Effects of Si addition on superelastic properties of Ti–Nb–Al biomedical shape memory alloys

Abstract

Effects of Si addition on microstructure, transformation temperature and mechanical properties of a Ni-free biomedical superelastic β-Ti alloy, Ti–24 at.%Nb–3 at.%Al (TiNbAl), were examined in the compositional range from 0 to 0.9 at.%Si. After the solution treatment at 1273 K, the constituent phases of the alloys with Si concentration lower than 0.7 at.% are parent β-phase (bcc) and athermal ω-phase. The alloys with Si concentration higher than 0.7 at.% contains incoherent (Ti, Nb) 3Si particles. Then, the solid solubility limit of Si in TiNbAl at 1273 K is estimated to be 0.7 at.%Si. Tensile tests revealed that the stress for inducing martensite is raised by Si addition. Thermomechanical analysis revealed that the reverse martensitic transformation finish temperature ( A f) becomes lower by Si addition by −250 K/at.%Si. The critical stress for slip deformation evaluated using residual strain increases with increasing Si concentration up to 0.7 at.%Si and the hardening rate is estimated to be 300 MPa/at.%Si. This hardening caused by Si addition is due to the solid-solution strengthening. When Si concentration exceeds 0.7 at.%, incoherent Ti 3Si-type precipitates are formed during the homogenization at 1273 K, and no further hardening occurs. Consequently, apparent transformation strain drastically decreases when Si concentration is higher than 0.7 at.%. The Ti 3Si-type particles are suggested to act as an obstacle for the stress-induced martensitic transformation. It was concluded that Si is effective to reduce M s and to raise critical stress for slip of TiNbAl, and that the formation of Ti 3Si-type precipitates exhibits negative effect for superelasticity.