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Hybrid molecular dynamics and Monte-Carlo simulations on the deformation behavior of nanocrystalline Pd-Au are presented. A semi-grandcanonical Monte-Carlo scheme is employed during straining to allow for local relaxation by chemical equilibration and to effectively shortcut diffusional processes. Altering the balance between an imposed straining and local relaxation reveals a strong correlation of the irreversible plastic deformation and the frequency of local relaxation. Using a novel method to quantify the amount of crystal slip strain from atomistic data, it is demonstrated how plastic deformation carried by dislocations changes as a function of the local relaxation. The results indicate that conventional molecular dynamics simulations overestimate the contributions of dislocation slip to the overall plastic deformation of nanocrystalline samples. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations are performed to investigate the impact of a coherent Σ3 (111) twin boundary on the plastic deformation behavior of Cu nanopillars. Our work reveals that the mechanical response of pillars with and without the twin boundary is decisively driven by the characteristics of initial dislocation sources. In the condition of comparably large pillar size and abundant initial mobile dislocations, overall yield and flow stresses are controlled by the longest, available mobile dislocation. An inverse correlation of the yield and flow stresses with the length of the longest dislocation is established and compared to experimental data. The experimentally reported subtle differences in yield and flow stresses between pillars with and without the twin boundary are likely related to the maximum lengths of the mobile dislocations. In the condition of comparably small pillar size, for which a reduction of mobile dislocations during heat treatment and mechanical loading occurs, the mechanical response of pillars with and without the twin boundary can be clearly distinguished. Dislocation starvation during deformation is more pronounced in pillars without the twin boundary than in pillars with the twin boundary because the twin boundary acts as a pinning surface for the dislocation network. Image, graphical abstract [ABSTRACT FROM AUTHOR]

We study the effect of solute segregation on pyramidal to basal transformation of 〈 c + a 〉 edge dislocations in a model Mg-Y alloy, using a hybrid Monte Carlo/Molecular Dynamics method. While a random solute distribution has no appreciable effect on this undesirable transition, a solute cloud around the dislocation slows and effectively prevents it in case of Mg-3 at % Y alloy. Comparing with the time scale required for diffusion of Y to the dislocation, suggests that segregation of Y and other fast diffusing elements can be engineered to prohibit this glissile to sessile transition, and stabilize the pyramidal glide of 〈 c + a 〉 dislocations. [ABSTRACT FROM AUTHOR]

We employ atomistic computer simulations to study the structure and migration behavior of a Σ11 symmetrical tilt grain boundary in a 4-component model FCC high-entropy alloy (HEA) (CuNiCoFe). The results are compared to grain boundaries in elemental metals and a so-called 'average-atom' sample. We find that the repeating structural units characterizing the static grain boundary structure show the same repeating structural units for all samples, while the high temperature equilibrium grain boundary structure is most strongly influenced by presence of stacking faults. Under an applied synthetic driving force, this GB migrates by a mechanism assisted by partial dislocations in all materials. For this reason the grain boundary mobilities and stacking fault energies are directly related. Moreover, the HEA sample and the average-atom sample show almost identical mobilities suggesting that local chemical fluctuations play a minor role. Solute segregation to the GB in the HEA suppresses GB migration up to very high temperatures and might be the main cause for reduced grain growth in FCC HEAs. [ABSTRACT FROM AUTHOR]

We introduce Interfacial Line Defect Analysis (ILDA), a method for identifying and extracting interfacial dislocations and disconnections with little-to-no input from the user on the nature of the interface. By simply providing the orientations and coherency strains for the crystals in a coherent reference state of the interface, ILDA provides exact Burgers vectors. Alternatively, these orientations and strains can be estimated using local atomic deformation gradients, making ILDA fully automated and providing estimated Burgers vectors. ILDA also determines the step height associated with each defect line segment, in case the associated defect is a disconnection. The heart of the method is the identification of atoms residing at co-incidence sites between the two crystals and the construction of a surface mesh connecting these sites that is used to compose Burgers circuits and insert defect line segments. We demonstrate the performance of ILDA in two test cases: a twist grain boundary and a phase boundary. [Display omitted] [ABSTRACT FROM AUTHOR]

