The growth rate Up of the new modification crystals depends on overcooling. With increase in overcooling ΔT the difference of chemical potentials Δμ grows and Uр increases. However with the temperature falls the diffusive mobility of atoms decreases. In case of large overcooling this reduction blocks influence of overcooling on Δμ and the crystals growth is slowed down; on the curve dependences of Uр from ΔT there is a falling branch (see Fig. 1) which isn’t on curves of metals crystallization.
If the rapid cooling is possible to prevent the β →α transformation up to low temperatures (for example Тв, see Fig. 2),
that high-temperature modification is long exists in the overcooled condition. In case of Т = Тв stable is α – modification, however because of the small speed of polymorphic transformation β modification remains in a metastable condition.
In some metals (Li, Co, Zr, Fe, etc.) the polymorphic transformation happens also at low temperatures at which the diffusive mobility of atoms is small. It is performed, however, not in the normal, but shift way. In this case, the atoms transitions from one modification to another are interconnected (cooperative). Such polymorphic transformation is called shift. As it isn’t thermally activated and passes with high speed at temperatures close to 0°К, it is called also athermic.
The ordered (coherent) reorganization of a lattice is the cornerstone of the shift mechanism of transformation. The normal inter phase border which is characterized by the distorted and friable packaging of atoms doesn’t arise. The lattices of both modifications are integrated (arranged). The ordered placement of atoms on border is possible only in case of the certain orientation of the integrated lattices.