Multiscale modeling of microstructure-dependent plastic deformation of SiC and tungsten

You Sung Han, Purdue University

Abstract

Tungsten (W) and silicon carbide (SiC) are leading candidate materials for plasma facing and structural components for nuclear fusion and advanced nuclear fission reactors due to their excellent high temperature properties. Irradiation-induced material damage involves multiscale phenomena, including defect formation, interaction of defects with impurities, void migration, and defect recovery. Such processes are affected by several factors, including irradiation energy, mechanical loading, temperature, and grain boundary (GB). All of these factors come into play together in determining material properties under irradiation. Hence, in this research multiscale modeling has been performed to capture several important phenomena which affect the material property changes and microstructure evolution of tungsten (W) and SiC during irradiation and to predict the failure limit of the materials under irradiation correctly. In the present study, it is hypothesized that between fractal dimension and strength of irradiated materials can be uniquely determined for a given equilibrated structure after irradiation-induced impact. If the correlation of fractal dimension with strength of irradiated materials can be established, such relationship can be extended to possibly other irradiated microstructures. In this sense, use of fractal dimension is pursued in the present work to establish (1) the correlation of fractal dimension and the change of atomic configuration features such as type of GB and presence of defects; (2) the correlation of fractal dimension and strength of irradiated materials. Fractal dimension based approach is used to link relevant length scale for multiscale simulation. Physical mechanism obtained from lower length scale can be passed to higher length scale using fractal dimension. Among the challenges of material selection for nuclear fusion and advanced nuclear fission power plant, the strength deterioration due to the helium (He) point defects that occur during irradiation and high level of irradiation damage have been the main issues in nuclear material research. The effect of He point defects on peak tensile strength of W grain boundaries (GBs) and the effect of radiation energy on tensile strength of SiC are investigated using ab initio simulations framework based on Car-Parrinello Molecular Dynamics (CPMD). A fractal dimension based approach is employed to characterize the different GB geometries and atomic configurations of the disordered materials. A linear relationship between structural fractal dimension and tensile strength is observed in all examined GBs. Based on observed correlations, an empirical relation is developed to predict the peak tensile strength of W and SiC GBs. The proposed GB strength relation was used for higher length scale simulation such as the phase-filed modeling by taking into account of interfacial energy in terms of fractal dimension. The evolution of voids in W polycrystalline under irradiation were studied using the phase-field modeling, including nucleation and growth of voids and sink efficiency for vacancy annihilation in vicinity of GB. Finally, finite element (FE) simulations have been performed using the Anand viscoplastic model to investigate the deformation behaviors and the yield failure limit of W microstructures at high temperature under the influence of irradiation. The FE analyses suggest that the yield strength of irradiated W microstructure is directly correlated with temperature and irradiation dose. Based on the simulation results, the empirical equation to predict yield strength as a function of temperature and irradiation dose was proposed.

Degree

Ph.D.

Advisors

Tomar, Purdue University.

Subject Area

Mechanics

COinS