Texture evolution of Zircaloy-4 tube during cold pilgering: Operating mechanism of Q factor

Abstract

Cold pilgering has proved to be an effective rolling fabrication method for tubular materials, especially for difficult-to-deform such as Zircaloy-4 (Zr-4) tube. In this paper, the influence of the Q factor defined as strain ratio between wall thickness and tube external diameter on the texture evolution is investigated during cold pilgering of Zircaloy-4 cladding tubes. Distinctive texture characteristics are detected under electron backscattered diffraction (EBSD) with Q factors of 1.0 and 1.3. Finite element simulations are performed to analyze the complex stress states during cold pilgering. Then the stress states are fully characterized with stress ratios to predict the local triaxial stress distribution under a wide range of Q factors. Lastly, the effect of complex stress state on the selection of deformation mode for Zr-4 tube is well interpreted with the calculation of effective Schmid factor (ESF). The correlation between Q factor and activation of slip modes with its related texture evolution for Zr-4 is unveiled. In summary, high ESF values for basal 〈a〉 slip was found to promote its activation under a larger Q factor, which in turn contributes a faster development of texture towards radial direction (RD). A thorough understanding of complex stress states and slip modes under various Q factors will be significantly valuable to guide the cold pilgering process with adjusting and controlling texture.