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 (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. [ABSTRACT FROM AUTHOR]