Steady-state creep in alpha iron as described in terms of effective stress and dislocation dynamics

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Abstract
Using the strain transient dip test technique the mean effective stress [sgrave]* was measured in steady-state creep of alpha iron in a temperature interval 400 to 700°C. At temperatures 400 to 450°C the mean effective stress was found to increase with increasing temperature, while at temperatures 550 to 700°C it is temperature-independent. At intermediate temperatures a more complicated behaviour of mean effective stress was observed. The parameter m* = (∂ ln v g/∂ ln [sgrave]*) describing the effective stress sensitivity of dislocation glide velocity v g was estimated, assuming the moving dislocation density ρ m to be a constant fraction of the density ρ of dislocations unbound in sub-boundaries. The contribution to the effective stress dependence of steady-state creep rate of the effective stress dependenoe of ρm was found to be considerably smaller—at least at high effective stresses—than that of v g. In the temperature interval 550 to 700°C the apparent activation energy of dislocation glide Q g is close to the activation enthalpy of self-diffusion ΔH SD. Non-conservative motion of jogs was, therefore, suggested as the mechansim controlling the dislocation glide. This suggestion is supported by the magnitude of the activation area as well as by its effective stress dependence. In the temperature interval 400 to 450°C the apparent activation energy of dislocation glide is lower than that of recovery, i.e. than ΔH SD. The mechanism controlling the dislocation glide in this temperature interval has not been identified. In the intermediate temperature interval both mechanisms controlling dislocation glide operate; at low effective stresses and higher temperatures that characteristic for the interval 550 to 700°C predominates, while at higher effective stresses and lower temperatures the mechanism controlling glide in the interval 400 to 450°C becomes dominant.