High temperature creep and deformation microstructures in recrystallized Zircaloy-4

Uniaxial creep tests were performed on recrystallized Zircaloy-4 tensile specimens with an average grain size of 8.5 μm at the stress range of 2.56 × 10^-4E-1.29 × 10^-3E, where E is the elastic modulus, at three temperatures: 500, 550 and 600 °C with an objective to uncover the rate-controlling mechanism(s). A transition in creep mechanism was observed as the stress exponent increased from a value close to 1 at the low stress range to 4.1 at high stresses with the calculated activation energies (Q_c) of 196 kJ/mol and 241 kJ/mol, respectively. Possible creep mechanisms have been shortlisted based upon the values of n and Q_c. Transmission electron microscopy analyses of the dislocation structures in the crept specimens as well as validation of the experimental results with the predictions by standard creep models were undertaken to ascertain the underlying rate-controlling mechanisms. While Coble creep was noted to be the dominant mechanism in the n ~ 1 regime at low stresses, dislocation climb was identified to control the creep rate in the n ~ 4.1 regime at high stresses.