Role and effectiveness of plastic deformation and stress relief in zirconium: origin of residual stress

Abstract

Industrially produced Zircaloy-4 clads were subjected to pressurized water in standard closed end burst test and shown to possess different ductilities. The latter was not determined by microstructure and crystallographic texture: but was shown to have a clear scaling with hydrostatic component (Ph) of the measured tri-axial residual stress matrix. The study then tried to bring out the origin of residual stresses during plastic deformation and stress relief annealing. Zircaloy-4 sheets were cold rolled to different percentages of deformation 20%, 40% and 60% and were well electro polished before undergoing measurement in X-ray diffraction and electron backscattered diffraction (EBSD) for measuring residual stress and plastic strain. A clear gradient of elastic and plastic strains were predicted through finite clement simulation. Such predictions were verified against experimental data on microtexture and bulk crystallographic texture. Microfocused X-ray also provided clear distinction in residual stress developments between different features of the deformed microstructures. For example deformed Zirconium grains were classified as fragmenting and non-fragmenting. The latter, mostly basal, had strongest signatures of residual stresses. During recovery, microstructure dependent stress-relief was established. Initial recovery or stress relief was primarily through reductions in orientation gradients in non-fragmenting grains. Only during the latter stages, recrystallization and stronger orientation sensitive stress-relief was noted for the fragmenting grains