In the present work, the dislocation structure evolution around and underneath the spherical indentations in (001) oriented single crystalline strontium titanate (STO) was revealed by using an etch-pit technique and molecular dynamics (MD) simulations. The 3D defect structure at various length scales and subsurface depths was resolved with the help of a sequential polishing, etching, and imaging technique. This analysis, combined with load-displacement data, shows that the incipient plasticity (manifested as sudden indenter displacement bursts) is strongly influenced by preexisting dislocations. In the early stage of plastic deformation, the dislocation pile-ups are all aligned in 〈100〉 directions, lying on {110}45 planes, inclined at 45° to the (001) surface. At higher mean contact pressure and larger indentation depth, however, dislocation pile-ups along 〈110〉 directions appear, lying on {110}90 planes, perpendicular to the (100) surface. MD simulations confirm the glide plane nature and provide further insights into the dislocation formation mechanisms by tracing the evolution of the complete dislocation line network as function of indentation depth. [ABSTRACT FROM AUTHOR]

Nanoindentation into single-crystalline Si is modeled by molecular dynamics simulation using a modified Tersoff potential. We observe that the high stress produced during indentation leads to three processes occurring consecutively in the substrate: (i) phase transformation of the original cubic diamond (cd) to the bct5 phase; (ii) generation of dislocations; and (iii) amorphization. The bct5 phase develops along {1 1 1} planes of the cd phase; when these meet, the enclosed volume of cd phase transforms to bct5. The particular role played by a stable tetrahedral structure formed by bct5 {1 1 1} planes and {1 1 1} intrinsic stacking faults in the cd structure is highlighted. The phase transformation to bct5 is partially reversed when dislocations nucleate in the cd phase and locally relieve stresses. The generation and reactions of the uncommon dislocations 1 4 〈 1 1 1 〉 and 1 3 〈 1 1 2 〉 are discussed. [ABSTRACT FROM AUTHOR]

We present a study of the elastic and plastic behavior of nanoporous gold in compression, focusing on molecular dynamics simulation and inspecting experimental data for verification. Both approaches agree on an anomalously high elastic compliance in the early stages of deformation, along with a quasi immediate onset of plastic yielding even at the smallest load. Already before the first loading, the material undergoes spontaneous plastic deformation under the action of the capillary forces, requiring no external load. Plastic deformation under compressive load is accompanied by dislocation storage and dislocation interaction, along with strong strain hardening. Dislocation-starvation scenarios are not supported by our results. The stiffness increases during deformation, but never approaches the prediction by the relevant Gibson–Ashby scaling law. Microstructural disorder affects the plastic deformation behavior and surface excess elasticity might modify elastic response, yet we relate the anomalous compliance and the immediate yield onset to an atomistic origin: the large surface-induced prestress induces elastic shear that brings some regions in the material close to the shear instability of the generalized stacking fault energy curve. These regions are elastically highly compliant and plastically weak. [ABSTRACT FROM AUTHOR]

Thermally-activated 1 / 2 〈 111 〉 screw dislocation motion is the controlling plastic mechanism at low temperatures in body-centered cubic (bcc) crystals. Dislocation motion proceeds by nucleation and propagation of atomic-sized kink pairs in close-packed planes. The atomistic character of kink pairs can be studied using techniques such as molecular dynamics (MD). However, MD’s natural inability to properly sample thermally-activated processes as well as to capture { 110 } screw dislocation glide calls for the development of other methods capable of overcoming these limitations. Here we develop a kinetic Monte Carlo (kMC) approach to study single screw dislocation dynamics from room temperature to 0.5 T m and at stresses 0 < σ < 0.9 σ P , where T m and σ P are the melting point and the Peierls stress. The method is entirely parameterized with atomistic simulations using an embedded atom potential for tungsten. To increase the physical fidelity of our simulations, we calculate the deviations from Schmid’s law prescribed by the interatomic potential used and we study single dislocation kinetics using both projections. We calculate dislocation velocities as a function of stress, temperature, and dislocation line length. We find that considering non-Schmid effects has a strong influence on both the magnitude of the velocities and the trajectories followed by the dislocation. We finish by condensing all the calculated data into effective stress and temperature dependent mobilities to be used in more homogenized numerical methods. [ABSTRACT FROM AUTHOR]

