5.1 Introduction

Shape memory alloy (SMA) is the generic name given to alloys which exhibit the unusual property of a strain-memory which can occur either at constant temperature, where large yet recoverable strains are possible (superelasticity), or on changes in temperature, where apparently permanent strains can be fully recovered (thermal shape memory).

These memory effects have their origin in a particular type of phase transformation (change in internal crystal structure) which produces a microstructural constituent known as martensite. A martensitic transformation is displacive, that is it occurs through a shearing of the crystal structure from the so-called parent-phase to that of the martensite. This is illustrated schematically in the two-dimensional analogue shown in Fig. 5.1. Such transitions are diffusionless, resulting in no change in chemical composition, and form the basis of heat treatments in many metallic materials, including the familiar transformation of austenite to martensite in ferrous (iron-based) alloys. Thermally induced martensitic transformations occur over a falling temperature interval (Fig. 5.2). The transformation proceeds at the M _s (martensite start temperature) where the martensite phase first appears, and progresses athermally (occurring over a falling temperature interval) until at the M _f temperature transformation is complete. On heating, at the A _s temperature the last martensite formed during forward transformation begins to revert to parent-phase, and there is then continuous reversion until at the A _f temperature the...