Microstructure characterization and nano & micro hardness of tri-modal microstructure of titanium alloy under different hot working conditions

Highlights: • The contents and sizes of α_p and α_l are determined by the processing parameters of first and second step. • The morphology of α_s embedded in β phase is determined by the accumulated distortional strain energy and dislocations. • α_p becomes harder as the strain rate of the first step increases due to the constantly enhanced work hardening effect. • β_t is always harder than α_p and α_l due to the interfacial-strengthening effect. • The hardness of α_l makes the greatest contribution in the integrated hardness of tri-modal microstructure. - Abstract: In this work, the dependences of tri-modal microstructure parameters and corresponding nano & micro hardness on through-process processing parameters were quantitatively studied during the three-step thermo-mechanical processing of TA15 titanium alloy. It is found that the processing parameters of first step, especially for the deformation temperature and strain rate, mainly affect primary equiaxed α (α_p) through the α → β phase transformation and the competition between dynamic recovery and dynamic recrystallization. The second processing step primarily affects the content and thickness of lamellar α (α_l). In the third processing step, compared with low-temperature aging, normal annealing provides sufficient driving force for α_l and secondary lamellar α (α_s) growing, which leads to thicker α_l and α_s. As for the nano & micro hardness, in one sample undergoing different process, transformed β matrix (β_t) is always harder than α_l and α_s due to the interfacial-strengthening effect. In addition, with increasing strain rate of the first step, α_p becomes harder due to the constantly enhanced work hardening effect while the hardness of β_t varies little because of the competition between interfacial strengthening and distribution disorder degree. However, the nano hardness of α_l decreases firstly and then increases with strain rate, which presents the same trend with the micro hardness of integrated hardness at different processing conditions.