Viscous creep of β-Co

Abstract

The creep of β-Co was studied at stresses of up to 5.0 MN m⁻² over the temperature range 973-1176 K (0.55T_m - 0.67T_m) and for grain sizes from 55 to 379 μm, conditions favourable for viscous creep mechanisms to dominate. The creep of helical specimens (self-heated by an electric current) due to their own weight was measured as follows: the deflection of each coil was monitored in situ as a function of time by direct observation through a transparent vacuum chamber using a cathetometer. Within the stress range investigated, the creep rates were observed to be proportional to stress for stresses in excess of a threshold value; below this threshold stress, no detectable creep occurred. Analysis of the results not only demonstrated the transitions from Coble to Nabarro-Herring creep and from Coble to Harper-Dorn creep but also established the regimes of grain size and temperature within which Coble, Nabarro-Herring or Harper-Dorn viscous creep is the dominating deformation process. Although the threshold stress generally decreases with increasing temperature, in the grain size and temperature regime of Nabarro-Herring creep dominance it is more strongly dependent on temperature than when Coble creep dominates. A measurable threshold stress also exists under Harper-Dorn creep conditions. On the basis of our present data and those reported by Sritharan and Jones, Ashby-Langdon-type and Langdon-type creep mechanism maps were constructed for β-Co.