Effect of Sn Content on the Microstructural Features, Martensitic Transformation and Mechanical Properties in Ti-V-Al-Based Shape Memory Alloys

In the present study, it is expected to tailor the microstructural features, martensitic transformation temperatures and mechanical properties of Ti-V-Al shape memory alloys through adding Sn alloying elements, which further expands their applications. Sn addition results in the monotonous rising of average valence electron number (e/a). In proportion, the single alpha '' martensite phase directly evolves into merely beta parent phase in present Ti-V-Al-based shape memory alloys, as Sn content increases from 0.5 to 5.0 at.%. Meanwhile, Sn addition causes the reduction in the grain size. Combined with transmission electron microscopy (TEM) observation and d electron theory analysis, it can be speculated that Sn addition can suppress the precipitation of omega phase. With increasing Sn content, fracture strength invariably decreases from 962 to 792 MPa, whereas the yield strength firstly decreases and then increases. The lowest yield stress for the stress-induced martensitic transformation of 220 MPa can be obtained in Ti-V-Al shape memory alloy by adding 3.0 at.% Sn. By optimizing 1.0 at.% Sn, the excellent ductility with a largest elongation of 42.1% can be gained in Ti-V-Al shape memory alloy, which is larger than that of the reported Ti-V-Al-based shape memory alloys. Besides, as a result of solution strengthening and grain refinement, Ti-V-Al-based shape memory alloy with 5.0 at.% Sn possesses the highest yield strength, further contributing to the excellent strain recovery characteristics with 4% fully recoverable strain.