The polymorphic transformations in metals (Part 3).

The required for homogeneous nucleation energy fluctuation is so great that it is in solid metals are not usually found. Before overcooling (or overheating) reaches the value at which it is possible to homogeneous nucleation, the germs has originated in the defective areas: on the surface of the crystals, in clusters of dislocations or vacancies on the inclusions of impurities, defects in the packaging. If defects in the polymorphic metal aren’t enough or they have insufficient excess energy, the initial modification strongly overcooled. Especially strongly faultless threadlike crystals overcool.

The origin on the free surface of initial crystals doesn’t demand energy of the deformation. The germs of the α-phase which are formed here have the disk-shaped form and remind two-dimensional germs.

The change of the thermodynamic potential in this case will be

ΔZ=Z0+Zn=-Δμ(πr2h/Va)+ πr2a+ γa-b– γβ),

where r and h — radius and height of the disk-shaped germ of the α-phase; γa and γβ — the superficial tension α-and β-phases.

The germ’s formation on a free surface is followed by small increase in superficial energy. In the time and cracks which are available in crystals, origin is also facilitated. Also inclusions of impurity can be effective.

The efficiency of linear and superficial defects is explained by reduction of the germs formation work (in comparison with homogeneous origin) at a size equal to excess energy of the used part of defect.

In the defective sites also lattices of movement of atoms, necessary for reorganization, are facilitated that also promotes education and germs growth.

The influence of the volume changes is shown also with the crystals growth. If tension arising on the surface in the course of polymorphic transformation doesn’t manage to relax, the crystal growth can stop. The role of volume changes at the polymorphic transformation of tin caused by change like interatomic communications is especially large.

The white tin (β-Sn) has tetragonal packaging of atoms and metal type of communication. Gray (α-Sn) — diamondlike, with small coordination number and covalent communication. At transformation β → α the volume of tin increases by 25% and in spite of the fact that temperature of balance of T0 of both modifications + 18°C, with a noticeable speed undergoes transformation only at — 20°C. On the surface of the white tin crystals the gray hillocks appear α-Sn modification. In crystals of white tin α-modification isn’t found. After the plastic deformation leading to accumulation of defects, speed polymorphic of transformation increases and α-Sn appears in β crystals.

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