CrCoNi alloy exhibits a remarkable combination of strength and plastic deformation, even superior to the CrMnFeCoNi high-entropy alloy. We connect the magnetic and mechanical properties of CrCoNi, via a magnetically tunable phase transformation. While both alloys crystallize as single-phase face-centered-cubic (fcc) solid solutions, we find a distinctly lower-energy phase in CrCoNi alloy with a hexagonal close-packed (hcp) structure. Comparing the magnetic configurations of CrCoNi with those of other equiatomic ternary derivatives of CrMnFeCoNi confirms that magnetically frustrated Mn eliminates the fcc-hcp energy difference. This highlights the unique combination of chemistry and magnetic properties in CrCoNi, leading to a fcc-hcp phase transformation that occurs only in this alloy, and is triggered by dislocation slip and interaction with internal boundaries. This phase transformation sets CrCoNi apart from the parent quinary, and its other equiatomic ternary derivatives, and provides a new way for increasing strength without compromising plastic deformation.

Random solid solution alloys are a broad class of materials that are used across the entire spectrum of engineering metals, whether as stand-alone materials (e.g. Al-5xxx alloys) or as the matrix in precipitatestrengthening materials (e.g. Ni-based superalloys). As a result, the mechanisms of, and prediction of, strengthening in solid solutions has a long history. Many concepts have been developed and important trends identified but predictive capability has remained elusive. In recent years, a new theory has been developed that builds on one historical model, the Labusch model, in important ways that lead to a well-defined model valid for random solutions with arbitrary numbers of components and compositions. The new theory uses first-principles-computed solute/dislocation interaction energies as input, from which specific predictions emerge for the yield strength and activation volume as a function of alloy composition, temperature, and strain-rate. Being a general model for materials that otherwise have a low Peierls stress, it has broad application and has been successfully applied to Al-X alloys, Mg-Al, twinning in Mg alloys, and recently fcc High-Entropy Alloys. Here, the new theory is presented in a general and systematic manner. Approximations and limiting cases that reduce the complexity and facilitate understanding are introduced, and help relate the new model to various physical features present among the historical array of models. The quantitative predictions of the model in the various materials above is then demonstrated.

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High-entropy alloys (HEAs) are an exciting new class of multi-component alloys some of which haveunusual and remarkable properties. As of yet, little is understood about dislocation core structure and stacking fault energies in these alloys. For this study, a five-component, equiatomicalloy (CrMnFeCoNi) was deformed to 5% plastic strain at room temperature. Posttest observations using diffraction contrast scanning transmission electron microscopy (DC-STEM) analysis provide evidence for numerous planar slip bands composed of ½<110> dislocations. More detailed analyses of dislocation separation distances were performed using high-order diffraction vector DC-STEM and atomic resolution high angle annular dark field (HAADF) STEM on ½<110> dislocations in 60° orientation. Large variations in dissociation distances are found, leading to the concept of a local stacking fault energy (SFE). This finding issupported through embedded-atom-method (EAM) calculations of a model, concentrated, three-element solid solution. For the first time, the Nye tensor and center of symmetry analysis were used collectively to accurately determine dissociation distance. Lastly, using highresolution energy dispersive X-ray spectroscopy, no ordering or segregation was observed,

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Solute strengthening of twin dislocation motion along an existing twin boundary in Mg-X (X=Al, Zn) is investigated using a new Labusch-type weak pinning model. First, the (1012) twinning dislocation structure is computed using first-principles methods. Second, the interaction energies of Al and Zn solutes with the twin boundary and twin dislocation are computed. Interaction energies are computed for all solute positions for Al and a subset of sites for Zn, and it is shown that the interaction energies of Zn solutes scale with the Al solute energies in propotion to the misfit volume plus an additional "chemical" interaction piece and this scaling is used to compute the Zn solute interactions at all other sites around the twin dislocation. Third, the solute/dislocation interaction energies are used in a new Labusch-type model to predict the overall solute strengthening of the twinning dislocation. New features emerge in the application of the model to twinning because of the very small Burgers vector of the twin dislocation, leading to a new functional form for the dependence of the strengthening on concentration, temperature, and strain rate. Fourth, application of the model leads to parameter-free predictions that agree well with available experimental data on various Mg-Al-Zn alloys. The predicted strengthening is not large, e.g. ≈ 10 MPa for the AZ31 alloy at room temperature, but is larger than the strengthening of basal slip by the same solutes. Overall, this work contributes to a growing quantitative understanding of alloying effects on various deformation modes in Mg.

The average effect of a single 500 eV incident argon ion on a silicon surface is studied using molecular dynamics simulations. More than 10 3 ion impacts at random surface points are averaged for each of seven incidence angles, from 0°to 28°off normal, to determine a local surface height change function, or a crater function. The crater shapes are mostly determined by mass rearrangement; sputtering has a relatively small effect. Analytical fitting functions are provided for several cases, and may serve as input into kinetic Monte Carlo calculations or stability analyses for surfaces subjected to ion bombardment.