Effect of Low-temperature Annealing on the Mechanical Properties of Step-rolled Ultra-fine Grained Ti-6Al-4V alloy

Abstract

The effect of annealing on mechanical properties of step-rolled ultrafine-grained (UFG) Ti-6Al-4V alloy at room and elevated temperatures was established. The initial coarse-grained microstructure (dα = 20~30 μm) was refined to UFG structure (dα = 0.7 μm) through a temperature-decreasing step rolling, and then a part of the as-step rolled plate was subsequently annealed at 500°C and 600°C. A series of room and high temperature tensile tests were carried out for as-step rolled specimen and the annealed specimen. The high temperature test conducted at strain rates of 2 × 10−4 to 10−2 s−1 in the temperature range of 650-750°C. The annealed specimen resulted in a significant enhancement of strength-ductility balance at room temperature due to the precipitation of nano-sized α grains, the formation of subgrain structure, and the decrease in internal dislocation density. In case of high temperature deformation, as-step-rolled plate exhibited higher superplasticity compared to other processed Ti-6Al-4V alloys due to effective grain refinement and high dislocation density along grain boundaries. The subsequent annealing degraded superplastic elongation; as-step-rolled microstructure showed much higher superplastic elongation than the annealed microstructures. It was mainly attributed to the active occurrence of continuous dynamic recrystallization in as-step-rolled microstructure, leading to the smaller grain sized microstructure compared to the annealed microstructure during high temperature deformation. To obtain higher superplasticity, it was suggested to design microstructure containing high dislocation density. In conclusion, the subsequent annealing showed ambivalent effect on mechanical properties of UFG Ti-6Al-4V alloy at room and high temperature.