Atomistic Simulations of Basal Dislocations Interacting with Mg17_{17}Al12_{12} Precipitates in Mg

The mechanical properties of Mg-Al alloys are greatly influenced by the complex intermetallic phase Mg$_{17}$Al$_{12}$, which is the most dominant precipitate found in this alloy system. The interaction of basal edge and 30$^\text{o}$ dislocations with Mg$_{17}$Al$_{12}$ precipitates is studied by molecular dynamics and statics simulations, varying the inter-precipitate spacing ($L$), and size ($D$), shape and orientation of the precipitates. The critical resolved shear stress $\tau_c$ to pass an array of precipitates follows the usual $\ln((1/D + 1/L)^{-1})$ proportionality. In all cases but the smallest precipitate, the dislocations pass the obstacles by depositing dislocation segments in the disordered interphase boundary rather than shearing the precipitate or leaving Orowan loops in the matrix around the precipitate. An absorbed dislocation increases the stress necessary for a second dislocation to pass the precipitate also by absorbing dislocation segments into the boundary. Replacing the precipitate with a void of identical size and shape decreases the critical passing stress and work hardening contribution while an artificially impenetrable Mg$_{17}$Al$_{12}$ precipitate increases both. These insights will help improve mesoscale models of hardening by incoherent particles.Comment: 13 pages with 9 figures and 2 tables. Supplementary materia

Systematic Atomic Structure Datasets for Machine Learning Potentials: Application to Defects in Magnesium

We present a physically motivated strategy for the construction of training sets for transferable machine learning interatomic potentials. It is based on a systematic exploration of all possible space groups in random crystal structures, together with deformations of cell shape, size, and atomic positions. The resulting potentials turn out to be unbiased and generically applicable to studies of bulk defects without including any defect structures in the training set or employing any additional Active Learning. Using this approach we construct transferable potentials for pure Magnesium that reproduce the properties of hexagonal closed packed (hcp) and body centered cubic (bcc) polymorphs very well. In the process we investigate how different types of training structures impact the properties and the predictive power of the resulting potential

The influence of pre-deformation on the fracture toughness of chromium, studied by microcantilever bending

Cr is bcc metals, which has a high melting point and high strength. However, its fracture toughness at room temperature is low. This is due to their rather high ductile to brittle transition temperature. At room temperature the fracture toughness is limited by dislocation mobility or by the inability to activate nucleation sources. While this behavior is well characterized for W, there are only few studies for Cr. Please click Additional Files below to see the full abstract

Deformation mechanisms of twinned nanoparticles and nanowires

The plastic deformation of nanoscale metallic specimens has recently attracted a lot of interest due to the reported changes of deformation mechanisms with reduced size. Similarly, the interaction of dislocations with twin boundaries has received lots of attention in the context of the ultrahigh strength and ductility of nanotwinned metals. Here, we present experiments and atomistic simulations of compression test on twinned gold nanoparticles to study dislocation processes and -storage in nanosized volumes and dislocation-twin interaction mechanisms and compare them with the deformation behavior of twinned silver and gold nanowires. Compression experiments were performed on triangular shaped, facetted particles using a nanoindenter with a flat punch tip. During compression along the [111] direction, all particles assume a characteristic asymmetric “mushroom” shape, which has not been reported in the case of uniaxially compressed single crystalline Au nanoparticles. Post-mortem TEM-analysis in cross-sectional and plan-view geometry reveal the storage of full dislocations. Dislocations were also observed on the (111) plane parallel to the twin plane, which should not experience any resolved shear stress during compression. Molecular Dynamics simulations of Au nanoparticles of same shapes as in the experiments were performed using different types of indenters, boundary conditions, strain rates and potentials. The processes of dislocation nucleation, interaction with the twin boundary, dislocation-dislocation reactions, cross-slip and dislocation escape through the free surfaces are studied in detailed and analyzed in terms of the stress state. Comparison with the experimental microstructure of the compressed particles allows to draw conclusions about the dominating dislocation processes during the deformation of the twinned nanoparticles. In particular, the presence of dislocations on the (111) planes provides indirect evidence for transmission of dislocations through the twin boundary onto {100}-type planes. The dislocation – twin interaction mechanisms are compared to single and multitwinned gold and silver nanowires. The results highlight the importance of boundary conditions and internal interfaces on the nucleation, escape, storage and interactions of dislocations in nano-objects

Wedge-shaped twins and pseudoelasticity in fcc metallic nanowires under bending

AbstractMolecular dynamics simulations were performed to study the deformation mechanisms of 〈110〉-oriented, faceted Cu and Au nanowires under bending along three different crystallographic directions. Independent of the bending direction, the stress field is characterized by a highly nonlinear elastic response, leading to a shift of the neutral fiber away from the central wire axis. The nanowires show ultra-high yield strengths, and the achievable large elastic strains directly influence the dislocation nucleation through the change of the unstable stacking fault energy. In agreement with theory and experiments on face-centered cubic 〈110〉 nanowires under uniaxial load, the tensile part of the wires exhibit deformation twinning, while plastic deformation in the compressed part takes place by slip of perfect dislocations. Independent of the bending direction, wire size, temperature and bending rate, all wires showed the formation of wedge-shaped twins. Upon instantaneous load removal, wires bent in two of the three directions showed spontaneous, pseudoelastic unbending. The findings of this study could be relevant for the design of flexible electronics and mechanical energy storage applications at the nanoscale

Unveiling the mechanisms of motion of synchro-Shockley dislocations in Laves phases

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Diffusive Molecular Dynamics and its Application to Nanoindentation and Sintering

The interplay between diffusional and displacive atomic movements is a key to understanding deformation mechanisms and microstructure evolution in solids. The ability to handle the diffusional time scale and the structural complexity in these problems poses a general challenge to atomistic modeling. We present here an approach called diffusive molecular dynamics (DMD), which can capture the diffusional time scale while maintaining atomic resolution, by coarse-graining over atomic vibrations and evolving a smooth site-probability representation. The model is applied to nanoindentation and sintering, where intimate coupling between diffusional creep, displacive dislocation nucleation, and grain rotation are observed