Dislocation density evolution in cold-rolled Zr-2.5%Nb pressure tubes under thermal treatments by high energy XRD and neutron TOF diffraction peak profile analysis

Abstract

Diffraction line profile analysis was done to characterize the change in the dislocation density at different stages of the processing route of a Zr-2.5%Nb pressure tube. Two diffraction experiments were carried out: High Energy X-Ray Diffraction (HE-XRD) and Neutron Time of Flight Diffraction (TOF-ND). On samples in the extruded, cold-rolled and post-annealed conditions, the HE-XRD experiment in transmission geometry allows a detailed characterization of the dependence of the line shape on the direction of the scattering vector. Important variations of the FWHM (Full Width at Half Maximum) were observed, which was interpreted as the non-uniform distribution of dislocations among grains with different orientations. Average values of the dislocation density and subgrain size were obtained after applying the Warren-Averbach method. The effect of post deformation annealing for soaking temperatures from 400° to 450°C was studied using TOF-ND. Peak profile analysis was performed on in-situ neutron time-of-flight diffractograms to obtain peak positions and widths to calculate residual stresses and dislocation densities. The results show that the dislocation density decreases largely at early times and it is followed by a slow diminution over time. Dislocation density decreases from ∼8 × 1014 m−2 for the cold-rolled sample to values close or under 2 × 1014 m−2. A remnant dislocation density after long annealing times was detected, with a value that decreases as the soaking temperature increases. Dislocation type was also characterized. It was found that dislocations with 〈a〉 Burgers vectors represent approximately 90% of the population, while the other 10% are 〈c+a〉. An empirical recovery kinetics model was proposed where dislocation mobility is assumed as a thermally activated mechanism. This model can be used as a processing tool to estimate the dislocation density expected for different soaking temperatures and times. The effect of residual stresses on peak broadening was also discussed. It was estimated that this contribution represents around 10–20% to the observed physical line broadening.