Strain Rate Dependence of the Transition of Dislocation Sources from Crack Tips to Grain Boundaries Based on the Development of Internal Defect's Stress Field
Tomotsugu SHIMOKAWA, Masaki TANAKA and Kenji HIGASHIDA
Abstract:In order to investigate the mechanism behind the improvement of fracture toughness in ultra-fine grained metals at low temperatures, the strain rate dependence of the transition of dislocation sources from crack tips to grain boundaries is studied by the combination of molecular dynamics simulations and the linear elastic theory. As the strain rate decreases, grain boundaries become another stress consented site due to the pile-up of dislocations against the grain boundaries. The amount of stress concentration became larger than that of crack tip as the number of dislocations emitted from the crack tip increases. It was clearly indicated that dislocations were impinged into the grain boundary when a new dislocation was emitted from there. It indicates the transition of dislocation sources from the crack tip to grain boundaries at lower applied stresses. The distribution of dislocations between the crack tip and grain boundary is strongly related to the strain rate; namely, a larger number of dislocations are distributed very close to the crack tip as the strain rate increases. It induces the smaller stress concentration at the grain boundary since the number of piling-up dislocations is decreased around the grain boundary. Consequently, as the strain rate increases, the material becomes brittle, indicating that it will fail in a brittle mode and no longer deform plastically. Key Words:Molecular dynamics, Mechanical property, Grain boundary, Dislocation, Crack-tip shielding, Fracture toughness