Phase transformation characteristics of laser gas nitrided NiTi shape memory alloy

Abstract

Laser gas nitriding (LGN) process is an efficient technique for modifying the surfaces of Ti and NiTi alloys. The process inevitably alters the composition, the microstructure and the properties of the alloyed surface. For components with thick cross sections, the changes of composition or microstructure at the surface may not affect the bulk properties of the materials. However, this may not be the same case if the cross-section thickness is small as compared to the thickness of the modified region. In this instance, when the cross section of the NiTi component is thin, the shape memory behaviour of NiTi alloy may be influenced as a result of the surface modification process. The present work aims to study the effect of the existence of the laser gas nitrided (LGN) layer of NiTi upon the phase transformation temperatures of the alloy. The changes in compositions in the LGN layer were chemically analysed using X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy. The microstructure of the layer was analysed by X-ray diffraction. Differential Scanning Calorimetry (DSC) was used to measure the reversible B19′ to B2 transformation temperatures. The results showed that the matrix of LGN layer retains the same composition as the substrate alloy. The LGN process did not alter the phase transformation characteristics of the alloy. The LGN layer itself has the same transformation characteristics as the parent metal.