YIELDING AND DYNAMIC STRAIN AGING BEHAVIOR OF ZIRCALOY-4 TUBE

Abstract

The expanding copper mandrel test is performed at three strain rates (3.2 x 10(-5)/s, 2.0 x 10(-6)/s and 1.2 x 10(-7)/s) in the temperature range of 553-823 K by varying the heating rates in air and in vacuum. Precise measurements and exact analyses of test results are made to improve the accuracy and reliability of the expanding copper mandrel test. Yield pressures at the different strain rates and temperatures are calculated from experimental results of diametral plastic expansion and are used to determine activation parameters of yielding and dynamic strain aging. The activation energy for dynamic strain aging obtained from the yield stress peak temperatures and strain rates is 228 kJ/mol and this value is in good agreement with the activation energy for oxygen diffusion in alpha-zirconium and Zircaloy-2 (207-220 kJ/mol). Therefore, oxygen atoms are considered to be responsible for the dynamic strain aging which appears between 573 and 673 K. The expanding copper mandrel test is proved to be a convenient and quantitative testing method with simple devices for the study of yielding and Strain aging behavior of thin-walled tube materials under biaxial stress conditions.