Dislocation Cross-Slip Controlled Creep at High Stresses and Transitional Creep Mechanisms in Zircaloy-4

Uniaxial creep tests were performed on Zircaloy-4 sheet in the temperature range of 500–600 °C at high stresses (>10⁻³E, E is the elastic modulus) to uncover the rate-controlling mechanism. Stress exponents and stress-dependent activation energies, respectively, in the range of 9.3–11 and 220–242 kJ/mol were obtained from the steady state creep rate data. TEM analyses on the deformed specimens revealed extensive hexagonal screw dislocation networks on the basal planes indicating recovery of screw dislocations by cross-slip to be the dominant mechanism. Furthermore, analysis of the creep data in the light of Friedel’s cross slip model for HCP metals and the activation volume of the operating deformation mechanisms measured using stress relaxation tests favor cross-slip of screw dislocations as the rate controlling mechanism in the creep testing conditions employed in this study. In addition, transitions in creep mechanisms of Zircaloy-4 are presented along with its application to the evaluation of the total strain accumulated in Zircaloy-4 fuel cladding during dry storage.