Effect of second phase precipitation on martensitic transformation and hardness in highly Ni-rich NiTi alloys

Abstract To clarify the origin of the high hardness of the 55NiTi alloy, the relationships between precipitation, martensitic transformation and hardness were investigated in three different highly Ni-rich NiTi alloys after different heat treatments. The results showed that, independent of the overall Ni content, the hardness of highly Ni-rich NiTi alloys depended only on the start temperature (M s ) of the B19′ martensitic transformation. The hardness at room temperature increased as the M s temperature was lowered due to the increasing difficulty of stress-induced B19′ martensitic transformation. The pre-occurrence of an overlapped R-phase transformation lowered the hardness and the effect of a separated R-phase transformation on the hardness was weak. The much higher hardness of the 55NiTi alloy resulted from the difficulty of the stress-induced B19′ martensitic transformation.