Theory of the martensitic transformation in cobalt

Abstract

A phenomenological theory of the martensitic fcc-hcp transformation is proposed and applied to the illustrative example of cobalt. The fcc and hcp structures are shown to result from different ordering mechanisms from a disordered polytypic structure and to be intrinsically faulted. The three, fcc, hcp, and disordered polytype, structures are inserted in the framework of the segregation process which leads to the formation of close-packed structures from the melt. The essential features reported for the fcc-hcp transformation in cobalt are explained within the preceding model, namely, the asymmetry of the interphase region, the phonon spectrum, the \ensuremath{\delta}-shape of its specific heat anomaly, and the existence of an intermediate modulated structure. The property of the transformation enthalpy to be different on heating and cooling is related to the different degree of order of the hcp and fcc structures. The partial dislocation mechanism currently assumed for the transformation is deduced from the secondary shear strains involved at the transformation.