Deformation mechanisms and transients in creep of zircaloys: Applications to nuclear technology

Zirconium alloys are commonly used as cladding tubes in water reactors and undergo complex biaxial creep deformation. The anisotropic nature of these hcp metals makes it relatively complex to predict their dimensional changes in-reactor. These alloys exhibit transients in creep mechanisms as stress levels change. The underlying creep mechanisms and creep anisotropy depend on the alloy composition as well as the thermomechanical treatment. We describe here, first the anisotropic biaxial creep of cold-worked and recrystallized Zircaloy-4 in terms of Hill's generalized stress formulation. The temperature and stress dependencies of the steady-state creep-rate are described followed by predictive models for transient and transients in creep (due to sudden stress changes) which incorporate anelastic strain. The effect of recrystallization on creep anisotropy is pointed out along with an extension of the model to radiation creep of irradiated cladding. While Zircaloy-4 exhibited class M creep, Nb additions resulted in class-A creep behavior with the characteristic stress exponent of 3. Nb-modified Zircaloy sheet revealed changes in creep mechanism to class-M type at high stresses, and viscous creep at low stresses. implications of these various features in the predictions of cladding dimensions during reactor operation as well as during the dry storage of spent fuel will be discussed.