Gas nitriding of an equiatomic TiNi shape-memory alloy Part I: Nitriding parameters and microstructure characterization

Ti50Ni50 shape-memory alloy is gas-nitrided to modify the surface condition . The phases and microstructures of the nitrided surface are studied by usi ng X-ray diffraction, electron probe microanalysis and scanning electron mi croscopy. Experimental results indicate that the specimen nitrided at 900 d egrees C exhibits a smooth surface morphology, but for the specimen nitride d at 600 degrees C, a large number of surface cracks exist. The weight gain s of specimens nitrided at temperatures below 650 degrees C are much higher than those of specimens nitrided at temperatures above 700 degrees C. Thes e weight gains are found to increase gradually with an increase in nitridin g time, and then approach saturated values at 24 h. The 900 degrees C nitri ded specimen consists of TiN and Ti2NiH0.5, two distinct layers. The surfac e layer of the 600 degrees C nitrided specimen has two regions: one is a ra ndom mixture of TiN, Ti2NiH0.5 and a nickel-rich phase, the other is a colu mnar-like structure of mixed TIN and nickel-rich phase. Reaction mechanisms to explain the formation of gas-nitrided layers at 600 degrees C and 900 d egrees C are also examined in this study. (C) 1999 Elsevier Science S.A. Al l rights reserved.