Effect of precipitation phase morphology on mechanical properties of Ni-Ti shape memory alloy

Aging heat treatment was applied to the Austeinitic NiTi shape memory alloy at 300 and 500°C, and two typical microstructures with grainy precipitation phase and acicular precipitation phase were obtained, respectively. Static and dynamic mechanical property testing were conducted for both the samples with different morphologies of precipitation. Results indicate that under static tension loading, the strain-induced martensite transformation of the both samples has an obvious strain rate effect. The initial stress for strain-induced martensite transformation of samples with grainy precipitation is higher than that of samples with acicular precipitation, while samples with acicular precipitation phase show better plasticity and higher flow stress of plastic deformation. Under static compression loading, the two kinds of samples exhibit similar mechanical behaviors. Under dynamic loading, the samples with acicular precipitation manifest much higher yielding strength, indicating a better strain rate hardening effect. However, the plastic deformation stage of marstensite transformation for samples with acicular precipitation displays more evident strain rate hardening effect, resulting in close failure strength with that of the other samples. The corresponding mechanism was also discussed.