted.

Neutron and ion radiation damage materials used in radioactive environments, such as nuclear reactors. This type of radiation can cause atoms to be moved around in the atomic structure or removed entirely, causing defects and damage to accumulate in the material, which leads to lower mechanical strength. Since these defects are on the atomic level, it can be hard to detect. We propose examining two different types of interfaces: bilayer interfaces, specifically Silicon-Aluminum and Silicon-Gold, as well as crystalline-amorphous interfaces to investigate whether these types of interfaces can help develop radiation-resistant materials. Using molecular dynamics, we will model these bilayer and amorphous structures as well introduce radiation in the form of Frenkel pairs to our system. To analyze the mechanical strength at different levels of radiation damage, yield surface calculations will be performed by subjecting the samples to biaxial and uniaxial tension and compression tests to determine the yield points. Different defect absorption mechanisms at the interface will also be discussed.

Creep cavitation is an important degradation mechanism for metallic components used at an elevated temperature. Stress, temperature and microstructure all play a role in the initiation of creep cavities, and the formation mechanism of cavities can depend heavily on local phase and chemistry. In this work, the relationship between microstructural evolution due to long term thermal ageing and creep cavitation is explored in an ex-service AISI type 316H stainless steel sample from an advanced gas-cooled nuclear reactor boiler header after 65,000 hours at 490 to 530°C. The microstructure of the ex-service specimen has been investigated by secondary electron microscopy, focused-ion beam cross sectioning, electron backscatter diffraction and transmission electron microscopy. M₂₃C₆ precipitates, α-ferrite precipitates and creep cavities are observed at grain boundaries, with increased precipitation and cavitation at random grain boundaries, and an absence of creep cavities at coincidence site lattice (CSL) 3 boundaries. The α-ferrite precipitates grow during in-service exposure at grain boundaries with a Kurdjumov-Sachs orientation relationship to at least one neighbouring austenite matrix grain. The creep cavities were observed to initiate at the intergranular M₂₃C₆ and α ferrite precipitates. Based on the orientation relationship of the ferrite precipitates with the austenite matrix, some creep cavities are initiated after the ferrite precipitates have formed. The influence of microstructure changes on creep cavitation of materials during in-service aging and the implications for more reliable predictions of lifetime for components operating at high temperature are discussed.

In the field of atomic-scale modeling, semi-empirical potentials represent a computationally
affordable method to describe complex phenomena occurring at time and space scales which are
unreachable through ab initio approaches. They play a central role in uncovering deformation
mechanisms, and in understanding the mechanical properties of metals that are often driven by
defect-defect interactions (e.g. interactions between dislocation and other defects). Semi-Empirical potentials are defined by specific functional forms and a fixed number of parameters that need to be accurately identified on targeted Quantities Of Interest (QOIs). Both aspects affects the model’s accuracy/reliability, and this results in parameters’ uncertainties. Even if the use of parametric potentials is widely established in the literature, very few studies discuss the quantification of such uncertainties [1, 2].
This work represents the starting point for the development of a new methodology aiming at a
robust management of the uncertainty in the calibration of semi-empirical potentials. A Tight
Binding Second Moment Approximation (SMA) potential [3,4] is used to setup the methodology; this choice is led by its relatively simple formulation and the fact that it is defined by four physical parameters. α-zirconium, a broadly used material in nuclear industry, is chosen to illustrate and develop the approach, focusing in particular on its ability to reproduce the energetics and structure of irradiation defects, and various dislocation-related properties.
A critical study is firstly carried out in order to identify the relevant QOIs to be used as input for the Uncertainty Quantification (UQ) of the SMA potential parameters. Such QOIs and their scattering are obtained through Density Functional Theory calculations [5,6], using different approximations of the exchange-correlation energy functional. Bayesian inference and polynomial chaos based techniques are then implemented to estimate the probability distribution functions (PDFs) of the uncertain parameters. In order to validate and compare the performances of the UQ methods, the obtained posterior PDFs are finally used to propagate uncertainty both in QOIs and in properties that were not used as QOIs for the UQ. The outcome establishes an objective framework leading to the choice of the most appropriate UQ technique for the methodology under development, that is finally applied to the case of α-zirconium.
References
[1] G. Dhaliwal, P. B. Nair, and C. V. Singh. In: Computational Materials Science 166 (2019), pp. 30–41.
[2] F. Rizzi et al. In: Multiscale Modeling and Simulation 10.4 (2012), pp. 1428–1459.
[3] Goringe et al. . In: Reports on Progress in Physics 60 (12) (1997), pp. 1447–1512.
[4] V Rosato, M Guillope, and B Legrand. In: Philos. Mag. A 59 (1989), pp. 321–336.
[5] E. Clouet. In: Phys. Rev. B 86.14 (2012), p. 144104.
[6] C. Varvenne, Olivier Mackain, and Emmanuel Clouet. In: Acta Mater. 78 (2014), pp. 65–77.

Titanium-Aluminum alloys (TiAl) are very interesting for applications in turbomachines because of their low density (3,8 - 4,2 g.cm^-3) and of their high mechanical strength at high temperatures. However, when exposing TiAl alloys to high temperatures, a catastrophic loss of ductility is observed. Here, we study the role of oxygen in this phenomenon. For this purpose, we investigate the nature of the TiAl-oxygen solid solution (diluted O atoms, or forming clusters), we determine the oxygen diffusion mechanisms and study the interactions between the dislocations and the oxygen atoms. The nature of the solute solution and the diffusion mechanisms have been investigated theoretically using atomistic calculations (DFT), and the dislocation-oxygen interactions have been studied experimentally by characterizations at the microscopic scale (TEM).
In the experimental part, tensile test on Ti₄₈Al₄₈W₂B_0.08 alloys have been carried out on samples exposed in air at 800°C for 500h, to characterize the embrittlement phenomenon. It was shown that the embrittlement comes from oxygen penetration in a surface layer 10 µm to 100 µm thick. Then, to evaluate the influence of oxygen on the bulk mechanical properties, tensile tests on alloys with low and high oxygen concentrations have been performed. Results showed that the alloy with high bulk oxygen concentration has a lower ductility than the alloy with lower oxygen content, as classically observed in TiAl alloys. TEM observations showed the presence of pinning points on the dislocations, which may have an impact on the alloy ductility. Based on these results, our hypothesis is that TiAl embrittlement at high temperature occurs by a loss of ductility of the superficial diffusion layer.
In the numerical part, DFT simulations were used to study the interactions between clusters of one to 6 oxygen atoms with lattice defects such as vacancies and antisites at the atomic scale. We identified which type of complex defect (defect + oxygen cluster) would be the most probable and in which concentration, depending on the temperature. However, these clusters of 1 to 6 oxygen atoms are much smaller than the clusters pinning the dislocations, which should be composed of about 10⁹ oxygen atoms, according to the first evaluations based on the TEM images. Nevertheless, both the two clusters may have an impact on the mechanical properties. The small clusters are likely to decrease the diffusivity of oxygen in TiAl by acting as traps, which would lead to decrease oxygen penetration, and thus to diminish embrittlement. On the contrary, the big clusters, which pin the dislocations, probably contribute to increase the strength and to lower the ductility in the surface layer, which increase embrittlement.

