Potential Superelastic Nickel-Free Titanium Alloy

A titanium-25 wt.% niobium- 21 wt.% hafnium alloy was produced based on optimization of molecular orbital calculations of electronic structures towards low modulus of elasticity and the potential for superelasticity.  Thin 127 µm wires were produced with varying cold work and heat-treated at 650°C in an Argon filled tube furnace.  Characterizations were made of as-drawn and heat-treated wires using electron beam, nanoindentation, axial tensile testing, and rotary beam fatigue techniques in order to inform structural-functional relationships.  In general, the microstructure comprised a nanoscale, highly twinned substructure at room temperature which converted to a non-twinned state upon axial load application or a temperature increase.  A modulus of elasticity of less than 60 GPa and axial recoverable strain of greater than 3% were observed with stress hysteresis resembling a reversible stress-induced martensitic transformation at a test temperature of 150°C.  Fatigue strain amplitudes in excess of 0.5% were observed at a cycle life of greater than 107 cycles in a 37°C saline environment.  These results are discussed with an emphasis on optimization of the product for nickel-free service as a superelastic material for medical device applications.