Global and local texture development during initial plastic deformation of cold-pilgered Zircaloy-4 tubing

Abstract

This work presents a thorough characterization of the texture development during the initial step of a cold pilgering process of a Zircaloy-4 TREX tube, on which the axial cross section is reduced by 80% over a 40cm length with an overall Q-ratio of 2.34, followed by a recrystallization thermal treatment. Texture across a tapered specimen representing this working zone was characterized by neutron diffraction and high energy synchrotron x-ray diffraction at 28 positions (4 locations along its length and 7 locations across its thickness with 0.3 mm spatial resolution), and on the deformed end after the thermal treatment. Results show that the starting material (TREX tube) presents a texture gradient through its wall thickness, with the c poles aligned to the hoop direction at the outer surface (with radial and hoop Kearns factors: fR=0.36, fH= 0.50), and gradually rotating around the axial direction to become a quasi-radial orientation (∼35o out of the radial direction) at the inner surface (fR=0.43, fH=0.35). The overall texture becomes more radial due to the cold pilgering, but the texture gradient remains during the whole deformation process, being always more radial at the inner surface, (fR=0.53, fH=0.37 at 80%) than at the outer surface (fR=0.41, fH=0.50at 80%). The most remarkable changes in texture produced by the rolling process is a 30o rotation of the grains around their c-axis, producing a marked increase of the <112¯0> // axial fiber texture at expenses of the original <101¯0> // axial fiber texture observed in the TREX tube. The observed changes in texture during deformation were correctly explained by visco-plastic self-consistent (VPSC) modelling. This was achieved after the inclusion of the basal slip system, which plays a central role to explain the 30o rotation around the c-axis during deformation. The texture evolution was correctly described after the inclusion of a drag effect to grain rotation due to the interaction with neighbouring grains. The through thickness texture differences are greatly reduced by the recrystallization thermal treatment, with similar Kearns factors at the inner surface (fR=0.50,fH=0.28) and the outer surface (fR=0.48, fH=0.39). The predominance of the <101¯0> // axial fiber texture is recovered after recrystallization.