Gas nitriding of an equiatomic TiNi shape-memory alloy

Abstract

Abstract Ti 50 Ni 50 shape-memory alloy is gas-nitrided to modify the surface condition. The phases and microstructures of the nitrided surface are studied by using X-ray diffraction, electron probe microanalysis and scanning electron microscopy. Experimental results indicate that the specimen nitrided at 900 °C exhibits a smooth surface morphology, but for the specimen nitrided at 600 °C, a large number of surface cracks exist. The weight gains of specimens nitrided at temperatures below 650 °C are much higher than those of specimens nitrided at temperatures above 700 °C. These weight gains are found to increase gradually with an increase in nitriding time, and then approach saturated values at 24 h. The 900 °C nitrided specimen consists of TiN and Ti 2 NiH 0.5 , two distinct layers. The surface layer of the 600 °C nitrided specimen has two regions: one is a random mixture of TiN, Ti 2 NiH 0.5 and a nickel-rich phase, the other is a columnar-like structure of mixed TiN and nickel-rich phase. Reaction mechanisms to explain the formation of gas-nitrided layers at 600 °C and 900 °C are also examined in this study.