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

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 (αp) 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 (αl). In the third processing step, compared with low-temperature aging, normal annealing provides sufficient driving force for αl and secondary lamellar α (αs) growing, which leads to thicker αl and αs. As for the nano & micro hardness, in one sample undergoing different process, transformed β matrix (βt) is always harder than αl and αs due to the interfacial-strengthening effect. In addition, with increasing strain rate of the first step, αp becomes harder due to the constantly enhanced work hardening effect while the hardness of βt varies little because of the competition between interfacial strengthening and distribution disorder degree. However, the nano hardness of αl decreases firstly and then increases with strain rate, which presents the same trend with the micro hardness of integrated hardness at different processing conditions.