Effect of thermo-mechanical treatment on mechanical properties and shape memory behavior of Ti–(26–28) at.% Nb alloys

The effect of thermo-mechanical treatment on mechanical properties and shape memory behavior of Ti–(26–28) at.% Nb alloys was investigated in order to develop biomedical shape memory alloys. For the solution treated specimens, superelastic behavior was observed in the temperature ranges between 293 and 313 K in Ti–26 at.% Nb, 193 and 313 K in Ti–27 at.% Nb and 163 and 233 K in Ti–28 at.% Nb alloys. However, perfect superelastic behavior with a strain larger than 2% could not be obtained at room temperature because of the low critical stress for slip deformation in the solution treated alloys. Low temperature annealing (at 873 K) increased the critical stress for slip and stabilized the superelastic behavior. An aging treatment at 573 K after annealing at 873 K further improved the superelastic properties of the Ti–(26–28) at.% Nb alloys. Both the tensile strength and the critical stress for inducing the martensitic transformation increased with increasing aging time. However, the elongation decreased with increasing aging time. The increase of the tensile strength and the aging embrittlement were due to the formation of thermal ω phase. The aging effect increased with decreasing Nb content. In particular, perfect superelastic behavior was obtained with the strain up to 3% in the Ti–26 at.% Nb alloy annealed at 873 K followed by aging at 573 K for 3.6 ks.