3C-SiC (the only polytype of SiC that resides in a diamond cubic lattice structure) is a relatively new material that exhibits most of the desirable engineering properties required for advanced electronic applications. The anisotropy exhibited by 3C-SiC during its nanometric cutting is significant, and the potential for its exploitation has yet to be fully investigated. This paper aims to understand the influence of crystal anisotropy of 3C-SiC on its cutting behaviour. A molecular dynamics simulation model was developed to simulate the nanometric cutting of single-crystal 3C-SiC in nine (9) distinct combinations of crystal orientations and cutting directions, i.e. (1 1 1) <-1 1 0>, (1 1 1) <-2 1 1>, (1 1 0) <-1 1 0>, (1 1 0) <0 0 1>, (1 1 0) <1 1-2>, (0 0 1) <-1 1 0>, (0 0 1) <1 0 0>, (1 1-2) <1-1 0> and (1-2 0) <2 1 0>. In order to ensure the reliability of the simulation results, two separate simulation trials were carried out with different machining parameters. In the first trial, a cutting tool rake angle of -25°, d/r (uncut chip thickness/cutting edge radius) ratio of 0.57 and cutting velocity of 10m s-1 were used whereas a second trial was done using a cutting tool rake angle of -30°, d/r ratio of 1 and cutting velocity of 4m s-1 . Both the trials showed similar anisotropic variation. variation of up to 45%, while the resultant cutting forces showed a variation of 37%. This suggests that 3C-SiC is highly anisotropic in its ease of deformation. These results corroborate with the experimentally observed anisotropic variation of 43.6% in Young's modulus of 3C-SiC. The recently developed dislocation extraction algorithm (DXA) [1, 2] was employed to detect the nucleation of dislocations in the MD simulations of varying cutting orientations and cutting directions. Based on the overall analysis, it was found that 3C-SiC offers ease of deformation on either (1 1 1) <-1 1 0>, (1 1 0) <0 0 1>,or (1 0 0) <1 0 0> setups. [ABSTRACT FROM AUTHOR]

[Display omitted] The influence of surface stresses on the deformation behavior of nanoporous metals is considered to be the main reason for the tension-compression asymmetry observed in atomistic simulation studies of nanoporous metals. While it is difficult to differentiate between the contributions of linear and non-linear elasticity, dislocation activity, topology and surface stresses from atomistic simulations, continuum mechanics allow to disentangle these effects by systematically varying the applied constitutive relations. In this study, we investigate the elastic response of a nanoporous Au and Ag structure, both by molecular statics simulations and an elastic continuum model, which includes surface stresses and elastic anisotropy. The 3-dimensional periodic atomistic nanoporous structure is triangulated and transferred to the continuum model, where periodicity, elastic anisotropy, and surface stresses are considered. The results show that surface stress acts as a mechanical pre-load and that the elastic response is sensitive to the loading direction. An asymmetric behavior is observed in the elastic regime. Moreover, we show that sites, where dislocation activity is starting, show high von Mises stresses. [ABSTRACT FROM AUTHOR]

Abstract: We present quasicontinuum simulations of three-dimensional (3D) edge cracks in single crystal bcc-Fe. Crystals with different thicknesses and crystallographic orientations have been investigated and loaded in modes I, II and III. We study incipient plasticity at the crack tip and the onset of crack propagation, and calculate the critical stress intensity factors for crack propagation and dislocation emission. The active slip systems and the character of attendant dislocations have been identified and compared with the standard 2D behavior. We find that the observed type of propagation depends on the loading mode and crystallographic orientation. Mechanisms comprise twinning, phase transformation, void formation and dislocation emission. [Copyright &y& Elsevier]

