Effect of Si contents on the heterogeneous structure and mechanical properties in Ti-V-Al-based shape memory alloys

In the present study, the design of a heterogeneous structure was proposed to simultaneously enhance the comprehensive mechanical properties of Ti-V-Al shape memory alloys by introducing the Si element. The results showed that Si addition not only induced the formation of Ti₅Si₃ secondary phase and O′ phase, but also facilitated the appearance of β′ matrix phase, in addition to the β parent phase. The morphology of Ti₅Si₃ secondary phases gradually evolved from rod-like to globular shape, which was controlled by the terminal migration model from the viewpoint of thermodynamic state. Moreover, the presence of β′ phase, having a distinctive chemical composition and microhardness different from those of the β phase, led to the formation of a heterogeneous structure. In particular, the Ti-V-Al-Si shape memory alloys showed excellent mechanical and functional properties, including the highest hardness of 378 HV, compressive yield strength of 1040 MPa, and elongation of 14.5% by optimizing 3.0 at.% Si. In short, the superior comprehensive mechanical properties can be attributed to the synergistic effect of solution strengthening, precipitation strengthening, as well as the heterogeneous deformation-induced strengthening effect.