Martensitic Transformation and Enhancement of Mechanical Property of Nb-Ni-Ti Alloy by Healing Treatment

The aim of the present study is to enhance the stress recovery capability of the β Nb phase in NbNi-Ti alloy by using the shape memory effect (SME) of the NiTi phase. The concept of healing treatment to recover the mechanical property has been newly introduced for Nb-Ni-Ti alloy. Nb-29.2at%Ni-40.8at%Ti alloy samples with a Ti/Ni ratio of 1.4 were prepared by the following sequences after casting; homogenization treatment – cold rolling – recrystallization treatment – pre-straining – healing heat treatment. The microstructure consisted of primary β Nb and β Nb-NiTi eutectic structure. The alloy exhibited twinned to detwinned martensite transformation during a room temperature tensile test. After pre-straining up to 600 MPa, the martensite to austenite transformation temperature increased significantly due to an increase in dislocation density. The healing treatment of the pre-strained samples at 120oC, 170oC and 200oC for 1 h followed by rapid cooling down to -20oC induces the reverse transformation, from detwinned to twinned martensite, leading to enhanced mechanical property as compared with the sample without healing treatment. As the healing treatment temperature increased, the mechanical property improved, indicating that reverse-transformed twinned martensite plays an important role in the recovery of mechanical property after pre-straining. The present study shows that the healing treatment using the SME of twinned NiTi martensite can contribute to the stress recovery of Nb which is a possible candidate for a hydrogen permeable membrane.