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The ability to experimentally synthesise ceramic materials to incorporate nanotwinned microstructures can drastically affect the underlying deformation mechanisms and mechanics through the complex ...

Virtual X-ray diffraction (XRD) and atomistic simulations are used to probe the relationship between XRD phenomena and the strain accommodation methods associated with deformation. Twinned and untwinned nanocrystalline (NC) samples loaded in tension reveal distinct XRD responses. In initially untwinned samples, peak splitting occurs precisely as dislocation mediated deformation mechanisms initiate at approximately 2.9% strain. However, initially pre-twinned samples reveal less dislocation mediated deformation and no observable XRD peak splitting. XRD responses from control sets of ideal defect structures representing bulk and unloaded NC samples are analyzed. This study shows that the peak splitting during deformation of the initially untwinned NC sample can be traced to both the high density of planar defects and the complex internal strain state present under external load.

The mechanical properties of the weld regions of a 6061-T6 resistance spot welded lap joint are determined. The change in mechanical properties resulting from RSW are linked to the changes observed in the microstructure. Processing currents and strain rates are varied to probe the effects of processing temperature at strain rates from 10−3 to 103 s−1. Results show that material strength decreases within the heat affected zone (HAZ) and fusion zone due to precipitate dispersion. Further, decreased ductility results at quasi-static strain rates from accelerated crack growth arising near voids formed during weld formation, but the short time scale at higher strain rates limits the ability for crack growth from these voids allowing the material to exhibit higher ductility. Overall, significant changes in the mechanical behavior across the weld resulting from a change in microstructure congruent with precipitate dispersion are apparent for all processing conditions.

Magnesium alloys are good candidates for a number of applications due to their high strength-to-weight ratio, but other properties like corrosion resistance, formability, and creep are still a concern. In magnesium-aluminum alloys, Mg17Al12 phase precipitates at the grain boundaries (GBs) can have important implications on the mechanical and corrosion behavior. In order to better understand the effects, atomistic segregation of aluminum to GBs must be evaluated first. This study uses atomistic simulations to quantify aluminum segregation energetics for training a machine learning model. Aluminum atoms were iteratively placed at various atomic sites near 30 different 〈 0001 〉 symmetric tilt grain boundaries (STGBs) in magnesium. The results show how aluminum segregation is affected by GB structure and the local atomic environment. The ability to compute grain boundary physical properties of interest using machine learning techniques can have broad implications for the area of grain boundary science and engineering.

Capturing twin nucleation in full-field crystal plasticity is a long-standing problem in materials science modeling. Pronounced efforts have been deployed to understand the nucleation event at the atomic level. Yet, it remains very challenging to appropriately scale up the inherently discrete mechanisms to the continuum scale. However, of significant implications for modeling of hardening and damage is not the embryonic/lamellar nucleation of an individual twin per say, but the spacing of the twins and their concomitant interactions with each other and active defects in the lattice. Thus, knowing when and where the next twin would nucleate or grow is certainly of significant consideration to crystal plasticity and would have been perhaps the primary goal of any model which would capture early site-specific nucleation. In this study, we develop an analytical model based on a micromechanical formulation, which is able to pinpoint where a second twin nucleates in an idealistic scenario in a homogeneous material. The model was probed with a three-point bending boundary value problem and predictions were compared to in-situ bending tests of an AZ31 Mg alloy. The alloy was chosen because it exhibited sharp textures which tend to develop twin patterns similar to single crystals. The analytical expression for the stress field was obtained for an ellipsoidal twin in an isotropic half-space. We show that the twin spacing depends primarily upon the height of the twin band, and the stress relaxation from twinning depends primarily upon the thickness of the twin domain. The solution compared well to experimental results.

Basal slip and dynamic recovery of dislocations have been long assumed as the main mechanisms accounting for the sharp texturing of magnesium alloys upon dynamic recrystallization at high temperature. Basal slip allows the basal plane to mitigate stress by reorienting the 〈 c 〉 -axis normal to the loading direction, while dynamic recovery allows dislocations to rearrange into subgrains while maintaining the basal plane parallel to the main loading axis. These phenomena, though pertaining to unquestionable laws of plasticity and thermodynamics, do not fully explain the preferred selection of crystal orientation during nucleation and growth of dynamically recrystallized grains. For example, during extrusion, the basal plane reorients itself in one of two directions, where either the 〈 11 2 ¯ 0 〉 axis or the 〈 10 1 ¯ 0 〉 axis is parallel to the extrusion direction. Also, only a few (grain boundary) misorientation relationships are established between the recrystallized and the parent grains. In this paper, electron backscattered diffraction (EBSD) characterization on partially recrystallized microstructures, molecular dynamic simulations, and interfacial defect theory are used to uncover the mechanisms leading to the phenomena of texture formation during dynamic recrystallization. Simulations show that grain boundary energy and mobility emerge as the key governing effects in the experimentally-observed preferred orientation selection during nucleation and growth. Due to the low crystal symmetry inherent to the hexagonal close-packed structure of magnesium, certain grain boundaries possess both low interfacial energies and high mobilities relative to other boundaries, making them prime candidates for accommodating subgrain rotation. In particular, a new experimentally-observed { 13 4 ¯ 0 } twin boundary is associated with the texture formation; simulations and theory show that the twin boundary has a highly mobile b 2 / 2 disconnection with a wide dislocation core, presumably causing the affected grains to dominate the final texture.

