CREEP IN METALS AT INTERMEDIATE TEMPERATURES AND LOW STRESSES - A REVIEW

In the present paper, some recent results on viscous creep in alpha-Fe and alpha-Zr are reviewed and some results of investigations of creep in a Cu-14Al solid solution alloy at intermediate temperatures and low stresses are presented and discussed. The "low stresses" are specified as those causing steady state creep rates lower than 10(-10) s-1 at "intermediate temperatures" ranging from 0.4 T(m) to 0.6 T(m) (T(m) is the melting temperature). It is shown that the creep in alpha-Fe and alpha-Zr results from stress-directed diffusional transport of matter via the grain boundaries (Coble creep) up to an intercept grain size of about 120-mu-m, while at the intercept grain sizes above this value Harper-Dorn creep controlled by dislocation core diffusion operates. In creep of a Cu-14Al solid solution alloy, two flow processes act in parallel: (i) the viscous flow process identified with Coble creep and (ii) the non-viscous flow process characterized by a strain rate proportional to applied stress squared, inversely proportional to the intercept grain size cubed, and by the activation energy close to the activation enthalpy of grain boundary diffusion. The latter flow process was identified with the creep due to grain boundary sliding accommodated by slip in thin zones adjoining the sliding boundaries. In the solid alloy under consideration, Harper-Dorn creep does not take place up to an intercept grain size of 270-mu-m at least.