Definition of a Shape Memory Alloy

Shape Memory Alloys (SMAs) are a unique class of metal alloys that can recover apparent permanent strains when they are heated above a certain temperature. The SMAs have two stable phases - the high-temperature phase, called austenite and the low-temperature phase, called martensite. In addition, the martensite can be in one of two forms: twinned and detwinned, as shown in Figure 1. A phase transformation which occurs between these two phases upon heating/cooling is the basis for the unique properties of the SMAs. The key effects of SMAs associated with the phase transformation are pseudoelasticity and shape memory effect.

Different phases of an SMA

Figure 1. Different phases of an SMA.

Upon cooling in the absence of applied load the material transforms from austenite into twinned (self-accommodated) martensite. As a result of this phase transformation no observable macroscopic shape change occurs. Upon heating the material in the martensitic phase, a reverse phase transformation takes place and as a result the material transforms to austenite. The above process is shown in Figure 2. Four characteristic temperatures are defined in Figure 2: martensitic start temperature (M0s) which is the temperature at which the material starts transforming from austenite to martensite; martensitic finish temperature (M0f), at which the transformation is complete and the material is fully in the martensitic phase; austenite start temperature (Aos) at which the reverse transformation (austenite to martensite) initiates; and austenite finish temperature (Aof) at which the reverse phase transformation is completed and the material is the austenitic phase.

Temperature-induced phase transformation of an SMA without mechanical loading

Figure 2. Temperature-induced phase transformation of an SMA without mechanical loading.