Fatigue-crack growth behavior in the superelastic and shape-memory alloy nitinol
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NITINOL is a thermoelastic alloy with a composition of approximately 50 at. pct Ni and 50 at. pct Ti, which is capable of two successive athermal martensitic phase transformations on cooling from its higher temperature austenite (B2-CsCl crystal structure) phase. The first of these is observed below ⬃25 ⬚C and results in the “R phase” (rhombohedral crystal structure); the second occurs typically below ⬃0 ⬚C and results in a monoclinic structure, often with a fine lath morphology. These transformations provide Nitinol with its shape-memory behavior, which results from a unique set of material properties, including thermoelasticity,* a low *Thermoelasticity is a property of a material that undergoes a martensitic transformation where the laths, which grow continuously on cooling, revert along the same crystallographic path when the alloy is heated.
driving force to nucleate a second phase, and a prominent role of twin deformation. In addition, Nitinol can exhibit superelasticity, with recoverable “elastic” strains up to ⬃8 pct, at temperatures slightly above the austenite finish temperature, Af ; this effect is attributed to a stress-induced reversible martensitic transformation. Because of its shape-memory properties, Nitinol has been used for such applications as pipe couplings and actuators. Its superelastic properties, on the other hand, are used in eyeglass frames, mobile phone antennas, dental braces, and most recently in the biotechnology industry, principally for endovascular stents. All these applications involve repetitive loading, yet surprisingly little information exists on the fatigue-crack growth properties of Nitinol; indeed, to our knowledge, the literature contains only three previous studies.[1,2,3] Moreover, none of these studies provide a systematic
A.L. McKELVEY, formerly Graduate Student, Department of Materials Science and Mineral Engineering, University of California, is Research Scientist, Materials Science Department, Ford Research Laboratory, Dearborn, MI 48121-2053. R.O. RITCHIE, Professor, is with the Department of Materials Science and Engineering, University of California, Berkeley, CA 94720-1760. Manuscript submitted April 27, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
examination on the role of temperature, microstructure, and constitutive behavior on crack-growth rates in NiTi.* *Dauskardt et al.[3] did attempt to study the effect of constitutive behavior on rates of fatigue-crack propagation; however, this was achieved by comparing the properties of alloys with different compositions, rather than in the same material.
Accordingly, the focus of this work is to evaluate the effects of temperature, microstructure, and the presence of the reversible stress-induced martensitic phase transformation on the growth of fatigue cracks in a biotechnology Nitinol alloy, specifically by comparing behavior in the stable austenite, superelastic austenite, and martensite structures. II. EXPERIMENTAL PROCEDURES A. Material The Nitinol alloy, which was received as 41.3-mm-diameter rod wi
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