TRANSITIONS IN CREEP MECHANISMS AND CREEP ANISOTROPY IN ZR-1NB-1SN-0.2FE SHEET

The creep characteristics of a Zr-1Nb-1Sn-0.2Fe alloy sheet were inves tigated at temperatures from 773 to 993 K and at stresses ranging from 9 to 150 MPa along both the rolling and transverse directions. Transi tions in creep mechanisms are noted, with diffusional viscous creep at low stresses, viscous-glide-controlled microcreep in the intermediate stress regime and the climb of edge dislocations at high stresses. Th e creep anisotropy decreases with a decrease in the stress exponent an d the creep rates differ by only 30% in the viscous creep regime, whil e an order-of-magnitude difference is noted at high stresses. The solu te-strengthening effect of Nb addition is evident in the stress regime where appropriate data are available. These transitions in creep mech anisms clearly reveal the dangers in blind extrapolation of short-term high stress data to low stresses and long times relevant to in-reacto r conditions. The creep behavior of these materials is similar to that noted in Class I alloys, while the transitions in deformation mechani sms in Zircaloy-4 resemble those found in pure metals or Class II allo ys with no viscous glide mechanism.