This talk will describe the results of computational studies of the energetics of twin boundaries in hcp transition metals and alloys. The calculations employ a recently developed method for modeling interfacial energies in substitutional alloys from first-principles calculations based on the special quasirandom structure approach. The calculations reveal anomalously low values for hcp metals and alloys with d-band filling near that of the group VII metal rhenium. The origins of this behavior are investigated and linked to the theory of the trends in bulk structural stability across the transition-metal series, which predicts the stability of topologically close-packed phases containing atomic coordination polyhedra similar to those found in the stable twins in this region of the periodic table. The results are discussed in light of experimental observations of deformation microstructures, and the implications for alloy design are described. This research was supported by the Office of Naval Research under Grant N00014-11-1-0886.

For magnesium and some other hexagonal-close-packed metals, twinning on the {10-12} plane is a common mode of plastic deformation. Recently, we have used in situ transmission electron microscope (TEM) to monitor the deformation of submicron-sized single-crystal magnesium, in quantitative compression and tension tests (B-Y. Liu et al., Nature Commun. 2014). We have observed the reorientation of the parent lattice to a “twin” lattice, producing an orientational relationship akin to that of the conventional {10-12} twinning. However, aberration-corrected TEM observations reveal that the boundary between the parent lattice and the “twin” lattice is composed of many segments of semi-coherent basal-prismatic (B-P) interfaces, instead of the {10-12} twinning plane. Both TEM and molecular dynamics simulations suggest that the migration of this boundary is accomplished by B-P interfaces undergoing basal-prismatic transformation, in addition to the migration of the boundary of the {10-12} extension twin. This deformation mode mimics conventional deformation twinning, but is distinct from the latter. It is a form of boundary motion coupled to stresses, but produces plastic strain that is not simple shear. The basal-prismatic transformation appears to be important under deformation conditions when the availability and/or mobility of twinning dislocations/disconnections are limited. As such, this interface-mediated lattice reorientation accompanying deformation twinning enriches the known repertoire of plasticity mechanisms.

Understanding the competition between twinning and dislocation slip is of significant importance for the fundamental understanding of plasticity in hexagonal-close-packed (HCP) materials. Towards developing such an understanding, a unified discrete dislocation dynamics (DDD) framework for modeling dislocations, twinning, and their interactions in magnesium (Mg) polycrystals has been developed. A systematic interaction model between dislocations and {10-12} tension twin boundaries (TBs) was introduced into the DDD framework, and a nominal grain boundary (GB) model based on experimental results was also introduced to mimic the GB&’s barrier effect. This new framework is then utilized to study grain size effects on the competition between dislocation slip and {10-12} twinning in the deformation of micro-twinned polycrystalline Mg. It is shown that twinning deformation exhibits a strong grain size effect; while dislocation mediated slip in untwinned polycrystals displays a weak one. This leads to a critical grain size at 2.7 mm, above which twinning dominates, and below which dislocation slip dominates. Furthermore, the predicted evolution of the twin boundary, the twin boundary velocity, and the morphology of the twin boundary are all compared to experimental results and predictions from molecular dynamics simulations.

Plasticity in zirconium, as well as in many other hexagonal close-packed (hcp) metals, is mainly due to the glide of dislocations with a Burgers vector along the axis. Those dislocations can however not explain any deformation along the axis. When submitted to a strain along the axis, the two possible and often competitive ways for the metal to accommodate deformation are glide of dislocations on the one hand and twinning on the other hand. At low temperatures
twinning can explain the main part of the observed plastic deformation whereas at higher temperatures dislocations appear and become predominant even if twinning is still active.
The present study focuses on the mechanisms controlling twin nucleation and growth. Atomistic simulations are performed using either an empirical interatomic potential of the EAM type or ab-initio calculations. The EAM potential is used to understand size effects due to elastic interactions between the different defects before performing ab-initio calculations.
The approach above is first applied to study perfect twin boundaries for the four twinning systems experimentally observed in zirconium. This study allow us to compare the structures and energies of the different twin boundaries.
The understanding and modeling of the nucleation and growth of twins in zirconium involves the study of disconnections. To this end we model disconnection glide along twinned planes. Studying this process with a similar combined EAM-ab inito approach allows us to calculate the energy variation of a disconnection dipole as a function of its length in order to extract the dipole core energy. We also use the NEB method to determine the energy barrier opposing disconnection glide. This study considers several possible disconnections to gather a broad knowledge of the conditions of twin nucleation and growth for different twinning systems.

HCP metals are widely used as structural materials in many industries, ranging from transport and energy to biomedical applications due to their low density, high specific strength. However, HCP metals show pronounced anisotropy in their mechanical properties and less number of slip systems compared to FCC and BCC metals. In the absence of sufficient number of slip systems in HCP metals, twining is found to be one of the important deformation modes during plastic deformation. At present, we do not have very clear knowledge of properties of twin boundaries in HCP metals at atomic length scale. Understanding atomic structure and chemistry of twin-associated boundaries is crucial to improve mechanical properties of HCP metals. In this work, using first-principles density function theory, we study twinning-associated boundaries (TBs), coherent twin boundaries (CTBs) and coherent basal-prismatic boundary (CBP) in six hexagonal metals (Cd, Zn, Mg, Zr, Ti and Be), with a focus on structure and solute&’s solubility at twin boundaries. We find that the formation of TBs is associated with creation of an excess volume. All the six metals show positive excess volume associated with and CTBs, but the excess volume associated with CTBs and CBP can be positive or negative depending on metal. To understand solubility at TBs, we calculated solubility of solute atoms in Mg, Ti, and Zr for solute positions in bulk, CTB and CBP boundaries and show that, in general, solute atoms have better solubility at CTB and CBP than in bulk. We also found solubility of solute atoms linearly changes with normal strain at CBP, increasing with the normal strain for solute atom with a greater metallic radius than the matrix, and decreasing with the normal strain for solute atom with a smaller metallic radius than the matrix. This suggests that the distribution of solute atoms in bulk, CTB, and CPB varies with stress state, and in turn affects mobility of CTB and CPB boundaries.

Recent studies have shown that doped interfaces can have intriguing structures and that, in some cases, thermodynamically-stable interfacial complexions can form. In this talk, we will explore the usage of nanoscale amorphous intergranular films as structural features that can alter mechanical behavior. Molecular dynamics and Monte Carlo simulations are used to show that amorphous grain boundaries can act as high-capacity sinks for dislocations, and to identify the processing conditions which promote the formation of such boundary structures. Experimental validation will be provided by high resolution transmission electron microscopy in conjunction with energy dispersive x-ray spectroscopy, showing segregation of Zr to the boundaries of Cu-Zr alloys created with mechanically alloying and providing evidence for the formation of amorphous films. Microcompression and in-situ bending experiments are then used to quantify the effect of doping on mechanical behavior, showing that strength, strain-to-failure, and failure mode can be controlled with segregation engineering.

Despite the good ductility exhibited by pure metals, their application as structural materials has been limited due to their poor specific strength. Strengthening methods such as solid solution, precipitation, dispersion, and grain-size reduction hardening have all been employed to improve the strength of metals. In particular, the reduction of grain-size to the nanometer scale has been widely used to produce both pure nanocrystalline metals and nanostructured alloys which exhibit ultra-high strength. In this study, we introduce novel Cu-based ternary nanostructured alloys with a unique microstructure. Cu-Zr-Ti and Cu-Zr-Al composites were synthesized by rapid solidification techniques. The composites show an ultra-fine distribution of three different phases: a ductile Cu crystalline phase; an intermetallic compound and nanocrystalline phase. Quasi-static uniaxial compression tests reveal yield strengths near 2 GPa, comparable or higher than those of Cu-base bulk metallic glasses. Additionally, extensive ductility over 10% was observed. The alloy's exceptional mechanical properties are the result of interface-dominated deformation, such as phase boundary sliding and crack deflection. These unique deformation mechanisms were investigated by use of scanning and transmission electron microscopy.

One strategy to retard grain growth in nanocrystaline alloys is the intentional addition of grain boundary segregating alloying elements. However, the segregation of impurities to grain boundaries is known to strongly affect mechanical behavior in bulk alloys, often leading to embrittlement. As grain sizes decrease, accompanied by increases in yield strengths, the cohesion of grain boundaries is expected to play an increasingly important role in determining overall mechanical properties. To design nanocrystalline alloys with enhanced mechanical performance, there is thus a need to systematically understand the effects of impurities on boundary cohesion. We systematically compiled a large database of existing ab initio simulations on the effects of impurities on grain boundary cohesion to determine global trends in the solutes&’ effects on boundary cohesion. Our analysis of the resulting database shows that the majority of the variation in impurities&’ effects on boundary cohesion is quantitatively predicted by simple bond breaking arguments, and that these trends are robust to the choice of solvent, and computational method. Implications for the design of nanocrystalline alloys with enhanced mechanical properties will be discussed.

Impact-based mechanical surface treatments are commonly used in industry to improve the service lifetime of materials through increasing the local mechanical properties in the near-surface. In this process, the material is exposed to repeated mechanical loadings, producing a severe plastic deformation and local refinement of the microstructure. Such Tribologically Transformed Surfaces (TTS) confer better local mechanical properties against wear or fatigue. In this project, two different procedures are implemented to obtain TTS surfaces on pure iron samples; shot-peening treatments and micro-percussion tests. For the first technique, the whole surface of the sample is impacted repetitively by metallic balls. For the second procedure, every impact is done on the same position by a rigid conical indenter, with control over the number, angle and velocity of the impacts. The main purpose of this work is to establish a complete description of the transformed microstructures for both techniques, and to understand the mechanisms involved on the formation and evolution of TTS and how it influences the material mechanical properties. EBSD crystal orientation mapping and HR-EBSD have been used to determine the size of the plastically deformed zone, the grain size distribution and to estimate the density of geometrically necessary dislocations in the cross section of the impacted zone. Nano-indentation and in-situ micro-pillar compression tests have also been performed to quantify the evolution of mechanical properties starting from the near-surface. A correlation between the grain size, dislocation density, and the mechanical properties in the deformed zone has been done and will be presented.

Metals and alloys used in industry are generally strongly heterogeneous since they include a wide variety of multi-scale physical mechanisms. For example, during the damage of thin-film deposited on substrates, micro-cracking and plasticity occurring at the nanoscale modify the evolution of buckles standing themselves at the microscale [1].
Numerically, in strategies consisting in describing the damage processes (cracks, pore formation, dislocationshellip;) and their effects on the evolution of structures, atomistic simulations usually fail to account for realistic systems with high enough space and time scales. Continuum methods, such as the phase-field thus appear as potentially well adapted alternatives.
Indeed, since the beginning of the last decade, these methods - already used in the study of phase transformations - have been extended to describe crystalline defects, such as cracks [2] and dislocations [3]. They are now used to solve problems involving cracks, voids and dislocations taken independently, often coupled with other kinds of fields related to other physical mechanisms. Recently, we have proposed a new model to explicitly couple cracks and dislocations, within a finite-strain continuum formalism [4].
In this presentation, the latest improvements of the actual model will be exposed (face-cubic centered symmetry, root-finding algorithmshellip;) and illustrated in the buckling context. In particular, we will show how interface plasticity modifies the shape of circular buckles and their destabilization. Since the model is general enough to address other damage problems, additional examples illustrating its potentialities should also be exposed.
[1] L.B. Freund and S. Suresh, Thin film materials: stress, defect formation, and surface evolution, Cambridge University Press, (2003)
[2] H. Henry and H. Lévine, Phys Rev Lett 93 (10), 105504, (2004).
[3] D. Rodney and A. Finel, Acta Mater 51, 17-30, (2003).
[4] A. Ruffini and A. Finel, Acta Mater 92, 197-208, (2015)

For plastic deformation at grain sizes around or below 10 nm, one of the central issues that remains to be resolved is to identify the relative importance of the variety of different possible deformation modes and assign and quantify in which manner the relevant/dominant modes contribute with rising stress to overall strain. In a case study on NC PdAu alloys with grain sizes below 10 nm, we determined activation energy, shear activation volume and shear dilatation to identify and discriminate deformation mechanisms. In-situ deformation and diffraction has been exploited to unravel the interplay of relevant mechanisms and their evolution up to large plastic strains. The unexpected observation of solid solution softening can be shown to has its origin in deformation modes related to grain boundaries and in particular to their inherent shear softening. Grain boundary mediated deformation seems to dominate the overall deformation behavior of metallic nanoscale microstructures at the low end of the nanoscale. Mechanistically, it bears great similarities with shear transformations, the generic flow defect of metallic glasses.

During last decades large experimental and modeling efforts have been made to investigate the rate-limiting deformation mechanisms in nanocrystalline metals. For grain sizes below 50nm these mechanisms usually involve grain boundary mediated processes, such a dislocation nucleation and interaction with grain boundaries, grain boundary sliding and grain boundary migration mechanisms. An important aspect that is often overlooked is the role of recovery. This, however, may play an important role in the constant deformation resistance that is often observed for nanocrystalline metals.
Transient testing has proven to be a suitable tool to gather information on the rate limiting deformation mechanisms that are activated during the deformation path. In particular in stress reduction experiments, after intermediate/large stress reduction thermally activated dislocation slip is suppressed so that recovery mechanisms are brought into foreground during subsequent transient creep. In this work, we combine this testing with in situ x-ray diffraction at the Swiss Light Source. Therefore, the transient responses are captured in terms of evolution of strain rate as well as diffraction peak broadening. The constant flow stress reached during uniaxial deformation of electrodeposited nanocrystalline metals reflects a quasi-stationary balance between dislocation slip and recovery mechanisms. The latter plays an essential role in producing plastic strain [1].
To gain further knowledge on the interplay between dislocation mechanism and grain boundary recovery mechanisms, molecular dynamics simulations (MD) have been performed. Stress reduction experiments have been simulated followed by creep simulations up to 1.7 ns. The simulations confirm the interpretation of the behavior of the peak broadening during in-situ diffraction experiments: after a stress drop strain is produced by grain boundary accommodation mechanisms. Depending on the magnitude of the drop, dislocation mechanism can start again after a certain amount of strain produced by accommodation.
REFERENCE
[1] Z. Sun, S. Van Petegem, A. Cervellino, K. Durt, W. Blum, H. Van Swygenhoven, Acta Materialia 2015; 91: 91-100

Stress-assisted grain growth in nanocrystalline metals transpires collectively with a number of competing deformation mechanisms including lattice and grain boundary mediated dislocation plasticity, grain boundary sliding, and grain rotation. While a number of pioneering studies have experimentally demonstrated mechanical grain growth with complementary simulations confirming simultaneous operation of the aforementioned mechanisms, questions remain concerning their role in facilitating grain boundary migration and its implications for plastic strain accommodation. In this study, molecular dynamics simulations of surface nanoindentation were performed to quantify the distribution of plastic strain among competing deformation mechanisms during stress-assisted grain growth in nanocrystalline nickel as a function of grain size and temperature. The role of segregated solute was also investigated using a Ni-1 at. % P alloy where the vast majority of the P atoms segregated to the grain boundaries following a Monte Carlo energy minimization procedure. Under identical conditions of rate and temperature in nominally the same grain size structure, stress-assisted grain growth found to be prevalent in pure nanocrystalline Ni was virtually absent in the Ni-P alloy. A reduction in the deformation temperature also quelled mechanical grain growth in both nanocrystalline Ni and Ni-P, suggesting thermal activation was inherent to the governing physics. The distribution of plastic strain among the disparate mechanisms was quantified using continuum deformation metrics. From this analysis, plastic strain was found to be highly localized in the grain boundaries during nanoindentation and dislocation activity, while present, did not represent the dominant carrier of plasticity. The addition of P reduced the magnitude of slip in the grain boundaries, thus confirming the ability of segregated solute to stabilize boundaries against mechanical grain growth via grain boundary migration.

