| Zirconium alloys are widely used in nuclear fission energy systems due to their low thermal neutron absorption cross-section, good mechanical and aqueous corrosion properties and good compatibility with nuclear fuel (UO2). The uptake of hydrogen and the introduction of helium in zirconium alloys can result in significant changes on the microstructures and properties, severely reducing their reliability and useful life in service condition. The complexities of the Zr-H systems bring much difficulty on their studies. There are still some controversies on the relative stability of different phases of zirconium hydrides and the phase transitions among them. Furthermore, many external factors, like heat treatment states, stresses, the sizes, distribution, structures and composition of the second phase particles (SPPs), can influence the formation of zirconium hydrides. It is generally considered that small amount of helium can hardly affect the structures and properties of zirconium alloys. However, under certain circumstances, the amount of helium generated in zirconium alloys can be high enough that cannot be ignored. There is still a lack of studies on the effects of helium on the microstructures and properties of zirconium and its alloys.In this paper, we have studied on some unsolved issues concerning the effects of hydrogen and helium on the structures and properties of pure zirconium and N18alloy both theoretically and experimentally. We have tried to provide some references to improve the resistance of zirconium alloys to hydrogen and helium embrittlement. We have mainly studied on the relative stability of various zirconium hydrides, their phase transitions as a function of hydrogen concentration, the interactions of SPPs and hydrogen, the effects of added yttrium on the formation of zirconium hydrides, the effects of the processing of high pressure torsion (HPT) on the formation of zirconium hydrides, the effects of helium on the microstructures and mechanical properties.In the present study, first-principles calculations have been performed to investigate the relative stability of the various phases of zirconium hydrides combined with the special quasirandom structures (SQS) method. For the ordered phases, it is found that:The ζ-Zr2H,"diamond-like" γ-ZrH,δ-[111] and fcc ZrH2phase are all not stable. For the disordered phases, the SQS method has been used to construct supercells to mimic random alloys. It is found that the structure of the random alloy gradually transforms from an hcp into an fcc and then an fct (c/a<1) one with the increasing H concentration. The equilibrium state between the hcp and δ phase is between ZrH0.25and ZrH0.375, while the equilibrium state between the δ and γ phase is between ZrH and ZrH1.5. The γ phase is energetically not favorable at all H concentrations.First-principles calculations have also been performed to investigate the interactions between second phase particles and hydrogen combined with the SQS method. Firstly, we studied the binary ZrCr2systems; it is found that among the seven different tetrahedral interstitial sites, H prefers to occupy the H5-2Zr2Cr-type one. The formation energy of H in those tetrahedral interstitial sites is all below zero except that of the H2-4Cr-type. Then, we studied the complex pseudo-binary Zr(FexCr1-x)2systems; it is found that with the increasing Fe concentration, the lattice constants and bulk moduli decrease linearly. When occupying the H5-2Zr2Cr-type tetrahedral interstitial sites, the formation energy of H is the lowest. With the increasing Fe concentration, the formation energy of H increases apparently. Comparing with the perfect crystal, the displacement of Zr, Fe and Cr atoms can result in lower H formation energy.The influences of added yttrium elements on the microstructures and properties of zirconium alloys were characterized systemically by using optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). We found that in the N18+Y alloy, after the hydrogenation, there are both intergranular and intragranular zirconium hydrides. The sizes of these hydrides increase apparently. The SPPs in the N18alloy are mainly Zr(Fe,Cr)2and Zr(Fe,Cr,Nb)2, while those in the N18+Y alloy are mainly Zr(Fe,Cr)2and partially oxidized Y particles. Around these partially oxidized Y particles, we found the enrichment of hydrides.The influences of HPT on the microstructures and properties of zirconium alloys were characterized systemically by using optical microscopy, nanoindentation, transmission electron microscopy, X-ray diffraction and secondary ion mass spectroscopy. It is found that in both the N18and N18+Y alloy, there is no co and (3phase zirconium after the HPT processing. Their Vickers hardness increase about10%. We also found the enrichment of hydrides forming some networks. The EDS (Energy dispersive spectrometry) and SIMS spectra both indicate that the distribution of Fe in the SPPs does not change while partial Cr diffuses into the Zr matrix. The results of first-principles calculations indicate that the bonding of Fe is stronger than that of Cr. That is why Cr tends to diffuse into the matrix, and leads to the enrichment of hydrogen.First-principles calculation has also been conducted to investigate the structural stability, electronic and mechanical properties of Zr-He systems. For the perfect a-Zr crystal, the He BO interstitial site is most energetically favorable, while for a not perfect one with pre-existing vacancies, the S site is most stable. The analysis on the DOS and charge density gives a better understanding about the structural stability issue. It is found that the introduction of He changes the elastic constants in an anisotropic way, leading to a significant increase in elastic anisotropy. And this effect is further enlarged while He-V complexes form, resulting an intensive deterioration of the mechanical properties of Zr. The implantation of helium ions was conducted using a200KeV beam, resulting a damage peak depth of about600to650nm and helium concentration peak depth of about700nm calculated using the SRIM code. With a dose of1017ions/cm2, the helium peak concentration is about0.78%and the peak damage is about4dpa. The microhardness and elastic modulus were measured by using nanoindentation. With the increasing helium ions fluence, the microhardness of both pure Zr and N18alloy increase significantly. The elastic modulus of pure Zr decreases slightly while that of N18alloys decreases more apparently.The above results can help in better understanding of the complex behavior of hydrogen and helium in zirconium alloys under reactor operating environment. We have obtained the relative stability of various zirconium hydride phases as well as their phase transitions as a function of hydrogen concentration. We have also obtained the effects of the interactions between SPPs and hydrogen, zirconium and helium on the microstructures and mechanical properties of zirconium and its alloys. And we have proposed the possible mechanism of the accumulation of zirconium hydrides when yttrium is added or the processing of HPT is conducted. Our study can promote our understanding and provide some technical support to improve the resistance of zirconium alloys to hydrogen and helium embrittlement. What’s more, our work can provide the methodology to study the synergistic behavior of hydrogen and helium in other materials, especially for structural materials in future fusion reactors。... |
PDF Full Text Request