Beyond the dilute limit, solute segregation to grain boundaries is known to be concentration dependent. The classic explanation for this effect comes from the Fowler-Guggenheim approach, which solely attributes this dependence to increasing solute-solute interactions at grain boundaries e.g. as solute concentration increases, a strong solute-solute repulsion at grain boundaries will reduce the segregation tendency of additional solute atoms. However, there is a second, less understood, contribution to this concentration dependence: the gradual exhaustion of favorable segregation sites. Grain boundaries have a complex environment with a variety of site-types available for segregation, and a spectrum of solute binding “segregation” energies. Segregation will first occur at the most favorable sites, forcing the next round of solute atoms to compete for less favorable sites, which reduces their tendency to segregate. In this work, we outline a computational and analytical framework to delineate, for the first time, both contributions to the concentration dependence of solute segregation: 1) the spectrality of site energies in grain boundaries and 2) solute-solute interactions.

We present our latest results mainly focusing on elevated temperature microstructural stability of Cu/Nb nanocomposites containing 3-D interfaces. Cu/Nb nanocomposites with atomically sharp 2-D interfaces that exhibit outstanding thermomechanical stability have been examined intensively for over two decades. Their strength is enhanced by defect-interface interactions such that dislocations are blocked from traversing Cu/Nb sharp interfaces. We have investigated the relationship between atomic structure, mechanical behavior, and thermal stability in Cu/Nb nanocomposites with novel 3-D interfaces. We have shown that the mechanical properties (such as the strength) of Cu/Nb nanocomposites can be further improved by the introduction of chemical, crystallographic, and/or topological variations in all 3 dimensions at heterophase interfaces. Structural characterization employed a combined atom probe tomography (APT) and transmission electron microscopy (TEM) approach. These techniques complement each other very well. While it is possible to observe chemical inhomogeneities in 3-D interfaces with APT, it does not render atomic-resolution information and is prone to magnification artifacts caused by differences in atomic vaporization rates in multi-component samples. However, high-resolution TEM provides atomic-scale information to probe the existence of amorphous regions, metastable alloys, or third-crystal phase precipitates in the 3D interfaces, and help to identify the relevant structure and possible deformation mechanisms. Additionally, we carried out thermal stability studies, incorporating static annealing and differential scanning calorimetry (DSC) scans. Annealed samples were investigated by TEM to uncover the structural evolution associated with thermally perturbed 3D interfaces, while enthalpy changes by DSC analysis were used to identify the onset of thermally-induced structural changes. These correlated studies determined the thermal stability of 3-D interfaces, and how post-deposition annealing can be used to manipulate 3-D interface morphology and possibly enable optimal mechanical performance.

Materials with heterogeneous microstructures have been shown to exhibit a superior combination of strength and toughness compared to homogeneous nanostructured or coarse-grained materials.However, only a limited progress has been made in producing heterogeneous microstructures with good repeatability and robust control of key microstructural parameters such as grain size dispersion, texture or spatial distribution of grains. In this work, we report a novel technique to synthesize metallic films with highly tailored heterogeneous microstructures using magnetron sputtering. The technique is capable of precisely controlling the volume fraction, in-plane texture, spatial arrangement, morphology and connectivity of two or more sets of grains. Using this technique, we synthesized bimodal films of pure metals like aluminum, copper, iron and nickel where the mean size of smaller grains is around 30-300 nm while the coarser grains have a mean size exceeding 5 µm. The microstructure of these films was characterized by a combination of X-ray diffraction, electron backscattered diffraction and transmission electron microscopy to reveal the grain size dispersion, texture and orientation distribution. Mechanical characterization via uniaxial tensile loading-unloading was performed on bimodal Fe films with varying volume fraction and spatial arrangement of nanocrystalline and coarse grains using MEMS stages. Films with higher volume fraction of coarse grains showed enhanced strain hardening capability and a better combination of strength, ductility and toughness. This method enables the optimization of microstructural parameters (volume fraction of fine and coarse grains, texture, morphology, etc.), to obtain films with superior mechanical properties.

A series of compression tests on chromium single crystals at 77 K was carried out in order to investigate the slip activity in the first 2-4% of plastic strain. Macroscopic samples of about 3×3×10 mm were polished mechanically and electrolytically and their orientations prior to and after deformation were determined by electron backscatter diffraction to assess the magnitudes and directions of crystal rotations. The crystals compressed in the direction close to [001] deformed primarily by slip on the most highly stressed (-101)[111] and (101)[-111] systems. Loading the crystals in the directions close to the [011]-[-111] edge of the stereographic triangle resulted in preferential twinning on {112}<111> systems. For loading in the center-triangle direction, the slip morphology was dominated by anomalous slip on the low-stressed (0-11) plane in contrast to the prediction of the Schmid law. Diffraction contrast imaging in TEM reveals the presence of [100] screw junctions between intersecting 1/2[111] and 1/2[-111] screw dislocations. Using molecular statics simulations, we show that this junction is not removed under the applied stress. Instead, the three dislocations move on the (0-11) plane, which results in the anomalous slip.

