Influence of Pre-deformation on Phase Transformations and Microstructures in Near β Ti Alloy During Aging

The influence of pre-deformation on phase transformations, microstructures and hardening response in near β Ti alloy Ti-5Al-3Mo-3V-2Cr-2Zr-1Nb-1Fe (wt%) during aging treatment was studied. The results show that obvious α phase refinement and stronger age hardening effect can be achieved when the alloy is slightly deformed before aging treatment. Because the formation of intermediate phases (O′, ω and O′′) suppresses long-range stress induced martensitic transformation and mechanical twinning, the alloy is mainly deformed via dislocation slipping during loading. Large numbers of crystal defects are generated during pre-deformation. With increasing the pre-deformation, the number density of dislocations increases gradually. These crystal defects generated by pre-deformation may partly annihilate upon early aging, but the precipitation of α can also be promoted, resulting in refined α precipitates. In the sample with 5% pre-strain, the average thickness of α precipitates decreases by 57% after aging at 600 °C compared with the sample without pre-strain, and the number density increases from 7.0±1 laths/μm² to 22.0±3 laths/μm². Some platelet-shaped α phases form when the samples experience comparably large pre-strains such as 12% and 20%. It proves that the refined α precipitates and better hardening effect can be achieved by pre-deformation plus aging treatment for titanium alloy.