The localization of stress at dislocation intersections with high angle grain boundaries in FCC polycrystals was studied through a combination of experimental methods and molecular dynamics modeling. Areas of highly localized stress are generally observed in the vicinity of the interaction area. Both, atomistic models and experiments show significant stress localization in the adjacent grain, decaying with distance from the grain boundary. The implications of these results for the transfer of slip to the adjacent grain are discussed.

Modern high-strength steels (Dual-Phase, low-alloyed TRIP) are generally multi-phase steels, in which the martensitic phase plays a key role.
In a multi-phase steel, the generally brittle martensitic phase clearly improves the strength but it may induce a detrimental effect on the ductility. However, recent literature has shown evidence of ductile fracture of martensite. The mechanism behind this ductile fracture is of great interest, since it may help to considerably improve the ductility of high-strength steels in general. The observed ductility may not be an intrinsic property of a martensitic lath, but rather a composite effect at the subgrain scale, where another phase may be retained. For this reason, the term 'apparent ductility' is used, since the small amounts of retained phases are difficult to observe and identify experimentally.
Using martensite's crystallographic substructure, this contribution aims to identify a physically based mechanism contributing to the apparent ductility of martensite. The proper description of the crystallographic characteristics at the micro-scale and the identification of small amounts of entrapped phases constitutes the basis of the computational model. A martensite grain reveals a well defined internal heterogeneous (crystalline) substructure, i.e. packets, blocks, laths. Thin layers of retained austenite may also be present between the laths, whereby small volume fractions (5\%) of interlath retained austenite may already explain the apparent ductility of martensitic subgrains. A conventional crystal plasticity is used to model austenite as FCC and martensitic laths as BCC phases. The entrapped thin FCC phase is shown to act like a greasy plane on which stiff BCC laths can slide. The role of the orientation relationship between FCC and BCC phases is shown to be of fundamental importance for the observed effect. The typical dimensions of martensite crystals and interlath austenite films range from hundred nanometers down to few nanometers. Therefore, Molecular Dynamics simulations have been carried out to analyse the small scale behaviour of a martensite-austenite bicrystal.
A two-scale framework results, in which the mesoscale behaviour is obtained by homogenization of the microscale bicrystal by means of a lamella rule of mixture. The model has been validated with experimental results on a martensitic steel. By accounting for the presence of interlath austenite, the main features of the experimentally observed deformation behaviour (stress-strain curve, slip activity, roughness pattern) are qualitatively well reproduced by the model. It is also shown that, by neglecting the presence of interlath austenite, the observed experimental stress-strain response is not captured. The presence of interlath retained austenite can remarkably enhance the local deformation of martensite. Finally, the potential influence of the thin layers retained austenite on the damage and fracture behaviour of martensite is explored.

Dual phase (DP) steels composed of ferrite and martensite are widely used in industries because of their high strength, adequate ductility and formability. It is expected that grain refinement can be an effective way to enhance mechanical properties of DP steels. In the present study, tensile behavior of a low-carbon DP steel with different grain sizes ranging from about 60 to 4 micrometers was investigated. The fine-grained DP steel was fabricated through a thermomechanical process including repetitive phase transformation and plastic deformation. It was found that the fine-grained DP steel had higher strength and larger total elongation than those of coarse-grained DP steel. In order to clarify details of the mechanical features in tensile tests, local strain distributions between ferrite and martensite phases were investigated by Digital Image Correlation (DIC) method. The strain value of the ferrite phase was always higher than that of the martensite phase due to the difference of the strength. On the other hand, it was found that the local strain distributions of two phases became more homogeneous in the DP steel having finer grain sizes. The results indicate that the change in local strain distribution in the DP steels with different grain sizes affects their macroscopic mechanical properties.

It has recently been reported that nano-crystalline pearlitic steel can exhibit an extreme tensile strength of up to 7 GPa, making it one of the strongest materials known in the world. Such excellent performance can be attributed to its lamellar microstructure, resulting in many interfaces that hinder the motion of dislocations. The combination of ferrite and cementite in this microstructure is a result of the lower C solubility in ferrite as compared to the high-temperature austenitic phase. However, an atomistic picture of its decomposition into ferrite and cementite at the cooling front and therefore the nucleation of the ferrite-cementite interface is so far missing. In this work we have identified a metastable intermediate structure (MIS) which allows a proper orientation relationship with austenite, ferrite and cementite, simultaneously. Our ab initio calculations suggest that the MIS acts as a buffer layer between austenite and ferrite and has the flexibility to evolve into all three phases triggered by elastic strain, magnetic transition and C diffusion. Since the MIS is structurally equivalent to a periodic arrangement of Σ3 twin boundaries in bcc Fe, it triggers the accumulation of C and consequently the nucleation of cementite. These insights, therefore, provide the mechanism for the formation of complex microstructures such as pearlite.

Conventional martensitic steels have limited ductility due to insufficient microstructural strain hardening and damage resistance mechanisms. In this work, a Fe-9Mn-3Ni-1.4Al-0.01C (mass %) medium-Mn TRIP maraging steel is produced and heat treated under different reversion conditions to introduce well-controlled variations in the austenite-martensite nano-laminate microstructure. Uniaxial tension and impact tests are carried out and the microstructure is characterized using scanning and transmission electron microscopy based techniques and post-mortem synchrotron X-ray diffraction analysis. The results reveal that (i) the strain partitioning between austenite and martensite is governed by a highly dynamical interplay of dislocation slip, deformation-induced phase transformation (i.e. causing TRIP effect) and mechanical twinning (i.e. causing twinning-induced plasticity (TWIP) effect); and (ii) the nano-laminate microstructure morphology leads to enhanced damage resistance. The presence of both effects results in enhanced strain hardening capacity and damage resistance, hence, the enhanced ductility.

Describing grain boundary (GB) crystallography requires five independent parameters. By combining electron and optical microscopy, we have developed a new hybrid method to assess—non-destructively—the 5-D GB crystallography in polycrystalline aluminum samples. Our method allows for the direct measurement of local properties at hundreds of fully characterized GBs at once, and thus represents a powerful alternative to techniques, such as X-ray diffraction, which are more cost- and time-intensive. As a case study, we apply our method to assess GB susceptibility to intergranular corrosion and show how the data acquired can be used to build GB crystallography-property maps. These results may enable higher accuracy lifetime prediction of engineering alloys and inspire novel designs of corrosion-resistant microstructures through GB engineering.
This work was supported by the US Department of Energy, O#64259;ce of Basic Energy Sciences under award No. DE-SC0008926; and by the MISTI Seed Fund “Progetto Roberto Rocca”.

Laminate microstructures contribute to strength of materials in a wide range of systems. We review some recent results on pearlitic steels, Cu-Nb and nano-twinned metals. The Cu-Nb laminates are made by two methods, namely accumulated roll bonding (ARB) and physical vapor deposition (PVD) with lamellae thicknesses in the range 10 µm to 10 nm. Nano-twinned copper and silver are electrodeposited, for which a considerable range of texture type and strength is accessible. The pearlitic steels are self-evidently a naturally occurring nano-laminate composite. The strength of the composites, as measured by nano-indentation (and other methods) increases with decreasing lamella thickness as has long been known.
The analysis of the pearlitic steels used the viscoplastic FFT model and focused on a) the influence of the different orientation relationships that have been postulated for this system and b) the influence of the geometry of the two phases e.g. lamellar versus globular cementite. The main findings are that a) the deformation is unaffected by the choice of the orientation relationship whereas b) the properties are strongly sensitive to the phase morphology, which suggests that load transfer across the hard (cementite) phase is an important feature.
Similar strengths in the Cu-Nb composites are seen in ARB and PVD material across the length scale, with ARB strength being slightly lower. The PVD composites are tested as-deposited whereas the ARB materials are the result of substantial plastic deformation, so it is surprising that the latter are less hard. Strength varies with average layer thickness following a Hall-Petch relationship. The nano-twinned copper & silver follow a similar trend, albeit at substantially lower strengths. The more physically based confined layer slip (CLS) model describes the variations in strength across the range of thicknesses. However there are other models that also can explain the variations.

The laminates Ti6Al4V/AA2519 were fabricated by explosive welding in two versions: with and without an aluminium alloy 1050 interlayer plate. In the present communication, the focus is on interface phenomena such as diffusion and phase transformations during bonding and subsequent annealing as well as mechanical behaviour of the laminate structures. The results demonstrated that both Ti6Al4V/AA2519 and Ti6Al4V/AA1050/AA2519 composite plates exhibit good quality bonding. Scanning electron microscopy observations and energy-dispersive X-ray spectrometry (EDS) results revealed that bimetallic compounds of Ti/Al form at the bonding interface in AA 1050 interlayer. Ti/Al interlayer had a thickness of about 5 µm and consisted of nano-sized grains formed along entire bonding interface. In the samples without aluminium interlayer the thickness of bimetallic Ti/Al interlayer was significantly smaller. However, near the joint one can see the grid in the grain boundaries rich in copper. These intermetallic inclusions were often accompanied by nano-cracks. The value of microhardness decreased when the distance from the interface increased. Tensile tests revealed that in both types of joints the interfacial zone wasn&’t the weakest part of this composites.

In many interface-dominated nanostructured materials the role of interfaces during deformation is not yet completely clarified. Very fine spacing of interfaces leads to a competition between dislocation controlled and grain boundary sliding based plasticity. To improve our understanding of this competition we have to investigate the atomistic origin of deformation in the interface region.
We present the results of molecular dynamics simulations of sliding at γ/γ interfaces in lamellar TiAl alloys, which can explain existing, seemingly contradictory, experimental results on the role of interfacial sliding during creep. We suggest that the origin of the controversy lies in the pronounced in-plane anisotropy of the shear strength of the individual interfaces, which is observed in the simulations. Experimentally, the orientation of in-plane directions with respect to the loading axis has not been monitored so far.
A multi-scale concept is introduced to capture effects of both the electronic and the atomistic level. On the one hand we carried out quasi-static calculations of multi-planar generalized stacking fault energy (MGSFE) surfaces of the interface plane as well as the adjacent layers. On the other hand molecular dynamics simulations guided by ab initio GSFE-surface calculations were carried out for different bicrystal cells under different shear loading conditions. The critical stresses for shearing some of these interfaces, which were derived from these bicrystal shear simulations, are of the same order of magnitude or even lower as those for dislocation motion in a γ-single crystal, showing that these mechanisms are competitive.
In total four shear mechanisms, twin nucleation and migration/absorption, interfacial partial dislocation nucleation, rigid grain boundary sliding and grain boundary migration were observed. The comparison with the MGSFE surfaces allows to create a link between (quasistatic) physical properties such as stacking fault energies, and the (dynamic) deformation mechanisms, and hence between the results of ab-initio calculations and molecular dynamics simulations.

Magnesium and its alloys, the lightest structural materials, have a huge potential as structural materials in the aerospace and automotive industries and have been actively investigated for the past few decades. However, Mg with a hexagonal close packed structure lacks the ductility and formability of cubic materials due to scarcity of easy slip systems and the localized shear characteristic of twinning. Therefore, for their applications as structural materials, there is a strong desire to improve the deformability while maintaining the high flow strength of Mg and Mg alloys. Multilayered metallic materials containing a high density of interfaces show promising enhancements in both the yield strength and ductility of layered composites. The mechanical strength of the Mg/Nb multilayers measured according to hardness can achieve a high value of ~1.0 GPa when the layers are a few nanometers thick. Besides such high strength, we also demonstrated such layered structure can enhance fracture toughness associated with highly activated basal slip and interface shearing between Mg and Nb interfaces. We performed in situ three-point bending experiments in SEM and found that Mg/Nb interfaces could block crack propagation accompanying with blunting crack tip. Stress concentration at crack tip is significantly released by shearing interfaces parallel to Mg basal planes.

Magnesium and its alloys, the lightest structural materials, have attracted a lot of attention of the automotive industry for reducing the vehicle&’s weight to improve its fuel efficiency. However, Mg has a hexagonal close packed (HCP) structure, which lacks the ductility and formability of cubic materials due to scarcity of easy slip systems to accommodate an arbitrary plastic deformation mode. Layered composites are promising because presence of high density of interfaces in such materials that can enhance both yield strength and ductility. In this work, we present first-principles study of the energetics and atomic solubility of coherent interface in Mg/Nb multilayers. We find that solutes Zn, Cd, Sc, Ti and Zr show better solubility at Mg/Nb interface compared to the bulk Mg and the bulk Nb and would segregate at the interface. We also search for solutes that can stabilize Mg/Nb interfaces and suppress deformation twinning in Mg to improve the mechanical properties of Mg/Nb multilayer systems.

Nanolayered composites such as Cu-Ag, Cu-Nb and Zr-Nb with layer thicknesses less than 100 nm exhibit enhanced material properties such as ultra-high strength, high ductility, and radiation damage resistance. At such fine length scales, the atomic structure of the interface plays a dominant role in its response to defects and dislocations. In this work, a combined approach of experiment and atomistic and mesoscale modeling elucidates the effects of the atomic structure of bimetal interfaces on the propensity for deformation twinning. We utilize Accumulative Roll-Bonding (ARB) to process bulk Cu-Nb and Zr-Nb nanolamellar composites from 1 mm thick high-purity polycrystalline sheet into bulk nanocomposites with individual layer thicknesses below 100 nm. The ARB process imposes thousands of percent strain to refine the microstructure of ordinary coarse-grained composite metals down to submicron and nanoscales. TEM analysis showed that the stable ARB interface structure at layer thicknesses less than 50 nm maintains the conventional Kurdjumov-Sachs (KS) orientation, but joins the Cu and Nb crystals at interface planes different from those observed in Physical Vapor Deposited materials investigated previously: {112}Cu || {112}Nb and <110>Cu || <111>Nb. In addition, twin formation was experimentally observed in copper on a plane that lies 19.5 degrees from the {112} interface plane, which had not been previously observed in rolled physical vapor deposited (PVD) nanolayered Cu-Nb composites. Moreover, the ability of interfaces in the Cu-Ag system to aid in twinning processes at fine length scales is investigated and compared to those found in Cu-Nb. Cu-Ag nanoscale multilayers, in bulk cast eutectic form, have a cube-on-cube or twinned orientation relationship with interfaces consisting of common {111} planes. These differing interface types will be discussed in terms of the influence of interfacial geometry, dislocation kinetics, interfacial energetics, and atomic structure on the propensity for different deformation mechanisms.

