Dissemination of IT for the Promotion of Materials Science (DoITPoMS)

DoITPoMS Teaching & Learning Packages Superelasticity and Shape Memory Alloys Superelasticity - Strain Accommodation by Martensite Formation

Superelasticity - Strain Accommodation by Martensite Formation

Superelasticity (SE), sometimes termed “pseudo-elasticity” or “pseudo-plasticity”, occurs without any change in temperature. SE takes place at temperatures above As - although usually only slightly above  -  where the austenitic phase is the more stable of the two thermodynamically, although not by very much.  When a mechanical strain is imposed, this can stimulate the transformation of austenite to martensite, sometimes termed “stress-induced martensite”.  The associated shear of local regions accommodates the imposed macroscopic shape change, while the lower strain energy component ensures that the overall free energy is now lower than it would be if the austenitic phase were still predominant.

Relatively large strains (up to about 8%) can be accommodated in this way.  As the strain is increased, the proportion of the specimen that has transformed to martensite progressively rises.  This occurs without much increase needed in the applied stress, giving rise to a characteristic “superelastic plateau”.

These strains are much higher than would normally be possible during conventional elastic deformation (up to ~0.5% for most metals).  Nevertheless, they are recoverable when the applied load is removed. When this is done, the material reverts to the austenitic phase. Since this tends to occur by the individual martensite crystallites shearing back to the austenite crystals from which they were formed, the original specimen shape is recovered.  However, this is only possible if all of the deformation (apart from that due to conventional elasticity) has been achieved by martensitic phase transformations.  If an excessive strain is imposed, then it is likely that some conventional plastic flow (dislocation glide) will occur, and of course this will be irreversible.