A Lightweight Ti-Al-Based Shape Memory Alloy with B2-Ordered Structure

The β-type Ti-based alloys have recently attracted much attention due to their exceptional properties such as low Young’s modulus, shape memory effect and gum-metal-like behavior. Among them, β-type Ti-based shape memory alloys such as Ti-Nb-Ta-Zr and Ti-Mo-Al, which undergo thermoelastic martensitic transformation from a <i>β</i> (body-centered cubic) parent phase to an <i>α</i><i>’’</i> (orthorhombic) martensite phase, show great promise for applications in medical devices due to their superelasticity and biocompatibility. However, the transformation strain by superelasticity is usually small to be less than 3% in these conventional β-type Ti-based shape memory alloys. In this talk, we report a novel B2-structured Ti-Al-Cr shape memory alloy with a greater superelastic strain. The crystal structure and lattice parameters of both parent phase and martensite phase were determined using the X-ray and neutron diffraction methods. The mechanical properties of the alloy were evaluated by uniaxial tensile or compressive tests on single-crystal samples at room temperature. It was found that the alloy exhibits excellent superelastic behavior with a recoverable strain over 7%. The experimentally observed transformation strain is consistent with the calculated one using the lattice parameters on the base of lattice deformation theory. This newly-developed Ti-Al-Cr shape memory alloy is a promising candidate for medical applications. This alloy is also characterized by its lower density than that of the conventional β-type Ti-based alloys containing high-density Nb or Mo, having potential as a lightweight shape memory alloy.