Interfaces in materials can play an important role in determining mechanical performance of the materials through the complicated interaction between defects and interfaces. In metallic nanolayered composites, interface barrier strength (IBS), which refers to the resistance for dislocations to cross an interface, is a key factor to determine the ultrahigh strength of nanolayered composites. Although several theoretical models have been proposed to describe strengthening mechanisms in multilayers and primary contributions to the IBS, the variation of interface properties caused by element interdiffusion at atomic scales and its influence on the IBS of nanolayered composites are still not well understood. In this talk, we will present experimental and theoretical investigations of the IBS of Cu/Au multilayers, emphasizing effects of interface properties and structures. The Cu/Au multilayers with individual layer thickness ranging from 25 to 250 nm were annealed at different temperatures to modulate Cu/Au interface properties. Experimental results from nanoindentation testing show that the Hall-Petch slope in the relation between the strength and the individual layer thickness of the multilayers gradually decreases with increasing annealing temperature, indicating a decrease in the IBS. TEM and HRTEM characterization reveals that element interdiffusion between the Cu layer and the adjacent Au layer leads to a compositional gradient at the interface. A detailed analysis for effects of the compositional gradient on the resistance to dislocation crossing the interface was conducted. It is demonstrated that the interface structure of the Cu/Au multilayers has become the most important factor in governing the IBS.

The thermal stability of Mg/Ti multilayer nanofilms was investigated by examining their microstructure and nanoindentation hardness after annealing at various temperatures and time periods. The multilayers with individual layer thickness hge;5 nm exhibit excellent capability of maintaining the lamellar microstructure and high strength up to 200 °C for annealing time up to 2.0 hours. The annealed multilayer films with h=2.5 nm are still highly textured but characterized with discontinuous layer interfaces, in which the transition of atomic arrangement from hexagonal close-packed (HCP) to body-centered cubic (BCC) structure was observed at columnar boundaries. The degradation of uniform lamellar microstructure is related to the decrease of hardness with annealing temperature at this size scale. A diffusion based instability mechanism was proposed for this typical HCP-based nanoscale multilayer system.

Tri-component multilayers, such as Cu/Ni/Nb. that contain both coherent and incoherent interfaces have a greater capacity for strain hardening than a tri-component bilayer (Cu-Ni/Nb) system, in which only incoherent interfaces are present. However, the thermal stability of these systems is not as well studied as other bi-metallic multilayer systems. The current study examines both the mechanical response and the microstructure and interfacial structure of sputtered Cu/Ni/Nb tri-layers and Cu-Ni/Nb in the as-deposited and annealed conditions. In general it may be expected that coherent interfaces would provide relatively easy diffusion and homogenization and hence softening of the multilayer was expected following annealing at 573 and 773 K for several hours. However, post-annealing nanoindentation of both the bi and tri-layer films showed an increase in hardness of the tri-layer film after elevated temperature testing had been conducted, indicating the likelihood of a change in the microstructure. X-ray diffraction experiments suggest that microstructural changes are taking place in the coatings following even a modest anneal; peaks from Cu and Ni in the tri-layer system are observed to begin to merge and there is some evidence for new peaks forming. Transmission electron microscopy revealed that while the grain size is relatively stable, a Ni-Nb intermetallic forms at both the Ni/Nb, Cu-Ni/Nb and Cu/Nb interfaces. It was also observed that considerable interdiffusion of Cu and Ni took place above 700K. This formation of precipitates appears to add a strengthening component above and beyond that predicted from the confined layer slip model.

Multi-layered aluminum clad sheets are used for a variety of applications in airplane structural parts and automobile heat exchangers. Different alloys with different properties can be joined together to one sheet and it show improved properties or additional properties rater than each alloy. In this study, 6022/5023/6022 aluminum clad sheets were fabricated by roll bonding process and the mechanical properties of the annealed sheets were examined. The bonding strength of the sheets increases with increasing a thickness reduction rate of roll bonding, 6022/5023/6022 clad sheets can be fabricated successfully by roll bonding at room temperature with the thickness reduction rate per pass of 50%. After annealing at 500 ^oC for 1 hour, the sheets have a well bonded interface without a void and 2^nd phase and the microstructure of the sheets consisted of fully recrystallized grains. Stress-strain curves show same strain hardening characteristics with core Al5023 alloy sheets. Meanwhile, the elongation of the sheets was higher than that of commercial Al6022 or Al5023 alloy sheet. The best thickness combination for the elongation was 1:2:1 thickness ratio of 6022:5023:6022. In addition, the sheets had very low tensile anisotropy due to different deformation characteristics of each layer. The low anisotropy of the sheets brought with excellent drawability and the minimum earing at cupping test. It comes from a combination of different texture characteristics of each layered sheets. Numerical simulation results on the formability were corresponded well with the experimental result.

The development of grain-oriented silicon steel sheet products processing a lower core loss and higher magnetic induction has become increasingly important. Recently, an ultra-low iron loss could be accomplished by using ceramic coating such as TiN, CrN and TiC applied by the PVD or CVD method after the forsterite (Mg₂SiO₄) film of silicon steel surface was removed and polished. However, this technique suffers two major shortcomings: (i) low deposition rate, typically in the range of 0.01~0.1 mm min^-1, and (ii) low power efficiency.
Cathodic arc deposition allows the production of a highly adherent deposit due to the condensation of numerous ions on the substrate. To address the problems of sputtering and EB deposition systems, in particular low productivity, poor surface properties, we aim to make TiN, AlN, Al₂O₃ coatings using a DC arc system. Additionally, due to the ease of scaling to large areas and industrial availability, we also consider reactive magnetron sputtering system. With the arc deposition system, high deposition rates were achieved when compared to the sputtering system (> 10~ 90 times).

Due to the lack of valuable experimental techniques, epitaxial strains developed at the metal // oxide interphase can only be study using numerical approaches [1-2]. In addition, most of the techniques, i.e. Raman spectroscopy, can gives only qualitative information about the developed stress field [3]. Moreover, obtained data is burdened with high measurement error and very often difficult to interpret. Therefore influence of epitaxial strain at the metal/oxide interphase is still under investigation [4]. In this work, developed at the zirconium / zirconia interphase epitaxial strains have been study using generalized Bollmann method. Based on the SEM/FIB investigation performed on oxidized at high temperature zirconium // zirconia cross section, numerical approach have been applied to the complex hexagonal vs monoclinic (case I) and hexagonal vs tetragonal (case II) systems [5]. The objective of this work is to study the sensitivity of the “best fit criterion” to two numerical parameters. Special attention has been given to the sensitivity of the cell parameter value and the initial orientation of the metal and the oxide (theta;=0^#7506;). Theoretical strain field created between zirconium and zirconia considering both crystallographic cases have been given. This study is presented as a tool to understand the phenomena of epitaxial strain accumulation in the zirconia layer after isothermal heating at high temperature. Reported results, for the first time quantitatively describe mechanical stress field generated due to the mismatch between metallic substrate and growing oxide.
Literature:
[1]. W. Bollmann, Crystal defects and crystalline interfaces (Springer - Verlag, 1970).
[2]. L. Kurpaska, J. Favergeon, L. Lahoche, G. Moulin, Jean-Marc Roelandt, Materials Science Forum 696 (2011) 176 - 182
[3]. L. Kurpaska, J. Favergeon, L. Lahoche, G. Moulin, M. El-Marssi, Jean-Marc Roelandt, Oxidation of Metals 79 (2013) 261 - 277
[4]. I. Salles-Desvignes, T. Montesin, C. Valot, J. Favergeon, G. Bertrand, A. Vadon, Acta Materialia 48 (2000) 1505 -1515
[5]. L. Kurpaska, J. Favergeon, L. Lahoche, M. El-Marssi, Jean-Luc Grosseau-Poussard, G. Moulin, Jean-Marc Roelandt, Journal of Nuclear Materials (2015) doi:10.1016/j.jnucmat.2015.06.005

Solute segregation plays a key role in a broad range of phenomena in multiphase materials containing a high density of interfaces, yet the diverse nature of these interfaces makes quantifying and predicting segregation a difficult task. Here we report on the simultaneous segregation of Au atoms to four different interfaces in Fe/Au particles on sapphire - two distinct metal surfaces, a metal-ceramic interface, and a metal-metal grain boundary - resulting in complete encapsulation of the particles. We accessed all of these interfaces simultaneously and found substantial differences in their segregation behavior. The metal-ceramic interface exhibited the strongest segregation tendency, followed by the two surfaces, and the grain boundary. The results were analyzed quantitatively by combining experimental, theoretical and ab-initio computational methods, leading to the new synergetic insights into such systems. We then demonstrated how segregation can be directly employed to design the morphology and properties of thermodynamically-stable nanoparticles and thin films.

A Ge CVD film is expected a lower temperature of bonding, and increasing the bonding reliability and bonding strength, and can be applied to wafer-level-bonding process with a patterned wafer. In this study, the Ge CVD thin film was studied to bond the wafers and to obtain a uniform thin film between the Si wafer and Au film deposited wafer.
The Ge films were deposited on Si wafers at substrate temperature of 400°C, 450°C, 500°C and 570°C, respectively. GeH₄ flow rate was 60sccm and deposition pressure was 4x10^-4 torr for 5min. A two step-deposition-method was tried to reduce a surface roughness, first a 30 ~ 60 nm-thickness film was deposited at lower temperature of 400°C and after the second film was deposited at higher temperature of 500°C. This is possible to obtain a lower surface roughness and uniform film thickness of about 30nm or more, deposited at a lower temperature could reduce the lattice mismatch during the growth of Ge film, also which can be known that the planar growth is possible for above 100nm-thickness film (target thickness).
We have observed Arrhenius plot of deposition rate vs. deposition temperature, and the surface morphology of the thin film by atomic force microscopy (AFM). The deposition rate and a surface roughness were increased with increasing deposition temperature. The higher deposition temperature showed the higher crystallinity at deposition temperature range from 400°C to 500°C. The X-ray diffraction (XRD) peak of Ge (004) was observed at deposition temperature of 400°C.
The Ge thin film was bonded between a about 100nm-thickness Ge CVD deposited in an optimized process condition on the Si substrate wafer and an Au (200nm)/Ti (10nm)/Si wafer. The deposition of Ge films were carried out at 400°C, and the bonding were performed at temperatures of 400°C (1hr), 450°C (1hr) and 500°C (1hr), respectively. Scanning acoustic microscopy (SAM) was used to analyze the status of bonding. From results of a SAM observation and a sectional SEM sample preparation, the good adhesion between Ge and Si was observed at higher bonding temperature of 450°C. To observe the bonding strength of the bonded films and inter-diffusions of Si-Ge-Au atoms, the upper wafer of the bonded wafers was grinded by using a back-grinder tool up to from a 480mu;m-thickness (4inch wafer) to a about 10mu;m-thickness. Then the bonding element distribution was confirmed by using a secondary ion mass spectroscopy (SIMS) and Electron energy loss spectroscopy (EELS) of transmission electron microscope (TEM). In this study, we knew that the Au-Ge bond was possible at about 450°C, and the results show that the Ge CVD film can be adaptable for the application of wafer level package (WLP).

Mg-based long-period stacking ordered (LPSO) phases formed in ternary Mg-TM (transition metal)-RE (rare-earth metal) alloys have recently attracted significant attention owing to their use in the strengthening of Mg alloys and as promising components for designing advanced lightweight structural materials. In such phases, the two-dimensional stacking-fault (SF)-type interfaces containing solute atoms can play a significant role in the formation of a nanostructured state with low-energy boundaries. To understand the fundamental mechanism of the formation of LPSO phases, it is important to elucidate the controlling factors influencing the segregation and ordering of solute atoms at SF-type interfaces, i.e., the solute-solute binding, solute interaction with SFs, etc. In this study, we investigated the interaction energies and trapping effects of substitutional Zn and Y atoms in the vicinity of the SFs in Mg using first-principles calculations based on density functional theory (DFT). As no reliable empirical potential exists at the moment to reproduce faithfully the interactions of the elements, we developed an effective interaction model based on the DFT-calculated energies, which will provide the theoretical framework to find the energetically favorable configurations from the bottom-up point of view. As examples, the parameterizations for the Mg-Zn-Y systems were conducted. We applied this to atomistic and/or coarse-grained Monte Carlo modeling to predict the solute segregation and nanocluster evolution at a SF in the Mg-Zn-Y systems. We confirmed that our approach can successfully describe the formation and alignment of the Zn-Y clusters in the systems, which provides an insight into the short- and medium-range chemical order in the Mg-based LPSO structures.
This study was supported by Grant-in-Aid for Scientific Research on Innovative Area (No. 23109004) and the Elements Strategy Initiative for Structural Materials (ESISM).

A high density (asymp; 1/10 nm) of twin boundaries have been reported parallel to close-packed directions in thin FCC and HCP films made using atom-by-atom deposition processes. Such nanotwinned materials are of interest because of the potential to create materials with high strength and high ductility. However, process control of twin boundary spacing has been elusive. In this work, we show that thin Ag films can be created with a range of twin boundary spacings by varying the deposition rate in e-beam evaporation. We explain these variations using a simple model of nucleation and growth from the vapor. Results suggest that twin structure and mechanical properties can be tuned with good precision.

The motion of grain boundaries is crucial for the evolution of microstructure both in processing and in application. In experiments, the grain boundary motion is usually studied at relatively high homologous temperature during long times under small mechanical loads. But grain boundary motion is also shown to be a deformation mechanism occurring at high stresses and relative low temperatures, as for example in deformation studies of nanocrystalline metals and shock loading.
Using molecular dynamics simulations the interface motion of grain boundaries is used to elucidate the role of atomistic structure (morphology) and velocity-driving force relation (mobility) for two different driving force mechanisms: elastic anisotropy and shear coupled motion. As function of temperature and driving force, the grain boundary mobility shows a complex nonlinear behavior beyond the conventional conjecture of Arrhenius-like temperature-dependence in mobility and linear velocity-driving force relation. The velocity-limiting mechanisms range from the pinning-depinning transition at low temperature, through rare-event dynamics of critical “kink- pair” disconnection nucleation along intrinsic grain boundary dislocations, to fluctuating randomly diffusive grain boundary motion at low driving forces and high temperatures.
The GB dynamics regimes are discussed in context of grain growth at high temperatures and stress-driven coarsening in nanocrystalline metals at low temperatures in the presence of triple junctions.

