Effects of cryogenic pre-deformation on the microstructure and mechanical properties of Ti-Mo alloys

In the present study, pre-deformation at liquid nitrogen temperature (77 K) was applied to introduce twins into Ti-Mo alloys with different contents of molybdenum. The microstructural evolution and mechanical properties with different cryogenic pre-deformation amounts (from 0 to 9 %) were characterized and analyzed. The results show that the twinning activities and the evolution of twinning systems are mainly influenced by beta phase stability and cryogenic pre-deformation amount. After cryogenic pre-deformation, the primary and secondary twinning of {332}<113>twins are observed in Ti-12Mo and Ti-15Mo alloys but absent in Ti-20Mo alloy. With the cryogenic pre-deformation amounts increasing, the width and quantity of twins increase constantly. Moreover, {112}<111>twinning is activated in Ti-15Mo alloy pre-deformed by 6 % and 9 %. Twins and high-density dislocations induced by cryogenic pre-deformation cause a significant enhancement in strength of all these three alloys. However, the effect of cryogenic pre-deformation on the elongation of alloys is complicated. For Ti-12Mo alloys, the yield strength increases monotonically (from 527 to 778 MPa) with the increase of cryogenic predeformation reduction, while the elongation is completely opposite (from 47 to 20 %). For Ti-15Mo alloys, the optimal matching of strength and ductility is achieved when the amount of cryogenic pre-deformation is 6 %, with a tensile strength of 815 MPa and an elongation of 24 %. Compared with the other two alloys, Ti-20Mo alloy gets a smaller increase in strength and a slight decrease in elongation with the amount of cryogenic predeformation increasing.