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Introduction to Shape Memory Alloys Shape memory alloys (SMAs) are metallic alloys which can recover permanent strains when they are heated above a certain temperature.The key characteristic of all SMAs is the occurrence of a martensitic phase transformation.
The parent austenitic phase (indicated by A in Figure 2) in the absence of applied stress will transform upon cooling to multiple martensitic variants (up to 24 variants for the cubic-to-monoclinic transformation) in a random orientation and in a twinned configuration (indicated by B).
As the multivariant martensitic phase is deformed, a detwinning process takes place, as well as growth of certain favorably oriented martensitic variants at the expense of other variants.
By applying mechanical loading to force martensitic variants to reorient (detwin) into a single variant, large macroscopic inelastic strain is obtained.
After heating to a higher temperature, the low-symmetry martensitic phase returns to its high-symmetry austenitic phase, and the inelastic strain is thus recovered. As a result of the martensitic phase transformation, the stress-strain response of SMAs is strongly non-linear, hysteretic, and a very large reversible strain is exhibited.
Also, note in Figure 2 that, in going from A to B many variants will start nucleating from the parent phase, while in going from D to E there is only one variant of the parent phase that nucleates from the single remaining martensitic variant indicated by D. Schematic representation of the thermomechanical loading path demonstrating the shape memory effect in an SMA.