Periodic Boundary Conditions (PBCs) in one or more directions of the real space is the most popular simulation setup for numerical calculations in materials science. Indeed, it allows to eliminate surfaces and mimic a bulk environment. However, for simulation boxes containing point defects and/or small clusters such as dislocation loops, i.e. that induce long range elastic fields, all calculated energies and elastic fields are those of a periodic arrangement of defects. This quantitatively impacts both the convergence rate and the accuracy of ab initio calculations of isolated point defect energetics, and mesoscopic simulations (e.g. Object Kinetic Monte Carlo) of the evolution of a population of interacting defects, e.g. point defects, clusters and/or dislocation loops. Getting rid of these artefacts requires the computation of infinite conditionally convergent sums. Various regularization schemes have been proposed in the literature, either inspired from the seminal work of Cai and Bulatov for dislocations [1], or more recently using integral formulations [2].

In this work, we first show the formal equivalence between the various numerical regularization techniques that are based on a summation in the real space [1,2], focusing on the case of constant volume calculations. We then discuss the direct construction of physically acceptable solutions for the elastic fields in the reciprocal space. Accuracy and numerical efficiency of the different approaches are illustrated on the evaluation of the interaction energy of a periodic array of point defects, that is significant for ab initio calculations where the number of atoms is limited [3]. Metals and defects having respectively various crystalline structures and point symmetries are considered for this application: several configurations of self-interstitial atoms in hcp Zr, and carbon solute in fcc Ni. Finally, the discussion is broadened to the cases of (i) constant stress calculations, (ii) charged point defects, and (iii) linear defects such as dislocations.

[1] W. Caï, V. V. Bulatov, J. Chang, J. Li and S. Yip, Philos. Mag. 83, 539 (2003),
[2] T. Jourdan, JMPS 125, 762 (2019), P. W. Ma and S. L. Dudarev, PRM 3, 013605 (2019),
[3] C. Varvenne et al., PRB 88, 134102 (2013); E. Clouet et al. CMS 147, 49-63 (2018).

Grain boundary (GB) migration is a prevalent plastic deformation mode in nanocrystalline and polycrystalline materials, and a systematic insight of GB migration is vital to the development of novel materials through GB engineering. However, current understanding on the atomistic mechanism of GB-dominated plasticity remains largely elusive. Here, we combine state-of-the-art in situ TEM nanomechanical testing and atomistic simulations to investigate the atomistic dynamics of shear-induced GB migration in Au nanocrystals. Using the <110> tilt GBs as examples, we demonstrate that the shear-induced GB migration is fundamentally accommodated by GB defects, including disconnections (high angle GB) and geometrically necessary dislocations (low angle GB). In the high angle range (as exemplified by the Σ11(113) GB), we unambiguously reveal a disconnection-mediated GB migration mechanism under shear loading, where the nucleation, propagation and dynamic interactions of various disconnections dominate the GB migration. Moreover, the migrating Σ11(113) GB can readily accommodate intragranular lattice defects (including dislocation and stacking fault), where the pre-existing disconnections interact with the residual disconnections generated on the GB. This disconnection-mediated dynamic is further proved to be universal among different high angle GB structures, where triple junctions can serve as effective nucleation and annihilation sites of disconnections. In contrast, low angle GBs with dislocation characters typically migrate via conservative gliding of intrinsic GB dislocations. The fully reversible motion of such coherent GBs can be achieved under shear loading cycles in Au nanocrystals, due to the suppression of heterogeneous surface nucleation of lattice defects and robust structural stability throughout GB motion. Inspired by these scientific insights, we propose a GB engineering protocol to realize controllable plastic reversibility in metallic nanocrystals. This reversible deformation via conservative GB migration is retained in a broad class of face-centered cubic metals with low stacking fault energies when tuning the GB misorientation, external geometry and loading conditions over a wide range. Above results enable us to establish a full deformation map of <110> tilt GBs, providing novel insights into the GB-dominated plasticity in nanocrystalline materials. This talk is based on our recent works: In situ atomistic observation of disconnection-mediated grain boundary migration. Nat. Commun. 10, 156 (2019); Metallic nanocrystals with low-angle grain boundary for controllable plastic reversibility. Nat. Commun. (2020, accepted); In situ atomistic observation of grain boundary migration subjected to defect interaction. (under review).

Comprehensive microscopic scale studies bring valuable information for extrapolating to the macroscopic mechanical response of materials and they can feed advanced multiscale crystal plasticity models. However, fundamental questions on the representativeness of observed phenomenon must be raised while extrapolating microstructural observations to the macro-scale. In this framework, macroscopic mechanical testing of bulk specimens has been successfully combined with a dislocation-scale characterization technique: Accurate Electron Channeling Contrast Imaging (A-ECCI). In this study, the focus will be on BCC titanium where 48 slip systems are potentially active and pencil glide occurs and hence serves as a challenging benchmark for the proposed methodology. In-situ macroscopic mechanical testing combined with A-ECCI methodology was performed on bulk Ti21S. The orientation of several grains of interest and their surroundings were digitized using EBSD data and were used as an input for the crystal plasticity models. An automation routine was set-up for slip line detection from the A-ECCI images of the deformed region of interest. This was used in tandem with the EBSD data to determine the slip activity distribution for the three different slip system families and the influence of grain boundary proximity on the observed slip activity.
EBSD data was used next as an input for the crystal plasticity models. Crystal plasticity simulations were carried out using three different models (a) Visco-plastic self-consistent (VPSC) model, (b) Affine elasto-visco-plastic self-consistent model (EVPSC) and (c) full field crystal plasticity models using FFT algorithm. Model predictions of the activated slip systems were statistically confronted with A-ECCI data mining for two extreme plastic strain states. Such a statistical confrontation aids in defining the limits for the validity of different models in being able to capture the complex interaction between grains undergoing plastic deformation. Particular attention is paid to the accuracy of the different models in being able to predict the slip system activity observed experimentally. The case for uniform CRSS ratio for all three slip system families (1:1:1) is compared with the non-uniform CRSS ratio (1:1:1.1) and stress hotspots from the models are compared with the experimental observed slip system activity. The slip system distribution from different regions of interest are compared with the distribution from the models to optimize the anisotropy ratios for a given strain state to best match the experimental data. These results highlight the importance of such data mining procedures to predict plastic events and stress hotspots and also to pinpoint different micro-structural features causing the same in polycrystalline microstructures.
In this work, fundamentals on the defect contrasts and the experiment procedure will be presented. Second, the full potentiality of A-ECCI for following the evolution of deformation microstructures will be highlighted [1,2]. Micro-structural information available from ECCI has been used for examining the effect of anisotropic elastic and plastic properties on the local stress field and dislocation activity distribution within grains at different stages of deformation and gather statistical information for identifying relevant micromechanical and microstructural variables that influence the material behavior.
References:
[1] M. Ben Haj Slama, N. Maloufi, J. Guyon, S. Bahi, L. Weiss, and A. Guitton, “In Situ Macroscopic Tensile Testing in SEM and Electron Channeling Contrast Imaging: Pencil Glide Evidenced in a Bulk β-Ti21S Polycrystal,” Materials, vol. 12, no. 15, p. 2479, Aug. 2019.
[2] M. Ben Haj Slama, V.Taupin, N. Maloufi, A.D.Rollett, S.Berbenni, K. Venkatraman , and A. Guitton “Bridging the gap between dislocation-scale and polycrystalline plasticity by coupling statistical in situ ECCI and modeling, Application to BCC titanium.” To be submitted, 2020.