Metals that are extensively used in electronics manufacturing, such as tin, zinc, and related alloys, often show electrically conductive hair-like crystalline structures on their surfaces, referred to as whiskers. These whiskers can lead to current leakage and short circuits in sensitive electronic equipment causing significant losses and, in some cases, catastrophic failure in the automotive, airspace and other industries. The mechanism responsible for whisker formation and growth remains mysterious after decades of research. Some theories attribute the whisker growth to stress relieving phenomena, but those never lead to any quantitative estimate of whisker parameters. A very recent electrostatic theory [1] provides, for the very first time, the quantitative estimates of the whisker nucleation along with their growth rates and length distributions, which is very consistent with many observations on whiskers growth and their morphology. It proposes that the imperfections on metal surfaces can form small patches of net positive or negative electric charge leading to the formation of the anomalous electric field which governs the whisker development in those areas.
In this work, which is still in progress, we report on the possibility of initiating and controlling the growth of whiskers by using surface plasmon polaritons (SPPs). As predicted in [1], localized high electric fields generated by the SPPs may play a crucial role in whisker onset and growth. The Otto and Kretschmann methods are very well established and studied attenuated total reflection geometries for implementing SPPs on the surface of metals and metal films. Generating SPPs on low-loss metal films such as Ag and Au is generally straightforward. However, generating SPPs on the surfaces of lossy metals such as Sn and Zn is challenging as the energy of the incident field is largely absorbed in the process of coupling to the SPPs. We employed a 100mW cw red light emitting laser to excite SPPs on optically smooth Zn and Sn films and then followed the evolution of the exposed surfaces by optical and electron microscopy techniques. This work has important implications: first, it would verify experimentally some aspects of the electrostatic theory and second, the controllable formation of whiskers would allow the development of accelerated failure testing methods of electronic components, which is currently not possible due to the random and unpredictable nature of whiskers formation. As a somewhat longer-term goal, we would like to explore the potential of controllable intentional whiskers growth in applications that involve microwire fabrication such as 3D electronic circuit wiring and sensors.
1. Karpov, V.G. , Phys. Rev. Applied, vol. 1, no. 4, # 044001, 2014

For over 40 years, SrRuO₃ has been the subject of extensive studies due to the coexistence of metallic conductivity and ferromagnetism.[1] Especially, as SrRuO₃ has an orthorhombic structure, there are six possible domain structures and orientations for epitaxial growth when deposited on cubic substrates (e.g. SrTiO₃). [2] It often happens that different domains simultaneously occur forming multidomain thin films under certain growth mode. Since SrRuO₃ with single domain shows optimal physical properties compared to multidomain structures, there have been many attempts to suppress the formation of multidomains.[3] Up to now, the most widely accepted claim is that vicinal SrTiO₃ substrate(ge;2°) plays a critical role in determining single domain structure originating from the growth mode change, but it is still remained controversial.[4]
In this study, we observe atomic scale structural evolution of SrRuO₃ on SrTiO₃(001) focusing on the interface. The SrRuO₃ thin films are deposited on SrTiO₃(001) substrates with various miscut angles by pulsed laser deposition technique with in situ high-pressure reflection high-energy electron diffraction (RHEED). We monitor the changes of growth mode by RHEED intensity oscillation in real time. Overall domain distribution is determined with high-resolution X-ray diffraction (HRXRD) and detailed domain composition is obtained by transmission electron microscopy (TEM) via direct imaging of microstructure. Precise atomic positions around the interface are detected by the help of annular bright field(ABF)-high angle annular dark field(HAADF) imaging technique. Based on ABF-HAADF images, we confirm that the degree of octahedral rotation is affected by domain configuration. Mechanical properties at the interface is also calculated in order to confirm the effect of interface atomic structure on interface properties. Our results demonstrate that there are close links between structure and physical property and that domain formation is strongly dependent on the substrate surface.
[1] G. Koster et al., Rev. Mod. Phys. 84, 253 (2012).
[2] J. C. Jiang et al., Appl. Phys. Lett. 72, 2963 (1998).
[3] G. Herranz et al., Phys. Rev. B. 71, 174411 (2005).
[4] Q. Gan et al., Appl. Phys. Lett. 70, 1962 (1997).

Nanometalic materials have interesting mechanical, electrical, and magnetic properties, but generally exhibit low thermal stability due to the higher interface area which acts as a high diffusivity path, and is a suitable place for grain boundary stabilization by kinetic or thermodynamic mechanisms (segregation). Specifically, nanometalic multilayers (NMMs) are an attractive solution to enhance thermal stability, as they enable local control over concentration of alloying elements, and surface energy distribution.
Low thermal stability of NMMs at high temperatures is a serious drawback for their further application. For miscible systems as Cu-Ni degradation of the multilayered structure and grain growth after annealing have been previously reported. The multilayered geometry enables grain size control (layer thickness correlates with grain size), and the alternating composition allows control over kinetic phenomena (kinetic barrier), as the initial microstructure can be tailored to be close to the equilibrium condition.
This study presents the evolution of the interlayer interface of Hf-Ti NMMs samples prepared by magnetron sputtering, and heat treated at vacuum conditions, at different annealing times and temperatures. FIB lift out was performed to prepare the samples for imaging, microstructures and grain sizes of as-sputtered and heat treated samples were analyzed by TEM, and EDX profiles were used to track Hf and Ti composition. The loss of layered structure and grain boundary segregation for various layer thicknesses is presented.

Recent studies have shown that growth of eta;-Cu₆Sn₅ on unidirectional Cu followed specific patterns with regular spatial arrangement, but the crystallographic origin and formation mechanism of such a microstructural feature is still unclear. Here we develop a phase field model incorporated with the microelasticity theory to investigate microstructure evolution during heteroepitaxial growth of eta;-Cu₆Sn₅ on unidirectional (001) Cu. The generation of eta;-Cu₆Sn₅ variants arising from symmetry reduction is determined by group theory and the microstructural evolution is simulated through phase-field modeling with the influence of epitaxial strain. Simulation results show that each eta;-Cu₆Sn₅ variant elongates along the direction of minimum atomic mismatch, which agree with experimental observations. The condition of kinematic compatibility across the interface of eta;-Cu₆Sn₅ variants is analyzed and validated by simulations. In addition, possible extension to heteroepitaxial growth on (111) Cu or other chemical systems are discussed in an analogous manner.

Recent works by Hemphill et al have shown encouraging fatigue resistance characteristics of a high entropy alloy (HEA) Al0.5CoCrCuFeNi, which compares favorably with a number of conventional alloys, including steels, titanium alloys and bulk metallic glasses. Due to the complex nature of the microstructure in HEA, the microstructural reasons to this excellent fatigue resistance is unclear at the moment. Here, we apply a novel technique in transmission electron microscopy (TEM) called scanning electron nanodiffraction (SEND) to characterize the microstructure development along the fatigue crack in Al0.5CoCrCuFeNi. SEND enables a complete structure characterization from which we can further analyze the crystal orientation and structure.
Arc-melted Al0.5CoCrCuFeNi (molar ratio) ingots were annealed at 1000 °C for 6 h, followed by water quenching and cold rolling. Four point bending fatigue testing was run at a maximum stress of 900 MPa corresponding to a stress range of 810 MPa. A sample which failed after 98,256 cycles was used in the present characterization. TEM samples were prepared by focused ion beam (FIB) technique at different locations of the fatigue crack. TEM investigation was carried out on a JEOL 2100 equipped with a LaB6 electron beam source operating at 200kV. Nanometer-sized electron beam of 2~4 nm in diameter was used to generate the diffraction patterns at each location in a serial manner. SEND patterns were acquired using a customized Gatan DM script which automates beam scanning and diffraction pattern recording. Typical exposure time for each diffraction pattern recording is 0.5 seconds.
A scanned area covering 1000x1000 nm2 using 20x20 sampling points was applied for each investigated site, where overall 400 diffraction patterns were recorded. A correlative analysis was carried out over these diffraction patterns. The regions with similar diffraction patterns was defined by a high correlation coefficient (> 0.75, 1 is the maximum). Using this technique, we are able to identify the regions in the sample that show similar diffraction patterns. The fatigue initiation region, the propagation region and the final cracking region have been all characterized. The results are compared directly with those obtained in the region far away from the fatigue crack. The microstructure evolution along fatigue crack is discussed based on the SEND results.

This talk will present our recent progress in understanding the fundamental mechanisms of plasticity and fracture in face-centered cubic metals and alloys strengthened by coherent twin boundaries (CTBs) using atomistic simulations and nanoscale experiments. The ability of CTBs in strengthening and maintaining ductility has been well documented; yet most understanding of the origin of this mechanical behavior relies on a perfect interface assumption. This presentation will focus on the important role played by intrinsic kink-like twin boundary defects in controlling strength and plasticity in nanotwinned Cu and Ag-Cu alloys. First, using simple bicrystal models, it is found that kink-like twin boundary steps have a profound impact on screw dislocation - CTB interactions, resulting in significant strengthening behavior. Second, massively-parallel molecular dynamics simulations will be used to describe new defect-dependent plastic deformation processes in columnar-grained and nanocrystalline microstructures containing inherently defective CTBs with kink-like steps. New Monte Carlo simulations will also be presented to discuss the influence of atom segregation on deformation mechanisms and twin size effects in nanotwinned Ag-Cu alloys.

Thin films having high densities of twin boundaries parallel to close-packed directions in thin FCC and HCP films made using atom-by-atom deposition processes have been reported. Such nanotwinned structures are of interest because of the potential to create materials with high strength and high ductility. However, to achieve, these properties, such structures must be stable during thermal annealing. Twin boundaries can represent significant stored energy and thus driving force for microstructural transformations. In this work we produced thin Ag films with a range of twin boundary spacings and studied the effect on structural transformations during annealing. We found that the rate and extent of grain growth scaled with the twin boundary density and showed that elimination of twin boundaries can provide both the driving force and the orientation selection mechanism for the well-known thickness-dependent (111) to (100) texture transformation. Conditions required for the stability of nanotwinned structures will be discussed.

Although nanoscale twinning is an effective means to enhance yield strength and tensile ductility in metals, nanotwinned metals generally fail well below their theoretical strength limit due to heterogeneous dislocation nucleation from boundaries or surface imperfections. Here we show that Au nanowires containing angstrom-scaled twins (0.7 nm in thickness) exhibit tensile strengths up to 3.12 GPa, near the ideal limit, with a remarkable ductile-to-brittle transition with decreasing twin size. This is opposite to the behaviour of metallic nanowires with lower-density twins reported thus far. Ultrahigh-density twins (twin thickness < 2.8 nm) are shown to give rise to homogeneous dislocation nucleation and plastic shear localization, contrasting with the heterogeneous slip mechanism observed in singlecrystalline or low-density-twinned nanowires. The twin size dependent dislocation nucleation and deformation represent a new type of size effect distinct from the sample size effects described previously.

High energy particles can introduce severe radiation damage in metallic materials especially those with low stacking fault energy. Twin boundary (TB) has recently been shown to enable the reduction of defect density in heavy ion irradiated nanotwinned Ag. However, the defect-twin boundary interaction mechanisms in nanotwinned metals remain poorly understood. This presentation will focus on the study of TB affected zone wherein time accumulative defect density and defect diffusivity are substantially different from those in twin interior. Furthermore the in situ studies also reveal surprising resilience and self-healing of TBs in response to radiation [Nano Letters, 2015, 15 (5), pp 2922]. The impressive capture-recovering capability makes TBs attractive defect sinks for the design of radiation tolerant metallic materials. This study is supported by NSF-DMR-Metallic Materials and Nanostructures Program under grant no. 1304101.
Key words: nanotwinned Ag; in situ radiation; defect migration kinetics; self-healing

The nanotwinned premartensitic state of fcc Fe₂Pd is known as ‘tweed&’. It is also known that it is shear-elastically extremely soft: The elastic constant C&’ approaches zero at the fcc→fct transition temperature, Tc. Formally, one would thus expect that the magnetostriction lambda;₁₀₀ (Tc)→infin;. In this presentation we will report very high and temperature independent values of lambda;₁₀₀ asymp; 600ppm in the temperature interval 1.05le;T/Tcle;1.4. The anisotropy of lambda; is non-Julian and consistent with a nanotwinned magnetoelastic state, which represents the minimum geometrical mean of the relevant magnetoelastic volume and twin boundary energies. We will also show that the linear and non-hysteretic magnetization curve of fcc Fe₂Pd represents the untwining of this state by the external magnetic field.

Over the years, two-phase nanolaminate composites have demonstrated an unusually broad number of desirable properties, such as high strength, high strain to failure, thermal stability, and resistance to light-ion radiation. Recently we have shown that nanolaminates with similar exceptional properties can be made via severe plastic deformation (SPD) in bulk sizes suitable for structural applications. While the cause of the attractive properties of SPD nanolaminates can easily be associated with a high density of bimetal interfaces, how the interfaces physically control structural properties remains an area of intense research. In this pursuit, we aim to use SPD to fabricate nanolaminates with different types of interfaces. Thus far, we have found that it is possible to change the crystallographic character of the bimaterial interfaces through changes in the SPD processing path. Most importantly, we observe that such differences can further improve some macroscopic properties. With these results, we are motivated to understand 1) the connection between interface character evolution and the severe plastic deformation process and 2) how interface character affects its interactions with dislocations and other defects generated during deformation. This presentation will highlight our recent modeling and experimental efforts to understand the effects of interfaces on texture evolution, interface character evolution, dislocation motion, strength, and plastic anisotropy, during deformation.

Thin films and multilayers exhibit strong size effects in their mechanical behavior such as increasing yield strength or hardness with decreasing film or layer thickness. When the film and layer thicknesses approach the nanometer scale, the interfaces and grain boundaries dominate the deformation behavior. Different deformation mechanisms have been suggested for different layer thicknesses and types of interfaces such as dislocation pile-up, confined layer slip, dislocation transmission across the interfaces or co-deformation of the layers.
In this study, Cu/Au-multilayers with individual layer thicknesses in the range from 25 to 250 nm were investigated at temperatures up to 93°C using nanoindentation and characterized by scanning and transmission electron microscopy. As expected, the hardness and the strain rate sensitivity of the multilayers increased with decreasing layer thickness. With increasing temperature the hardness decreased while only small changes in the strain rate senstivity were observed for temperatures up to 93°C. The deformation microstructures were studied using scanning electron microscopy cross sections prepared by focused ion beam. Co-deformation was identified for layer thicknesses >50 nm and the individual layer thickness reduction was more pronounced for the nanoindents at 93°C than at room temperature. The 25 nm thick layers exhibited mechanical mixing in the strongly deformed layers while for 250 nm layer thickness changes of the grain size were observed.
The differences in deformation microstructures for the different layer thicknesses will be illustrated and discussed in the context of the active deformation processes. In this study observed co-deformation processes can lead to homogeneous plastic strains of the individual layers up to 80% without delamination of the layers. Understanding deformation and failure of nanoscale multilayers will contribute to their future applicability in small-scale mechanical and functional devices.

