Some Observations on Time-Hardening and Strain-Hardening Rules for Creep in Zircaloy-2

A study was made of the applicability of time-hardening and strain-hardening rules to describe creep deformation in Zircaloy-2 under variable stress and temperature conditions. Variable stress and variable temperature creep data were compared to isotonic (iso-stress) and isothermal data in the stress regime 69 to 172 MPa and the temperature regime 325 to 400°C. It was observed that creep deformation under these variable conditions does not follow a time-hardening rule. A strain-hardening rule, on the other hand, described well the variable temperature creep deformation at temperatures up to 375°C. At 400°C, however, the strain-hardening rule broke down because of a nonnegligible recovery rate. Consequently, for conditions in which recovery is significant, an explicit treatment of recovery rates may be necessary for accurate creep predictions.