Abstract: Plastic deformation of nanocrystalline Pd–Au is studied by means of atomic-scale computer simulations. The distribution of solutes is equilibrated in model structures of different grain sizes and compositions using a combination of Monte Carlo and molecular dynamics methods. The resulting samples are deformed under uniaxial load. The role of grain boundary solute segregation is analyzed in detail by comparing chemically and structurally relaxed samples with model structures that were only structurally relaxed or reloaded. By analyzing dislocation activity, site occupancy, atomic free volume in the grain boundaries, stacking and twin fault densities, we make a connection between composition-dependent properties of the miscible alloy and the observed stress–strain behavior. [Copyright &y& Elsevier]

Atomic simulations are widely used to study the mechanics of small contacts for many contact loading processes such as nanometric cutting, nanoindentation, polishing, grinding and nanoimpact. A common assumption in most such studies is the idealisation of the impacting material (indenter or tool) as a perfectly rigid body. In this study, we explore this idealisation and show that active chemical interactions between two contacting asperities lead to significant deviations of atomic scale contact mechanics from predictions by classical continuum mechanics. We performed a testbed study by simulating velocity-controlled, fixed displacement nanoindentation on single crystal tungsten using five types of indenter (i) a rigid diamond indenter (DI) with full interactions, (ii) a rigid indenter comprising of the atoms of the same material as that of the substrate i.e. tungsten atoms (TI), (iii) a rigid diamond indenter with pairwise attraction turned off, (iv) a deformable diamond indenter and (v) an imaginary, ideally smooth, spherical, rigid and purely repulsive indenter (RI). Corroborating the published experimental data, the simulation results provide a useful guideline for selecting the right kind of indenter for atomic scale simulations. [ABSTRACT FROM AUTHOR]

We use molecular dynamics simulation to study the mechanisms of plasticity during cutting of monocrystalline and polycrystalline silicon. Three scenarios are considered: (i) cutting a single crystal silicon workpiece with a single crystal diamond tool, (ii) cutting a polysilicon workpiece with a single crystal diamond tool, and (iii) cutting a single crystal silicon workpiece with a polycrystalline diamond tool. A long-range analytical bond order potential is used in the simulations, providing a more accurate picture of the atomic-scale mechanisms of brittle fracture, ductile plasticity, and structural changes in silicon. The MD simulation results show a unique phenomenon of brittle cracking typically inclined at an angle of 45°–55° to the cut surface, leading to the formation of periodic arrays of nanogrooves in monocrystalline silicon, which is a new insight into previously published results. Furthermore, during cutting, silicon is found to undergo solid-state directional amorphisation without prior Si–I to Si-II (beta tin) transformation, which is in direct contrast to many previously published MD studies on this topic. Our simulations also predict that the propensity for amorphisation is significantly higher in single crystal silicon than in polysilicon, signifying that grain boundaries eases the material removal process. [ABSTRACT FROM AUTHOR]

The article presents information on the requirement of the Monte Carlo algorithm for the purpose of studying the atomistic simulations of the precipitation in the alloys. The transmutation of the simulations of the multicomponent systems is allowed by the semi-grand-canonical ensemble. Information on the study of the precipitation of copper in the nanocrystalline iron through the algorithm is also presented.

Abstract: The optimal structure and element distribution of Si x Ge1−x clusters was investigated in terms of free energy. The methods employed were computational simulations based on classical molecular dynamics. Clusters obtained in our previous work were further simulated through annealing at different temperatures. In addition, a combination of molecular dynamics and a semi-grand-canonical Monte Carlo algorithm was used to find a free-energetically favorable element configuration for the clusters. The results show that annealing at conventional temperatures improves the clusters’ sphericity only slightly, and they remain much more amorphous than clusters cut out from crystalline bulk; only at extreme annealing temperatures are the sphericity and crystallinity notably improved. Furthermore, Ge atoms are found to segregate to the surface of the clusters, which greatly reduces the free energy of the clusters. [ABSTRACT FROM AUTHOR]

