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

DoITPoMS Teaching & Learning Packages Superelasticity and Shape Memory Alloys Limits of Superelasticity
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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Limits of Superelasticity

There are limits to the temperature and stress ranges within which superelastic deformation can occur.  The stress needed to initiate the austenite-martensite transformation rises with increasing temperature (ie as the austenite phase becomes thermodynamically more stable).  This dependence is predicted by a form of the Clausius-Clapeyron equation. \[\frac{{{\rm{d}}\sigma }}{{{\rm{d}}{M_{\rm{s}}}}} = \frac{{ - \Delta H}}{{T{\varepsilon _0}}}\]

where ΔH is the latent heat of the transformation and ε0 is the associated strain.  {this eqn. needs checking }  At temperatures well above the stress-free value of Ms, quite substantial stresses may be needed to stimulate martensite formation.  Furthermore, the stress needed to induce dislocation motion (in the austenite phase) is likely to fall with increasing temperature.

The temperature at which these two processes (slip and martensite formation) require the same applied stress will be the upper limit for both superelastic deformation and the shape memory effect (since slip will occur preferentially above this temperature).  Superelastic behaviour requires a minimum temperature of Af, since the specimen should be fully austenitic initially.  The shape memory effect not only requires heating to above Af, but also cooling down to Mf

Therefore SE occurs at temperatures between Ms and Md where Md is the temperature at which slip becomes easier than the formation of the martensitic phase.

Others limits

It’s also possible for local defects to accumulate during repeated transformation, which can reduce the achievable strain and the force that can be exerted by the transformations.  Also, excessive deformation, beyond that which can be accommodated by transformation to martensite, will lead to irreversible strain (plastic deformation by slip).

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