Ab-initio modelling of Zr and Be alloys for nuclear applications

Abstract

Zr and Be alloys are crucially important for a number of applications in extreme environments. Here, density functional theory simulations were carried out to investigate the effect of alloying additions and second phase particles (SPPs) on the physical, mechanical and corrosion properties of the alloys. First the partitioning of H between Zr SPPs and Zr metal is investigated. Zr(Cr,Fe)₂ is not expected to getter hydrogen from the Zr matrix but it may act as a bridge for locally enhanced H diffusion across the oxide barrier layer. On the other hand, Zr₂(Fe,Ni) may getter some H from Zr solution if the Fe/Ni ratio is low. Fe always decreases the H affinity of SPPs, whilst Nb increases H affinity of Laves phases and decreases that of β-(Zr,Nb) SPPs. Following irradiation induced SPP amorphisation, Fe and Cr dissolve and cluster in the Zr matrix. Both substitutional and interstitial accommodation are relevant to Fe and Cr additions and two new low-energy interstitial sites were identified. Local stress states affect the stability of point defects and, in turn, these cause highly anisotropic lattice strains in Zr-Fe and Zr-Cr solid solutions, which deviate from Vegard’s law. The solubility of Fe and Cr, which is remarkably limited in pristine Zr, is increased dramatically by pre-existing Zr vacancies. Strong binding was predicted for the clustering of V_Zr and Feᵢ/Crᵢ defects. Furthermore, up to four Fe or three Cr atoms may be accommodated on or around a single V_Zr, with lower solution energies and relaxation volumes than dilute solutions. Al and Fe are the most common impurities found in commercial Be alloys, yet the binary and ternary phases of the Be-Al-Fe system are poorly characterised. First, the Be-Fe phase diagram is considered and a new structure and composition were identified for the Be-rich ε phase. Phonon density of states calculations indicate that ε-Fe₂−xBe₁₇+x phase is only stable up to ∼ 1500 K, while δ-FeBe₅ is stable only above ∼ 1100 K and ζ-FeBe₂ is stable at all temperatures below melting. Non-stoichiometry, elastic and magnetic properties of the intermetallics were also evaluated. Small additions of Al stabilise the δ-FeBe₅ intermetallic over ε-Fe₂−xBe₁₇+x and ζ-FeBe₂. Increasing amounts of Al lead to the formation of a disordered (Al,Fe)Be₂ phase. Finally, the solubilities of selected extrinsic elements in Be metal and its SPPs were investigated. It was found that Si, Al, Li and H are preferentially dissolved in δ-FeBe₅, ζ-FeBe₂ and (Al,Fe)Be₂ over Be metal. The ability of Fe-bearing SPPs to absorb Al and Si is thought to be beneficial for the mechanical properties of Be alloys. On the other hand, if a sufficient volume fraction of SPPs is present, their high affinity for H may aggravate tritium retention in Be-based plasma facing components used in fusion reactors. SPPs were shown to not interact strongly with He. O, Mg and C preferentially form other SPPs (BeO, MgBe₁₃ and Be₂C respectively).