The Effect of Phase Change on the Mechanical Properties of Cobalt Near Its Transformation Temperature

The phase transformation of cobalt is studied from observations of the changes in the work‐hardening, creep and fracture characteristics of cobalt near its critical temperature. The parabolic work‐hardening coefficient, Xp, is found to be about four times greater for the h.c.p. than for the f.c.c. phase at the same temperature. However, the temperature coefficient of Xp remains fairly constant over the phase change and has an average value of about −0.0027 deg−1. In the presence of the two phases simultaneously during the transition stage, this thermal coefficient is increased by an order of magnitude. The phase transition is exhibited as a change in slope of the curve of the logarithm of steady state creep versus the reciprocal of the absolute temperature. Dislocation climb controlling of the steady state creep is activated by 5.6 eV in the h.c.p. phase, which is comparable with double the activation energy for self diffusion. This is attributed to the fact the height of an elementary jog in the hexagonal structure is twice an interatomic spacing. In the cubic phase, the climb process requires only 2.0 eV which is smaller than that for self diffusion by nearly double the vacancy‐dislocation binding energy.