Small-scale shear punch testing has been applied to a butt joint created by friction stir welding of two adjoining AA2139-T8 plates. Advantages of this technique include the ability to perform a large number of independent tests on a given volume of material and the ability to measure site-specific differences and variations in local material properties. As such, combined with a simultaneous evaluation of the weld morphology, a series of 144 shear punch tests were carried out in a 12×12 grid pattern on the retreating half of the weld. The overlay of the grid pattern onto the etched surface allowed a correlation of the microstructure and mechanical properties measured across the weld at each shear punch site. Two-dimensional color enhanced property maps were generated to provide a powerful site specific visualization of the unique or distinctive microstructural features and how they correlate with the local mechanical response across the weld. One of the more insightful discoveries was the weld nugget region undergoing 2.5 times more strain-hardening than the base plate material, while simultaneously experiencing the Portevin-LeChatelier effect. Aspects of the technique and results of our experiments are described.

The origin of texture components often associated with rare-earth element (REE) additions in wrought magnesium alloys is a long-standing problem in magnesium technology. While their influence on the texture is unquestionable, it is not yet clear why certain texture components, such as $$ \langle 11\bar{2}1 \rangle ||{\text{extrusion}}\;{\text{direction}}, $$ are favored over other components typically observed in traditional magnesium alloys. The objective of this research is to identify the mechanisms accountable for these RE textures during early stages of recrystallization. Electron backscattered diffraction and transmission electron microscopy analyses reveal that REEs in zinc-containing magnesium alloys corroborate discontinuous dynamic recrystallization. REEs promote isotropic growth for all nuclei generated through the bulging mechanism. During nucleation, the effect of REEs on orientation selection was explained by the diversified activity of both $$ \langle 10\bar{1}0 \rangle $$ and [0001] Taylor axes in the same grain with a marked preference for [0001] rotations to occur first. During nuclei growth, no growth preference was observed when sufficient REEs are added in the zinc-containing magnesium alloys, instead isotropic nuclei growth across all grain orientations occurs. This phenomenon is attributed to REEs segregating to grain boundaries (GBs), in agreement with prior computational and theoretical results (Barrett et al., Scripta Mater 146:46–50, 2018) that show a more isotropic GB energy and mobility after segregation.

Symmetric and asymmetric tilt grain boundaries in Cu and Al were generated using molecular statics energy minimization in a classical molecular dynamics code with in-plane grain boundary translations and an atom deletion criterion. The following dataset (NIST repository, http://hdl.handle.net/11256/358) contains atomic coordinates for minimum energy grain boundaries in three-dimensional periodic simulation cells, facilitating their use in future simulations. This grain boundary dataset is used to show the relative transferability of grain boundary structures from one face-centered cubic system to another; in general, there is good agreement in terms of grain boundary energies (R2 > 0.99). Some potential applications and uses of this tilt grain boundary dataset in nanomechanics and materials science are discussed.

Microstructure characterization continues to play an important bridge to understanding why particular processing routes or parameters affect the properties of materials. This statement certainly holds true in the case of directionally solidified dendritic microstructures, where characterizing the primary dendrite arm spacing is vital to developing the process–structure–property relationships that can lead to the design and optimization of processing routes for defined properties. In this work, four series of simulations were used to examine the capability of a few Voronoi-based techniques to capture local microstructure statistics (primary dendrite arm spacing and coordination number) in controlled (synthetically generated) microstructures. These simulations used both cubic and hexagonal microstructures with varying degrees of disorder (noise) to study the effects of length scale, base microstructure, microstructure variability, and technique parameters on the local PDAS distribution, local coordination number distribution, bulk PDAS, and bulk coordination number. The Voronoi tesselation technique with a polygon-side-length criterion correctly characterized the known synthetic microstructures. By systematically studying the different techniques for quantifying local primary dendrite arm spacings, we have evaluated their capability to capture this important microstructure feature in different dendritic microstructures, which can be an important step for experimentally correlating with both processing and properties in single crystal nickel-based superalloys.

Atomistic simulations have shown that the grain boundary (GB) structure affects a number of physical, mechanical, thermal, and chemical properties, which can have a profound effect on macroscopic properties of polycrystalline materials. The research objective herein is to use atomistic simulations to explore the role that GB structure and the adjacent crystallographic orientations have on the directional asymmetry of an intergranular crack (i.e. cleavage behavior is favored along one direction, while ductile behavior along the other direction of the interface) for aluminum grain boundaries. Simulation results from seven 〈110〉 symmetric tilt grain boundaries (STGBs) show that the GB structure and the associated free volume directly influence the stress–strain response, crack growth rate, and crack tip plasticity mechanisms for middle-tension (M(T)) crack propagation specimens. In particular, the structural units present within the GB promote whether a dislocation or twinning-based mechanism operates at the crack tip during intergranular fracture along certain GBs (e.g., the ‘E’ structural unit promotes twinning at the crack tip in Al). Furthermore, the crystallography of the adjacent grains, and therefore the available slip planes, can significantly affect the crack growth rates in both directions of the crack – this creates a strong directional asymmetry in the crack growth rate in the Σ 11 (113) and the Σ 27 (552) STGBs. Upon comparing these results with the theoretical Rice criterion, it was found that certain GBs in this study ( Σ 9 (221), Σ 11 (332) and Σ 33 (441)) show an absence of directional asymmetry in the observed crack growth behavior, in conflict with the Rice criterion. The significance of the present research is that it provides a physical basis for the role of GB character and crystallographic orientation on intergranular crack tip deformation behavior.

