Study of crystal deformation and inter-granular stress in cold rolled pressure tube material using neutron diffraction

Abstract

The scope of the present work is to study the effect produced by plastic deformation in the cold rolling process of Zr-2.5%Nb pressure tube material, one of the most important structural components containing the fuel channel of PWR CANDU type nuclear power reactors. The deformation process propitiates changes in the microstructure and generates inter-granular stresses that might be orientation dependent, representing an important technological topic of study. In order to accomplish this, experiments of neutron diffraction (ND) were performed in the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL), USA. The instrument used was VULCAN time-of-flight diffractometer, specially designed for studies in material sciences. A series of six samples were measured, starting with the pressure tube material (as_received condition), and continuing with deformed material (systematic steps of cold rolling), having 5%, 10%, 15%, 20% and 30% percentage of accumulated plastic deformation, correspondent to the transversal area reduction. In this paper, the analysis of the microstructure, the texture, the lattice strain and the inter-granular stresses in the pressure tube material are presented, showing the changes in the material through the plastic deformation process. Micrographs were obtained using a Scanning Electron Microscope FEI-Quanta 200. The texture, the lattice strain and the inter-granular stresses, were characterized using the neutron diffraction experiments. From the construction of conventional experimental pole figures, the calculation of the Orientation Distribution Function (ODF) allows us to characterize the evolution of the texture. All the results indicate that the changes in orientation of crystals are subtle and the main result is the sharpening effect of the hoop texture. The inter-granular stresses were characterized introducing a model based on the construction of the Stress Orientation Distribution Function (SODF), obtained through the experimental strain pole figures construction. The SODF was calculated as a mean stress field in the Euler orientation space and represents a novelty in the inter-granular stresses field of study for the particular case of hexagonal materials. A detailed study of the SODF evolution is presented in terms of the accumulated plastic deformation in the material, and the principal line profiles of the six components of the inter-granular stress tensor are presented, giving the range of values of [-700, 270] MPa for all samples measured. As a general conclusion, meanwhile the changes in texture, d-spacing strain and defect strain are gradual with the increase of plastic deformation in the material, the change in lattice strain produced by inter-granular stresses from extruded material and deformed samples is quite relevant, and the first step in the rolling process is enough to print the inter-granular stresses mean field in the pressure tube material.