Martensitic transformation, shape memory effect and superelasticity of Ti–Nb binary alloys

Shape memory and superelastic properties associated with the martensitic transformation from β to α″ martensite were investigated in Ti–(15–35) at.% Nb alloys. The transformation strain and transformation temperature linearly decreased with increasing Nb content. The low critical stress for slip deformation resulted in only a small superelastic strain in solution-treated Ti–Nb binary alloys. Fine and dense ω precipitates formed during aging in the temperature range between 573 and 673 K were effective in increasing the critical stress for slip deformation in a Ti–26 at.% Nb alloy. An intermediate-temperature annealing at 873 K for 600 s without solution treatment was also effective in increasing the critical stress for slip deformation due to the fine subgrain structure. The higher critical stress for slip deformation resulted in a larger recovery strain and stable superelasticity. Excellent superelasticity was achieved by annealing at 873 K for 600 s followed by aging at 573 K due to the combined effect of work hardening and age hardening.