Microstructure Evolution and Mechanical Properties of Ti-6Al-4V Alloy Prepared by Multipass Equal Channel Angular Pressing

Multipass ECAP was applied to prepare ultra-fine-grained Ti-6Al-4V alloy. The microstructure evolution and mechanical properties of Ti-6Al-4V alloy during ECAP process were discussed in this study. Grain size of Ti-6Al-4V alloy decreased significantly with the increase in the number of ECAP passes, and numerous grains with size of 200-300 nm were observed in the four-pass ECAP sample. The twinning structure plays an important role in the plastic deformation coordination mechanism, which can be reflected by the gradually increasing twins during multipass ECAP process. Moreover, the Vickers hardness and room-temperature compression strength increased with the increase in the number of ECAP passes due to the high dislocation density, grain refinement and the large number of twins. The hardness and compression strength increased from 355 HV and 1296 MPa to 392 HV and 1432 MPa, respectively. However, the ultimate strain of the four-pass ECAP sample is even higher than that of the two-pass and three-pass samples because of its uniformly refined microstructure and high twinning density. Uniform microstructure with average grain size of 1-2 μm was obtained by four-pass ECAP and following annealing treatment at 923 K for 30 min, which indicates that static recrystallization occurs in a relatively low temperature and short time. The compression experiments results show that Ti-6Al-4V alloy prepared by four-pass ECAP and following annealing treatment exhibits high comprehensive properties in both strength and plasticity at room temperature, with corresponding values of 48.8% in ultimate strain and 1400.3 MPa in strength.