Effect of grain shape and texture on equi-biaxial creep of stress relieved and recrystallized zircaloy-4

Abstract

Biaxial creep behaviors of stress relieved and recrystallized thin-walled tubing of Zircaloy-4 are considered under equal hoop and axial stresses by internal pressurization superimposed with axial load. Both hoop and axial strains were monitored and the ratio of the strain rates along the hoop to axial directions is considered to represent the degree of anisotropy. The slightly stronger hoop direction of the recrystallized material became weaker compared to the axial direction following cold work and a stress-relief anneal. Crystallographic texture was considered in terms of X-ray pole figures from which the c rystallite o rientation d istribution f unctions (CODF) were derived. A crystal plasticity model based on slip on representative systems was combined with the CODF to predict the creep anisotropy. It was found that the textural differences between the recrystallized and stress-relieved materials do not account for the observed variations. Rather, the columnar grain shape of the stress-relieved material is believed to invoke anisotropic grain boundary sliding leading to stress enhancement in the hoop direction. This stress enhancement is shown to account for the observed differences in creep behavior between the present equiaxed and columnar grain structures.