The Interaction of Point Defects with the Martensitic Transformation: A Prototype of Exotic Multiscale Phenomena
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The Interaction of
Point Defects with the Martensitic Transformation: A Prototype of Exotic Multiscale Phenomena
Xiaobing Ren and Kazuhiro Otsuka Abstract The martensitic transformation has so far been studied without considering its interaction with point defects. In this article, we shall show that such interaction, which stems from a universal symmetry property of point defects, can create a rich spectrum of exotic multiscale phenomena in martensitic materials. These phenomena include unique short-range diffusion at the atomic or nano level, remarkable domain-pattern memory at the mesoscopic level, and peculiar rubber-like behavior and aging-induced two-way shape memory at the macroscopic level. Exotic multiscale phenomena may also be found in a wide range of transforming materials, such as ferroelastic, ferroelectric, and ferromagnetic materials. These novel effects may provide new opportunities for these important materials. Keywords: atomic diffusion, intermetallic alloys, phase transformation, shape memory.
Introduction Point defects, such as vacancies, solute atoms, impurities, or antisite defects in ordered compounds, are ubiquitous in crystalline solids and are important in controlling many physical and mechanical properties of materials. Their critical role in diffusional transformation has long been recognized and well understood; in sharp contrast, their role in martensitic/diffusionless transformation is largely unknown and has been neglected for a long time. Traditionally, it has been considered that all features of martensite can be understood once the lattice correspondence between the parent phase and the martensite is es-
MRS BULLETIN/FEBRUARY 2002
tablished because the martensitic transformation involves only a diffusionless lattice distortion. In fact, as martensitic alloys always contain point defects, they can produce many unexpected interesting effects through their interaction with the martensitic transformation and during the subsequent aging/diffusion process. In this article, we shall give a concise review of recent advances in the understanding of the important role of point defects in martensitic transformations and associated exotic multiscale phenomena. We shall show that all these interesting effects stem from an important and general
symmetry property of point defects.1,2 Due to the generality of this symmetry property, we expect that novel multiscale phenomena may also be found in a wide range of materials.
Historical Background Before explaining the central idea of the symmetry property of point-defect configurations, we first give a brief historical account of the field closely related to the present topic, because the symmetry concept of point defects actually developed out of this background. Since 1932,3 it has been known that a number of martensitic alloys (including Au-Cd,3,4 Au-Cu-Zn,5 Cu-Zn-Al,6,7 Cu-Al-Ni,8 In-Tl,9,10 Ti-Pd,11 Ti-Pd-Ni,12 and Ni-Ti-Hf13) show an unusual kind of deformation behavior in the martensitic state. These alloys, after aging for some time
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