Experimental studies on tribological properties of pseudoelastic TiNi alloy with comparison to stainless steel 304
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INTRODUCTION
NEAR equiatomic TiNi alloy is a well-known shape memory alloy.[1–5] The shape memory effect of this alloy results from a thermoelastic martensitic transformation occurring around room temperature. On cooling, the parent phase (b) having an ordered B2 (CsCl) structure transforms to a martensite phase that has a monoclinic structure of B19 type.[6,7] This martensitic transformation is thermoelastic or reversible and can be induced either by changing temperature or by applying a stress. When temperature decreases to the starting temperature of martensitic transformation, Ms , the martensite begins to form and the transformation completes when the temperature is below the martensite finishing point, Mf. The martensitic transformation, b → M, may occur above Ms when the alloy is under an external stress. This process is reversible and the initial b phase can be recovered as the stress is removed. As a result of the reversibility of the martensitic transformation, TiNi alloy exhibits a special mechanical behavior termed pseudoelasticity.[8] With the pseudoelasticity, the TiNi alloy behaves like an elastic spring and the recoverable strain is usually in the range of 5 to 8 pct or even higher.[9,11] Theoretically, all shape memory alloys may exhibit pseudoelasticity, but whether a shape memory alloy has good pseudoelasticity depends on many factors, such as crystal structures of the martensite and the parent phase, microstructure, and thermal history.[2,5] For example, dislocations and inclusions would hinder the martensitic transformation, thus deteriorating the pseudoelasticity. Recently, the TiNi alloy was found to exhibit excellent wear resistance to erosion and abrasive wear.[12–20] This high wear resistance makes the TiNi alloy more attractive, in addition to its shape memory effect that has been extensively RONG LIU, Postdoctoral Fellow, and D.Y. LI, Assistant Professor, are with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada T6G 2G6. Manuscript submitted July 28, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
studied and used for many medical, electrical, and mechanical applications.[9,11] Different from conventional wear-resistant materials, hardness is not the main parameter that dominates the wear resistance of this alloy.[14,15] It was observed that the resistance of TiNi alloy to erosion or abrasion was strongly dependent on its chemical composition as well as microstructure. When the composition of a TiNi alloy falls into the range in which the reversible martensitic transformation takes place, this TiNi alloy exhibits superior wear resistance. It was suggested that the pseudoelasticity played an important role in producing high wear resistance of TiNi alloy.[12–15] Liang et al.[12] investigated the performance of TiNi alloy during sliding wear, impact abrasion, and sand-blasting erosion. They demonstrated that TiNi specimens with good pseudoelasticity exhibited higher wear resistance than those having no pseudoelasticity. Shida and Sugimoto[1
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