Behavior of the 2nd phase particles in irradiated Zircaloy-4: Amorphization and re-precipitation

High resolution DRX experiences held in the 'Laboratorio Sincrotron de Campinas' (LNLS) detected the re-precipitation of the Laves C14, ZrFe1.5Cr0.5 phase in irradiated Zircaloy-4 with neutrons from the CNA-1. Studies carried out with TEM in the last decade showed that second phase precipitates (SPP) of Zircaloy-4 are unstable at high neutron flows, suffering a gradual process of amorphization that ends with the dissolution of the precipitate in the α-Zr matrix. Due to its low concentration in the alloy (≅0.3w%) this phase had not been detected in Zircaloy-4 using conventional diffractometers or other techniques for obtaining information about these microstructural transformations in the volume. The diagrams obtained in the LNLS for this work clearly show the 5 most intense peaks of this hcp structure, ≤110≥θ, ≤103≥θ, ≤112≥θ, ≤201≥θ and ≤004≥θ together with some of lesser intensity in non irradiated samples. In samples irradiated at high flows (≥1021neutrons/cm2) these peaks are not seen at all, while in samples with similar accumulated flows treated at 24h or 72h at 600oC the peaks reappear with a gradual recovery of the structure, in other words, the re-precipitation of the phase. In samples annealed at intermediate temperatures (400oC) significant changes are not observed compared with the non annealed irradiated samples, indicating that the re-precipitation occurs at 400 to 600oC. For material irradiated at moderate doses (1020neutrons/cm2), the peaks for the phase ZrFe1.5Cr0.5 are still observed in the non annealed samples, confirming the fact that the amorphization process only appears for accumulated neutron high flows. Calorimetric experiences undertaken with high flow samples show a peak between 475 and 510oC during heating. This peak is irreversible, can be seen only in the first heating cycle up to 600 and does not appear in later cycles, so that it has been associated with the crystallization process of the SPP amorphisized by irradiation (CW)