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

DoITPoMS Teaching & Learning Packages Superelasticity and Shape Memory Alloys Martensitic Phase Transformations - Basic Thermodynamics
Superelasticity and Shape Memory Alloys AimsBefore you startIntroductionMartensitic Phase Transformations - A Simple ExampleMartensitic Phase Transformations - Basic ThermodynamicsMartensitic Phase Transformations - Hysteresis CharacteristicsSuperelasticity - Strain Accommodation by Martensite FormationSuperelasticity - Hysteresis in the Stress-Strain BehaviourShape Memory Effect - "Training" of the TransformationShape Memory Effect - The "Ferris Wheel" ExperimentMicrostructural Changes during Thermo-Mechanical TreatmentLimits of SuperelasticityApplicationsSummaryQuestionsGoing further
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Martensitic Phase Transformations - Basic Thermodynamics

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.

It is important to be clear that this terminology is generic – i.e., 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.

Footnote: Although commonly presented in this form, it's not strictly correct for the free energy to be shown as decreasing with increasing temperature. It actually increases, despite the -TS term, because the enthalpy increases approximately linearly with temperature (with the proportionality constant being the specific heat of the material).  However, when comparing two phases (with different entropies, but similar specific heats) in this way, the changes in enthalpy are often neglected.  Both plots then decrease with increasing T (due to the -TS term), with the decrease being at a higher rate for the (more disordered) phase with higher entropy. (This is why highly disordered phases, such as liquids and gases, tend to be stable at higher temperatures.)

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