The microstructural evolution and thermo-mechanical behavior of UFG Ti processed via equal channel angular pressing

Abstract

Equal channel angular pressing (ECAP) was applied to grade 1 Ti for up to four passes and the microstructure and thermo-mechanical behavior were determined. ECAP was performed with a channel angle of 90°, at a pressing temperature of 275°C, and using route BC. Electron backscatter diffraction (EBSD) was used to characterize the microstructure. After the first pass of ECAP, {1 0 1¯ 1} twins were present, but significant slip had also occurred. The microstructure was highly anisotropic, consisting of relatively undeformed grains to grains less than a micrometer. Increasing the number of passes decreased the grain anisotropy, and by the fourth pass a relatively homogeneous microstructure was present. After all numbers of passes, there was evidence for continuous dynamic recrystallization as the mechanism of grain refinement, even when twinning had occurred. This refinement accelerated as the number of passes increased. The response of UFG Ti was determined at −196, 22, 200 and 375°C, and at strain rates from 10−4 to 1100s−1. It was found that the ECAP process greatly increases the yield strength versus the as-received condition, and UFG Ti has similar work hardening as the as-received material. The work hardening does not decrease much as the number of passes is increased. Additionally, the strain rate sensitivity tends to increase with the temperature and decrease as the number of passes is increased.