Martensitic Transformation And Shape Memory Effects Of Ti-Ta-Zr High Temperature Shape Memory Alloys

Ti-Ta alloy exhibits good plasticity and excellent workability, which make it the most potential high temperature shape memory alloy for applications. However, the ω phase precipitates from Ti-Ta alloy during thermal cycling, leading to an unstable transformation temperature which becomes the obstacle for development and application. In this study, the precipitation of ω phase has been suppressed and the thermal stability has been improved by addition of Zr. The shape memory effect has also been enhanced by thermal-mechanical treatment.The influence rules and mechanisms of Zr content and thermal-mechanical treatmenton the microstructures, martensitic transformation behavior, mechanical behavior and shape memory effect of Ti-Ta-Zr alloys have been investigated systematically by means of SEM, TEM, XRD, DSC and tensile tests.The research finds that when the Zr content is not more than20at%, βâ†'α″martensitic transformation occurs during cooling, and transformation temperature increase with the increasing of Zr content.When Zr content is25at%, βâ†'α′martensitic transformation occurs during cooling. Most of martensite in solution-treated alloy is self-accommodation, the martensite substructure are mainly (111) twins.There are thick crossed″martensite variants with high density of dislocations in the cold-rolled Ti70Ta15Zr15alloy. The cold-rolled Ti70Ta15Zr15alloy is recrystallized after annealing at973K for0.5h, the morphology and substructure of α″martensite are similar to the solution-treated specimens.There are low density and regular dislocations in the martensite variants.The ω phase precipitates from Ti70Ta30alloy during thermal cycles, transformation temperature decreases with increasing number of thermal cycles and drops80K after20thermal cycles. The addition of Zr effectively surpresses the precipitation of ω phase and enhances the stability of martensitic transformation temperature.The transformation temperature of Ti70Ta15Zr15alloy decreases less than5K in the first five cycles then stays unchanged in the following thermal cycles, which shows good thermal stability.The Zr content and thermal-mechanical treatment can affect tensile deformation behavior. When Zr content is between5at%and15at%, the stress-strain curves of solution-treated specimens show two clear stress stages, which is corresponding to the martensite variants reorientation and plastic deformation, respectively. The stage stress increases with increasing Zr content. The stress-strain curves of solution-treated Ti70Ta30and Ti70Ta10Zr20alloy show only one clearly stress stage. The stress-strain curves of Ti70Ta30alloy show two stress stages after annealing at873K and973K.The stress-strain curves of Ti70Ta15Zr15alloy show two stress stage when the annealing temperature is not less than873K. And stress-strain curves of Ti70Ta15Zr15alloy show only one stress stage when annealing temperature is less than873K.The Zr addition and thermal-mechanical treatment increase the critical stress for slip and effectively enhance the shape memory effect of Ti-Ta alloy. The recovery strain of solution-treated Ti70Ta30-xZrx alloys first increases then decreases with the increase of Zr content. When Zr content is15at%, the alloy shows the maximum recovery strain of5.6%. The recovery strain of Ti70Ta15Zr15alloy reaches6.7%after cold-rolling and annealing at973K for0.5h.