Solubility and partitioning of impurities in Be alloys

The most energetically favourable accommodation processes for common impurities and alloying elements in Be metal and Be-Fe-Al intermetallics were investigated using atomic scale simulations. Fe additions, combined with suitable heat treatments, may scavange Al and Si through their incorporation into the FeBe₅ intermetallic. In the absence of Fe, Al and Si will not be associated with Be metal. Li and Mg are also not soluble, but may react with other impurities if present (such as Al or H). Mg may also form the MgBe₁₃ intermetallic phase under certain conditions. He and H exhibit negligible solubility in all phases investigated and whilst He will tend to form bubbles, H can precipitate as BeH₂. Similarly, C additions will form the stable compound Be₂C. Finally, oxygen exhibits a strong affinity to Be, exhibiting both some degree of solubility in all phases considered here (though especially metallic Be) and a highly favourable energy of formation for BeO

Predicting the formation and stability of single phase high-entropy alloys

Crown Copyright © 2015 Published by Elsevier Ltd on behalf of Acta Materialia Inc. A method for rapidly predicting the formation and stability of undiscovered single phase high-entropy alloys (SPHEAs) is provided. Our software implementation of the algorithm uses data for 73 metallic elements and rapidly combines them - 4, 5 or 6 elements at a time - using the Miedema semi-empirical methodology to yield estimates of formation enthalpy. Approximately 186,000,000 compositions of 4, 5 and 6 element alloys were screened, and ∼1900 new equimolar SPHEAs predicted. Of the 185 experimentally reported HEA systems currently known, the model correctly predicted the stability of the SPHEA structure in 177. The other sixteen are suggested to actually form a partially ordered solid solution - a finding supported by other recent experimental and theoretical work. The stability of each alloy at a specific temperature can also be predicted, allowing precipitation temperatures (and the likely precipitate) to be forecast. This combinatorial algorithm is described in detail, and its software implementation is freely accessible through a web-service allowing rapid advances in the design, development and discovery of new technologically important alloys

Formation and structure of V-Zr amorphous alloy thin films

© 2014 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. All rights reserved. Although the equilibrium phase diagram predicts that alloys in the central part of the V-Zr system should consist of V2Zr Laves phase with partial segregation of one element, it is known that under non-equilibrium conditions these materials can form amorphous structures. Here we examine the structures and stabilities of thin film V-Zr alloys deposited at room temperature by magnetron sputtering. The films were characterized by X-ray diffraction, transmission electron microscopy and computational methods. Atomic-scale modelling was used to investigate the enthalpies of formation of the various competing structures. The calculations confirmed that an amorphous solid solution would be significantly more stable than a random body-centred solid solution of the elements, in agreement with the experimental results. In addition, the modelling effort provided insight into the probable atomic configurations of the amorphous structures allowing predictions of the average distance to the first and second nearest neighbours in the system

The formation and structure of Fe-Mn-Ni-Si solute clusters and G-phase precipitates in steels

Solute clustering and G-phase precipitation cause hardening phenomena observed in some low alloy and stainless steels, respectively. Density functional theory was used to investigate the energetic driving force for the formation of these precipitates, capturing temperature effects through analysis of the system's configurational and magnetic entropies. It is shown that enrichment of Mn, Ni and Si is thermodynamically favourable compared to the dilute ferrite matrix of a typical A508 low alloy steel. We predict the ordered G-phase to form preferentially rather than a structure with B2-type ordering when the Fe content of the system falls below 10–18 at. %. The B2 → G-phase transformation is predicted to occur spontaneously when vacancies are introduced into the B2 structure in the absence of Fe

Synthesis and DFT investigation of new bismuth-containing MAX phases

The M(n + 1)AX(n) phases (M = early transition metal; A = group A element and X = C and N) are materials exhibiting many important metallic and ceramic properties. In the present study powder processing experiments and density functional theory calculations are employed in parallel to examine formation of Zr(2)(Al(1−x)Bi(x))C (0 ≤ x ≤ 1). Here we show that Zr(2)(Al(1−x)Bi(x))C, and particularly with x ≈ 0.58, can be formed from powders even though the end members Zr(2)BiC and Zr(2)AlC seemingly cannot. This represents a significant extension of the MAX phase family, as this is the first report of a bismuth-based MAX phase

