Constrained phase-transformation of a TiNi shape-memory alloy
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I. INTRODUCTION
PSEUDOELASTICITY, the shape-memory effect, and a high damping capability have made shape-memory alloys (SMAs) attractive as functional materials in engineering and medical applications.[1,2] If a prestrained SMA is prevented from shape recovery during the transformation from martensite to austenite, a large stress of up to several hundred milli Pascals against the external constraint is generated. This stress is called recovery-stress, which is typically characterized by an almost linear increase with increasing temperature.[3–6] For SMAs as actuators, the recovery stresses are usually used to overcome some external stress or strain. The SMAs have been applied not only as actuators, but also as reinforcement elements in smart composites or structures in recent years.[7–13] It is well known that the thermoelastic martensitic transformation is sensitive to both stress and temperature. Thus, the constrained phase transformations of SMAs associated with recovery-stress are more difficult to understand than unconstrained transformation. The transformation induces the recovery-stress, which, in turn, affects the transformation. Up to now, a clear understanding of such constrained transformations and their influence on the subsequent thermomechanical characteristics of SMAs is still lacking. The purpose of the present work is to investigate phase transformations and associated mechanical characteristics of a prestrained TiNi SMA during constrained thermal cycling. The constrained condition here refers to a constant uniaxial Y. LI, Postdoctoral Fellow, and H.B. XU, Professor, are with the Department of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing, 100083, People’s Republic of China. Contact e-mail: li [email protected] L.S. CUI, Professor, is with the Department of Materials Science and Engineering, University of Petroleum, Beijing, 102200, People’s Republic of China. D.Z. YANG, Professor, is with the Department of Materials Engineering, Dalian University of Technology, Dalian, 116024, People’s Republic of China. Manuscript submitted January 14, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
mechanical strain, which is kept constant and does not allow for shape recovery. II. EXPERIMENTAL PROCEDURES A commercial polycrystalline Ti-50.2 at. pct Ni alloy wire of 0.48 mm in diameter was supplied by the General Research Institute for Non-Ferrous Metals (Beijing, China). The material was annealed at 973 K in vacuum for 1.2 ks and then cooled in air. The martensite start and finish temperatures (Ms and Mf , respectively) and the reverse-transformation start and finish temperatures (As and Af , respectively) of the material, as measured by a differential scanning calorimeter (DSC), are 302, 287, 318, and 338 K, respectively. In order to study the constrained transformation behaviors of a TiNi SMA, tensile deformation and constrained thermal cycling were imposed, as illustrated in Figure 1. The wires in the martensitic state were first deformed in tension and unl
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