Determination of the Mechanical Properties of NiTi + Ni4Ti3 and Ni3Ti in Hardened 60NiTi Using Nanoindentation

The mechanical properties of the NiTi + Ni₄Ti₃ region and Ni₃Ti precipitates of hardened 60NiTi were measured using nanoindentation. Scanning electron microscopy and energy dispersive spectroscopy were used to image and identify the phases present in the material. A cube corner diamond tip with a 100-nm tip radius was then used to image and indent on the NiTi + Ni₄Ti₃ region and the microscale globular Ni₃Ti precipitates by utilizing the in-situ scanning probe microscopy capability of the nanoindenter. The Ni₃Ti phase was found to have a significantly higher modulus of elasticity and hardness compared to the NiTi + Ni₄Ti₃ region, which has properties likened to those of bulk 60NiTi. The results alluded that the Orowan strengthening enabled by the Ni₄Ti₃ nanoscale precipitates is largely responsible for the bulk hardness of the material. It is important to note that, despite their high elastic modulus and hardness, the microscale Ni₃Ti precipitates do not enhance the mechanical properties of 60NiTi.