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

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 438; pp. 835 - 838
• Main Authors: Masumoto, K., Horiuchi, Y., Inamura, T., Hosoda, H., Wakashima, K., Kim, H.Y., Miyazaki, S.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.11.2006
• Subjects: Mechanical properties; Si addition; Superelasticity; Ti 3S; Ti–Nb–Al; β-Titanium; Si addition; Mechanical properties; β-Titanium; Superelasticity; Ti 3S; Ti–Nb–Al
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2006.02.060

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.