Effect of Surface Oxidation on Shape Memory Behaviour of NiTi Alloy

摘要

Heat treatment of Nickel Titanium (NiTi) shape memory alloy at high temperature will result in formation of titanium oxide (TiO_2) layer at the surface. TiO_2 is a ceramic with higher value of hardness and does not exhibit martensitic phase transformation, thus affects the shape memory behaviour of the NiTi core. This work studies the effect of formation of TiO_2 layer on the surface towards the shape memory behaviour of near-equiatomic NiTi alloy. The NiTi plate with composition of Ti-50.2at%Ni was subjected to heat treatment at temperature of 723K to 1173K for 30 and 60 minutes. The formation of TiO_2 layer was observed using scanning electron microscope. The change in surface hardness of heat treated NiTi plate was characterized by Vickers hardness test and nano-indentation. The results showed that surface hardness increased as the thickness of the TiO_2 layer increased. Subsequently, the austenite finish and martensite start temperatures were also shifted to higher temperature to correspond to the effect of TiO_2 presence at the surface.