On the Detwinning Mechanism in Shape Memory Alloys

Abstract

Why do some materials exhibit shape memory effect while others do not? Taking steel and NiTi, for example, in both materials a martensitic transformation takes place when the temperature is lowered, and a reverse transformation occurs when they are subsequently heated [1–6]. Upon heating, steels normally do not show a well defined shape memory effect while NiTi does. One of the major differences between these two materials is the deformation mechanism. When steel is stressed beyond its yield limit in martensite, dislocations are generated and are responsible for the observed plastic deformation. However, when a martensitic NiTi is stressed beyond its yield limit, a “detwinning” process is responsible for the observed inelastic deformations [7–12]. This inelastic deformation can reach about 6% strain without a significant increase in dislocation density. The critical stress for activating the detwinning process is lower than that for dislocation generation and, owing to an insignificant dislocation process, the associated shape change can be partially or even fully restored through a reverse phase transformation. This phenomenon is widely observed in shape memory alloys (SMAs). What makes SMAs so attractive is their unique combination of various novel properties including the shape memory effect, superelasticity, high damping capacity, good fatigue and wear resistance, high kinetic output per volume and, of significant importance, an excellent biocompatibility of NiTi. As listed in Figure 1, most of these properties are somehow related to the detwinning process.