Dislocation cross-slip controlled creep in Zircaloy-4 at high stresses

Uniaxial creep tests were performed on Zircaloy-4 sheet in the temperature range of 500-600°C at high stresses (>10^-3E), to uncover the rate-controlling mechanism. A stress exponent of 9.3-11 and a stress-dependent activation energy in the range of 220-242kJ/mol were obtained from the steady state creep data. TEM analyses revealed an extensive presence of hexagonal screw dislocation network on the basal planes indicating recovery of screw dislocations by cross-slip to be the dominant mechanism. The creep data was therefore analyzed in the light of Friedel[U+05F3]s cross slip model for HCP metals according to which the stress-dependency of the activation energy determined from the creep data was written in the form,U=(150±4)+(2236±124τ)kJ/molThe constriction energy of screw dislocations of 150. kJ/mol is in agreement with the values reported in the literature for zirconium and other HCP metals. Further analysis of the yield strength and the activation volume data obtained from stress relaxation tests in the temperature range 500-600. °C favors cross-slip of screw dislocations as the rate controlling mechanism over the test conditions.