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

Influence of intrinsic strain on irradiation induced damage: the role of threshold displacement and surface binding energies

International audienc

Atomistic simulations of focused ion beam machining of strained silicon

International audienc

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

A new method for microscale cyclic crack growth characterization from notched microcantilevers and application to single crystalline tungsten and a metallic glass

The lifetime of most metals is limited by cyclic loads, ending in fatigue failure. The progressive growth of cracks ends up in catastrophic failure. An advanced method is presented for the determination of cyclic crack growth on the microscale using a nanoindenter, which allows the characterization of > 10,000 loading cycles. It uses focused ion beam fabricated notched microcantilevers. The method has been validated by cyclic bending metallic glass and tungsten microcantilevers. The experiments reveal a stable crack growth during the lifetime of both samples. The metallic glass shows less plasticity due to the absence of dislocations, but shows shearing caused by the deformation. The crack growth rates determined in the tests follow Paris' power law relationship. The results are reliable, reproducible and comparable with macroscopic setups. Due to the flexibility of the method, it is suitable for the characterization of specific microstructural features, like single phases, grain boundaries or different grain orientations

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