Gas nitriding of an equiatomic TiNi shape memory alloy

Abstract

Ti 50 Ni 50 shape memory alloy was gas nitrided to modify the surface conditions. The surface hardness, wear characteristic, transformation temperature and shape memory ability of gas-nitrided Ti 50 Ni 50 alloy were investigated. Experimental results indicate that the surface hardness is increased owing to the formation of TiN and Ti 2 NiH 0.5 compounds. The Ti 50 Ni 50 specimens nitrided at 700–900°C show improved wear characteristics, but those nitrided at 600°C cannot be effectively improved owing to surface cracks appearing in nitrided layers. Martensitic transformation temperatures are depressed slightly owing to the constraining effect originating from the nitrided layers, and/or the penetration of N and H atoms into the Ti 50 Ni 50 matrix. The shape recovery is also slightly reduced because the nitrided layers do not exhibit a shape memory effect, and the constraining effect will also depress the shape recovery of the Ti 50 Ni 50 matrix.