The dynamic evolution and interaction of defects under the conditions of shock loading in single crystal and nanocrystalline Cu are investigated using a series of large-scale molecular dynamics simulations for an impact velocity of 1 km/s. Four stages of defect evolution are identified during shock simulations that result in deformation and failure. These stages correspond to: the initial shock compression (I); the propagation of the compression wave (II); the propagation and interaction of the reflected tensile wave (III); and the nucleation, growth, and coalescence of voids (IV). The effect of the microstructure on the evolution of defect densities during these four stages is characterized and quantified for single crystal Cu as well as nanocrystalline Cu with an average grain size of 6 nm, 10 nm, 13 nm, 16 nm, 20 nm, and 30 nm. The evolution of twin densities during the shock propagation is observed to vary with the grain size of the system and affects the spall strength of the metal. The grain sizes of 6 nm and 16 nm are observed to have peak values for the twin densities and a spall strength that is comparable with the single crystal Cu. [ABSTRACT FROM AUTHOR]

The dynamic evolution and interaction of defects under the conditions of shock loading in single crystal and nanocrystalline Cu are investigated using a series of large-scale molecular dynamics simulations for an impact velocity of 1 km/s. Four stages of defect evolution are identified during shock simulations that result in deformation and failure. These stages correspond to: the initial shock compression (I); the propagation of the compression wave (II); the propagation and interaction of the reflected tensile wave (III); and the nucleation, growth, and coalescence of voids (IV). The effect of the microstructure on the evolution of defect densities during these four stages is characterized and quantified for single crystal Cu as well as nanocrystalline Cu with an average grain size of 6 nm, 10 nm, 13 nm, 16 nm, 20 nm, and 30 nm. The evolution of twin densities during the shock propagation is observed to vary with the grain size of the system and affects the spall strength of the metal. The grain sizes of 6 nm and 16 nm are observed to have peak values for the twin densities and a spall strength that is comparable with the single crystal Cu. [ABSTRACT FROM AUTHOR]

Tracer diffusion of all constituting elements is studied at various temperatures in a series of (CoCrFeMn) 100 − x Ni x alloys with compositions ranging from pure Ni to the equiatomic CoCrFeMnNi high-entropy alloy. At a given homologous temperature, the measured tracer diffusion coefficients change non-monotonically along the transition from pure Ni to the concentrated alloys and finally to the equiatomic CoCrFeMnNi alloy. This is explained by atomistic Monte-Carlo simulations based on a modified embedded-atom potentials, which reveal that local heterogeneities of the atomic configurations around a vacancy cause correlation effects and induce significant deviations from predictions of the random alloy model. [ABSTRACT FROM AUTHOR]

In this work we present molecular dynamics simulations on the creep behavior of Cu 64 Zr 36 metallic glass composites. Our results reveal that all composites exhibit much higher creep rates than the homogeneous glass. This is because the glass–crystal interface acts like a weak interphase, where the activation of shear transformation zones is easier than in the surrounding glass. We observe that the creep behavior of the composites does not only depend on the interface area but also on the orientation of the interface with respect to the loading axis. We propose an explanation in terms of different mean Schmid factors of the interfaces, with the amorphous interface regions acting as preferential slip sites. [ABSTRACT FROM AUTHOR]

Nanoglasses consist of glassy grains connected by an amorphous interface. While internal interfaces in nanoglasses help to prevent brittle failure, they are usually not beneficial to the glasses overall strength. In this molecular dynamics study, we manipulate the glass–glass interfaces of a Cu–Zr nanoglass, such that they are replaced by stronger crystalline interphases. Analogous to grain boundary strengthening in crystalline materials, we show that it is possible to reinforce the nanoglass without compromising its ductility. [ABSTRACT FROM AUTHOR]

Highlights: [•] Molecular dynamics simulation of thermal spray of copper was performed. [•] A parameter “flattening aspect ratio” is proposed to characterize the process. [•] Reynolds number increases only up to a certain critical impact velocity. [•] Impact velocity dominates impact temperature to influence recrystallization. [ABSTRACT FROM AUTHOR]