Intergranular residual stresses can exist in zirconium alloys, especially when there is a large distribution of grain orientations. The stresses result from the anisotropic plasticity and thermal expansion of the hexagonal close-packed crystal structure of α-zirconium. Apart from complicating the characterisation of materials using lattice parameter measurements, the intergranular stresses can significantly affect material behaviour, especially in nuclear reactor environments, and there is therefore a great deal of interest in their measurement.
The effects of specimen preparation and surface relaxation on X-ray diffraction measurements of lattice parameters of zirconium alloys have been investigated by comparing bulk neutron diffraction with X-ray diffraction on identical materials. The results show that: (i) intergranular or interphase residual stresses exist in dual-phase Zr-2.5Nb pressure tubes; (ii) the stresses normal to the surface of an X-ray diffraction specimen are not relieved completely when there are intergranular residual stresses in the material. One can conclude that intergranular stresses have to be considered when determining chemical compositions from lattice parameter measurements and also when measuring macroscopic residual stress using X-ray diffraction.