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The extraordinary work hardening ability and fracture toughness of the face-centered cubic (fcc) high-entropy alloys render them ideal candidates for many structural applications. Here, the deformation and failure mechanisms of an equiatomic CrCoNi medium-entropyalloy (MEA) were investigated by powerful laser-driven shock experiments. Multiscale characterization demonstrates that profuse planar defects including stacking faults, nanotwins, and hexagonal nanolamella were generated during shock compression, forming a three-dimensional network. During shock release, the MEA fractured by strong tensile deformation and numerous voids was observed in the vicinity of the fracture plane. High defect populations, nanorecrystallization, and amorphization were found adjacent to these areas of localized deformation. Molecular dynamics simulations corroborate the experimental results and suggest that deformation-induced defects formed before void nucleation govern the geometry of void growth and delay their coalescence. Our results indicate that the CrCoNi-based alloys are impact resistant, damage tolerant, and potentially suitable in applications under extreme conditions.

Using molecular dynamics simulations, we assess the uniaxial deformation response of ice Ih as described by two popular water models, namely, the all-atom TIP4P/Ice potential and the coarse-grained mW model. In particular, we investigate the response to both tensile and compressive uniaxial deformations along the [0001] and [ 0 1 ¯ 10 ] crystallographic directions for a series of different temperatures. We classify the respective failure mechanisms and assess their sensitivity to strain rate and cell size. While the TIP4P/Ice model fails by either brittle cleavage under tension at low temperatures or large-scale amorphization/melting, the mW potential behaves in a much more ductile manner, displaying numerous cases in which stress relief involves the nucleation and subsequent activity of lattice dislocations. Indeed, the fact that mW behaves in such a malleable manner even at strain rates that are substantially higher than those applied in typical experiments indicates that the mW description of ice Ih is excessively ductile. One possible contribution to this enhanced malleability is the absence of explicit protons in the mW model, disregarding the fundamental asymmetry of the hydrogen bond that plays an important role in the nucleation and motion of lattice dislocations in ice Ih. [ABSTRACT FROM AUTHOR]

Irradiation by light ions may change the mechanical properties of nanofoams. Using molecular-dynamics simulation, we study the effect of irradiating a Au foam (porosity, 50%, and ligament diameter, 3 nm) with heavy ions: here, 10 keV Au ions up to a dose of 4 × 1016 m−2. We demonstrate that in consequence, the ligament morphology changes in the irradiated region, caused by local melting. The changes in mechanical properties are monitored by simulated nanoindentation tests. We find that the foam hardness is only around 1/3 of the hardness of a bulk Au crystal. Irradiation increases the hardness of the foam by around 10% in the central irradiated area. The plastic zone extends to only 1.5 ac, where ac denotes the contact radius; this value is unchanged under irradiation. The hardness increase after irradiation is attributed to two concurring effects. To begin with, irradiation induces melting and annealing of the ligaments, leading to their coarsening and alleviating surface stress, which in turn increases the dislocation nucleation threshold. In addition, irradiation introduces a stacking fault forest that acts as an obstacle to dislocation motion. [ABSTRACT FROM AUTHOR]

This review covers areas where our understanding of the mechanisms underlying nanoindentation has been increased by atomistic studies of the nanoindentation process. While such studies have been performed now for more than 20 years, recent investigations have demonstrated that the peculiar features of nanoplasticity generated during indentation can be analyzed in considerable detail by this technique. Topics covered include: nucleation of dislocations in ideal crystals, effect of surface orientation, effect of crystallography (fcc, bcc, hcp), effect of surface and bulk damage on plasticity, nanocrystalline samples, and multiple (sequential) indentation. In addition we discuss related features, such as the influence of tip geometry on the indentation and the role of adhesive forces, and how pre-existing plasticity affects nanoindentation. [ABSTRACT FROM AUTHOR]

We investigate by molecular dynamics simulation the mechanical behavior of concentrated alloys under nanoindentation for the special example of single-phase fcc Fe[Formula: see text]Ni[Formula: see text] alloys. The indentation hardness is maximum for the equiatomic alloy, [Formula: see text]. This finding is in agreement with experimental results on the strength of these alloys under uniaxial strain. We explain this finding with the increase of the unstable stacking fault energy in the alloys towards [Formula: see text]. With increasing Fe content, loop emission from the plastic zone under the indenter becomes less pronounced and the plastic zone features a larger fraction of screw dislocation segments; simultaneously, the length of the dislocation network and the number of atoms in the stacking faults generated in the plastic zone increase. However, the volume of twinned regions in the plastic zone is highest for the elemental solids and decreases for the alloys. This feature is explained by the fact that twinning proceeds by the glide of dislocations on adjacent parallel lattice planes; this concerted motion is less efficient in the alloys. Finally, we find that surface imprints show increasing pile-up heights with increasing Fe content. The present results will be of interest for hardness engineering or generating hardness profiles in concentrated alloys., (© 2023. The Author(s).)

Using molecular dynamics simulations, we assess the uniaxial deformation response of ice I h as described by two popular water models, namely, the all-atom TIP4P/Ice potential and the coarse-grained mW model. In particular, we investigate the response to both tensile and compressive uniaxial deformations along the [0001] and [ 0 1 ¯ 10 ] crystallographic directions for a series of different temperatures. We classify the respective failure mechanisms and assess their sensitivity to strain rate and cell size. While the TIP4P/Ice model fails by either brittle cleavage under tension at low temperatures or large-scale amorphization/melting, the mW potential behaves in a much more ductile manner, displaying numerous cases in which stress relief involves the nucleation and subsequent activity of lattice dislocations. Indeed, the fact that mW behaves in such a malleable manner even at strain rates that are substantially higher than those applied in typical experiments indicates that the mW description of ice I h is excessively ductile. One possible contribution to this enhanced malleability is the absence of explicit protons in the mW model, disregarding the fundamental asymmetry of the hydrogen bond that plays an important role in the nucleation and motion of lattice dislocations in ice I h .

Using molecular dynamics simulation, we study the nanoindentation of three hcp metals: Mg, Ti, and Zr. Both the basal and two prismatic surface planes are considered. We focus on the characterization of the plasticity generated in the crystal. The similarities to, and the differences from, the behavior of the more commonly investigated fcc and bcc metals are highlighted. We find that hcp metals show a larger variety than the fcc and bcc metals studied up until now. The prolific emission of prismatic loops can lead to extended plastic zones. The size of the plastic zone is quantified by the ratio f of the plastic zone radius to the radius of the contact area. We find values of between 1.6 (an almost collapsed zone) and >5; in the latter case, complex dislocation networks build up which are extended in the direction of easy glide.