Nano-scale metallic multilayered materials, having alternate FCC-BCC layers, have shown great potential for management of injected helium (He) owing to their high density of heterophase interfaces. In this work, we aim to determine the effect of FCC-BCC interface properties on He precipitation. To this end, we perform atomistic modeling studies on three Cu-Nb bilayer systems containing interfaces differing in crystallographic orientations and interface planes and analyze how He precipitates at the interfaces. We observe that the He-precipitation patterns in the interfaces depend on He pressure and the wetting characteristics of the interfaces, which can be computed based on location-dependent interface energies. By comparing these three models, we show a direct correlation between the atomic structure of the interface and the He wetting patterns.

Interfaces and defects, such as grain boundaries and dislocations, play a crucial role in thin-film interdiffusion, because intermixing can be dominated by fast diffusion along these short circuit diffusion paths in nano-crystalline thin films. Point defects can strongly influence volume diffusion. Since substitutional interdiffusion in thin-film diffusion couples is generally accompanied by a net vacancy flux, due to the generally different mobilities of the components, vacancies are generated in the sublayer composed of the faster component and annihilated in the sublayer composed of the slower component during interdiffusion. Interfaces and defects can serve as vacancy sources and sinks and thus the equilibrium vacancy concentration-depth profile can be maintained only if enough vacancy sources and sinks are present (Darken regime). However, if the number of vacancy sources and sinks is too small, a non-equilibrium vacancy concentration-depth profile develops and interdiffusion is either accelerated (sufficient sources and lack of sinks) or decelerated (lack of sources and sufficient sinks or in the extreme case of no sources and sinks: Nernst-Planck/Nazarov-Gurov regime).
In this study, the influence of vacancy sources and sinks on interdiffusion in epitaxial single-crystalline Ag/Au bilayers (as verified by X-ray diffraction) has been investigated. The concentration-depth profiles after diffusion annealing have been measured by Auger electron spectroscopy sputter-depth profiling and corrected for sputter induced alterations. Measurements of the defect concentration-depth profile by in-situ positron annihilation Doppler broadening spectroscopy revealed that the defect distribution changes upon diffusion annealing which might be ascribed to the establishment of a non-equilibrium vacancy concentration-depth profile due to insufficient vacancy sources and sinks during interdiffusion, and/or to microstructural changes. A detailed evaluation of the measured concentration-depth profiles on the basis of either the Darken case of the Nernst-Planck case will be presented.

Widmanstatten microstructures have long been studied in physical metallurgy. Numerous observations have evidenced the growth of colonies composed of parallel lamellae, sharing a same crystalline orientation, starting from grain boundaries of the mother phase to the grain interior. Such microstructures, resulting from a diffusion-controlled process at high temperatures, have been observed in many metallic alloys such as steels, brass or Ti-based alloys. Despite a large number of studies devoted to this microstructure, the understanding of Widmanstatten structures still remains incomplete.
In the present contribution, new insight into their growth is provided by phase field modeling. In particular, we will show that the occurrence of Widmanstatten, identified by common morphological and kinetic features, can be rationalized by the anisotropy of elastic energy due to the eigenstrains.
Finally, we will discuss the role of plasticity, as described by some isotropic model incoporating internal length, on the growth process.

Metallic nanoglasses have been prepared by compaction of amorphous nanoparticles with sizes ranging from 8 to 10 nm. These novel amorphous structures are constituted of two distinctly different amorphous components, the cores of the former nanoparticles and an interfacial component. For the investigated systems, the core component has been found to be structurally identical to the structure of amorphous materials prepared by rapid quenching from the melt (RQ). The interfacial component, however, is structurally different exhibiting a large amount of free volume and a medium range order that is substantially different from the core regions. The results of studies using Mössbauer spectroscopy as well as small and wide angle X-ray scattering indicate that amorphous structures, at least for metallic systems, allow for the introduction of a large fraction of free volume, resulting in amorphous structures being different from those in the RQ state. As a consequence of this difference in structure, the mechanical and magnetic properties are altered drastically. As an example, amorphous Fe₉₀Sc₁₀, which is paramagnetic at room temperature in the RQ state and ferromagnetic below 120 K, exhibits a Curie temperature (T_c) well above room temperature in the nanoglass state. In any case, nanoglasses offer a chance to alter the mechanical and magnetic properties of metallic amorphous structures. After the description of the structure of metallic nanoglasses, the mechanical properties of these novel materials will be presented. The most significant finding of the study is the absence of pop-in events in the load displacement curves measured using nanoindentation, indicating that shear localization in macroscopic shear bands does not occur. Furthermore, in situ micro-compression tests show homogeneous flow with large strain values. The results will be compared to results obtained using molecular dynamics simulations.

We present a combination of experiments and phase-field modeling used to investigate the morphological evolution of alloys under conditions where one alloy component is dissolving into a molten metal bath in which the other component is immiscible - so called liquid metal dealloying (LMD). LMD is analogous to electrochemical dealloying, which has been shown to be a useful self-organization technique that allows bulk nanostructured materials to be fabricated from the bottom up through self-organization. In our model system, parent alloys of Ti-Ta are immersed in a bath of molten Cu, which has a high Ti solubility but is immiscible with Ta. The molten Cu dissolves Ti out of the alloy while allowing the refractory component to diffuse along the metal/liquid interface and reorganize into a porous network. Copper remains in the pores and fills the dealloyed phase volume as it penetrates into the parent alloy. After the dealloying process is completed, a dense bicontinuous composite with a characteristic ligament diameter is formed upon cooling. The new material is polycrystalline, consisting of individual, single-crystal bcc refractory micropillar networks, and a rapidly solidified, polycrystalline Cu phase. Results are presented that clarify the kinetic processes associated with LMD such as dealloying velocity, network coarsening, and morphological instabilities, and how these vary with alloy composition and processing temperature.

Mg alloys containing long period stacking ordered (LPSO) structures have received considerable attention owing to their excellent mechanical properties for structural applications. Four polytypes, 10H, 14H, 18R and 24R have been reported for Mg-Y-Zn alloys. LPSO structures consist of Y- and Zn- enriched building blocks which have a local FCC stacking sequence of close packed planes. The 18R phase is formed during solidification of Mg-Y-Zn alloys, and is gradually transformed to 14H after appropriate heat treatments in the temperature range of 350-500°C. However, the detailed phase transformation mechanisms are still under discussion. We present an atomic scale structural evolution of (i) LPSO plate precipitates in α-Mg matrix grains and (ii) interdendritic LPSO phase grains for the materials solution treated at 500°C for 2.5h and 10h, respectively, followed by water quenching using high resolution high-angle annual dark-field scanning transmission electron microscopy (HAADF-STEM). Irregular LPSO structures with various numbers of Mg layers between LPSO building blocks were frequently observed during the solution treatment. In addition, direct evidence of the transformation from 18R to 14H was firstly obtained. A possible transformation sequence of LPSO precipitates in α-Mg is proposed as : single building block → various irregular LPSO structures → 14H while that of interdendritic LPSO grains is proposed as : 18R→ various irregular LPSO structures → 14H.

The formability of metallic alloys is enabled by atomic shear steps that are carried by linear defects called dislocations. Here we report on confined phase transformations of dislocations. In a body centered cubic Fe-9at% Mn alloy we found Mn segregation followed by nucleation of a face centered cubic phase on dislocations upon heating but no further phase growth occurred. The transformed regions are coherent subcritical austenite zones in equilibrium with the matrix. The phenomenon resembles the recently suggested complexions, i.e. interface-stabilized phases. The observations are based on atomic scale structural and chemical analysis, thermodynamic calculations and an analytical model which accounts for the free energy gain obtained when replacing the dislocation core by an undistorted new phase. We suggest extending the planar complexion concept phenomenologically to the linear case. The effect observed fulfills all characteristics of a linear complexion, namely, defect stabilization, geometrical confinement to the defect region, individual structural and compositional state, as well as kinetic and compositional stability without further growth. Considering that a cubic meter of strained alloy can contain a light-year of dislocation length suggests that linear complexions provide opportunities to nanostructure alloys via segregation and confined phase transformation.

Since the deleterious effect of hydrogen on mechanical properties in structural metals was first discovered over one century ago, hydrogen embrittlement has become one unsolved problem, restricting the design of materials with good mechanical properties. Specially, of great importance for hydrogen embrittlement is the interaction between hydrogen and grain boundaries. In this study, we utilized one novel algorithm based on space tessellation method to effectively identify the grain boundary structures on the basis of polyhedral structural units. Also, we used state-of-the-art first principle calculations, coupled with polyhedron geometry concept, to systematically investigate energetics associated with hydrogen trapping to symmetric tilt grain boundaries in several FCC materials, namely, Ni, Cu, γ-Fe and Pd.
Our results indicate that the hydrogen adsorption sites at grain boundaries in general fall into several well defined categories. The sites in each category exhibit the same local bonding characteristics in the pristine state. In addition, interestingly we find that for all adsorption sites, the energetics of hydrogen show a generic volumetric dependence on the local structure enclosing the site. An analytical expression is then developed to provide accurate predictive assessments of hydrogen energetics and segregation at grain boundaries. The above findings provide fundamental knowledge of hydrogen segregation at grain boundaries and valuable mechanistic insights towards rational grain boundary engineering in the development of embrittlement-resistant metals.

Hydrogen Embrittlement (HE) is one of the causes mainly evoked in premature rupture of industrial components exposed to aggressive environment. Many studies have been conducted in order to understand the mechanisms involved during this degradation, and the influence of metallurgical states. However, the contribution of grain boundary character on the damage assisted by hydrogen remains a little studied and point of controversy. In fact, the brittle intergranular fracture promoted by hydrogen ingress in the material depends on densities and organizations of defects near grain boundaries [1]. Particularly, we illustrate first the relation between the grain boundary character and the different defects and trapping sites stored, and their consequences on hydrogen transport and segregation [2]. High-angle Random grain boundaries (R) are considered as a disorganized phase where the hydrogen diffusion is accelerated, while the Special grain boundaries (Coincident Site Lattice, CSL) constitute a potential zone for hydrogen trapping due to the high density of trapping sites as dislocations and vacancies [2]. The predominance of one phenomenon depends on several parameters, such as the grain size, the probability of grain boundaries connectivity, the grain boundaries energy and the excess of free volume. Additionally, our experiments confirm that hydrogen promotes vacancies formation, probably in grain boundaries [2]. In a second part, we have explored the role of the Random grain boundaries on damage assisted by hydrogen. Tensile strengthening is reduced under hydrogen flux when the fraction of random grain boundaries increases. These results support the idea that hydrogen flux promotes intergranular fracture more than the hydrogen concentration [1].
[1] A. Oudriss, J. Bouhattate, C. Savall, J. Creus, X. Feaugas et al., Procedia Mater. Sci., 3 (2014), 2030-2034
[2] A. Oudriss, J. Creus, J. Bouhattate, E. Conforto, C. Berziou, C. Savall, X. Feaugas., Acta Mater., 60 (2012), 6814-6828.

Solute distribution dictates the stability and mechanical properties of nanocrystalline alloys, and is often measured by Atom Probe Tomography (APT). However, APT does not directly yield grain boundary features, and the capability to reconstruct nanocrystalline grain structures is required to study the correlations between solute distribution and grain boundaries. We have developed a local spatial autocorrelation-based modeling method to reconstruct nanoscale grain structures and establish solute segregation statistics in nanocrystalline alloys from APT data. Using nanocrystalline Ni-W alloys, we reconstruct the three-dimensional grain boundary network by carrying out two series of APT data quantization, the first probing the anisotropy in the apparent local atomic density and the second quantifying the local spatial autocorrelation. Similar procedures are carried out for local composition. Segregation of solute atoms is revealed, and its correlation with grain boundary locations is examined. This approach enables automatic and efficient quantification and visualization of grain structure and solute segregation in a large volume and at the finest nanoscale grain sizes.

Oxygen-assisted embrittlement of nickel and nickel-based superalloys has been a well-known phenomena extensively investigated both experimentally and computationally. Oxygen has been observed to preferentially segregate to the grain boundary (GB) in Ni, accompanied by the formation of nickel oxide and large voids, which results in intergranular crack nucleation and propagation and brittle fracture, Moreover, Σ3 GBs are found to be much more resistant to intergranular oxidation, compared with high index ones. A detailed and thorough knowledge of the energetics of the segregation of oxygen at the grain boundaries and the mechanisms of fracture at an atomistic level is essential in aiding in the improvement of the resistance of Ni for oxidation and embrittlement. This talk will present a systematic study of the energetics related to oxygen segregation, vacancy formation, and GB decohesion and fracture using density functional theory and molecular dynamics simulations. The energetics and mechanisms are investigated for 4 model GB systems: Σ3(111), Σ5(012), Σ5(013) and Σ11(113). Oxygen segregation energetics and the relevant decohesion effect are found to be highly correlated to GB structure, leading to remarkable differences in response to oxygen-assisted embrittlement. For example, the resistance to oxidation and embrittlement is observed to be much higher for the Σ3(111) GB in comparison to the other GBs investigated, in line with experimental findings. Additionally, the effect of GB structure and temperature on the preferential oxygen segregation and oxidation along the GB and the decohesion effect behavior will be discussed.

In this work the coupled motion of [111] symmetrical tilt boundaries in FCC metals was investigated using Molecular Dynamics and Phase Field Crystal simulations. We calculated coupling factor as function of misorientation for different inclinations and range of temperatures and shear rates. The coupling factor shows significant deviations from ideal behavior predicted by geometrical theory of coupling. Specifically, the coupling factor is multivalued and exhibits discontinuous transition with increasing misorientation from ideal coupling branch of a 3d material to that of 2d hexagonal lattice. Further, for some boundaries the coupling factor does not belong to either 2d or 3d branch and depends on the shear rate. Finally, we discover novel grain boundary structures composed of structural units that can be described as missing columns of atoms. Such GB structure allows for GB normal motion without producing shear and have high mobility. These boundaries exhibit multiple phases with different atomic densities.

