Optimization of a thermomechanical treatment of superelastic Ti-Zr-Nb alloys for the production of bar stock for orthopedic implants

Abstract

Ti-19Zr-14 Nb (at%) shape memory alloy was subjected to low- and high-temperature thermomechanical treatments to fabricate long-length bar stock for the production of load-bearing orthopedic implants. The phase composition, structure, texture, uniaxial tensile and three-point bending fatigue behavior were studied. Low-temperature thermomechanical treatment (LTMT), combining cold rotary forging and post-deformation annealing at 550 °C led to the formation in the peripheral zone of the bar cross-section of a statically recrystallized fine-grained structure with a relatively strong [001] crystallographic texture and of a mixed statically recrystallized and polygonized substructure, in the central zone of the cross-section. In this state, the alloy exhibited an excellent combination of the static functional and mechanical properties: relatively high strength (UTS=680 MPa), low Young’s modulus (E < 40 GPa), and high superelastic recovery strain (εrsemax=3.4%). High-temperature thermomechanical treatment (HTMT), consisting in hot rotary forging at 700 °C, led to the formation of a dynamically polygonized substructure with a uniform crystallographic texture close to the [101] direction; this direction corresponding to the maximum theoretical recovery strain limit. As compared to the LTMT bar stock, its HTMT equivalent manifested slighly inferiour static properties but superior fatigue resistance in bending, thus being an optimal candidate for the production of orthopedic implants.