Dynamic strain aging of zircaloy-4 in biaxial stress state

Abstract

The expanding copper mandrel test was performed at three strain rates($3.2\times10^{-5}S^{-1},\;\; 2.0\times10^{-6}S^{-1}$ and $1.2\times10^{-7}S^{-1}$) over 553-873 K temperature range by varying the heating rates ($8-10\,^\circ\!C /s,\; 1-2\,^\circ\!C /s$ and $0.5\,^\circ\!C /s$) in air and in vacuum($5\times10^{-5}$ torr). The observed yield stress peaks in air were 576 K for $\varepsilon=1.2\times10^{-7}$, 623 \& 723K for $\varepsilon=2.0\times10^{-6}S^{-1}$ and 673 \& 798K for $\varepsilon=3.2\times10^{-5}S^{-1}$, and these peaks in vacuum were 576 K for $\varepsilon=1.2\times10^{-7}S^{-1}$, 623 K for $\varepsilon=2.0\times10^{-6}S^{-1}$ and 673 K for $\varepsilon=3.2\times10^{-5}S^{-1}$. The strain rate sensitivity minimum and the activation volume peak appeared were at 623 K and 723 K in air, but these behaviors at 723 K were not observed in vacuum. The yield stress peak, the strain rate sensitivity minimum and the activation volume peaks could be explained in terms of the dynamic strain aging. The activation energy for dynamic strain aging obtained from the yield stress peak temperature and strain rate was 196 (KJ/mol) and this value was in good agreement with the activation energy for oxygen diffusion in $\alpha$-zirconium and Zircaloy-2(207-220KJ/mol). Therefore, oxygen atoms are responsible for the dynamic strain aging appeared between 573 K and 673 K. The small peak appeared between 723 K and 798 K only in air test is probably due to the Fe atoms inducing the dislocation pinning. The yield stress increase due to the oxidation was obtained by comparing the yield stress in air with that in vacuum and represented by the percentage increase of yield stress($\sigma_y^{\partial}-\sigma{y}/\sigma{y}$). the slower the strain rate, the greater the percentage increase occurs. In order to estimate the yield stress of PWR fuel cladding material under the service environment, the yield stress in water was obtained by comparing the oxidation rate in air and that ...