The polymorphic transformations in metals (Part 6).

Like a normal polymorphic transformation, the shear transition begins heterogeneously. The germs appear, apparently, in places where there are corresponding dislocation nodes. The crystals grow rapidly at a speed of> 103m/sec. Since the thermal motions of atoms do not play an important role in the shear transformation, it occurs at a great speed and at temperatures close to absolute zero. The rate of polymorphic transformation is particularly large in a defect-free crystal. If a growing crystal meets a boundary, a sub-boundary, or a cluster of dislocations, growth by a shear is usually stopped: partial dislocations stop at the defect. The violation of the fit (joints) of lattices and the appearance of a normal interphase boundary with the distorted packing of atoms can also occur as a result of thermal displacements of atoms. Thus, in violation of the conjugacy of the lattices, further ordered cooperative transition of atoms from one modification to another becomes impossible, and it occurs as a result of disordered transitions of atoms in the process of thermal motion. If the supercooling is high, then growth practically ceases.

The structure resulting from the polymorphic transformation depends on the supercooling. With the small supercoolings and coarse-grained initial structure, the little new-phase nuclei are formed. The growing crystals from them usually have the equal axial form. If the supercooling is increased, many centers of recrystallization arise. They are located on the grain boundaries, and during the polymorphic transformation around the grains of the initial phase, a shell of a new phase is created. With even greater supercoolings, the numerous germs of a new modification can appear inside the crystals of the original modification.

The recrystallization can change the shape of the crystals. If the germs of the new modification are orientationally connected with the lattice of the initial phase, then their growth rate in different crystallographic directions is not the same and plates and needles are formed. Their mutual orientation in the volume of one original grain is natural. This structure is called Widmanstatten pattern.

In the polymorphous transformations under conditions of large supercoolings, usually achieved by quenching, the crystals of the initial and newly formed phases are strengthened. The resulting hardening, in contrast to deformation hardening of phases, can be called a transformation hardening. This kind of hardening is due to defects in the atomic-crystal structure and stresses arising during the transformation of gratings under conditions of rapid cooling. An even more hardening (to a greater accumulation of defects) is the multiple polymorphic transformations of the lattices (cooling-heating-cooling).

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