Dislocation, generalized-disclination densities and the associated elastic strain, rotation and curvature fields are derived in grain boundary interfaces from the atomic structure of the relaxed and un-relaxed configurations. These defect density fields allow characterizing the incompatibility of the lattice strain and second-distortion arising from discontinuities of the elastic displacement and distortion (rotation and strain), respectively [1]. The method is first applied to a copper symmetrical tilt boundary as a benchmark [2]. Its accuracy is validated by comparison with a similar recent technique [3]. The core structure of the tilt boundary is then shown to contain edge dislocations and dipoles of generalizeddisclinations (including standard wedge-disclination dipoles). The latter reflect in particular localized shear and stretch discontinuities across the interface, in addition to the rotation discontinuity. The method is then applied to grain boundaries in Si, and in olivine [4].
[1] Acharya, A., Fressengeas, C., 2012. Coupled phase transformations and plasticity as a field theory of deformation incompatibility. Int. J. Fract. 174, 87-94.
[2] Fressengeas, C., Taupin, V., Capolungo, L., 2014. Continuous modelling of the structure of symmetric tilt boundaries. Int. J. Solids Struct. 51, 1434-1441.
[3] Zimmerman, J.A., Bammann, D.J., Gao, H., 2009. Deformation gradients for continuum mechanical analysis of atomistic simulations. Int. J. Solids Struct. 46, 238-253.
[4] Cordier, P., Demouchy, S., Beausir, B., Taupin, V., Barou, F., Fressengeas, C., 2014. Disclinations provide the missing mechanism for deforming olivine-rich rocks in the mantle. Nature 507, 51-6.

We developed a new method to determine the unique reference state in which the Burgers vectors of misfit dislocations in semicoherent interfaces are defined. Following Vattré, our method requires that the sum of the coherency stresses with the far-field stresses of the misfit dislocations must vanish. However, it does not require the definition of transformation pathways between the materials at the interface. Moreover, it is applicable to all types of interfaces including interfaces with three independent sets of dislocations (the general case). We examined the accuracy of our method by comparing it with previous calculations on interfaces with one, two, and three sets of dislocations.

Precipitates are very effective in strengthening metallic alloys because they act as obstacles to dislocation motion. One of the key factors that controls the mechanical properties of precipitate-hardened alloys is the morphology of the precipitates i.e. their size and shape. In Nb-containing zirconium alloys, currently employed in nuclear power plants, niobium precipitation occurs under irradiation. Coherent β-Nb nano-precipitates (b.c.c. structure) presenting a platelet shape lying in the basal plane appear in the α-Zr matrix (h.c.p). The purpose of the present work is to investigate the different factors that might control the morphology of the β-Nb precipitates.
In microstructure presenting such nanosized precipitates, heterophase interface properties play an important role underlying the stability and morphology of the microstructure. Moreover, the equilibrium shape of small coherent inclusions, characterized by the orientation relationship together with interface orientations, is determined by both interfacial and elastic strain energies generated between the precipitate and matrix.
In a first step, we will show the nature of the Zr/Nb interfaces to provide insights into the factors responsible for their stability. An ab-initio study on coherent α/β interfaces has been carried out: we have analyzed different interfacial atomic configurations, alignments and orientations to evaluate the magnitude and anisotropy of the associated interface energies. The favored interfaces are compared to the different precipitate/matrix interfaces evidenced by high resolution electron microscopy. Then, as a competing energetic contribution controlling the precipitate morphology, the elastic coherency strain energy for the Zr/Nb system has been determined from the continuum inhomogeneous and anisotropic elasticity theory. The results obtained will give us the necessary energetic informations to predict the equilibrium morphology of the β-Nb precipitates as a function of size. A comparison with experimental observations will be done to assess deviations from the equilibrium shape.

Grain boundaries (GBs) influence a wide array of properties, such as diffusivity, conductivity, microstructure stability and intergranular failure resistance, in both structural and functional materials. One of the key steps in building structure-property relationships of interfaces lies in the ability to quantify their structures at the atomistic length scale. From a thermodynamic point of view, the structure of GBs depends on five macroscopic degrees of freedom - the misorientation and the boundary-plane orientation. Traditionally, the atomic structures have been identified using the Structural Unit model, which is a one-dimensional model for quantifying GB structures and is applicable only for very symmetric interfaces. In this talk, we will present a three-dimensional polyhedral unit model for quantifying GB structures applicable for both the usual tilt and twist interfaces and those with mixed (or general) character. The polyhedral unit model, proposed by Ashby et al. for describing atomic packing in metallic glasses, will be utilized for representing the atomic packing at the interfaces. A novel image-processing algorithm, for automatically identifying these polyhedral units in the GB structures obtained through atomistic simulations, will be introduced. We will also propose strategies for quantifying GB structures in the complete five-dimensional crystallographic phase space.

In many systems such as whiskers, wires and pillars the reduction of the mean free path of dislocations below the micron scale produces significant increase of the mechanical strength. In small-grain polycrystals, the constraint to the motion of dislocation is due to grain boundaries (GB). By absorbing dislocations, GBs contribute to shut down dislocation activities [1]. At that point, other alternate plasticity mechanisms are needed.
Shear-migration coupling is one of them and is the focus of many theoretical and experimental studies. At variance from dislocation-based plasticity, the shear produced by a moving GB can result in different values, depending on a parameter called the coupling factor Beta. Recent results obtained by in-situ Transmission Electron Microscopy (TEM) in ultra fine-grained Aluminum, show that many deformation modes are activated, including shear migration coupling. The coupling factor can be measured experimentally using image correlation analysis and therefore confronted to what has been predicted by models such as the one from Cahn and Mishin [2]. Although solid statistical data are still missing, beta appears smaller than what has been predicted. A reason could lie in the atomic-scale mechanisms that guide the migration of GBs. The Cahn and Mishin model assumes collective motion of GB dislocations, while Rae and coworkers insist on the role of steps propagation [3]. High resolution imaging of bicrystals shows that steps decorate GBs and that the motion of imperfect steps could result in the migration of the GB associated with a shear. To take in account these observations we also proposed a geometrical model for the shear migration coupling of grain boundaries [4], based of the shuffling of atoms within extended cells around the GB. Finally, recent simulations show that step dislocations (disconnections) are probably the basic mechanism leading to grain boundary migration [5]. Those disconnections are found in non-ideal GBs and can be created from interactions between lattice dislocations and GBs [6]
References:
[1] F. Mompiou, D. Caillard, M. Legros, H. Mughrabi, Acta Mat. 60/8 (2012) 3402.
[2] J.W. Cahn, Y. Mishin, A. Suzuki, Acta Mat. 54/19 (2006) 4953.
[3] C.M.F. Rae, D.A. Smith, Philosophical Magazine 41/4 (1980) 477.
[4] F. Mompiou, D. Caillard, M. Legros, Acta Mat. 57/7 (2009) 2198.
[5] A.Rajabzadeh, F. Mompiou, M. Legros, N. Combe, Phys. Rev. Lett., (2013) 110,265507
[6] A.Rajabzadeh, F. Mompiou, N. Combe, M. Legros, D. A. Molodov, S. Lartigue-Korinek, Acta Mat. 2014

Severe plastic deformation (SPD) is widely used to process bulk nanocrystalline metals. The most accepted models to describe the microstructural evolution, e.g., the grain refinement, during SPD are based on dislocation activities and the influence of grain boundary motion has not been fully integrated to any of the existing models yet. However, due to the large volume fraction of grain boundaries in nanocrystalline metals, it is expected that grain boundary motion has significant influence on the microstructural evolution. In this work the interaction of grain boundary motion with cracks in nanocrystalline metals during shear deformation has been studied by atomistic simulations. It is shown that based on metal type, temperature and grain boundary structure different mechanisms, namely crack healing, grain boundary decohesion and sub-grain formation can happen, which could be used to tailor the overall microstructures in nanocrystalline metals.

Nanocrystalline (NC) metals have shown potential for improved wear response over their bulk counterparts, however the propensity for grain evolution under wear could limit their applicability for tribological applications. In this work, electroplated Ni-40at%W --- a binary NC alloy with demonstrated improvement in thermal stability over pure Ni --- was studied under wear as a function of applied force and number of cycles. Microstructural evolution in each wear track was then analyzed via cross-sectional transmission electron microscopy (TEM). Results indicate an increased propensity for grain coarsening under sliding contact with increasing contact force. At low contact force (1 mN), a low friction wear regime (friction coefficient below 0.3) is maintained up to 10000 cycles. However, at higher contact forces (100 mN and 1000 mN), a shift in wear regime from low to high friction occurs within a few hundred cycles. Cross-sectional TEM analysis shows that in the 1 mN case, only minimal grain coarsening occurs and is confined to a depth of 200 nm. In contrast, for applied forces of 100 and 1000 mN, significant grain coarsening is observed down to depths of 400 and 800 nm. We hypothesize that material softening due to grain coarsening gives rise to the observed shift to a high friction regime at higher contact forces.

The study of interactions between grain boundaries (GBs) and second phase precipitates is crucial to enhance our understanding of microstructural evolution in crystalline materials. We show both analytically and computationally that a planar symmetric GB contained within a second phase lamellar precipitate is unstable against long wavelength perturbations. The instability is mediated by the elastic interactions between the GB and compositional domain boundaries.
A simple relationship between the growth rate and wavelength of this instability is derived analytically for an elastically isotropic material in the case of a symmetrical GB centered in a second phase lamella infinite in the GB direction. This dispersion relation reveals that short wave-lengths are stabilized by the surface tension while larger wave-lengths are unstable due to elastic interactions. In particular, we show that the growth rate is maximum for a given wave-length, revealing the pattern-forming character of this instability.
In a second step, we derive a similar dispersion relation in the case of anisotropic elasticity for cubic symmetry. The anisotropy is shown to inhibit the instability by reducing the growth rate and increasing the wave-length corresponding to the maximum growth rate.
Simulations using both an elastically periodic model for dislocations [Geslin et al., Acta. Mater. 71, p.80-88, 2014] and amplitude equations derived from the phase-field-crystal model [Spatschek, Karma, PRB 81, 214201, 2010] confirm the key predictions of the linear stability analysis in both the isotropic and anisotropic cases and shed light on the subsequent nonlinear stages of this instability. In particular, we show that this instability can lead to the break-up of low angle GBs, thus changing their properties and mobilities.
Finally, simulations performed with a circular precipitate in the vicinity of the GB show that elastic interactions between the precipitate and the GB lead to a similar instability.

One-dimensional nanocrystalline metallic nanowires with extremely high mechanical strength are believed to be promising building blocks for a wide range of emerging applications in electronic, optical and nanoelectromechanical devices, etc. Metallic nanowires, however, experience severe degradation of mechanical behavior in the presence of minute impurities, such as oxygen, especially along the grain boundary. Various mechanisms have been proposed, whereas these phenomena are still poorly understood on an atomic level. To find solutions to these problems, it is crucial to develop an in-depth understanding of the deformation mechanisms of metallic nanowires, and degradation effects of oxygen along the grain boundary, which is hard to probe experimentally. A thorough investigation of the tensile deformation and failure mechanisms of pure nanocrystalline Ni and nanocrystalline Ni with varying levels of oxygen impurities is therefore carried out using molecular dynamics simulations. The evolution of defect structures and the mechanisms of failure are characterized for nanowires of 24 nm and 48 nm diameter. It is observed that segregation of varying levels of atomic oxygen impurities along the grain boundaries have a great impact on the deformation and failure modes of nickel nanowires. The effect of the concentration of oxygen impurities on the evolution of defect structures, densities and the micromechanisms of failure for the various nanocrystalline Ni nanowires will be discussed.

Grain boundaries (GBs) play an important role in the plastic deformation of metals. They are sites for dislocation adsorption, transmission, and emission and sliding between grains. There are many ad hoc approaches to accounting for each of these effects and several have been applied in discrete dislocation dynamics (DDD). In this presentation, we discuss which grain boundary features are essential for a DDD model of the deformation of polycrystalline metals. We then discuss an implementation of such a model in 2D DDD. The resultant model is self-consistent in how it treats GBs and dislocation plasticity. Next, we demonstrate that this model is able to account for several of the well known features of the plastic deformation of nanocrystalline metals including the transition from smaller-is-stronger to smaller-is-weaker with decreasing grain size. Finally, we provide some insight into the development of a continuum formulation of a GB constitutive model that accounts for the same features as this Poly-DDD model as well as discussion some outstanding issues in Poly-DDD.

Some grain boundaries act as a clear obstacles to dislocation motion and result in a blocked array of dislocations. This array leads to a high stress concentration at the tip, extending from the slipping grain into the neighbour shearing grain. This is important when considering failure and deformation as these hot spots are likely nucleants of future damage. In this study we have selected one grain boundary that has many blocked slip planes and investigated this in 3D with differential aperture Laue micro-diffraction (DAXM). Our results show that sub-surface and 3D measurements of the stress intensity ahead of the blocked array of dislocations. This analysis provides exciting new insight into how grain boundaries impart strengthening behaviour in polycrystals and links well with surface based measurements, such as high resolution electron backscatter diffraction (HR-EBSD) measurements. 3D quantitative stress distributions were used to calculate a stress intensity factor derived from a pile up of dislocations associated with the Eshelby, Frank, and Nabarro (EFN) model. Our analysis reveals that the 3D microstructure can be important, as there are differences along the line of intersection between the impinging slip plane and the grain boundary, as well as between lines of intersections. These results contextualise individual measurements of stress intensities at grain boundaries and offer new insight into modelling methods.

The strive for advanced materials with well-defined microstructures has also led to an increasing effort towards a physically based description of the motion of dislocations as the cause of plastic deformation and the origin of materials failure. Several dislocation based continuum theories have been introduced, but only recently rigorous techniques have been developed for performing meaningful averages over systems of moving, curved dislocations, yielding evolution equations for a higher order dislocation density tensor, see [1].
In order to construct a self-consistent coarsening in a numerical implementation, several issues have to be resolved including calculation of the stress field of a system of dislocations, correlation functions, and boundary conditions. Accurate solutions have been found for one dimensional systems [2]. Fully two- and three-dimensional systems will be compared to ensemble averages over discrete dislocation distributions. A continuous field approach including stress interactions perpendicular to the slip planes is introduced and an overview of results for a distribution of one-dimensional glide planes in two-dimensional elastic media is presented. Several aspects of numerical homogenization are analyzed and discussed. Using comparisons with Discrete Dislocation Dynamics (DDD) in a few simple systems, the multi-component stress field which must be considered for dislocation density motion is discussed and enhanced by a statistical model for the representation of dipole formation according to [3] and a formulation of dipole dissolution in the continuum formulation.
REFERENCES
[1] Hochrainer, T., Sandfeld, S., Zaiser, M., & Gumbsch, P. (2014). Continuum dislocation dynamics: towards a physical theory of crystal plasticity. Journal of the Mechanics and Physics of Solids, 63, 167-178.
[2] Schulz, K., Dickel, D., Schmitt, S., Sandfeld, S., Weygand, D., & Gumbsch, P. (2014). Analysis of dislocation pile-ups using a dislocation-based continuum theory. Modelling and Simulation in Materials Science and Engineering, 22(2), 025008.
[3] Dickel, D., Schulz, K., Schmitt, S., & Gumbsch, P. (2014). Dipole formation and yielding in a two-dimensional continuum dislocation model. Physical Review B, 90(9), 094118.