Both grain boundaries and dislocations have been recently shown to experience structural transitions between defect states known as complexions. In this talk, we discuss how one can manipulate the natural variations in stress, chemistry, and atomic structure near grain boundaries and dislocations to stabilize these defects and use them as tailorable features. We also demonstrate how different complexions can alter the mechanical properties of materials on the nanoscale. For grain boundary complexions, we focus on nanocrystalline metals, where such features have the largest effect due to the high grain boundary density. We isolate the materials and processing conditions which can cause the strength and ductility of these materials to be simultaneously augmented, as well as report on small-scale mechanical testing and atomistic modeling techniques to study grain boundary mechanics. For linear complexions, our work uncovers how nanoscale phases restricted to the region along a dislocation line direction can affect plasticity, encompassing both initial yield and subsequent flow. Alloy choice is shown to be very important, as various responses ranging from softening to strong hardening are predicted. As a whole, this work demonstrates that our field is on the precipice of a new age, where defects can be manipulated and designed rather than simply tolerated.

This talk reviews recent work complex systems local structure evolution in high entropy alloys (HEAs). Despite their nominal chemical disorder, several studies have reported short range order (SRO) in HEAs – i.e. preferential bonding, local elemental enrichment and/or clustering – and such SRO may have broad implications for HEA performance. To tackle this problem, a suite of spatially resolved, electron imaging, diffraction, and spectroscopy techniques is leveraged to correlate local order with microstructural evolution and related dislocation phenomena. Microstructures of HEAs subjected to a variety of deformation regimes and quantified using diffraction-based techniques. In these samples, the extent to which SRO controls localized dislocation-based phenomena during microstructural evolution is discussed. 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 HEA development.

Dynamic properties of dislocations are both of fundamental importance in understanding and modelling plasticity and mechanical deformation of solids and are required for constructing mesoscale dislocation dynamics simulations. However, these properties are not easily accessible by experimental means due to the short time-scales. Molecular dynamics provides a tool for studying the dynamics of dislocations at the short time and length scales, which are difficult to access by experimental means, but requires careful convergence for physical meaning. We have studied the dynamics of dislocation glide and of dislocation cross-slip in FCC crystals of Al, Cu, Ni and Ag to elucidate the effect of chemical species on the dislocation properties. Highly converged simulations were performed on long dislocations in large FCC crystal supercells with full periodic boundary conditions, incorporating a dislocation dipole with zero net Burgers vector. Kinematics were studied by accelerating dislocations at constant stress and temperature. The relation between stress and terminal velocity at cryogenic and room temperatures for both screw and edge dislocations was obtained. A transition from viscous behaviour to near sonic asymptotic behaviour was identified and the velocity dependence of the drag force was characterised. At higher stresses a shift into the transonic regime occurs and the mechanism and stability of this transition is examined. The thermally activated process of cross-slip was simulated multiple times creating statistical data from which the kinematics and kinetics were deduced and from which the activation parameters were calculated. The effects of stresses in the cross-slip and glide planes on the activation volumes were determined. Finally, the rate-controlling step of the cross-slip process was identified producing insight into the cross-slip mechanism.
[1] E. Oren, E. Yahel, G. Makov, Kinetics of dislocation cross-slip : A molecular dynamics study, Comput. Mater. Sci. 138 (2017) 246–254. doi:10.1016/j.commatsci.2017.06.039.
[2] E. Oren, E. Yahel, G. Makov, Dislocation kinematics : a molecular dynamics study in Cu, Model. Simul. Mater. Sci. Eng. 025002 (2017). doi:10.1088/1361-651X/aa52a7.

Interfaces, such as grain boundaries, are crucial for the mechanical properties of metallic materials. Especially in refractory metals like tungsten and molybdenum, grain boundaries are often the weakest link regarding the materials’ strength and ductility. These metals are inherently susceptible for intergranular failure and suffer from a limited deformation capability at low temperatures. An advanced approach for the improvement of the interfaces is segregation engineering, e.g. the addition of cohesion enhancing elements segregating to the grain boundaries.
In this work, three-point-bending tests are conducted on recrystallized commercially pure and microdoped molybdenum at temperatures between -40°C and room temperature. The early stages of intergranular crack formation are examined on the specimen surface which is under tension close to the final fracture plane. The occurring grain separations are investigated regarding their distinct features, for instance the crystallography of the adjacent grains or the frequency of open grain boundaries. Doping of molybdenum with elements such as carbon and/or boron is known to suppress intercrystalline failure. Therefore, the presented grain boundary characterization methods will be applied to extract mechanical changes caused by these segregations.

Microstructural plastic strain distribution evolution is highly heterogeneous even in single-phase alloys. One of the important factors that govern this heterogeneity is slip/twin transfer across grain/phase boundaries. In this regard, the fundamentals of transfer across grain boundaries have drawn significant attention in the literature, while the understanding of phase boundaries remains comparatively limited. (α+β) titanium alloys provide a profound platform to explore these phenomena, since: (i) both of the present phases can exhibit plastic deformation at similar microscopic strain levels; (ii) both slip and mechanically-induced twinning can be triggered to accommodate plastic strain. By integrating in-situ EBSD/SEM testing, crystallographic calculations, and microstructure-based strain mapping approach, we aim to address the following three propositions in this presentation: (1) could deformation transfer take place in the form between mechanical twinning in the α-phase and dislocation glide in the β-phase? (2) What parameters would be rational indicators to quantify the propensity for the incipience of this event? (3) What micro-mechanical consequences are associated with the slip-twin transfer activity? Broader indications for mechanistically-guided microstructural design concept will also be discussed.

