Microstructural study by XRD profile analysis and TEM observations on hydrided recrystallized Zircaloy-4

Zircaloy-4, used as cladding tube material in the nuclear reactors, may become brittle due to the precipitation of hydrides. During hydride formation, the anisotropic misfit strains between hydrides and the hexagonal-close-packed zirconium matrix results in a preferred orientation of the hydride platelets in the anisotropic stress field caused by non-relieved fabrication residual stresses and misfit stresses. To understand the mechanism of rupture and to predict the threshold stresses for hydride stress orientation, it is necessary to study the residual stresses, especially the microstrain caused by crystalline lattice misfit, in a hydrided specimen. The X-ray diffraction profile analysis is very sensitive to all the microstructure evolution in metallic materials. It is a non-destructive and voluminal technique compared with transmission electron microscope observation. The XRD peak broadening can be related closely with the microstrain in case of hydrided Zircaloy-4, because the hydride formation creates in general a great number of dislocations which contributes especially to the diminution of coherent domain size and to the increase of microstrain. To calibrate the internal microstrain due to precipitation effect of hydrided specimens, XRD profile analysis has also been realized on the non-hydrided specimens deformed by uniaxial tension. In this paper the authors restrict to analyzing the results about the recrystallized state, because more informations about the anisotropic elasticity, plasticity, thermal expansion, neutron diffraction measurement and the crystallographic texture results are available