Some simple features of the thermodynamics relevant to phase transformations are given here - see Ellingham Diagram TLP. The quantity of primary concern here is the Gibbs free energy, G.
The animation below shows the stability of two phases as a function of temperature. For superelastic and shape memory alloys, the two phases are normally termed “austenite” (stable at higher temperatures) and “martensitic” phase (stable at lower temperatures). Sometimes, the austenitic phase is termed the “parent” phase.
Note: This animation requires Adobe Flash Player 8 and later, which can be downloaded here.
It is important to be clear that this terminology is generic – ie these phases are not any specific ones, but refer to a type of phase. In particular, it is important to avoid any confusion with the austenite and martensite phases that form in steels. As it happens, while the austenite-to-martensite transformation that occurs in the Fe-C system obviously is a martensitic transformation, it is crystallographically complex and exhibits certain rather special characteristics. Moreover, for reasons that need not be detailed here, superelasticity and shape memory behaviour are NOT normally exhibited by steels.
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