Gas nitriding of an equiatomic TiNi shape memory alloy II. Hardness, wear and shape memory ability

Ti50Ni50 shape memory alloy was gas nitrided to modify the surface conditio ns. The surface hardness, wear characteristic, transformation temperature a nd shape memory ability of gas-nitrided Ti50Ni50 alloy were investigated. E xperimental results indicate that the surface hardness is increased owing t o the formation of TIN and Ti2NiH0.5 compounds. The Ti50Ni50 specimens nitr ided at 700-900 degrees C show improved wear characteristics, but those nit rided at 600 degrees C cannot be effectively improved owing to surface crac ks appearing in nitrided layers. Martensitic transformation temperatures ar e depressed slightly owing to the constraining effect originating from the nitrided layers, and/or the penetration of N and H atoms into the Ti50Ni50 matrix. The shape recovery is also slightly reduced because the nitrided la yers do not exhibit a shape memory effect, and the constraining effect will also depress the shape recovery of the Ti50Ni50 matrix. (C) 1999 Elsevier Science S.A. All rights reserved.