F. Cardellini et G. Mazzone, THERMAL AND STRUCTURAL STUDY OF THE HCP-TO-FCC TRANSFORMATION IN COBALT, Philosophical magazine. A. Physics of condensed matter. Defects and mechanical properties, 67(6), 1993, pp. 1289-1300
The fc.c.-to-h.c.p. transformation in cobalt has been studied in sampl
es subjected by ball milling to different amounts of cold work. Differ
ential scanning calorimetry and X-ray diffraction have been used to ch
aracterize the enthalpy exchanges and the structure of the samples bef
ore and after heating at different temperatures. After milling for a f
ew hours, one observes that the h.c.p. phase transforms to a disordere
d 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. st
ructure. Heating the starting powder at different heating rates shows
that the transformation is thermally activated. The measurements in th
e calorimeter seem to indicate that the transformation to the f.c.c. p
hase, observed in all cases at a sufficiently high temperature, is rel
ated to the amount of disorder present in the stacking sequence of clo
se-packed planes.