Shape Memory Alloys and Their Applications in Power Generation and Refrigeration
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Shape Memory Alloys and Their Applications in Power Generation and Refrigeration Jun Cui Pacific Northwest National Laboratory, Richland, WA 99354, U.S.A. ABSTRACT The shape memory effect is closely related to the reversible martensitic phase transformation, which is diffusionless and involves shear deformation. The recoverable transformation between the two phases with different crystalline symmetry results in reversible changes in physical properties such as electrical conductivity, magnetization, and elasticity. Accompanying the transformation is a change of entropy. Fascinating applications are developed based on these changes. In this paper, the history, fundamentals and technical challenges of both thermoelastic and ferromagnetic shape memory alloys are briefly reviewed; applications related to energy conversion such as power generation and refrigeration as well as recent developments will be discussed. INTRODUCTION A shape memory alloy (SMA) is a functional material with great potential for various engineering applications. SMAs can recover their original shape from deformed configurations when conditions such as temperature, stress, magnetic field, electrical field, etc., are suitably changed. An SMA absorbs and releases a large amount of latent heat when undergoing a phase transformation induced by temperature, stress or a magnetic field. These physical properties enable SMAs for various applications such as sensing, actuation, heat engines for power generation, and heat sinks or active cooling for temperature control. A number of SMAs have been developed in the past several decades [1]. Important SMAs include AuCd, CuAlNi, CuZnAl, FePd, FePt, NiTi, NiTiCu, NiTiPd, NiMnGa, NiMnAl, NiMnIn, and NiMnSn. The most widely used SMA is NiTi with composition near Ni-50 at.%Ti. It was serendipitously discovered in 1961 by W. Buehler at the Naval Ordnance Laboratory— thus the name Nitinol—and later understood through the dedicated work of F. E. Wang [2]. Nitinol’s high-temperature parent phase (austenite) has an ordered cubic (B2) crystal structure, and its low-temperature product phase (martensite) has an ordered monoclinic (B19') crystal structure; it also has another intermediate rhombohedral phase (B2') often referred to as the R-phase. A martensitic phase transformation is diffusionless and involves crystallographic shearing deformation. It reduces the symmetry of the parent phase and results in formation of crystallographic domains with several possible geometric arrangements. These possible domains are referred to as martensitic variants. They can be described by the transformation stretch tensor. The exact forms of the variants can be derived if lattice parameters of both the austenite and the martensite phases are known [3]. For example, there are six martensitic variants for a transformation from cubic to orthorhombic. In the case of transformation through face-diagonal stretch, the exact forms of the variants are
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