THERMAL AND STRUCTURAL STUDY OF THE HCP-TO-FCC TRANSFORMATION IN COBALT

The fc.c.-to-h.c.p. transformation in cobalt has been studied in samples subjected by ball milling to different amounts of cold work. Differential scanning calorimetry and X-ray diffraction have been used to characterize the enthalpy exchanges and the structure of the samples before and after heating at different temperatures. After milling for a few hours, one observes that the h.c.p. phase transforms to a disordered phase composed of close-packed planes arranged in a random sequence. Further milling does not affect the stability of this phase up to the point where iron contamination induces the transition to the fc.c. structure. Heating the starting powder at different heating rates shows that the transformation is thermally activated. The measurements in the calorimeter seem to indicate that the transformation to the f.c.c. phase, observed in all cases at a sufficiently high temperature, is related to the amount of disorder present in the stacking sequence of close-packed planes.