The role that grain boundary (GB) structure plays on the plasticity of interfaces with preexisting cracks and on the interface crack dynamics was investigated using MD for both and aluminum STGBs. In simulations with a crack at the interface, this research shows how the maximum normal strength of the interface correlates with the respective GB energy, the GB misorientation, and the GB structural description. For instance, the normal interface strength for GBs containing D structural unit (SU) or stacking faults in the GB structural description ({\Sigma}13 (510) {\theta}=22.6{\deg} and {\Sigma}97 (940) {\theta}=47.9{\deg}) shows a noticeable decrease in interface strength, as compared to other evaluated GBs that contained favored SUs. In the case of interfaces, the presence of the E SU in the GB structure lowers the maximum normal interface strength by 35%. Further investigation of the deformation at the crack tip in GBs containing the E structure revealed that the E SU underwent atomic shuffling to accommodate intrinsic stacking faults (ISFs) along the interface, which in turn acts as a site for partial dislocation nucleation. Interestingly, regardless of GB misorientation, GB interfaces examined here containing the E structure in their structural period exhibited relatively small variation in maximum normal strength of interface. The GB volume ahead of the crack tip underwent structural rearrangement which, in turn, influenced the crack propagation mechanism. In most GBs, the crack propagation was due to alternating mechanisms of dislocation emission, followed by propagation of dislocation (blunting) and cleavage/crack advance. Moreover, the crack growth rates along the GB interface were strongly influenced by the initial free volume at the interface, i.e., faster crack growth was observed along interfaces with higher initial free volume.

porosity has typically been limited to � 40%. These relatively low porosity levels (compared to liquid state processes) constrain the applications of metal foams produced via solid state foaming. To extend the capabilities of solid state foaming, we have developed an additive means of creating porosity by intraparticle expansion. The current limitation of solid-state expansion by gas entrapment is dictated by voids formed between solid particles during consolidation. In this model, the initial gas pressure and foaming temperature determine the resulting porosity. However, if the expanding gas is not limited to just that which is trapped between particles, but is also located within particles, solid state foaming may assume a character more akin to expandable polymers which foam from the constituent pellets. This concept is a paradigm shift in terms of the solid state foaming process and the achievable levels of porosity. On the basis of this simple, powder feedstock expansion, there is universal application to powder metallurgy methods for foaming. In this work, we examine the microstructure and morphology of a Cu–Sb alloy that expands to porosities of close to 40% within individual particles, resulting in absolute porosity of 69% in sintered samples. The Cu–Sb alloy powder was formed by mechanically alloying Cu and Sb (Alfa Aesar, 99.9 and 99.5%, respectively) at cryogenic temperature(� 196°C) for 4h using a modified SPEX 8000M Mixer/Mill. The elemental powders were combined to achieve 5 at% Sb in Cu. The as-milled powders contained no appreciable porosity. Ball milling was used as a means to intimately mix the elements and refine and distribute preexisting oxides. Although oxygen exposure was controlled during milling and storage of powders, the manufacturersupplied precursors did contain appreciable oxygen content. The importance of this will be detailed later. The alloyed powder was annealed at 600°C for a period of 1h under 3% H2 (bal. Ar). During annealing, the powders underwent pore formation and expansion. When annealing was done in the absence of H2, it did not produce any expansion. Microscopic examination of the loose powders was carried out using an FEI Nova Nano Lab 600 dual beam microscope using scanning electron microscopy (SEM). Cross-sectional analysis was performed using a focused ion beam (FIB). The grain size and grain orientations were measured using focused ion beam

Al–Mn alloys with a solid-solution Mn content ranging from 0 to 3.1 at% were successfully prepared by high energy mechanical alloying at room temperature of an Al–8 at% Mn sample. The solubility level obtained is up to five times the equilibrium solubility limit of Mn in Al (from 0.62 at% Mn). In general, the observed microstructures are consistent with being a nanocomposite composed of an Al–Mn solid solution matrix with dispersed Mn particles. For alloys with solid solutions up to 3.1 at%, increasing the Mn content correlated with a decrease in the matrix grain size down to a minimum of 12 nm. High hardness values of ~4 GPa were obtained. The main strengthening mechanism of the Al–Mn alloys is attributed to the grain size reduction. Further attempts to increase the dissolved solute content resulted in the precipitation of the Al 6 Mn equilibrium intermetallic phase.

The surface mechanical attrition treatment (SMAT) process was applied to pure Cu at both cryogenic and room temperatures. The cryogenic SMAT process resulted in a 60% reduction of grain size in the polycrystalline microstructure compared to that at room temperature. The level of grain refinement is related to a transition in the dominant deformation mechanism during SMAT from a dislocation-mediated behavior at room temperature to a twinning/shear band-mediated behavior at cryogenic temperatures, which also helps to suppress thermally activated processes.

