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

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 (Af) 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 Ti3Si-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 Ti3Si-type particles are suggested to act as an obstacle for the stress-induced martensitic transformation. It was concluded that Si is effective to reduce Ms and to raise critical stress for slip of TiNbAl, and that the formation of Ti3Si-type precipitates exhibits negative effect for superelasticity.