Upon the shift transition of one modification to another the atoms move to small distances, smaller interatomic. These movements are studied in cobalt in detail. At Т > 425°С in cobalt it is stable face-centered cubic lattice modification (β-Со), is below 425°C —modification (α-Со). Both packaging can be provided as the atomic layers alternating, packed by the identical method. The transformation of one packaging to another can be performed if to displace the closely packed atomic layer in parallel, the octahedral plane (111) β-Со from в position into а. Such shifts need to be made on every second layer.
Schematically it looks as follows (the arrows have specified the layers movements):
абв
↓
абв
↓
абв
↓
абв
↓
аб. . .
As a result of shift alternation of atomic layers characteristic face-centered cubic lattice абабабаба……, inherent to cubic lattice. Such shifts may result in partial dislocation with a corresponding shift vector. The partial dislocation movement in the plane (111) β-Co atomic layer transforms from the в into а location.
The atomic movements which are necessary for transformation of one lattice into another, can be result of action the unipolar source of dislocations of Frank — Read (see Fig. 1).
In case of each turnover around motionless dislocation the partial dislocation passes into other plane parallel to the first, but remote from it to the distance equal to two interlunar distances. The similar mechanism of polymorphic transformation is similar to the mechanism of the twin deformation. As well as in case of the twin, the transition of atoms to the new provision is performed by movement of partial dislocations. The moving dislocation line reserves the layer of atoms transferred to the new provision inherent in the double (twin) or and – modifications (polymorphic transformation). In case of each turnover the partial dislocation leaves the sliding plane in which it made shift, and passes into other plane parallel to the first. If in case of a twin, the partial dislocation passed into each following atomic layer, then in case of transformation of the face-centered cubic lattice of the item packaging transition is performed through an atomic layer. These transitions to another come from one plane thanks to the fact that motionless (polar) dislocation is screw with the shift vector equal to doubled interlunar distance. Thus, the partial dislocation slides on some spiral surface, turning of face-centered cubic lattice of the item packaging. During the shift transformation in cobalt density of dislocations sharply increases. In some metals polymorphic transformation can happen in both ways. In case of small overcooling, the normal mechanism is implemented; in case of big is shift. At iron, for example, the small overcooling is lower То = 910°С can cause reorganization face-centered cubic lattice → to cubic lattice in the normal way. If overcooling exceeds hundreds of degrees, polymorphic transformation will become shift. The similar influence of overcooling on the mechanism of polymorphic transformation is observed also for other metals.