During the past two decades, twinning and slip in hexagonal close-packed structures have been extensively studied using molecular dynamics. However, the simulation methods and corresponding results have rendered different conclusions regarding the active twin modes and their mechanisms for nucleation and growth. The nucleation mechanisms for twinning in hexagonal close-packed polycrystalline materials are known to depend strongly on grain boundary orientations, but little is known of the exact mechanisms that occur. The variability in the experimental behavior of single crystals reported in early literature may result from the extreme sensitivity of twinning and slip to heterogeneities in the crystals and their complex dislocation cores. Therefore, both the boundary conditions and loading directions are likely to have profound effects on the mechanisms of deformation captured by molecular dynamics simulations. In an effort to rationalize the inconsistencies reported in literature and guide future molecular dynamics studies, we have performed a comprehensive molecular dynamics study on magnesium that encompasses effects of crystal orientation, boundary conditions, initial defects, and interatomic potentials. The general trends of the results support theories which advocate heterogeneous nucleation for both twinning and slip. In fact, the behavior of perfect crystals with periodic boundary conditions deviated substantially from previous experimental observations. Additionally, strong non-Schmid effects were identified and consistently correlated to non-Schmid stresses. Deviations from Schmid's law were strikingly reduced by introducing defects such as free surfaces and voids, but twinning was still influenced by non-Schmid stresses. Twin–twin interactions led to secondary twinning and nanovoids, which encouraged stress localization and damage.

Material strengthening and embrittlement are controlled by complex intrinsic interactions between dislocations and hydrogen-induced defect structures that strongly alter the observed deformation mechanisms in materials. In this study, we reported molecular statics simulations at zero temperature for pure α-Fe with a single H atom at an interstitial and vacancy site, and two H atoms at an interstitial and vacancy site for each of the 〈100〉, 〈110〉, and 〈111〉 symmetric tilt grain boundary (STGB) systems. Simulation results show that the grain boundary (GB) system has a smaller effect than the type of H defect configuration (interstitial H, H-vacancy, interstitial 2H, and 2H-vacancy). For example, the segregation energy of hydrogen configurations as a function of distance is comparable between symmetric tilt GB systems. However, the segregation energy of the 〈100〉 STGB system with H at an interstitial site is 23 pct of the segregation energy of 2H at a similar interstitial site. This implies that there is a large binding energy associated with two interstitial H atoms in the GB. Thus, the energy gained by this H-H reaction is ~54 pct of the segregation energy of 2H in an interstitial site, creating a large driving force for H atoms to bind to each other within the GB. Moreover, the cohesive energy values of 125 STGBs were calculated for various local H concentrations. We found that as the GB energy approaches zero, the energy gained by trapping more hydrogen atoms is negligible and the GB can fail via cleavage. These results also show that there is a strong correlation between the GB character and the trapping limit (saturation limit) for hydrogen. Finally, we developed an atomistic modeling framework to address the probabilistic nature of H segregation and the consequent embrittlement of the GB. These insights are useful for improving ductility by reengineering the GB character of polycrystalline materials to alter the segregation and embrittlement behavior in α-Fe.

Motivated by a need to characterize twinning and slip–twin interactions in hexagonal close-packed metals, we have developed a novel method that facilitates analyses of twin activities in molecular dynamics simulations. The basal plane vector method described herein calculates the basal plane orientation for each atom based on bond angles and first nearest neighbors to accurately identify crystallographic orientations. This method is able to unambiguously identify twin embryos and twin variants, calculate twin volume fractions and analyze grain evolution.

The use of acoustic emission for quantifying the microstructural damage evolution under tensile loading is studied for a 7075 aluminum alloy. First, the cracking of intermetallic particles present in the material was evaluated using interrupted tensile tests combined with digital image analysis of large optical image montages. Subsequent acoustic emission tests under tensile monotonic loading produced an in situ signature that correlated with the quantitative stereology results obtained destructively. Acoustic emission is a viable option for quantifying the evolution of microstructure damage in terms of particle cracking for the 7075 alloy.

Molecular dynamics simulations of 50 Fe grain boundaries were used to understand their interaction with vacancies and self-interstitial atoms at all atomic positions within 20 Angstroms of the boundary, which is important for designing radiation-resistant polycrystalline materials. Site-to-site variation within the boundary of both vacancy and self-interstitial formation energies is substantial, with the majority of sites having lower formation energies than in the bulk. Comparing the vacancy and self-interstitial atom binding energies for each site shows that there is an energetic driving force for interstitials to preferentially bind to grain boundary sites over vacancies. Furthermore, these results provide a valuable dataset for quantifying uncertainty bounds for various grain boundary types at the nanoscale, which can be propagated to higher scale simulations of microstructure evolution., 4 pages, 4 figures

Damage evolution in a wrought magnesium alloy under uniaxial tensile deformation is investigated. Sectioned specimens subjected to interrupted tensile deformation were examined under optical microscopy to quantify the number density of cracked intermetallic particles as a function of applied strain. Digital image analysis of the optical images was employed to quantify damage by separating cracked from non-cracked particles. Finally, an internal state variable damage model was shown to adequately capture the experimentally observed damage progression due to the intermetallics.

