Advances in Understanding and Predicting Inelastic Deformation in Zircaloy

The mathematical model MATMOD-Z has been built to permit computer simulation of in-reactor Zircaloy deformation behavior. The 250 to 450°C temperature range is covered together with a wide range of possible strain rates, accrued strains, and as-fabricated material conditions. MATMOD-Z is comprised of coupled time-rate equations in stress, strain, temperature, fast neutron flux, and certain material state variables. These latter quantities are: F_def, representing the friction stress due to isotropic deformation-induced obstacles; F_sol, representing the friction stress due to solute drag; R, the “rest stress,” or back stress due to directional obstacles; and f₂, the Kearns texture number in the direction bisecting the basal pole intensity peaks. Comparison of MATMOD-Z predictions to independent test data indicate a distribution of errors in predicted stress given complex strain-time-temperature history with standard deviation of the order of 17 percent. The most important source of error is thought to lie in an effect not treated in the current model, namely, increased solute drag with increasing dislocation density.