Phase mixture modeling of the strain rate dependent mechanical behavior of nanostructured materials

A phase mixture model was employed to simulate the deformation behavior of metallic materials covering a wide grain size range from micrometer to nanometer scale. In this model a polycrystalline material is treated as a mixture of two phases: grain interior material whose plastic deformation is governed by dislocation and diffusion mechanisms and grain boundary 'phase' whose plastic flow is controlled by a boundary diffusion mechanism. The main target of this study was the effect of grain size on the flow stress and its strain rate sensitivity as well as on the strain hardening. Conventional Hall-Petch behavior in coarse grained materials at high strain rates governed by the dislocation glide mechanism was shown to be replaced with inverse Hall-Petch behavior in ultrafine grained materials at low strain rates, when both phases deform predominantly by diffusion controlled mechanisms. A deformation mechanism map representing the ranges of predominance of the dislocation glide and the diffusion controlled plastic flow is presented. The model predictions are illustrated with examples from the literature.