Serial sectioning methods continue to produce a wealth of image data for quantifying the three-dimensional nature of material microstructures. In this work, we discuss a computational methodology for automated detection and 3D characterization of dendrite cores from images taken from slices of a production turbine blade made of a heat-treated single crystal Ni-based superalloy. The dendrite core locations are detected using an automated segmentation technique that incorporates information over multiple length scales and exploits the four-fold symmetry of the dendrites when viewed down the 〈100〉 growth direction. Additional rules that take advantage of the continuity of the dendrites from slice to slice help to exclude segmentation artifacts and improve dendrite core segmentation. The significance of this technique is that it may be extended to include any symmetric features.

Grain boundary evolution in copper bicrystals is investigated during uniaxial tension at 10 K. Grain boundary structures are generated using molecular statics employing an embedded atom method potential, followed by molecular dynamics simulation at a constant 1 × 109 s−1 strain rate. Interfacial free volume is continuously measured during boundary deformation, and its evolution is investigated both prior to and during grain boundary dislocation nucleation. Free volume provides valuable insight into atomic-scale processes associated with stress-induced grain boundary deformation. Different boundary structures are investigated in this work to analyze the role of interface structure, stress state and initial free volume on dislocation nucleation. The results indicate that the free volume influences interfacial deformation through modified atomic-scale processes, and grain boundaries containing particular free volume distributions show a greater propensity for collective atomic migration during inelastic deformation.

By exploiting the (prior) knowledge that certain microstructural features should, on average, have a certain symmetry, it was possible to develop an automated technique for identifying their locations within a complex microstructure. Subsequently, this technique is applied to a single-crystal Ni-based superalloy to identify dendrite core locations by using their fourfold symmetry as viewed along the 〈1 0 0〉 growth direction. Results of such a technique show good agreement with time-intensive manual identification of dendrite core locations.

A computationally efficient microstructure characterization technique is presented that separately identifies morphological texture and any orientation dependence of second-phase clustering via a concise visual representation. This technique, the Vector Multi-Scale Analysis of Area Fractions (VMSAAF), is then applied to computer-generated microstructures to understand the effects of second-phase area fraction, aspect ratio, alignment propensity, variant orientation, and degree of microstructure banding on the homogenous length scale—a metric used to quantify clustering—as well as the extent of representative volume elements for a microstructure.

Molecular dynamics simulations are used to investigate orientation and rate dependence of partial dislocation nucleation in Cu. As the strain rate is reduced from 109 to 107 s−1, the tensile stress required for homogeneous dislocation nucleation is reduced by at most 5%. Furthermore, mild orientation sensitivity is observed in the rate dependence of the critical tensile stress. The computed resolved shear stress for partial dislocation nucleation is consistent with previous ab initio calculations of the theoretical shear strength of Cu.

Atomistic simulations are used to investigate how grain boundary structure influences dislocation nucleation under uniaxial tension and compression for a specific class of symmetric tilt grain boundaries that contain the E structural unit. After obtaining the minimum energy grain boundary structure, molecular dynamics was employed based on an embedded-atom method potential for copper at 10 K. Results show several differences in dislocation nucleation with respect to uniaxial tension and compression. First, the average nucleation stress for all 〈1 1 0〉 symmetric tilt grain boundaries is over three times greater in compression than in tension for both the high strain rate and quasistatic simulations. Second, partial dislocations nucleate from the boundary on the {1 1 1} slip plane under uniaxial tension. However, partial and full dislocations nucleate from the boundary on the {1 0 0} and {1 1 1} slip planes under uniaxial compression. The full dislocation nucleation on the {1 0 0} plane for boundaries with misorientations near the coherent twin boundary is explained through the higher resolved shear stress on the {1 0 0} plane compared to the {1 1 1} plane. Last, individual dislocation nucleation mechanisms under uniaxial tension and compression are analyzed. For the vicinal twin boundary under tension, the grain boundary partial dislocation is emitted into the lattice on the same {1 1 1} plane that it dissociated onto. For compression of the vicinal twin, the 1/3〈1 1 1 〉 disconnection is removed through full dislocation emission on the {1 0 0} plane and partial dislocation emission parallel to the coherent twin boundary plane, restoring the boundary to the coherent twin. For the Σ 19 boundary, the nearly simultaneous emission of numerous partial dislocations from the boundary result in the formation of the hexagonal close-packed (HCP) phase.

Atomistic simulations are used to investigate how the stress required for homogeneous nucleation of partial dislocations in single crystal copper under uniaxial loading changes as a function of crystallographic orientation. Molecular dynamics is employed based on an embedded-atom method potential for Cu at 10 and 300 K. Results indicate that non-Schmid parameters are important for describing the calculated dislocation nucleation behavior for single crystal orientations under tension and compression. A continuum relationship is presented that incorporates Schmid and non-Schmid terms to correlate the nucleation stress over all tensile axis orientations within the stereographic triangle. Simulations investigating the temperature dependence of homogeneous dislocation nucleation yield activation volumes of ≈ 0.5 – 2 b 3 and activation energies of ≈ 0.30 eV . For uniaxial compression, full dislocation loop nucleation is observed, in contrast to uniaxial tension. One of the main differences between uniaxial tension and compression is how the applied stress is resolved normal to the slip plane on which dislocations nucleate—in tension, this normal stress is tensile, and in compression, it is compressive. Last, the tension–compression asymmetry is examined as a function of loading axis orientation. Orientations with a high resolved stress normal to the slip plane on which dislocations nucleate have a larger tension–compression asymmetry with respect to dislocation nucleation than those orientations with a low resolved normal stress. The significance of this research is that the resolved stress normal to the slip plane on which dislocations nucleate plays an important role in partial (and full) dislocation loop nucleation in FCC Cu single crystals.

