A theory of the transformation in pure cobalt

Abstract

The suggestion that the faults in hexagonal cobalt are in thermal equilibrium with the structure and that the phase change in pure cobalt is influenced considerably by surface energy is examined. It is shown that for the pseudo-equilibrium of a number of individual crystals surface energy effects could produce transformation over a temperature range but the other effects of the transformation are not explained. Quantitative results for the number of hexagonal faults show that these cannot be in thermal equilibrium. The transformation in cobalt is martensitic in nature and the mechanism is considered in terms of ‘half dislocations’ of the face-centred-cubic (f.c.c.) lattice. It is shown qualitatively that in the case of these dislocations reflexion at a free surface can lead to the production of a twinned structure or, under favourable conditions, to the transformation from f.c.c. to close-packed-hexagonal (c.p.h.). The faults in hexagonal cobalt can be considered to be the result of ‘wrong’ reflexions during the transformation. The characteristic features of martensitic transformations can be explained in terms of the dislocation mechanism.