Crystallographic Characteristics of Shape Memory Alloys

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Crystallographic Characteristics of Shape Memory Alloys Madangopal Krishnan Materials Science Division, Bhabha Atomic Research Centre, Mumbai 400 085, India.

ABSTRACT The martensitic microstructure of shape memory alloys is an aggregate of self accommodating plate groups. The principal character of this aggregate is its demonstration of pseudoplasticity, wherein a macroscopic shape change is brought about by extensive rearrangement and reorientation of the self accommodating martensite variants, and that of microstructural reversibility, wherein the polyvariant microstructure transforms back to the original grain of the parent phase after pseudoplastic deformation. These aspects of the shape memory intermetallic alloys are intriguing and, to a great extent, unsolved. The aim of this paper is to show that the underlying crystallographic interrelationships of the self accommodating microstructure of intermetallic alloys are responsible for the observed effect. The paper will discuss the relation between autocatalytic nucleation and self accommodation, the relation between microstructural reversibility and intervariant interfaces of the martensitic microstructure and the manifestations of microstructural irreversibility using results from the microstructural examination of the self accommodating microstructures. INTRODUCTION The schematic in figure 1 provides an explanation for the shape memory phenomenon in intermetallic alloys. In Ni-Ti alloys for instance, when a single crystal of the B2 phase of a nearequiatomic Ni-Ti alloy is cooled to room temperature, it transforms into a poly-variant microstructure made of 12 orientational variants of the monoclinic B19’ crystal. As seen in figure 2, the peculiar feature of such microstructures in various shape alloys is that the martensite variants are arranged in self-accommodating patterns. By this it is suggested that the formation of the martensitic microstructure is guided by a tendency to minimize the overall transformation strain. When such microstructures are strained, deformation is caused by the movement of interfaces between the orientational variants. In other terms, unlike most metallic alloys, wherein plastic deformation is dissipative and occurs by movement of crystal defects such as dislocations, the deformation in these alloys is essentially a macroscopic shape change brought about by rearrangement of the orientational variants of the martensite. This kind of deformation is known as pseudoplastic deformation. Thus, the martensitic condition is like putty, wherein every arrangement within the microstructure is equivalent to all others and the transformation between them is brought about by overcoming the frictional resistance to movement of the intervariant interfaces, without introducing actual shape setting plastic deformation. As shown in figure 1, the reverse transformation of the self-accommodating poly-variant microstructure of thermoelastic alloys gives rise to the original single crystal of parent austenite phase. Figure 3 shows that this is not usual

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