Effect of cold work and annealing on the structure and characteristics of NiTi alloy

A hot rolled NiTi alloy (50.7 at.% Ni) was subjected to cold rolling with reductions of 20%, 30% and 40%. Some of the 40% cold rolled samples were then annealed at 400 °C, 500 °C, 600 °C and 700 °C. X-ray results and microstructural analysis show that increasing the percent of cold rolling causes an increase in the volume fraction of martensite, which results in an increase in recoverable strain, tensile strength and hardness. On the other hand, this eliminates the superelasticity plateau and results in a reduction in ductility and shape memory recovery. After an initial sharp decrease in superelasticity with cold rolling, further increase in cold rolling results in relative improvement in the superelasticity. Annealing on the other hand causes a significant increase in austenite volume fraction, leading to a decrease in hardness and superelasticity in the temperature range of 400 °C–500 °C. Recrystallization starts at 500 °C and grains grow with annealing at higher temperatures while hardness and superelasticity increase.The results of this work indicate that it is best to use the NiTi alloy in the as hot rolled condition if maximum SME is desired. Cold rolled NiTi would be preferable if high strength, hardness and acceptable recoverable strains are desirable.