Ideally, leaving aside the possibility of phases forming with different compositions, the most stable phase (ie that with the lowest free energy) should be present at any given temperature. However, in reality a phase may persist beyond the temperature range in which it is thermodynamically stable. This is because a driving force is often required in order to form the new phase within the existing phase. For example, a common cause of this is a nucleation barrier, which is associated with the large interfacial energy contribution, per unit volume, for small transformed volumes.
A further potential source of such behaviour, which applies only when both phases are solid, and is particularly important for shear transformations, is the stored elastic strain energy that arises when a region undergoes a shape change as it transforms. Unlike the case of a nucleation barrier, the energy penalty associated with this elastic strain continues to rise as larger volumes of material transform. The upshot of this is that pronounced hysteresis is commonly observed in the phase transformations that occur during thermal cycling of SMAs.
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- The austenitic phase start temperature, As, which is the temperature where the martensitic phase begins to transform into the parent (austenitic) phase.
- The austenitic phase end temperature, Af, which is the temperature where the martensitic phase has completely transformed in the austenitic phase.
- The martensite start temperature, Ms, which is the temperature where the austenitic phase starts to transform into the martensite phase.
- The martensite end temperature, Mf, which is the temperature where the austenitic phase has completely transformed into the martensite phase.
These temperatures are not very well-defined, since they are dependent on experimental conditions (for example heating and cooling rates).