Understanding dislocation interactions at different length scales, from atomistic to macroscopic, is important in the study of work hardening, fatigue and crack formation. Colloidal systems, in which particles can be tracked by confocal microscopy in 3D-space and time during deformation, provide the opportunity to study dislocations and their interactions over a wide size scale, from the particle to the sample level. The colloidal crystals in this work consist of micrometer-size hard spheres dispersed in a liquid, which have been sedimented onto a template to form a face-centered cubic single crystal. When the crystal is sheared, dislocations nucleate, glide, react and often form Lomer-Cottrell locks. We supplement these observations with measurements of the local stress and strain fields and identify the zipping-unzipping mechanism of the locks.

The vessel of pressurized water reactors operated in nuclear plants consists mainly of ferritic steels (Fe-C alloys). During operation, these components are subjected to extreme conditions (600 K, 155 bar, irradiation, ...) that affect their microstructural evolution and induce hardening and embrittlement, which can significantly impact the safety of the plant. In particular, the carbon atoms tend to diffuse and segregate over time, forming Cottrell atmospheres around dislocations and modifying their core structure [1].

The aim of this work is to establish a link between microstructural evolution and plastic properties. This requires to refine our understanding of the glide of screw dislocations in ferritic steels, in the presence of carbon solute atoms and at different scales. A number of state-of-the-art Fe-C interatomic potentials from the literature were tested on different alloy and dislocation properties, including carbon-carbon and carbon-dislocation binding energies, the screw dislocation core structure and Peierls potential. While none of the potentials fully agrees with ab initio reference data, we propose a combination of the two that perform best [2] [3].
This new potential is used in molecular dynamics (MD) simulations to study the glide of screw dislocations pinned by random solute atoms. It is evidenced that (1) short range interactions between solute and dislocation core atoms prevail compared to mid-range elastic interactions, (2) that the dislocation adopts a 3D structure due to its interaction with carbon atoms in different neighboring octahedral positions and (3) that the dislocation is very strongly pinned when the distance between carbon atoms decreases below a few tens of Burgers vectors. The simulated time scale is then extended to model static and dynamic strain ageing with a kinetic Monte Carlo approach parameterized on the DFT and EAM calculations. This work paves the way for the development of a general mobility law for dislocations in aged steels.

[1] Ventelon L. et al. Phys. Rev. B. 91 (2015).
[2] Veiga, R. G., Becquart, C. S., & Perez, M. Comput. Materials Science, 82 (2014).
[3] Proville, L., Rodney, D., & Marinica, M. C. Nature Materials, 11 (2012).

Electromigration is a critical failure mode of metallic interconnects in the semiconductor industry, and in the emerging fields of flexible and wearable electronics. This failure can be attributed to the diffusion of metallic atoms due to momentum transfer of electrons flowing through the interconnect when a high current density is applied. The diffusion eventually leads to formation of voids and hence a time-dependent increase of electrical resistance of the interconnect. Since it is a diffusion process, the microstructure of the interconnect is one of the controlling factors of electromigration. Twin boundaries, in contrast to grain boundaries, can have the potential to reduce diffusion of atoms and consequentially improve electromigration performance both in electronic chips and in flexible electronics. Note that in nanostructured flexible electronics, it is common to have nanowires with twin boundaries, acting as interconnects. Unfortunately, very few studies have been conducted about the diffusion performance of these interconnects. No literature is available about the collective effect of twin boundaries throughout the interconnect. Moreover, some of the studies said that twinning slows down the diffusion and hence void growth [1], [2] and some said that twinning does not necessarily hinders void growth [3]. Given these conflicting reports, there is clearly a need for a systematic study of diffusion through twinned interconnects.

In this work, we investigated electromigration failure of penta-twinned silver nanowires, with diameters ranging from 50 nm to 80 nm to understand the diffusion process during electromigration of twinned interconnects. Samples were prepared by random deposition of nanowires on a substrate containing gold electrodes fabricated by photolithography. The nanowires were connected with the electrodes by e-beam lithography. The electrodes were then connected to a printed circuit board (PCB) through a wirebonding package. Under a fixed current density and temperature, the resistance was measured using 4-point measurement method so that it can be measured continuously without the influence of contact resistances. The currents were applied in an automated fashion to test a large number of nanowires. The failure criterion was set as 10% increase in resistance which is widely used in the electronics industry. Resistance vs. time plots were obtained in different temperatures, which were fitted to a mathematical model combining the theory of electromigration diffusion flux and the theory of void growth. The fit produces a diffusivity product, independent of temperature, for twin boundaries and the activation energy required for atomic fluxes through twin boundaries. These parameters enable a comparison between the diffusion through twinned interconnects and the published data for diffusion through other microstructures of interconnects currently used in the electronics industry. The results of this study should also allow reliability calculations to be made for novel electronic devices that utilize twinned metallic interconnects, such as wearable electronics that use nanowires as flexible conductors.

References

[1] K. Chen, W. Wu, C. Liao, L. Chen and K. Tu, "Observation of atomic diffusion at twin-modified grain boundaries in copper," Science, vol. 321, no. 5892, pp. 1066-1069, 2008.

[2] H. Chem, C. Huang, C. Wang, W. Wu, C. Liao, L. Chen and K. Tu, "Optimization of the nanotwin- induced zigzag surgace of copper by electromigration," Nanoscale, vol. 8, no. 5, pp. 2584-2588, 2016.

[3] Y. Oh, S. Kim, M. Kim, S. Lee and Y. Kim, "Preferred diffusion paths for copper electromigration by in situ transmission electron microscopy," Ultramicroscopy, vol. 181, no. Supplement C, pp. 160-164, 2017.

Unlike dislocation-driven size-effect in single-crystalline materials at the nanoscale, the effect of sample dimensions on the strength of twin-containing materials, especially that of ceramics, still lacks universal understanding. Previous work has demonstrated >5% plastic strain in Zirconia-based ceramics via twinning. Using diffraction methods and in-situ nanomechanical experiments, we uncover the microstructure-strength relationship in these twin-containing ionic crystals and reveal contributing factors to the competition between twinning and slip as the major plasticity carrier. The prominent twinning-governed size-dependence was compared to that reported for metals, where the uncanny similarities between these drastically different material systems shed light on an universal scaling relationship that can be tied back to the well-known “smaller-is-stronger” slip-governed size-effect.

