Effect of (α＋β) heat treatment on the mechanical properties of zircaloy-4

Abstract

The mechanical properties of Zircaloy-4 were investigated after quenched from the ($\alpha+\beta$) phase temperature. The resulted microstructure consisted of a mixture of hard primary $\alpha$ and soft transformed $\beta$ phases. At 1163K treatment the transformed $\beta$ phase showed isolated lath morphology, and at 1223K the transformed $\beta$ showed parallel plate Widmanstatten $\alpha$. The $\alpha/\beta$ transformation kinetics was comparable to Johnson-Mehl-Avrahmi equation and the activation energy for $\alpha\,\rightarrow\,\beta$ transformation was found to be 33Kcal/mole. Microhardness of constituent phases revealed that the strength of transformed $\beta$ phase was almost constant, while that of primary a increased due to smaller grain size and oxygen enrichment with increasing amount of transformed $\beta\,(f_{\alpha``}$) The 0.2\% yield and tensile strengths increased after the ($\alpha+\beta$) heat treatment. the plot of the yield strength against $f_{\alpha``}$ by the modified rule of mixtures showed that the yield strength was dependent on the strength and amount of primary $\alpha$ phase and on the interactions between the phases. For $f_{\alpha``}<0.3$, the calculated yield strength agreed well with the measured, showing that the interaction term was very small. For $f_{\alpha``}>0.3$, the calculated yield strength was lower than the measured and the deviation increased with increasing $f_{\alpha``}$ exhibiting that the interaction term was positive and increased with $f_{\alpha``}$. Two different stages of work hardening existed. Up to 0.015 total strain, the work hardening exponent n increased when the matrix was the hard primary $\alpha$ at 1163K treatment, but at 1223K n decreased as the soft transformed $\beta$ became the matrix and more continuous. Above the 0.015 strain, the work hardening exponent was similar to that of $\alpha$-annealed Zircaloy-4. Uniform elongation decreased remarkabley for small amount of transformed $\beta$ phase...