Relationship between Flow Behavior and Microstructure Evolution during Isothermal Compression of near β Titanium Alloy Ti-55531 with Acicular Starting Microstructure

Abstract

Near β titanium alloy Ti-55531 with an acicular starting microstructure was isothermally compressed at 750-825 • C and 10 −3 −1 s −1 . The microstructure evolution and its influence on the flow behavior of yielding and softening were investigated. Discontinuous or continuous yielding depends on the hindrance to the dislocation motion coming from the β grain boundary or α phase. At higher temperatures, the hindrance mainly comes from the β grain boundary. Its discontinuous action, including the piling-up and subsequent loosening of dislocations at the β grain boundary, leads to discontinuous yielding. At lower temperatures, the continuous hindrance to the dislocation motion, which is exerted by the β grain boundary and acicular α, causes continuous yielding. Sequentially, the substructures in acicular α are evolved from high-density dislocations or local shear bands, which depend on the orientation relationship between β and α. Then, the β matrix edges into the acicular α along substructure boundaries. The higher strain rate decreases the deformation time to carry out the fragmentation of acicular α, while the higher temperature decreases the dislocation density due to the recovery of β, which does not benefit the substructure formation and subsequent fragmentation of acicular α. Therefore, the retardation of acicular fragmentation and the as-resulted decreased flow softening rate are observed.