Ségrégation intergranulaire dans des alliages métalliques de structure hexagonale compacte (Zr).

Abstract

In this work, we studied the intergranular segregation of zircaloy 2 and zircaloy 4 alloy elements in zirconium on the basis of the first principles ab initio of quantum mechanics. Using the ABINIT computer code, we studied the bulk zirconium of hcp structure using the norm-conserved pseudopotentials with five approximations: two GGAs and three LDAs. The convergence and optimization results showed that the pseudopotentials GGA-PBE-HGH and GGA-PBE-TM are the optimal pseudopotentials allowing a good agreement with theoretical and experimental results. We have described the construction steps of the supercell containing the symmetric tilt grain boundary: Σ17 [θ=49.71°/[21�1�0],(03�37)] in the Zr. We calculated the total energies of the relaxed sup ercell to calculate the Sn segregation energies in the different sites near and far from the grain boundary. We have calculated the segregation energies of Fe, Cr and Ni atΣ17grain boundary. The calculation of the segregation energies shows that the segregation of Sn at the grain boundary sites is unfavorable, but it is favorable for the sites of the first plane parallel to the grain boundary plane. For the sites of the other plans, parallel to the grain boundary plane, the segregation is unfavorable. The segregation energiescalculation at Σ17 grain boundary for the Fe, Cr and Ni elements shows that the segregation is unfavorable for Fe and Ni, but it is favorable for the Cr. This leads to the enrichment of the boundary by chromium and the depletion of the bulk by this element and consequently to the degradation of the corrosion resistanc e pr operties. The oxidation kinetics of zircaloy 4 in the air was investigated experimentally by in situ x-ray diffraction at different temperatures, namely: 25, 350, 500, 830 and 1000 °C. The XRD spectrums show that the monoclinic and quadratic phases are formed at a temperature of 350 °C and their volume fractions increase with temperatur e tillKeywords: Intergranular segregation, ab initio calculation, DFT, zircaloy, transition metals, high temperature oxidation. to 1000 °C. In order to simulate the Loss of Coolant Accident (LOCA), we studied quenched Zry-4 samples after different aging times at 1050 °C. This quenching leads to the appearance of the "Widmanstätten" structure with poor mechanical properties. The analysis of the XRD spectrums and the microhardness curve show that the increase of the aging time from 10 seconds to 5 minutes leads to an increase in the volume fractions of the monoclinic and quadratic zirconia, accompanied by a rapid increase in the microhardness. After aging for 10 and 20 minutes, there is an appearance of chipping followed by severe cracks (br eak-away).