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

本研究的目的是利用分子动力学模拟来揭示某些纳米金属的应力状态相关的非弹性变形模式的本构描述形式，包括晶界洗牌/滑移、晶界位错成核、吸附和脱附以及三结晶界的自由体积交换。研究方法将侧重于模拟和定量描述三维纳米晶系综中晶界滑移、晶界位错成核、吸附和脱附以及晶心位错活动等竞争变形机制的不同贡献。为了实现上述目标，这项研究整合了材料科学/物理和工程科学，特别是材料力学之间的概念和方法。该项目的更广泛的成果将被用来获得对纳米晶材料行为的根本新的理解，这些可用于提供具有竞争变形模式的纳米晶材料的改进的连续介质模型；激励适当的原子一致的屈服面形式，在其中嵌入连续介质模型，包括拉伸-压缩不对称和一般应力状态效应；以及估计块体和晶界介导的非弹性变形模式的激活能和体积谱，这对于长时间尺度的动力学建模是有用的。这项研究将为研究生和本科生提供高级培训。此外，还将开展外展活动，以激励高中生从事工程研究和教育事业。

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.