Temperature-controlled plasma nitriding of 60NiTi alloy: Microstructure evolution and performance optimization

In the present study, the microstructural features of 60NiTi alloys were tailored by changing nitriding temperatures, further optimizing the performances. With rising of nitriding temperatures, the thickness of outermost TiN ceramic layer and Ni 3 Ti phase layer gradually increased. Moreover, the morphologies of Ni 3 Ti phase layer in interior of B2-NiTi matrix evolved from fine needle-like shape to coarse granular particles. Furthermore, Ni 4 Ti 3 precipitates can be found in nitrided 60NiTi alloy at the nitriding temperature of 540°C. All nitrided 60NiTi alloys are featured with one-stage B2↔B19ˊ martensitic transformation. With the increasing of nitriding temperature, the martensite starting temperature ( Ms ) increased from -33.6°C to -20.6°C owing to the altering Ti/Ni stoichiometry and the constraints effect of outermost TiN ceramic phase and Ni 3 Ti phase layer on the B2-NiTi matrix. For comparison, the maximum compressive strength of approximately 2800 MPa and the largest fracture strain of about 35% as well as the superior strain recovery rate higher than 94% under the pre-strain of 8% condition can be obtained in nitrided 60NiTi alloy at the nitriding temperature of 480°C. The microhardness increased continuously to 664 HV with the rising of 12%, when the nitriding temperatures increased from 480 °C to 560 °C, which is mainly attributed to the thicker harder TiN ceramic phase layer and the precipitation strengthening of Ni 4 Ti 3 precipitates in B2-NiTi matrix. The excellent tribological properties with the lower friction coefficient of 0.4 can be optimized by tailoring nitriding temperatures of 540°C. In proportion, the wear mechanism changed from severe adhesion/abrasive wear to mild oxidation/abrasive wear.