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

The scope of this work is to study the effect produced by plastic deformation in the cold rolling process of Zr-2.5 %Nb pressure tube material, one crucial structural component containing the fuel channel of PHWR CANDU¹ nuclear power reactors. The deformation process generates changes in the microstructure and inter-granular stresses that might be orientation dependent, representing a subject of technological importance. Experiments of neutron diffraction (ND) were performed at the VULCAN time-of-flight diffractometer, where six samples of pressure tube material were measured (as_received and deformed condition having 5 %, 10 %, 15 %, 20 %, 30 % of accumulated plastic deformation). This paper presents the study of the microstructure, the texture, the lattice strain and the inter-granular stresses, showing the changes in the material through the deformation process. SEM Micrographs were obtained, and Vickers micro-hardness was measured. The texture, lattice strain and inter-granular stresses, were characterized using ND. From the construction of conventional experimental pole figures, the calculation of the Orientation Distribution Function (ODF) allowed the characterization of texture evolution. The changes in crystal orientation are subtle, being the main result an increase in the frequency of crystals aligning their c-axis along the hoop direction of the tube. 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. The inter-granular stresses, represented by six tensor components with orientation dependence, constitute a novel contribution in the study of materials with hexagonal structure. As a general conclusion, meanwhile the changes in texture and intra-granular strain are gradual with the increase of plastic deformation, the change in lattice strain produced by inter-granular stresses from extruded material to the first step of deformed material is quite abrupt, being enough to generate the inter-granular stresses mean field in the pressure tube material.