Effects of thermo-mechanical parameters on microstructure and mechanical properties of Ti–50at.%Ni shape memory alloy produced by VAR method

Abstract

► The martensite content and tensile strength increased by increasing cold reduction. ► During annealing, softening was occurred by recrystallization at high temperature. ► Thermo-mechanically treated specimens showed a pseudo-yield plateau region. ► The thermo-mechanical treatment led to an increase in shape recovery of the alloy. ► Higher cold reduction and lower annealing temperature resulted more shape recovery. In this work, effects of a thermo-mechanical treatment including cold rolling and annealing on microstructure, mechanical properties and shape memory characteristics of Ti–50 at.%Ni alloy were studied. The vacuum arc remelted ingot was first homogenized followed by hot rolling and annealing to prepare the initial microstructure. The annealed specimens were then cold rolled to 10–40% thickness reduction at room temperature. Post deformation annealing was conducted at 400, 500 and 600 °C for 1 h. Phase transformation and microstructural evolution was studied by X-ray diffraction and optical and scanning electron microscopes. Tensile, hardness and three-point bending tests were conducted to determine mechanical and shape memory properties. Experimental results showed that volume fraction of martensite, hardness and tensile strength were increased by increasing cold reduction. More recrystallization was occurred at 40% reduction followed by annealing at 600 °C, leading to a significant decrease in hardness and the amount of remained martensite. Better shape memory characteristics were achieved by applying the thermo-mechanical treatment. Both a higher cold reduction and a lower annealing temperature caused more shape recovery. The specimen annealed at 400 °C after 20% cold rolling showed full shape recovery.