Grain boundaries (GBs) can act as a barrier for dislocation motion, leading to a pile-up of dislocations, which in turn might lead to decohesion at the GB. This decohesion can lead to the creation of voids, which may combine to form cracks, ultimately causing macroscopic failure. A better understanding of this chain of events is the goal of our study, as part of a larger multiscale research program.
In this work we use molecular dynamics simulations (MD) to let edge dislocations in bcc iron interact with a GB between Fe and a second crystalline material. This second material is a tuneable artificial material, functioning as a model of a precipitate. The properties of the second material were varied to study the individual influences of stiffness, GB structure, and GB strength on the GB-dislocation interactions. Fe is described with an EAM-potential (Ackland et al., J. Phys.: Condens. Matter, 16, 2004). The description of the second material, as well as the cross interactions, is based on the same potential.
The first results show that a high GB strength prevents dislocations from entering the GB, whereas a lower GB strength permits this entering. The stiffness difference between the individual grains is found to determine whether a dislocation that has entered a GB will cross the GB, remain dissolved or will be reflected. Results from simulations with multiple dislocations on the same slip plane will also be reported.
Insights from these MD simulations, including mixed mode cohesive laws, will be used in models such as discrete dislocation dynamics, to help developing a better description of GB-dislocation interactions and plasticity at a larger scale.

Plasticity in polycrystalline materials is largely controlled by the grain size, which is the key quantity in the Hall-Petch relation linking flow stress and grain size. The role of the grain boundary properties with respect to transmission of dislocations or their resistance against plastic flow is less clear. Different modeling techniques with varying level of details on the dislocation grain boundary interaction have been used to formulate constitutive laws. Some of them are based on purely geometrical factors of the grain boundary like the crystallopgraphic misorientation [1], others include a yield criterion of the grain boundary surface [2]. On the upper end of scale, strain gradient plasticity models describe this process phenomenologically, but the physical picture behind the model is not clear [3]. Therefore the current study focuses on the role of the elastic interaction of dislocations across grain boundaries with Discrete Dislocation Dynamics [4], varying the misorientation for twist and tilt grain boundaries. The results show, that physical transmission of dislocations might be less important in continuum descriptions than the transparency of the grain boundary to stress and displacement fields of dislocations adjacent to the grain boundary [5]. Due to the elastic interaction of dislocations across grain boundaries, the grain boundary effectively yields in a continuum sense.
References
[1] Sutton, A. P. & Balluffi, R. W. (1996): Interfaces in crystalline materials, Clarendon Press
[2] Wulfinghoff, S. et al. (2013): Int. J. Plast. 51:33
[3] Zhang, X. et al. (2014): J. Mater. Res. 29:2116
[4] Weygand, D. et al. (2002): Model. Simul. Mater. Sci. Eng. 10(4):437
[5] Stricker, M. et al. (2015): Meccanica, Springer Netherlands 2015: 1

The future development of Nickel-based superalloys for power and aerospace applications can benefit greatly, in terms of cutting the development costs and time, from a physics based understanding of the most active deformation mechanisms taking place when they are subject to high stresses and temperatures. In particular, the interaction of large collections of dislocations with arbitrarily shaped and distributed strengthening precipitates needs to be better characterized. In this work, the three-dimensional discrete dislocation (DDD) method is used to simulate the behavior of single crystal two-phase Nickel-based superalloy micropillars subject to monotonic loading. The DDD method has been extended to properly handle random collections of arbitrary shaped precipitates. Furthermore, dislocations (i.e. superdislocations and superpartials) interactions with precipitates are incorporated without any prior assumptions on their placement or dissociation within the simulation volume in an attempt to match experimentally observed microstructure. The anti-phase boundaries (APB) creation and destruction due to dislocation motion is explicitly tracked and the appropriate precipitate reactions are accounted for. The effects of the precipitate size, volume fraction, its APB energy and crystal size are all presented and analyzed. Different cases where dislocations show looping as opposed to shearing through the precipitates are identified and correlated with the precipitate microstructural features. The results are validated by direct comparisons with microcompression experimental studies of Nickel based superalloy microcrystals.

Nanocrystals and nanolayered composite materials consist of high ratio of interface to bulk as well as dislocation defects governing their mechanical behavior and radiation resistance. Dislocations provide sites for vacancies, interstitials, and charged carriers supplying fast pathways for mass and charge transport. Interfaces disturb the stress fields of dislocations and thus contribute not only to the mobility of dislocations but also to the polarization charge variations. While plastic deformation governs by dislocation defects in polycrystals, deformation in nanocrystals promotes and may be dominated by contributions of interface and charge. The elastic fields are important in the nucleation, emission, and mobility of dislocation defects as well as carrier transport. However, the elastic distortions for anisotropic crystals with interfaces are not equally partitioned and the solutions near dislocation cores require modification of classical Peierls model. Despite several models, the coupled elastic and electric fields of interface dislocation defects as well as partitioning of charge transport between different dislocation characters and glide/climb components are still not clear in anisotropic bicrystals. Understanding the elastic and electric interactions between extrinsic dislocations and grain boundaries (GBs) or triple junctions (TJs) requires advanced anisotropic elasticity modeling.
In this work, anisotropic elasticity theory and Green&’s functions methodology are utilized to determine the coupled elastic and electric fields around infinite dislocation lines and dislocation loops in bilayer systems consisting of two adjoining semi-infinite media. The solutions are applied to determine the elastic fields of nucleated dislocations from GBs by cross-slip into inclined glide planes. The charge partitioning for different dislocation characters and the contribution of charge on the elastic fields near and at the bilayer interface are presented using geometrical and electrostatic considerations. Half loops of dislocation defects nucleated at and emitted from the interface are elastically analyzed to understand the static interactions with interfacial GBs and adjacent TJs during extrinsic lattice dislocation emission. Similar methodology is presented to explore a bimetal system of copper and niobium with a heterophase interface having Kurdjumov-Sachs (KS) or Nishiyama-Wassermann (NW) orientation relations. This has been carried out by considering a half loop that nucleated from two adjacent dislocation intersections (DIs) at the interface. The results may offer mechanisms for emitted dislocation-interface interaction and its role in microstructure evolution and mechanical behavior in nanocrystals and nanocomposites.

Nickel aluminide (NiAl) has shown great promise for next generation high temperature structural use, due to its excellent properties such as high melting point, low density, high oxidation and corrosion resistance. Low ductility and fracture toughness at room temperature are considered to be restrictions for the use of NiAl. Directional solidification of NiAl alloyed with refractory metals resulting in highly aligned microstructures have shown to address these issues. The alloying addition as well as processing parameters greatly affect the final microstructure and properties of these alloys. Thus, it is important to investigate the mechanical behavior of these materials at varying length scales in order to understand in detail the relationship between microstructure, processing parameters and alloying additions on the macroscopic properties of the material.
For this study, NiAl-Cr eutectic alloys were prepared by directional solidification at three different solidification speeds, namely 20 mm/h, 50 mm/h and 80 mm/h, where NiAl forms the matrix and Cr the continuous fibers. Fiber diameter and spacing follow an inverse relation with solidification speed and are in the submicron dimension. In order to investigate the role of individual phases and the interfaces on the mechanical properties at room temperature, we have employed different micromechanical testing techniques such as nanoindentation, in-situ tensile test of individual fibers, micro-pillar compression and small scale fracture tests. Hardness and modulus of overall alloys are found to be independent of the solidification speeds and values for modulus correspond well with previously reported values. In-situ SEM tensile tests of the individual Cr fibers, extracted by chemical etching of the matrix, shows that they deform plastically to several percent and neck formation was observed before fracture. The fibers have high strength values of ~3 GPa which suggest that these fibers have initially a very small number of dislocations. Micro-pillars containing single fiber surrounded by matrix as well as single phase NiAl pillars were prepared by FIB milling and compressed with a flat punch in a nanoindenter. It is found that for comparable dimensions both single phase NiAl pillars and pillars with fibers show similar strength values of ~2.5 GPa. Moreover, cross sections across the deformed pillars show no sign of delamination or fracture at the fiber-matrix interface suggesting that the interface is strong enough to accommodate possible mismatch in deformation. First TEM results suggest that the fiber-matrix interface acts as a source for dislocations in the NiAl matrix providing additional plasticity during deformation. Fracture toughness tests with different fiber orientation with respect to loading axis are carried out. The experimental observations point to the important role of fiber-matrix interfaces, acting as source for dislocations, on the macroscopic properties of the alloy.

Plasticity in nanostructured materials is largely mediated by phenomena driven by the local stress state at or close to grain boundaries. It is proposed here to develop a constitutive model, relying on a higher order mathematical form of the Helmholtz free energy in the medium, allowing for a description of the internal stress state, and of its evolution, at material interfaces and applicable to polycrystal simulations. The approach relies on a continuous description of material interfaces as zones of incompatibilities whereby both strain and curvature fields are no longer necessarily simple first order derivatives of displacement and rotation vectors, respectively. From a defect theory standpoint it is found that the incompatibility fields are representative of aerial density dislocation and disclinations. While such descriptions can be numerically rendered at a fine scale (i.e. sub-atomic), their extensions to the mesoscale (i.e. ~10nm) are challenging. In addition one must acknowledge that the measure of incompatibilities is scale dependent.
The work will introduce an approach allowing to reconcile both fine scale viewpoint with the need of a mesoscale rendering of grain boundary plasticity. It builds on previous field dislocation and disclination mechanics (FDDM) models that have shown to allow for a structure sensitive rendering of excess energy and shear coupled boundary migration in symmetric tilt boundaries. The idea proposed lies in introducing plastic curvature laws consistent with simulations of plasticity at a fine scale. The so-called phenomenological FDDM (PMFDDM) approach therefore aims at mimicking results from FDDM in a numerically efficient fashion. A regularization procedure is introduced to impose that dissipation in FDDM and PMFDDM simulations are equal when simulating similar processes. The capabilities of the PMFDDM model are then demonstrated in the case of plasticity in bicrystal interfaces and nanopolycrystals.

we experimentally demonstrated that a fine-grained HCP bulk titanium exhibits clear yield drop behavior and extraordinary plastic elongation in tensile test only if the grain size is less than one micron owing to pyramidal slip system activation, while the pyramidal slip system has 2-3 times higher critical resolved shear stress than the other slip systems, like basal and prismatic slip systems. By considering size effects on a critical stress of intra-grain slip system activation and a critical stress of grain boundary dislocation nucleation, we theoretically and atomistically explained why the critical grain size for pyramidal slip system activation is one micron. The theory was also applied to Mg, and we found that Mg also has a critical grain size to activate non-basal slip systems which can produce c-axis strain, but the critical size is 10 times larger than that of Ti.

Recent interest in nanocrystalline alloys stems from their exceptional mechanical properties such as high strength and ductility. In a recent study by Pozuelo et al. ([PMKY11]), a Magnesium-based nanocrystalline alloy is synthesized using cryomilling to form nano-sized grains, followed by spark plasma sintering as a consolidation process. The resulting alloy is observed to have a bimodal distribution of grain size which is an extremely important feature that results in high strength and high ductility. The main focus of our study is to model the evolution of the microstructure throughout the consolidation stage, simulate the bimodal distribution of grains, and to study it&’s long term stability in the presence of various inclusions. Due to the presence of high temperature gradients during the sintering process, we develop a thermodynamically consistent phase field model and demonstrate the formation of a bimodal distribution of grain size. In addition, we set the stage for future work to include the effects of inclusions in the phase field model in order to stabilize the microstructure.
[PMKY11] Marta Pozuelo, Christopher Melnyk, Wei H. Kao, and Jenn-Ming Yang. Cryomilling and spark plasma sintering of nanocrystalline magnesium-based alloy. Journal of Materials Research, 26:904-911, 2011.

The instability of nanocrystalline metals against grain growth is a principal challenge in their production and use. Alloying can stabilize the nanocrystalline state against grain growth if the solute species energetically prefer to segregate to the grain boundary over forming bulk states, and in some cases it appears possible to stabilize such a structure against phase separation as well. In this talk, we describe our recent work extending the concept of nanocrystalline stabilization to systems with negative enthalpy of mixing that tend to form ordered compounds in equilibrium. Equilibrium microstructures are determined using a lattice-based Monte Carlo simulation that are tailored to study the thermodynamic competition between nanostructured states and bulk states, including the opportunity to form intermetallics. The results reveal nanocrystalline states with solute segregated grain boundaries as well as duplex nanostructured states that simultaneously exhibit ordered compounds, solid solution grains, and solute segregated grain boundaries at equilibrium. A thermodynamic stability map for negative enthalpy of mixing systems shows the predicted equilibrium microstructure based on the enthalpy of mixing and enthalpy of grain boundary segregation of the potential alloy candidates.

Advanced nuclear energy systems will require materials to perform for extended periods under conditions of elevated temperatures and extreme radiation. Conventional materials lack the required microstructural stability and typically exhibit excessive hardening and swelling induced by cumulative radiation damage. Our previous work shows that GB networks consisting of a high fraction of special boundaries can be stabilized against thermal coarsening and other interface-mediated degradation. The design principle then for a radiation-resistant microstructure is to have a high specific surface area of crystallographically open boundaries while still containing a high frequency of special boundaries that constrain thermally-induced coarsening. We will discuss these ideas based on experiments on nanotwinned copper samples and highlight the role of higher order correlations built into the grain boundary network.
This work performed under the auspices of the US DOE by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. The work was supported by the US DOE Office of Basic Energy Sciences, Division of Materials Science and Engineering.

We show that elastic interactions between point defects and semicoherent interfaces lead to a marked enhancement in interface sink strength. Our conclusions are based on simulations that integrate object kinetic Monte Carlo (OKMC) with anisotropic elasticity calculations of interface stress fields. Elastic interactions of interfaces with vacancies and interstitials are characterized using elastic dipole tensors computed from first principles. Surprisingly, the enhancement in sink strength is not due primarily to increased thermodynamic driving forces, but rather to reduced defect migration barriers, which induce a preferential drift of the defects towards the interface. The sink strength enhancement is highly sensitive to the detailed character of interfacial stresses, suggesting that “super-sink” interfaces may be designed by optimizing interface stress fields. Such interfaces may be used to create materials with unprecedented resistance to radiation-induced damage.

