Plasma nitriding temperature-dependent microstructure and properties of 60NiTi alloy: Tailoring mechanical, superelastic, and wear behavior

The combination of plasma nitriding and solution treatment has been confirmed as an effective method for enhancing the comprehensive properties of 60NiTi alloy. In the present study, the effects of plasma nitriding temperature on the microstructure, martensitic transformation, mechanical and tribological properties of 60NiTi alloy were investigated. The results showed that microstructural features of nitrided + solution treated 60NiTi alloy was composed of an outermost TiN layer, secondary layer Ni₃Ti phase as well as the NiTi matrix enhanced by higher density of Ni₄Ti₃ precipitates. The martensitic transformation in the nitrided + solution treated 60NiTi alloy was absent owing to the constraint effect of multiple non-transformation TiN ceramic phase as well as Ni₃Ti phase layer and the suppression effect of Ni₄Ti₃ precipitates. The thickness of the TiN layer and the volume fraction of Ni₄Ti₃ phase increase with the nitriding temperature rising, which was beneficial for the mechanical properties of nitrided + solution treated 60NiTi alloy. As the nitriding temperature increased from 480 °C to 560 °C, the fracture strength and elongation were increased by 48% and 28%, respectively. Meanwhile, the microhardness was increased gradually from 961 HV to 1008 HV. Additionally, the nitrided + solution treated 60NiTi alloy nitrided at temperature of 560 °C demonstrated the optimal superelasticity with the strain recovery rate of 98.23 % under 8 % pre-strain condition. As the nitriding temperature increased, the coefficient of friction also increased. In proportion, the superior wear performance with the lowest coefficient of friction of 0.352 can be optimized by tailoring the nitriding temperature of 480 °C, which was characterized by the abrasive wear.