Methods for Atomistic Input into Initial Yield and Plastic Flow Criteria for Nanocrystalline Materials

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The objective of this research is to employ molecular dynamics simulations to shed light on the proper forms for the constitutive description of stress-state dependent inelastic deformation modes for some nanocrystalline metals, including grain boundary shuffling/sliding, grain boundary dislocation nucleation, absorption and desorption, and free volume exchange of grain boundaries with triple junctions. The research approach will emphasize methods to simulate and quantitatively characterize distinct contributions of competing deformation mechanisms of grain boundary sliding, grain boundary dislocation nucleation, absorption and desorption, and grain core dislocation activity in three-dimensional nanocrystalline ensembles. To accomplish the above goals, the research integrates concepts and methods that bridge between materials science/physics and the engineering sciences, in particular mechanics of materials.The broader outcomes of this project will be used to gain fundamental new understanding of behavior of nanocrystalline materials, which can be used to inform improved continuum models of nanocrystalline materials with competing modes of deformation; to motivate proper, atomistically-consistent forms of yield surfaces in which to embed continuum models, including tension-compression asymmetry and general stress state effects; and to estimate the spectra of activation energy and volumes for both bulk- and grain boundary-mediated inelastic deformation modes, useful for modeling kinetics at long time scales. The research will provide advanced training for both graduate and undergraduate students. Moreover, outreach activities will be conducted to motivate high-school students to pursue careers in engineering research and education.