Effect of Zr on the martensitic transformation and the shape memory effect in Ti-Zr-Nb-Ta high-temperature shape memory alloys

The microstructures, phase transformations, mechanical properties and shape memory effect of Ti-xZr-10Nb-4Ta (x = 16, 18, 19 and 19.5 at. %) alloys were investigated. The X-ray diffraction and transmission electron microscopy results showed that the Ti-16Zr-10Nb-4Ta alloy is composed of single αʺ-martensite, while the others consisted of predominant orthorhombic αʺ-martensite phase and a small amount of the β phase. The reverse martensitic transformation starting temperature (As) was 477 K for the Ti-16Zr-10Nb-4Ta alloy and decreased non-linearly with increasing Zr content. The addition of Zr resulted in the appearance of the β phase, thus elevating the stress for martensite re-orientation. However, it weakened the plasticity and the shape memory effect. The highest shape memory strain and the largest tensile strain in Ti-16Zr-10Nb-4Ta alloy were found to be 4% and 16%, respectively.