The polymorphic transformation of the solid solution upon heating and cooling consists of the lattice rearrangement and diffusion redistribution of the components between the phases. It is usually called normal. The normal β → α recrystallization occurs as a result of disordered transitions of atoms across the interface and is accompanied by the redistribution of the atoms of the components, i.e., their diffusion over large distances. This is realized with the slow cooling.
In some cases, the accelerating cooling, β → α-recrystallization can be prevented and the alloy at room temperature will consist of crystals of the metastable β solution.
In many cases, however, the accelerated cooling succeeds in inhibiting only the diffusion redistribution of components between the initial and the resulting solid solutions. The lattice β → α reconstruction occurs. Such non-diffusion recrystallization becomes possible if the alloy is super cooled below the temperature at which the thermodynamic potentials of α- and β-solutions of the same composition are the same. In the alloy shown in Fig. 1, this is observed at Т=Тd.
With the accelerated cooling to temperatures below Td, the solution of β can turn into an α-phase without changing the composition, although this will lead to the smaller decrease in the thermodynamic potential of the alloy than conversion to the two-phase mixture of α + β. To complete the diffusion-free β → α transformation, it may not be necessary to cool the alloy below the temperature Te.
Above the temperature Te, the solution of this composition is metastable and the prolonged residence under these conditions will lead to the transition to the two-phase mixture of α + β. Thus, the accelerated cooling of the β-solution to the temperature in the range of Тd-Тe can lead to the formation of the metastable solid solution α of the same composition as the initial β-solution. If the diffusion less polymorphic transformation occurs at temperatures below Te, the resulting α-phase is stable. At the room temperature, it will cause the decrease in the thermodynamic potential Zт—Zд (see Fig. 1, e).
The diffusion less polymorphic transformations of solid solutions often occur due to the ordered shear collective transition of atoms from one phase to another. This transformation is also called martensitic, and the resulting solution is called martensite. The martensitic transformation begins when the alloy is cooled to the certain temperature ((Мн), depending on the composition of the initial solution, and leads to the formation of acicular and lamellar crystals.
In addition to the normal and martensitic polymorphic transformations, the intermediate transformations can occur in alloys.