The polymorphic metals atomic packing provide the minimum value of the free energy potential in some circumstances and become unstable in others circumstances. In this case at the temperature and pressure change there is a polymorphic transformation. The modifications of polymorphic metal passing one into another with increasing temperature, designated by the α, β, γ and δ letters.
Fig. 1 shows the phase-rule diagrams and changes of the free energy potential of polymorphic metal in a solid state at the constant pressure. In the temperature range lying above T0, the stable β is the modification as it has smaller free energy potential. The α modification exists in these conditions only in a metastable condition. If T <T0 the stable atomic packing inherent α modification because its free energy potential is lower than the β modification. At T = T0, both modifications have the same free energy potentials and stably exist. The transitions of one modification to another are caused by the pressure and temperature change. The temperature reduction below Т0 will change β modification into α. The temperature increase above Т0 will change α modification into β. The metal free energy potential in these transformations is reduced.
The recrystallization of polymorphic metals has much in common with the crystallization it also results from the germs formation and the subsequent growth of the new phase. The phase transformations in a solid state have the features as the original phase is not a liquid, but crystal.
The first feature is related to the metal specific volume change. If during the consolidation the volume changes considerably have no effect on the germs formation owing to the fluidity (pliability) of fusion, then at the recrystallization their role is large. The metal volume change is followed by the deformation of the original and the resulting phases, which requires the additional energy. As a result, the energy incentive transformation, the difference of the free energy potentials, is reduced by amount of the energy deformation.
Fig.1. The diagrams of the phase states (top) and the free energy potentials (bottom) of metal with two (a) three and (b) polymorphic modifications.