Atomistic simulations were used to investigate dislocation nucleation from Σ3 asymmetric (inclined) tilt grain boundaries under uniaxial tension applied perpendicular to the boundary. Molecular dynamics was employed based on embedded atom method potentials for Cu and Al at 10 K and 300 K. Results include the grain boundary structure and energy, along with mechanical properties and mechanisms associated with dislocation nucleation from these Σ3 boundaries. The stress and work required for dislocation nucleation were calculated along with elastic stiffness of the bicrystal configurations, exploring the change in response as a function of inclination angle. Analyses of dislocation nucleation mechanisms for asymmetric Σ3 boundaries in Cu show that dislocation nucleation is preceded by dislocation dissociation from the boundary. Then, dislocations preferentially nucleate in only one crystal on the maximum Schmid factor slip plane(s) for that crystal. However, this crystal is not simply predicted based on either the Schmid or non-Schmid factors. The synthesis of these results provides a better understanding of the dislocation nucleation process in these faceted, dissociated grain boundaries.

Atomistic simulations of dislocation nucleation from grain boundaries provide an insight into dislocation sources in nanocrystalline copper. Simulations show that dislocation sources emit single partial dislocation loops, with half absorbed into the boundary and half emitted into the lattice. The specific boundary dislocation content determines whether the absorbed half-loop annihilates pre-existing boundary dislocations or increases boundary dislocations. Atomistic studies of this type provide details of the emission sequence that enhance our understanding of dislocation sources in high angle boundaries.

Nanostructured Cu–Ta alloys show promise as high-strength materials in part due to their limited grain growth. In the present study, we elucidate the role of Ta on the transition from deformation twinning to dislocation-mediated slip mechanisms in nanocrystalline Cu through atomistic simulations and transmission electron microscopy characterization. In particular, computed generalized stacking fault energy curves show that as Ta content increases there is a shift from twinning to slip-dominated deformation mechanisms. Furthermore, heterogeneous twinnability from microstructural defects decreases with an increase in Ta. The computed effect of Ta on plasticity is consistent with the HRTEM observations. IMPACT STATEMENT We show for the first time using atomistic simulations and TEM that, similar to grain size, the Tanano-particles can be used to tailor the governing deformation mechanisms in NC-alloys.

Atomistic simulations are used to investigate how the stress required for homogeneous nucleation of partial dislocations in single crystal copper under uniaxial tension changes as a function of crystallographic orientation. Molecular dynamics is employed based on an embedded-atom method potential for Cu at 10 and 300 K. Results indicate that non-Schmid parameters are required to describe dislocation nucleation for certain single crystal orientations. Specifically, we find that the stereographic triangle can be divided into two regions: a region where dislocation nucleation is dominated by the conventional Schmid factor (the resolved shear stress in the direction of slip) and a region where dislocation nucleation is dominated by the normal factor (the resolved stress normal to the slip plane). A continuum relationship that incorporates Schmid and non-Schmid terms to correlate the stress required for dislocation nucleation over all tensile axis orientations within the stereographic triangle is presented. The significance of this work is that simulation results are cast into an atomistically inspired continuum formulation for partial dislocation loop nucleation in face-centered cubic single crystals.

Atomistic simulations were employed to investigate the structure and energy of asymmetric tilt grain boundaries in Cu and Al. In this work, we examine the Σ5 and Σ13 systems with a boundary plane rotated about the ⟨ 100 ⟩ misorientation axis, and the Σ9 and Σ11 systems rotated about the ⟨ 110 ⟩ misorientation axis. Asymmetric tilt grain boundary energies are calculated as a function of inclination angle and compared with an energy relationship based on faceting into the two symmetric tilt grain boundaries in each system. We find that asymmetric tilt boundaries with low index normals do not necessarily have lower energies than boundaries with similar inclination angles, contrary to previous studies. Further analysis of grain boundary structures provides insight into the asymmetric tilt grain boundary energy. The Σ5 and Σ13 systems in the ⟨ 100 ⟩ system agree with the aforementioned energy relationship; structures confirm that these asymmetric boundaries facet into the symmetric tilt boundaries. The Σ9 and ...

The objective of this research is to use atomistic simulations to investigate the energy and structure of symmetric and asymmetric Σ3 ⟨110⟩ tilt grain boundaries. A nonlinear conjugate gradient algorithm was employed along with an embedded atom method potential for Cu and Al to generate the equilibrium 0 K grain boundary structures. A total of 25 ⟨110⟩ grain boundary structures were explored to identify the various equilibrium and metastable structures. Simulation results show that the Σ3 asymmetric tilt grain boundaries in the ⟨110⟩ system are composed of only structural units of the two Σ3 symmetric tilt grain boundaries. The energies for the Σ3 grain boundaries are similar to previous experimental and calculated grain boundary energies. A structural unit and faceting model for Σ3 asymmetric tilt grain boundaries fits all of the calculated asymmetric grain boundary structures. The significance of these results is that the structural unit and facet description of all Σ3 asymmetric tilt grain boundaries m...

