Quantification of dislocations densities in zirconium hydride by X-ray line profile analysis

Abstract

Zirconium-based components in nuclear power plants are embrittled by precipitates of δ zirconium hydride, which involves a martensitic-type transformation of the hexagonal α-Zr lattice into the face-centered cubic Zr sublattice of the hydride. As a result, the hydride precipitates have a complex and heavily distorted internal structure that manifests as broad peaks in X-ray diffraction experiments. By a detailed analysis of the peak widths measured for different crystal planes we have found that most of this broadening is the result of dislocations. The analysis also showed that δ-hydride has very anisotropic mechanical elastic properties, in agreement with ab-initio simulations presented in the literature. Provided with this peak-broadening model, we have quantified dislocation densities within δ-hydrides precipitated in several Zr alloys, by analyzing previously published X-ray diffraction experiments performed at three synchrotron X-ray sources. The specimens investigated correspond to components affected by different hydride embrittling processes, namely: (i) samples from various components, charged in the laboratory with H contents in the ∼250 wt ppm range, (ii) laboratory-produced hydride blisters in Zr2.5%Nb pressure tubes; and (iii) Zircaloy-4 specimens machined from cooling channels of Atucha I nuclear power plant after 10 years in-service, containing ∼140 wt ppm of equivalent H content and subjected to an estimated fast neutron fluence of ∼1022 neutrons/cm2. Results show that dislocations densities in the δ-hydrides are large (5–20 × 1015 cm−2) and vary among the different specimens. We also found that dislocations densities in the hydride are proportional to the fraction of hydrides already formed in the matrix, which was interpreted as the effect of matrix hardness in the precipitate structure.