Correct option is C
The characteristic deformation in Shape Memory Alloys (SMAs) is known as thermo-elastic deformation, which involves a reversible phase transformation between:
Martensite (low temperature/low stress phase)
Austenite (high temperature/high stress phase)
This transformation is not due to slip or dislocation movement (as in plasticity), but due to shear-like lattice distortions. The entire process is thermally and stress driven, and thus termed thermo-elastic.
Shape Memory Alloys do not deform permanently like conventional metals. Instead, their primary deformation mechanism is a thermo-elastic martensitic transformation, which is a reversible, diffusionless, solid-state phase transformation:
Deformation Mechanism:
At low temperature (martensite state), deformation occurs by detwinning and reorientation of martensite variants.
When heated above the transformation temperature, martensite transforms back to austenite, and the material regains its original shape (Shape Memory Effect).
At higher temperatures, stress alone can induce reversible martensite, enabling superelasticity (large recoverable strains upon unloading).
Thus, the deformation is not permanent plasticity but a recoverable structural transformation.