Structural, elastic and thermal properties of cementite (Fe$_3$C) were studied using a Modified Embedded Atom Method (MEAM) potential for iron-carbon (Fe-C) alloys. Previously developed Fe and C single element potentials were used to develop an Fe-C alloy MEAM potential, using a statistically-based optimization scheme to reproduce structural and elastic properties of cementite, the interstitial energies of C in bcc Fe as well as heat of formation of Fe-C alloys in L$_{12}$ and B$_1$ structures. The stability of cementite was investigated by molecular dynamics simulations at high temperatures. The nine single crystal elastic constants for cementite were obtained by computing total energies for strained cells. Polycrystalline elastic moduli for cementite were calculated from the single crystal elastic constants of cementite. The formation energies of (001), (010), and (100) surfaces of cementite were also calculated. The melting temperature and the variation of specific heat and volume with respect to temperature were investigated by performing a two-phase (solid/liquid) molecular dynamics simulation of cementite. The predictions of the potential are in good agreement with first-principles calculations and experiments., Comment: 12 pages, 9 figures

Characterizing the spacing of primary dendrite arms in directionally-solidified microstructures is an important step for developing process-structure-property relationships by enabling the quantification of (i) the influence of processing on microstructure and (ii) the influence of microstructure on properties. In this work, we utilized a new Voronoi-based approach for spatial point pattern analysis that was applied to an experimental dendritic microstructure. This technique utilizes a Voronoi tessellation of space surrounding the dendrite cores to determine nearest neighbors and the local primary dendrite arm spacing. In addition, we compared this technique to a recent distance-based technique and a modification to this using Voronoi tesselations. Moreover, a convex hull-based technique was used to include edge effects for such techniques, which can be important for thin specimens. These methods were used to quantify the distribution of local primary dendrite arm spacings, their spatial distribution, and their correlation with interdendritic eutectic particles for an experimental directionally-solidified Ni-based superalloy micrograph. This can be an important step for correlating with both processing and properties in directionally-solidified dendritic microstructures.

Segregation of impurities to grain boundaries (GBs) plays an important role in both the stability and macroscopic behavior of polycrystalline materials. The research objective in this work is to better characterize the energetics and length scales involved with the process of solute and impurity segregation to GBs. Molecular statics simulations are used to calculate the segregation energies for carbon within multiple substitutional and interstitial GB sites over a database of 125 symmetric tilt GBs in Fe. The simulation results show that there are two energetically favorable GB segregation processes: (1) an octahedral C atom in the lattice segregating to an interstitial GB site and (2) an octahedral C atom and a vacancy in the lattice segregating to a grain boundary substitutional site. In both cases, lower segregation energies than appear in the bulk lattice were calculated. Moreover, based on segregation energies approaching bulk values, the length scale of interaction is larger for interstitial C than for substitutional C in the GB (?5?? compared to ?3?? from center of the GB). A subsequent data reduction and statistical representation of this dataset provides critical information about the mean segregation energy and the associated energy distributions for carbon atoms as a function of distance from the grain boundary, which quantitatively informs higher scale models with energetics and length scales necessary for capturing the segregation behavior of alloying elements and impurities in Fe. The significance of this research is the development of a methodology capable of ascertaining segregation energies over a wide range of GB character (typical of that observed in polycrystalline materials), which herein has been applied to carbon segregation to substitutional and interstitial sites in a specific class of GBs in ?-Fe.

The energetics and length scales associated with the interaction between point defects (vacancies and self-interstitial atoms) and grain boundaries in bcc Fe was explored. Molecular statics simulations were used to generate a grain boundary structure database that contained $\ensuremath{\approx}$170 grain boundaries with varying tilt and twist character. Then, vacancy and self-interstitial atom formation energies were calculated at all potential grain boundary sites within 15 \AA{} of the boundary. The present results provide detailed information about the interaction energies of vacancies and self-interstitial atoms with symmetric tilt grain boundaries in iron and the length scales involved with absorption of these point defects by grain boundaries. Both low- and high-angle grain boundaries were effective sinks for point defects, with a few low-$\ensuremath{\Sigma}$ grain boundaries (e.g., the $\ensuremath{\Sigma}3$${112}$ twin boundary) that have properties different from the rest. The formation energies depend on both the local atomic structure and the distance from the boundary center. Additionally, the effect of grain boundary energy, disorientation angle, and $\ensuremath{\Sigma}$ designation on the boundary sink strength was explored; the strongest correlation occurred between the grain boundary energy and the mean point defect formation energies. Based on point defect binding energies, interstitials have $\ensuremath{\approx}$80$%$ more grain boundary sites per area and $\ensuremath{\approx}$300$%$ greater site strength than vacancies. Last, the absorption length scale of point defects by grain boundaries is over a full lattice unit larger for interstitials than for vacancies (mean of 6--7 \AA{} versus 10--11 \AA{} for vacancies and interstitials, respectively).

