Effect of annealing on martensite stabilization due to deformation via cooling under stress in cold-worked NiTi thin wire

Most practical applications of NiTi shape memory alloys involve self-motivated shape recovery of the alloys associated with the reverse transformation. An increase of the reverse transformation temperature (martensite stabilization) implies a retardation, or complete avoidance, of the shape recovery, thus failure of the function. Therefore, understanding the mechanism of martensite stabilization in NiTi alloy is extremely important and still needs in-depth studies for its successful use. In the present paper the influence of annealing on martensite stabilization due to deformation via cooling under constant tensile stresses in cold-worked NiTi thin wires has been investigated. The annealed samples exhibit different degrees of martensite stabilization. The stabilization effect was much smaller in 550 °C and 650 °C annealed samples than that in 350 °C and 450 °C annealed ones. It increases with increasing annealing temperature up to 450 °C and decreases when annealed above 450 °C. The result implies that other mechanisms are involved apart from plastic deformation under such deformation mode.