As austenitic stainless steels are widely used in nuclear reactor core components, the irradiation induced degradation of them is a leading concern in nuclear applications. Therefore, understanding the time evolution of displacement cascades and the distribution of radiation-induced defects is important for the prediction of materials behaviour and investigation of radiation tolerance of materials under normal operating conditions. In this study, the influence of grain boundary structure on the resistance of austenitic iron to cascade damage was examined. In particular, comparisons were made between the interactions of low and high angle grain boundaries with defects produced from low energy (5 keV) primary knock-on atoms (PKA). Molecular dynamics (MD) simulations were performed using the embedded atom method (EAM) potential developed by Bonny, et al. (Modelling Simul. Mater. Sci. Eng., 21, 2013, 085004) and the LAMMPS simulation program (S. Plimpton, J Comp Phys, 117, 1-19, 1995). It was found that both low and high angle grain boundaries act as sinks for the radiation-generated vacancies and interstitials, and hence reduce the amount of defects in the bulk material. For a better understanding of the phenomena, the effects of grain boundaries were investigated in both nano-crystals and larger systems. Some aspects of the mechanical behaviour of the irradiated material were also investigated and are discussed.

Solid-state interfaces in metallic composites are preferential sites for impurity precipitation. We describe a phase-field model for simulating the growth, coalescence, and subsequent stability of He precipitates on planar interfaces with non-uniform energy distributions. Our work leads to interface design criteria that predict whether stable linear networks of He precipitates form at a given interface. These criteria are used to design and test multilayered metallic composites with increased resistance to He damage.

Development of metals and alloys capable of surviving high energy neutron environments and high temperatures (~ 500^oC) is crucial to the next generation nuclear reactor design. Use of nanocrystalline (nc) materials is one promising approach to solve the problem of structural damage caused by neutrons.
The superior mechanical properties of nc-metals and alloys are attributed to ultra fine grains according to Hall-Petch relationship. An increase in the grain size results in the deterioration of these mechanical properties. Nanocrystalline metals are thermally unstable due to their large grain boundary area. Effects of irradiation on grain size have been studied on pure nc-metals and other nc-alloys, i.e nc-Pd and nc- Zr-Fe, and the grains have been observed to grow.
It is presently unknown if thermally stabilized nc-alloys will exhibit increased resistance to grain growth. Also, it is an open question if the grains grow more slowly at high temperature after irradiation than common nc-alloys. In this work, we study the effects of self-irradiation on a 500 nm thick nc-Ni metal films that have been alloyed with 15% W (at.) in order to improve its thermal stability. The alloy is deposited using reverse pulse electroplating on copper substrates and have been shown to be thermally stable up to 450°C. Samples were ion-irradiated with 2 MeV Ni⁺ ions at room temperature at doses corresponding to 1, 3, 10 and 100 displacements per atom (dpa) in the NiW film. The samples were then observed by transmission electron microscopy (TEM) using hollow cone dark field (HCDF) imaging to determine NiW grain size. Irradiated samples were subsequently annealed at 300 ^oC for 24 hours in an inert atmosphere in order to investigate the effect of irradiation on thermal stability of the alloy. The annealed samples were also analyzed using the same TEM methods.
Results show an increase in grain size with radiation increasing from 1 dpa to 100 dpa. Formation of collision cascades that promote atomic diffusion and local thermal spike have been proposed as a grain growth mechanism in pue nc-metal alloys. Annealing of the irradiated samples reveals a curious trend of further grain growth related to the amount of irradiation on the sample. Thermal stability in nc-NiW occurs due to excess solute occupying the grain boundaries. Irradiation may cause these solutes to diffuse back into the interior of the grain, thereby destabilizing the alloy. We plan to map the tungsten content distribution inside the irradiated grains to gain an understanding of this irradiation-induced thermal instability.

Diffusional migration of an interphase boundaries in solids control microstructural evolution during most solid-state phase transformations, for which a detailed atomic picture does not exist. Using a combination of phase field crystal modeling and crystallographic analysis, we present here a complete atomistic description of the relationship between atomic structure and migration mechanism of a high-index planar interface during a hexagonal to square polymorphic (reconstructive or diffusional) phase transition. In particular we show that a terrace-step interface advances macroscopically in the form of lateral migration of a growth ledge rather than synchronous motion of structural ledges (i.e., the steps), without producing any shape strain. While microscopically its migration occurs by opposite shearing on the terraces and climbing of interfacial dislocations on the steps, following a unique one-to-two lattice site correspondence.

Atomistic modelling of the dynamics of phase transformations is a particularly challenging task. If the mechanism of the phase transformation is governed by so-called rare events then the time scale of interest will reach far beyond the capabilities of regular molecular dynamics simulations.
The atomistic rearrangements during solid-solid phase transformations in bulk systems involve massive structural changes including concerted multi-atom processes. The interface between two structurally different phases leads to a complex energy landscape that needs to be explored during the dynamical evolution of the interface. Here, we employ an adaptive kinetic Monte Carlo (AKMC) approach to investigate such processes at the interface between cubic and topologically close-packed phases in transition metals.
In particular we investigate the transformation between BCC and A15 in molybdenum. During the dynamical simulations a finite, disordered interface region evolves to compensate the structural mismatch between the two crystal phases. This disordered interface region makes the identification of a single transformation mechanism difficult. Still, from our simulations we extract a rate for the layer transformation which we relate to an effective barrier for the transformation mechanism and discuss the corresponding features of the complex energy landscape along the transformation path.

Microstructural properties play a vital role in defining maroscopic properties of a material like grain size effect on tensile stress by Hall-Petch relationship. During thermomechanical processing of steels recovery, recrystallization, phase transformation and grain coarsening are major phenomenons which influence the final microstructure of steel . Effect of morphlogy, phase distribution, solute drag should be understood to modify the microstructure based on macroscopic property requirements.
A multicomponent multiphase field model is formulated using Steinbach et al (Physica D,Volume 134 Issue 4 p 385-393,1999) and Abhik et al (Phys. Rev. E 85, 021602,2012) to study isothermal phase transformation and grain growth in a binary Fe-C system. Phase Field and Diffusion equation are derived from a free energy functional in a thermodynamically consistent way. Some salient features of the model are: (1) A condition of quasiequilibrium i.e. every position at the interface is at local equilibrium with similar chemical potential of each component, is imposed. (2) Free energy functional is formulated in a form of Grand potential function. Functional is formulated to describe compositions as a function of chemical potential. This approach has several benefits like chemical potential equivalence is not solved explicitly and excess chemical free energy, which is a numerical artefact in diffuse interface approach, is removed from interface. (3) Youngs force equilibrium at multijunctions holds true, interaction is decomposed into pairwise interaction between different orientations or phases. (4) Kim's Algorithm is modified to replace a constant number of fields stored at a particular point to dynamically change the number of present fields at any position.
Isotropic grain growth in a pure Fe system are studied. It is shown that grain size distribution after particular timesteps follows hillert distribution, which is also independent of initial distribution of grains.Isothermal transformation from austenite to ferrite is studied in a binary Fe-C system. Carbon concentration at interface,Volume fraction, grain size and morphology of austenite and ferrite are analysed.It is shown for finite interface width, chemical contribution to surface diffusion is negligible by using grand potential formulation.

Ni films of 40 nm in thickness were deposited on sapphire substrates. The films were polycrystalline, with a “mazed bicrystal” microstructure. Solid-state dewetting (agglomeration) and thermal grain boundary grooving in the films were studied at 700°C. Both the receding edge of growing holes in the film, and the groove shape exhibited an uncharacteristically-flat morphology. We proposed a model which explained this by considering Ni self-diffusion along the film-substrate interface and homogeneous thickening of the film.
Based on these results we propose a new method for measuring metal diffusion along metal/ceramic interfaces, for which the literature data is severely lacking. This is performed by deposition of an ultra-thin layer of a metal diffuser on partially-agglomerated films, followed by diffusion annealing and quantitative characterization by high-resolution transmission electron microscopy (HRTEM). We demonstrate this by studying Au hetero-diffusion along the Ni/sapphire interface. This method could be applied to a variety of metal and substrate combinations. We discuss the nature of the driving force for the interface diffusion, which is related both to entropy effects and to the concentration dependence of the interface energy.

The degradation and oxidation of nickel-based alloys are known problems for light water nuclear reactors (LWRs). A better understanding of the rates and mechanisms of oxide ingress and metal ion release would help to better predict corrosion and fouling rates in LWR cores. We have performed atomistic simulations to study cation and oxygen anion diffusion mechanisms in bulk Fe-Cr-Ni alloys and oxides known to form in LWR conditions. Vacancy formation and migration energies were explored numerically in bulk and on grain boundaries of Fe-Cr-Ni alloys. Self-diffusivities were calculated using an empirical Arrhenius-type equation.
We found that the concentration-dependent diffusivity is significant enough not to be neglected. In chromium oxide Cr₂O₃, four different point defects, namely the Cr vacancy, Cr interstitial, O vacancy, and O interstitial were investigated, and their corresponding temperature- and concentration-dependent diffusion coefficients were also computed. Simulations of Cr₂O₃ were carried out at various temperatures, to investigate an observed temperature-dependent diffusion anisotropy. Metal cations diffusion was found to be strong anisotropic at lower temperatures, which disappeared above 1800K. Migration energy barriers of point defects evaluated by Nudged Elastic Band (NEB) method show the origin of this anisotropic diffusion, and explain why it disappears at higher temperatures.

We produced the micro- and nanoparticles of Au, Fe, and Mo on sapphire substrate employing solid state dewetting (agglomeration) of thin films. We demonstrate that most of the faceted single crystalline particles do not evolve to their equilibrium crystal shape upon annealing at high temperatures. This was related to their perfect single crystalline nature, the lack of defects, and to the high energy barrier for the two-dimensional nucleation of the new atomic layer on the facet. We deformed plastically individual nanoparticles employing the atomic force microscope-based indentation, tapping, and flat punch compression. We demonstrate that during subsequent anneal, the deformed nanoparticles quickly evolve to their equilibrium shape, contrary to their pristine counterparts. Moreover, the deformed particles of equilibrium shape return to their original shape, demonstrating reversible “capillary shape memory effect”. The developed method was calibrated by statistically based determination of the Au {111}/sapphire {0001} interfacial energy, with the result (2.11±0.08 J/m²) being in a good agreement with the values reported in the literature. The method was further employed for determining the equilibrium crystal shapes of Fe and Mo, and of the energies of their respective interfaces with sapphire.

A thickness-dependent (111) to (100) texture transformation is well known in certain FCC thin films. During annealing, films that are deposited with good (111) texture remain (111) when sufficiently thin, but transform to (100) by an anomalous grain growth process when sufficiently thick. Intermediate films transform to stable mixed textures. In recent years we have shown that such transformations are driven by elimination of defects during grain growth; however, no current model can predict the final mixed texture. In the present work, we show that the stable mixed texture depends on the annealing temperature, and describe two possible mechanisms, grain boundary grooving and depletion of the driving force, by which the anomalous grain growth process can be stopped in a thickness-dependent manner leading to the observed mixed textures. Since the stresses and stress states in such films can vary dramatically with texture, understanding these structures is critical to improving the performance and reliability of devices containing such films.

Hollow metallic nanoparticles attract great deal of attention due to their possible applications in various fields of nanotechnology (drug delivery, energy production and storage, catalysis, etc.).
These particles have been fabricated in the past employing wet chemistry methods or/and Kirkendall effect during bulk interdiffusion in the core-shell nanoparticles. The latter process requires relatively high temperatures at which the bulk diffusion is active.
In this work we present a method of fabricating hollow nanoparticles based on surface and grain boundary diffusion, similar to the method proposed earlier for producing of hollow nanowhiskers [1, 2]. We produced an array of single crystal nanoparticles on sapphire employing the solid state dewetting process of a thin film deposited on ceramic substrate. A thin polycrystalline film was then deposited on the dewetted sample. During subsequent heat treatment core atoms of the core-shell nanoparticles diffused along the short-circuit diffusion paths (as demonstrated by energy filtered transmission electron microscopy analysis), leaving behind hollow nanoparticles composed mainly of the shell material. The hollowing kinetics determined by scanning electron microscopy and by a combination of focused ion beam with transmission electron microscopy are presented.
References:
1. G. Richter , Fabrication of freestanding gold nanotubes, Scripta mater. 63, 933, (2010).
2. S. Baylan, G. Richter, M. Beregovsky, D. Amram, E. Rabkin, The kinetics of hollowing of Ag-Au core-shell nanowhiskers controlled by short-circuit diffusion, Acta mater. 82, 145, (2015).

In polycrystalline metallic thin films, residual stress develops during their growth. The resulting stress varies dramatically with the processing parameters and film material. We try to understand the stress evolution by considering competing processes that happen at the triple junction where neighboring grains impinge to form new segments of grain boundary. Using wafer curvature, we measure the stress in real time during copper and nickel electrodeposition with varying growth rates and solution concentrations on uniform as well as lithographically patterned substrates. The results are compared with an analytical model that considers forces and kinetic processes that occur during grain boundary formation. The model predicts that the steady-state growth stress depends on the combined parameter D/RL, where D is effective atomic diffusivity, R is growth rate, and L is grain size or island spacing.

Understanding intergranular fracture in steels is of significant importance, especially given the great susceptibility of steels to hydrogen embrittlement. Hydrogen segregation and diffusion along different grain boundaries (GBs) typically control the susceptibility of those boundaries to fracture. Furthermore, during forming processes of polycrystals, the coincidence site lattice (CSL) of GBs typically can experience a deviation from their ideal configurations, which can subsequently alter the material&’s performance.
Here, molecular statics (MS) simulations are performed to study hydrogen diffusion and segregation along GBs in BCC iron. The effect of angular deviations from the ideal symmetry plane are investigated within the Brandon range. The pure GB energy, as well as the GB and free surface segregation energies are quantified. The current results show that the GB energy of ideal GBs is relatively high compared to the deviated GBs due to the sub-boundary superimposed dislocation networks, which are associated with the higher atomic disorder in the deviated GBs. Furthermore, the Rice-Wang model is used to assess the embrittlement impact for the range of deviation angles studied. It is shown that the ideal GB structure has the greatest resistance to embrittlement prior to GB hydrogen saturation, while the deviated GBs shows the highest susceptibility to embrittlement. Molecular dynamics (MD) simulations are then utilized to calculate hydrogen diffusivity within ideal and deviated GBs. It is shown that hydrogen diffusivity decreases significantly with increasing deviation angle from ideal GBs. In addition, the deviated GB is representing the local minimum for diffusivity results suggesting the existence of the highest atomic disorder and excessive secondary dislocation accommodation within this interface.