Experimental plasticity on single crystals has found substantial non-Schmid effects in both twinning and non-basal slip in pure magnesium. The deviation from Schmid’s law has been attributed to the strong sensitivity of both twinning and slip to small lattice heterogeneities [1] and the effect of pre-slip and non-planar dislocation dissociation [2]. However, most molecular dynamics simulations use heterogeneities so the effect of slip on twin nucleation and vice-versa has been shrouded. This has motivated us to investigate the influence of crystal loading orientation on homogeneous slip and twin nucleation using molecular dynamics. These simulations allowed us to appreciate the propensity and nature of twin nucleation when pre-existing defects are absent. Analyses of deformation mechanisms and stress-strain responses shows that homogeneous dislocation nucleation on the basal slip system is correlated with the highest Schmid resolved shear stress, while homogeneous nucleation of tensile twins did not always correlate with the highest Schmid resolved shear stress.

A computational characterization technique is presented for assessing the spatial heterogeneity of two reactant phases in a three-phase chemically reactive composite. This technique estimates the reaction yield on multiple microstructure length scales based on the segregation of the two reactant phases and the expected reaction stoichiometry. The result of this technique is a metric, quantifying the effectiveness of phase mixing in a particular microstructure as a function of length scale. Assuming that the proportionate mixing of reactant phases on multiple length scales will enhance reaction kinetics and the overall level of reaction completion, this tool can subsequently be used as a figure-of-merit for optimizing microstructure via appropriate processing. To illustrate this point, an example is shown where a bimodal three-phase microstructure has a higher reaction yield at every length scale when compared with a monomodal three-phase microstructure with the same constituent loading.

Digital image correlation of laser-ablated platinum nanoparticles on the surface of a polycrystalline metal (nickel-based superalloy Rene 88DT) was used to obtain the local strain behavior from an in situ scanning electron microscope tensile experiment at room temperature. By fusing this information with crystallographic orientations from EBSD, a subsequent analysis shows that the average maximum shear strain tends to increase with increasing Schmid factor. Additionally, the range of the extreme values for the maximum shear strain also increases closer to the grain boundary, signifying that grain boundaries and triple junctions accumulate plasticity at strains just beyond yield in polycrystalline Rene 88DT. In situ experiments illuminating microstructure-property relationships of this ilk may be important for understanding damage nucleation in polycrystalline metals at high temperatures., Comment: 16 pages, 5 figures

Molecular dynamics simulations are frequently used to study deformation mechanisms at the nanoscale. However, the lattice orientations of grains in the starting nanocrystalline (nc) configurations are typically based on either random orientations (Van Swygenhoven and Derlet [1]; Schiotz et al. [2]) or orientations specifically chosen to be high angle grain boundaries (Yamakov et al. [3]), which may not be either global or local minimum energy configurations. Our hypothesis is that global or local minimum energy configurations are more representative of actual nc grain structures.

Molecular dynamics (MD) and density functional theory (DFT) studies were performed to investigate the influence of vacancy defects on generalized stacking fault (GSF) energy of fcc metals. MEAM and EAM potentials were used for MD simulations, and DFT calculations were performed to test the accuracy of different common parameter sets for MEAM and EAM potentials in predicting GSF with different fractions of vacancy defects. Vacancy defects were placed at the stacking fault plane or at nearby atomic layers. The effect of vacancy defects at the stacking fault plane and the plane directly underneath of it was dominant compared to the effect of vacancies at other adjacent planes. The effects of vacancy fraction, the distance between vacancies, and lateral relaxation of atoms on the GSF curves with vacancy defects were investigated. A very similar variation of normalized SFEs with respect to vacancy fractions were observed for Ni and Cu. MEAM potentials qualitatively captured the effect of vacancies on GSF.

Serial sectioning methods continue to produce an abundant amount of image data for quantifying the three-dimensional nature of material microstructures. Here, we discuss a methodology to automate detecting and characterizing eutectic particles taken from serial images of a production turbine blade made of a heat-treated single crystal Ni-based superalloy (PWA 1484). This method includes two important steps for unassisted eutectic particle characterization: automatically identifying a seed point within each particle and segmenting the particle using a region growing algorithm with an automated stop point. Once detected, the segmented eutectic particles are used to calculate microstructural statistics for characterizing and reconstructing statistically representative synthetic microstructures for single crystal Ni-based superalloys. The significance of this work is its ability to automate characterization for analysing the 3D nature of eutectic particles.

Computer-generated 2D microstructures of varying second phase area fraction (5–30%), aspect ratio (1–16) and degree of alignment (where the reinforcement major-axis orientation is random, perfectly aligned or semi-aligned) are analyzed via the isotropic and directional forms of the computationally efficient multi-scale analysis of area fractions (MSAAF) technique. The impact of these microstructure parameters on the representative volume element (RVE) necessary to characterize a microstructure is ascertained with variations in isotropic and directional homogeneous length scales, derivative quantities of the MSAAF technique. Analysis of these results produces empirical expressions for the directional homogeneous length scale as a function of area fraction and aspect ratio for the limiting cases of random and 'perfect' second phase alignment. Generally, particle alignment is observed to increase the aspect ratio of a microstructure's RVE—a trend amplified by higher reinforcement aspect ratios and lower area fractions. Particle alignment also decreases the absolute size of such an element by reducing the directional homogeneous length scales transverse to the axis of alignment. Periodic boundary conditions on the perimeter of the synthetic microstructures are used to characterize the error in the MSAAF technique via multiple instantiations of the same microstructure, which further indicates that the statistical variation in the directional homogeneous length scale (measured by the directional MSAAF technique) can be an order of magnitude less than the variation in the isotropic homogeneous length scale (measured by the isotropic MSAAF technique).