Effect of morphology of dual-phase structure on inhomogeneous plastic deformation and ductile fracture in Ti-4%Cr Alloy

The morphology of (α+β) dual-phase structure in Ti-4%Cr alloy was controlled to be plate-like and equiaxed types by aging treatment and warm rolling, respectively. Tensile testing for the specimens with different morphology revealed that the equiaxed specimen was much superior to the plate-like one in elongation and reduction of area. The inhomogeneous and hierarchical strain distribution was quantitatively visualized for these specimens by DIC method, and it was found that the plastic strain is preferentially introduced into the softer phase of α, which results in a marked strain partitioning between α and β phases, particularly in the plate-like specimen. There were three conditions for preferential plastic deformation in the plate-like specimen: 1. α plate is aligned nearly in the direction of maximum shear stress, 2. the length and width of α plate is relatively large, and 3. Schmidt factor for prismatic slips is relatively large. As a result, the plastic strain is increased in such α plates and expanded along the plate, leading to a void formation at plate/plate or plate/β grain boundary junctions. On the other hand, the equiaxed specimen deforms uniformly with a less strain partitioning, and also, the strained regions are formed separately and hard to connect each other. Since the void formation is significantly delayed due to those reasons, the equiaxed specimen can continue plastic deformation to a higher strain regime.