Due to the small scale and high surface-to-volume ratio of nanometer-sized metallic crystals, it is generally believed that surface surface-related deformation mechanisms govern the plasticity of nanocrystals at atomic scale. As the sample size of the nanocrystals goes further went down to sub-ten nanometer, strong surface diffusion activities took place and mediated plastic deformation. However, thus far, much less is known about the yielding and plastic flow mechanisms at atomic scale in small-sized nanocrystals where surface atomic diffusion is activated. Here, by performing in- situ transmission electron microscope tensile tests of Ag NWs and Pt NWs, it was revealed that the yield strength-size relationship changed from ‘‘smaller is stronger’’ to ‘‘smaller is weaker’’ with decreasing the sample diameters of Ag NWs, while Pt NWs showed traditional “Hall-Petch” size-dependent behavior. This difference arose from surface atomic diffusion activities in Ag NWs, which may increase dislocation nucleation sites and the probability of surface defect interaction, thereby lowering critical surface dislocation nucleation stress. The coupled displacive-diffusive plasticity was quantitatively investigated by analyzing the lattice stress-applied strain evolution in nanowires at atomic scale. This work provides new insights into the atomic-scale mechanisms of diffusion-assisted dislocation nucleation and diffusion-mediated plastic deformation process with displacive plasticity in small-sized materials.

High strength Al alloys exploit solid state precipitation to tailor their mechanical response. This precipitation requires two ingredients: a thermodynamic driving force and atomic mobility. For a given alloy chemistry, the heat treatment (precipitation) temperature is chosen as a compromise between having sufficient driving force for precipitation and sufficient atomic mobility so that the precipitation reaction occurs in a reasonable time and results in a ‘not too coarse’ precipitate distribution. It is this compromise that frames the competition between nucleation, growth and coarsening that constrains the possible precipitate distributions and hence mechanical responses.

This talk demonstrates a new approach that uses small amplitude cyclic plasticity at room temperature as a means of continually pumping vacancies into the system to achieve atomic mobility under conditions of high thermodynamic driving force. The approach is self-regulating (in both space and particle size) and results in extremely fine-scale inhomogeneous solid solutions. Such structures lead to combinations of strength and elongation that exceed those of conventionally precipitate strengthened alloys, without the presence of precipitate free zones.

Since the approach does not use thermal treatments, it somewhat decouples control of the thermodynamic driving force and atomic mobility. This provides a means to fully alter the competition between precipitate nucleation, growth and coarsening and new microstructures, with new combinations of properties are obtained. Examples of both monotonic and cyclic properties are demonstrated.

Understanding the formation and evolution of dislocation networks and their effect on the mechanical properties of solids is challenging, as it spans a vast hierarchy of length and time scales. Hard-sphere colloidal suspensions provide a unique model system to address this difficulty: the micrometer size of the suspended particles both slows down their motion and allows them to be visualized by optical microscopy. At high density. these colloidal particles form face-centered cubic crystals with non-zero stiffness, which allows us to use them to study the principles of crystalline plasticity.
Here we present simultaneous multi-scale visualization of dislocation networks. At the single particle level the structure of the dislocations and their interactions are visualized by laser confocal microscopy. To visualize the large-scale collective dislocation dynamics we developed a laser diffraction imaging technique inspired by the classical TEM imaging methods in atomic systems. The flexibility in manipulating visible light, in contrast to electrons, allows us to image under multiple diffraction conditions almost instantaneously. We designed a deep convolutional neural network to address the inverse problem of inferring the structure of the dislocation network from the multiple complex diffraction images. We will show that the neural network reliably reconstructs the spatial positions of the dislocations and their Burgers vectors by testing the results with in-situ confocal microscopy data on a smaller portion of the sample. We demonstrate the success of our method by applying it to the network of misfit dislocations formed via during particle sedimentation on a strained substrate.

Strengthening of metals through nanoscale grain boundaries and coherent twin boundaries is manifested by a maximum strength—a phenomenon known as Hall-Petch breakdown. Different softening mechanisms are generally considered to occur for nanocrystalline and nanotwinned materials. Here, we synthesized nanotwinned Ag metals with hardness 3.05 GPa, i.e. 42% higher than the previous record, by segregating trace concentrations of Cu impurity (<1.0 wt.%) to grain boundaries and twin boundary defects. The grain diameter and twin spacings were 45 nm and 3.6 nm, respectively, which is well below the current limits for pure Ag, making it possible to explore plasticity mechanisms in nanostructures that approach those only studied by atomistic simulation. In this presentation, hybrid Monte-Carlo and Molecular-Dynamics simulations reveal three distinct strength regions as twin spacing decreases, delineated by positive Hall-Petch strengthening to grain-boundary-dictated (near-zero Hall-Petch slope) mechanisms, and to softening (negative Hall-Petch slope) induced by twin-boundary defects. Both our experiments and simulations find that an ideal maximum strength is reached for a range of twin spacings below 7 nm in pure and Cu-segregated nanotwinned Ag. We present a continuum theory predicting the softening behavior from kink motion in twin boundaries applicable to pure metals and segregated alloys containing nanoscale twins.

Quasicrystalline materials possess a remarkable structure that does not exhibit
crystallographically legitimate periodicity in certain crystallographic directions. Despite their
special configuration and many related useful properties, the major drawback of these
quasiperiodic structures is extreme brittleness of the materials at temperatures below about 0.75
T/Tm(Tm-the melting point) which hugely handicaps their practical implementation in the
moderate temperature region. Here, we study the mechanical behavior of a typical icosahedral
quasicrystal (i-Al-Pd-Mn) using a micro-thermomechanical technique over the temperature range
of 25-500 °C, which has never been explored before. A couple of interesting phenomena have
been observed, including micro-pillar shrinkage, phase transformations, grain refinement, and
thermally induced transition from brittle fracture to serrated and homogeneous plastic flows.
Furthermore, we discuss the multiple underlying mechanisms on the deformation behavior of the
quasicrystal in this temperature regime, including surface evaporation/diffusion, diffusion-
enhanced plasticity, dislocation activities, and grain boundary rotation/sliding.
Our study bridges the gap between room temperature and high temperature plasticity in quasicrystals and also demonstrates a new opportunity to study complex intermetallic phases in broad size and temperature regimes.

