The polymorphic transformations in metals (Part 2).

The second feature of the phase transformations in solid metals is connected with smaller, than in liquid, atoms mobility. The diffusion velocity in liquid metals is enormously higher, than in solid metals. Therefore, at the identical difference of chemical potentials (Δμ), the crystals growth as a result of individual transitions of atoms happens faster in liquid metal, than in solid metals. This distinction is most noticeable if the polymorphic transformation happens at temperatures much below than a melting point.

The specified features of phase transformations in a solid state lead to the fact that recrystallization comes at big deviations from equilibrium temperatures, than crystallization. They affect and that modifications may be long in a metastable state.

Let’s consider the polymorphic transformation of metal which equilibrium diagram is provided on Fig. 1.

In the initial state at Т = Та metal is in β modification. When cooling lower than temperature То β modification becomes metastable and there are germs of the a – modification. The overcooling size (ΔT = Т0 — Тб) which is required for the α, germs formation depends on the metal structure and the impurity content. The general change of the thermodynamic potential (ΔZ) at the α germs modification formation.

ΔZ = Z0 + Zп + Zд,

where Zo — a difference of thermodynamic potentials β-and α-modifications;

Zп — germ’s surface energy;

Zд — energy of deformation.

The size of the germ’s surface energy is defined by the work of a specific interphase tension (γαβ) and the general germ’s surface (S):

Zп = γαβ S.

This tension depends on mutual orientation of crystals. In the crystal latitudes of α-and β-modifications it is possible to find the atomic planes and the directions in which both lattices are well integrated. The small distinction of interatomic distances in the place of crystals contact leads to elastic lattice misstatement and emergence in certain sites of dislocations or accumulations of vacancies.

The deformation energy, with which the germs modification formation is connected, is defined by elastic properties and the difference of specific volumes of both modifications. If difference of specific volumes of phases (volume effect of polymorphic transformation) ΔV, then the deformation energy

Zп = P ΔV,

where Р — pressure on the interphase surface.

Using the above-stated equations, it is possible to determine the sizes of germs which formation requires the minimum energy. They will also be the most probable. From the thermodynamic analysis that germ should not be spherical and disk-like or needle-like shape, which provides the minimum value of the total energy section and surface deformation.

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