Tailoring the microstructure, martensitic transformation and strain recovery characteristics of Ti-Ta shape memory alloys by changing Hf content

In the present work, the microstructure features, martensitic transformation, mechanical properties and strain recovery characteristics of Ti-Ta based shape memory alloys were tailored by changing Hf contents. The single α´´ martensite phase was dominated in Ti-Ta alloy with 2 at.%H f. Upon Hf content exceeded 2 at.%, β phase started to appear. Moreover, the amount of β phase gradually increased with Hf content increasing. The martensitic transformation temperatures continuously decreased with the increased Hf content, which was attributed to the rising of valence electron concentration. Meanwhile, Hf addition improved the thermal cycling stability of Ti-Ta alloys due to the suppression of ω precipitation. The yield stress of Ti-Ta based alloys firstly decreased and then increased with Hf content increasing. In addition, the completely recoverable strain of 4 % can be obtained in Ti-Ta alloy with 6 at.% Hf as a consequence of the higher critical stress for dislocation slip. Besieds, the Ti-Ta based alloy containing 8 at.% Hf had the superior superelasticity behavior with the fully recoverable strain of 2 % at room temperature.