Effects of pre-strain and heat treatment temperature on phase transformation temperature and shape recovery stress of Ti-Ni-Nb shape memory alloys for pipe joint applications

Effects of pre-strain and heat treatment on shape recovery stress and phase transformation temperature in Ti-Ni-Nb shape memory alloys with various Nb content (6. 9. and 12mol%) were investigated by tensile testing. The recovery stress of Ti-Ni-Nb alloys increased with increasing the pre-strain and then decreased after reaching the maximum recovery stress of around 450-500MPa at about 9% pre-strain. Martensitic transformation temperatures (M_s, A_s, A_f) also increased with increasing the applied pre-strain. while the relation between A_s and pre-strain did not depend on the Nb contents and Ni/Ti ratio. Moreover, the recovery stress in the Ti-Ni-Nb alloy heat treated at 673 and 773 K after solution treatment was slightly higher than that in the solution-treated alloy. These behaviors were examined relative to the increased defect density due to pre-straining and the elimination of defects by heat treatment.