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 our recent work on in-situ nanomechanics of crystalline metallic nanowires inside transmission electron microscope (TEM) in three case studies. 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. 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. Finally, we found that the twin boundary can cause interesting recoverable plasticity and Bauschinger effect.

Grain boundary doping has been widely leveraged for the stabilization of nanocrystalline materials. Depending on the dopant species and concentrations, a range of mechanisms have been demonstrated for stabilizing the nanocrystalline grain structure that include thermodynamic contributions often driving alloy design criteria and kinetic pinning forces inhibiting grain boundary motion at elevated temperatures. The presence of deliberately designed solute heterogeneities has implications for the mechanical behavior with a number of studies reporting grain boundary segregated nanocrystalline alloys with significantly enhanced strength relative to their unalloyed counterparts. In this presentation, we use molecular dynamics simulations of nanocrystalline Al containing Mg doped grain boundaries to quantify the mechanisms responsible for this unexpected increase in strength in the context of fundamental changes in grain boundary properties upon alloying. We find that grain boundary microplasticity at low strains is suppressed by solute enrichment producing more stable interfacial configurations, which delays the onset of grain boundary-mediated dislocation slip and promotes an increase in strength. Following the yield point, the interaction of lattice dislocations with grain boundary solute atoms reduces the strain accommodated through dislocation plasticity, thus shifting plastic strain accommodation to the grain boundaries and augmenting flow behavior relative to pure nanocrystalline aluminum.

Simple aging of Magnesium alloys often yields inadequate precipitation hardening response compared to Aluminum alloys. Deformation induced precipitation is one viable strategy to overcome this problem. Using defects, one can severely alter the nucleation and growth of desired second phases and thereby tune their number density, size, shape, orientation, and spatial distribution. Through simple compression experiments on rolled, large-grained Magnesium-Aluminum alloys, we show how solid-solid phase transformations are altered in grain interiors and along grain boundaries after only 10% plastic strain and at four, relatively low temperatures (25C, 100C, 150C, and 200C). With ex situ TEM, we detail the nanoscale Mg₁₇Al₁₂ intermetallic precipitates that form with a fully solutionized Mg-9Al matrix after compression. With crystal plasticity simulations, we link the inhomogeneity of these nano precipitates to the heterogeneity of deformation gradients within the samples. Finally, we explore the underlying mechanisms behind this deformation induced precipitation.

Establishing the appropriate thermal pathways to tune a metal’s microstructure and generate the appropriate continuum response requires a quantitative view of high-temperature annealing. The dislocation motion and interactions that are known to dictate microstructural changes in metals depend strongly on temperature, however, the necessary time-resolved measurements have been elusive, especially near the melt. We use time-resolved dark-field X-ray microscopy to directly image dislocation motion at temperatures spanning the final 7% to T_m. Our real-time movies resolve the creep-dominated dislocation motion and interactions deep beneath any surface in single-crystal, showing dynamics that has previously been limited to theory. Quantitative analysis of the temperature-dependent dislocation mobility in these findings present opportunities to test and refine dislocation models that have previously relied on indirect measurements and multi-scale models for validation.
This work was performed in part under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Magnesium has great potential as a lightweight material, and there are great interests in designing Mg alloys with enhanced mechanical properties. However, precipitate strengthening in the Mg alloys is much less effective as compared to Al alloys, due to the low number density of the precipitates in the Mg alloys and their highly anisotropic morphology. One promising method to overcome this limitation is dynamic processing, e.g. hot compression and ECAE, which could enhance nanoscale precipitation to achieve a higher precipitation strengthening effect.[1] This enhancement is achieved through precipitate nucleation facilitated by defects generated during deformation, including dislocations and vacancies.

We performed molecular dynamics (MD) simulations on nucleation of gamma-Mg₁₇Al₁₂ precipitates in Mg-Al solid solution. The critical nucleus size, and the Gibbs free energy calculated for solids under non-hydrostatic stress field, were used to estimate the interfacial free energy using the classical nucleation theory (CNT). Dislocations and vacancy clusters significantly reduce the critical nucleus sizes and the nucleation barriers of the precipitates, mainly by reducing the precipitate-matrix interfacial free energies. A macroscopic strain hardening model to predict the dislocations multiplication for precipitate nucleation enhancement compared favorably with ECAE experiments.[2] Vacancy diffusion and clustering were also simulated using a continuum model parameterized using MD and kinetic Monte Carlo simulations. The temporal and spatial distributions of vacancy clusters are also consistent with the non-uniformity of the precipitate particles observed in dynamic processing experiments.

References:
1. Ma, X.L., et al., Dynamic precipitation and recrystallization in Mg-9wt.%Al during equal-channel angular extrusion: A comparative study to conventional aging. Acta Materialia, 2019. 172: p. 185-199.
2. Prameela, S.E., et al., Deformation assisted nucleation of continuous nanoprecipitates in Mg–Al alloys. Materialia, 2020. 9: p. 100583.

The design of new metallic materials is essential in fulfilling the promise of emerging and improving key technologies from efficient energy conversion over lightweight transport to safe medical devices. Over the last decades, two approaches in materials physics have proven immensely successful in the design of new metallic materials: Firstly, thermodynamic descriptions of crystalline phases have enabled materials scientists and engineers to tailor and process alloys to obtain a desired internal structure at the microscale. Secondly, better understanding and manipulation of the crystal defects, which govern the material’s strength, formability and corrosion resistance, has led to the development of new alloying and processing concepts that provide some of the most advanced high-performance alloys in operation today. However, to date, these two concepts remain essentially decoupled.
Our vision is to bridge the gap between these existing and powerful approaches by bringing them together in one new conceptual framework, which will consider defects and their thermodynamic stability in a holistic manner. Consequently, a new material design concept originating from the atomic scale will be accessible to materials physicists and engineers which jointly considers the local crystalline structure of defects (structural complexity), the atomic distribution of each element among the different types of phases and defects (chemical complexity) and the stability of defects under the given conditions, such as bulk composition, temperature and applied stress or electrode potential. We will show here, how these can be linked by defect phase diagrams, which will describe the transition between and the coexistence of defect phases. This structural and chemical complexity at the atomic scale naturally exists in all materials. On this basis, we believe that a new physical description of metallic materials will possible and will provide a powerful toolbox for future design of engineering materials with tailored properties regarding concurrent mechanisms and properties, such as mechanical and corrosion performance of engineering materials.

