Interpretation of high temperature creep in alpha-zirconium in terms of effective stress and dislocation dynamics

Abstract

Steady state as well as transient creep of alpha-zirconium in the temperature interval of 400° to 750° C was investigated. In steady state creep the stress sensitivity parameter m′ = ( ∂ ln e ̇ s /∂ ln σ) T equal to 6.6 ± 0.1 was obtained in the interval of 550° to 750°C. In the same temperature interval the apparent activation energy Q = [∂ ln e ̇ s /∂ (− 1/ kT] σ equal to 64.9 ± 1.4 kcal mol −1 was found. Lattice self-diffusion was identified as a creep rate controlling process in this interval. At lower temperature m′ increases and Q decreases with increasing external stress. Measuring mean effective stress σ ∗ by the strain transient dip test technique a linear relation between σ ∗ and the external stress σ was found for the temperature interval of 500° to 750°C, the σ ∗/σ ratio being equal to 0.53±0.2. Therefore, the effective stress sensitivity parameter m′ ∗ = (∂ ln e ̇ s /∂ ln σ ∗) T equals m′ and the apparent activation energy Q ∗ = [∂ ln ≐ s/∂ (−1/kT)] σ ∗ equals Q. Creep was, therefore, interpreted in terms of dislocation glide. It was found that the parameter m ∗ = (∂ ln v g /∂ ln σ ∗) T characterizing effective stress dependence of dislocation glide velocity v g is one order of magnitude higher than the parameter m ϱ ∗ = (∂ ln ϱ m /∂ ln σ ∗) T characterizing effective stress dependence of moving dislocation density ϱ m. Since m′ = m ∗ + m ϱ ∗ the dislocation glide velocity is a much more important parameter than moving dislocation density in effective stress sensitivity of steady state creep in alpha-zirconium in the interval of 550° to 750°C. Using a stress sensitivity technique the relations between transient creep rate and effective stress at various constant internal stresses and various given temperatures were obtained and relations between effective stress and primary creep rate determined. The effective stress sensitivity of transient creep rate was found to be weaker than that of steady state creep rate. Therefore the effective stress sensitivities of v g and ϱ m —or at least the effective stress sensitivity of one of these parameters—are changing in the course of transient creep as well as v g and ϱ m are changing themselves.