In specific conditions, grain boundary (GB) migration occurs in poly-crystalline materials as an alternative vector of plasticity compared to the usual dislocation activity. The shear-coupled GB migration, the expected most efficient GB based mechanism, couples the GB motion to an applied shear stress. For a given GB, several migration mechanisms referred as coupling modes can occur.
Both experimental and theoretical studies have evidenced that the migration of the GB occurs through the nucleation and motion of disconnections [1-3]. Disconnections are GB defects that have both a step and a dislocation (Burgers Vector) character. They can homogeneously or heterogeneously nucleates [4-6].
We use atomistic simulations (especially the Nudge Elastic Band method) on symmetric tilt grain boundary in FCC materials to evidence these disconnections and to characterize both their structural and energetic properties. In this study, we focus on homogeneous disconnections nucleation. Investigating the <100> and <110> coupling modes, we evidence the disconnections characteristics dependence on the grain boundary disorientation. Besides, we evidence the operation of specific disconnections involving coupling modes different from the usually observed <100> and <110> coupling modes.
[1] A. Rajabzadeh et al., Philosophical Magazine, 93, 1299 (2013)
[2] A. Rajabzadeh et al., Phys. Rev. Lett., 110,265507 (2013)
[3] A. Rajabzadeh et al, Acta Mat., 77, 223-235 (2014)
[4] N. Combe, et al., Phys. Rev. B, 93, 024109 (2016)
[5] N. Combe, et al., Phys. Rev. M, 1, 033605 (2017)
[6] N. Combe, et al. Phys. Rev. M,3, 060601 (2019)

High strength Al alloys exploit solid state precipitation to tailor their mechanical response. This precipitation requires two ingredients: a thermodynamic driving force and atomic mobility. For a given alloy chemistry, the heat treatment (precipitation) temperature is chosen as a compromise between having sufficient driving force for precipitation and sufficient atomic mobility so that the precipitation reaction occurs in a reasonable time and results in a ‘not too coarse’ precipitate distribution. It is this compromise that frames the competition between nucleation, growth and coarsening that constrains the possible precipitate distributions and hence mechanical responses.

This talk demonstrates a new approach that uses small amplitude cyclic plasticity at room temperature as a means of continually pumping vacancies into the system to achieve atomic mobility under conditions of high thermodynamic driving force. The approach is self-regulating (in both space and particle size) and results in extremely fine-scale inhomogeneous solid solutions. Such structures lead to combinations of strength and elongation that exceed those of conventionally precipitate strengthened alloys, without the presence of precipitate free zones.

Since the approach does not use thermal treatments, it somewhat decouples control of the thermodynamic driving force and atomic mobility. This provides a means to fully alter the competition between precipitate nucleation, growth and coarsening and new microstructures, with new combinations of properties are obtained. Examples of both monotonic and cyclic properties are demonstrated.

Hydrogen in steel is associated with a catastrophic failure mode, known as hydrogen embrittlement. This phenomenon is abrupt and is initiated at the atomic level. Although the exact mechanisms are still subject to debate [1], some mitigation strategies are available, including minimizing hydrogen ingress with surface coatings, or controlling hydrogen diffusion within via the introduction of microstructural ‘traps’, e.g. second phase precipitates such as niobium carbide [2]. It is believed that the incorporation of fine, distributed traps can reduce the mobility of detrimental hydrogen atoms and mitigate the macroscale embrittlement. However, as hydrogen is difficult to examine at fine scale, the experimental evidence is lacking for further optimization of the microstructural design.

Atom probe tomography is a powerful technique that can provide accurate 3D maps showing the position and identity of atoms. H is easily detected but, because it is so mobile, researchers are never sure whether it arises from the sample or the chamber itself. It has recently been demonstrated [3] that this issue can be tackled by charging samples with deuterium (D), the less common stable isotope of H, and cooling the specimen to cryogenic temperatures immediately after charging to slow diffusion. This approach allows the D to serve as a marker for H, so that the location of the H atoms can be determined unambiguously.

The University of Sydney has recently installed a suite of tools that allow the preparation and transfer of samples into and between instruments via ultra-high vacuum cryogenic transfer. These facilities include a purpose-built controlled-atmosphere glovebox (Microscopy Solutions), a Zeiss Auriga scanning electron microscope-focused ion beam (SEM-FIB) equipped with a custom-designed cryogenic stage (also Microscopy Solutions), and a CAMECA laser-assisted local electrode atom probe (LEAP) equipped with a Vacuum and Cryo Transfer Module (VCTM) on the load lock, all which are connected by a Ferrovac UHV suitcase. This new specimen treatment chamber allows specimens to be charged with D in a gaseous environment, at various temperatures, quenched to cryogenic conditions and then transferred to the atom probe for analysis. This talk will include an overview of this system as well as atom probe data from deuterated steel samples, including direct observation of deuterium at trapping sites, such as dislocations, grain boundaries and precipitates in martensitic steels [4].

References:
[1] I.M. Robertson et al., Metall Mater Trans A, 46a, 2323-2341 (2015).
[2] H. Bhadeshia, ISIJ International, 56, 24-36 (2016).
[3] Y.S. Chen et al., Science 355 (2017) 1196-1199.
[4] Y.S. Chen et al., Science 367 (2020) 171-175.

Both grain boundaries and dislocations have been recently shown to experience structural transitions between defect states known as complexions. In this talk, we discuss how one can manipulate the natural variations in stress, chemistry, and atomic structure near grain boundaries and dislocations to stabilize these defects and use them as tailorable features. We also demonstrate how different complexions can alter the mechanical properties of materials on the nanoscale. For grain boundary complexions, we focus on nanocrystalline metals, where such features have the largest effect due to the high grain boundary density. We isolate the materials and processing conditions which can cause the strength and ductility of these materials to be simultaneously augmented, as well as report on small-scale mechanical testing and atomistic modeling techniques to study grain boundary mechanics. For linear complexions, our work uncovers how nanoscale phases restricted to the region along a dislocation line direction can affect plasticity, encompassing both initial yield and subsequent flow. Alloy choice is shown to be very important, as various responses ranging from softening to strong hardening are predicted. As a whole, this work demonstrates that our field is on the precipice of a new age, where defects can be manipulated and designed rather than simply tolerated.