Swelling due to the partition of soluble fission products between the grey phase and uranium dioxide

The change in volume associated with the partition of soluble cations from uranium dioxide into the (Ba,Sr)ZrO3 grey phase has been investigated using atomic scale simulations. Here past work on the thermodynamic drive for the segregation of trivalent and tetravalent cations from uranium dioxide is built upon in the context of fuel swelling. Only small tetravalent cations segregate into the grey phase and this is predicted to result in an overall reduction in fuel volume. Individual trivalent cations that segregate, can cause either a contraction or an expansion of the overall fuel volume. Cr2O3 doped UO2 promotes co-partition forming mixed cation clusters in the grey phase and causing an overall reduction in fuel volume for all trivalent cations. This may have implications for fuel performance and may alter other fuel swelling mechanisms.© 2013 Elsevier Ltd

Crystal structure, thermodynamics, magnetics and disorder properties of Be–Fe–Al intermetallics.

The elastic and magnetic properties, thermodynamical stability, deviation from stoichiometry and order/disorder transformations of phases that are relevant to Be alloys were investigated using density functional theory simulations coupled with phonon density of states calculations to capture temperature effects. A novel structure and composition were identified for the Be–Fe binary ε phase. In absence of Al, FeBe5 is predicted to form at equilibrium above ∼1100 K, while the ε phase is stable only below ∼1500 K, and FeBe2 is stable at all temperatures below melting. Small additions of Al are found to stabilise FeBe5 over FeBe2 and ε, while at high Al content, AlFeBe4 is predicted to form. Deviations from stoichiometric compositions are also considered and found to be important in the case of FeBe5 and ε. The propensity for disordered vs ordered structures is also important for AlFeBe4 (which exhibits complete Al–Fe disordered at all temperatures) and FeBe5 (which exhibits an order–disorder transition at ∼950 K). © 2015 Elsevier B.V

Accommodation of excess oxygen in group II monoxides

Atomic scale simulations are used to predict how excess oxygen is accommodated across the group II monoxides. In all cases, the preference is to form a peroxide ion centered at an oxygen site, rather than a single oxygen species, although the peroxide ionic orientation changes from to to with increasing host cation radius. The enthalpy for accommodation of excess oxygen in BaO is strongly negative, whereas in SrO it is only slightly negative and in CaO and MgO the energy is positive. Interestingly, the increase in material volume due to the accommodation of oxygen (the defect volume) does not vary greatly as a function of cation radius. The vibrational frequency of peroxide ions in the group II monoxides is predicted with the aim to provide test data for future experimental observations of oxygen uptake. Finally, calculations of the dioxide structures have also been carried out. For these materials the oxygen vacancy formation energy is always positive (1.0–1.5 eV per oxygen removed) indicating that they exhibit only small oxygen defect concentrations. © 2012, The American Ceramic Society

Preferential formation of al self-interstitial defects in gamma-TiAl under irradiation

Empirical dynamic calculations were used to observe a distinct increase in aluminium interstitial defects compared to titanium interstitial species remaining after a displacement cascade (even though Frenkel formation energies for both species were found to be similar). Thermodynamic data from static ab-initio models support this interesting result. Calculations were then used to determine whether these interstitials are free to move and therefore have the possibility of migrating to a surface where a passive Al2O3 layer can be formed. © 2012, Elsevier Ltd

Modelling the thermal conductivity of (UₓTh₁-ₓ)O₂ and (UₓPu₁-ₓ)O₂

The degradation of thermal conductivity due to the non-uniform cation lattice of (UₓTh₁-ₓ)O₂ and (UₓPu₁-ₓ)O₂ solid solutions has been investigated by molecular dynamics, using the non-equilibrium method, from 300 to 2000 K. Degradation of thermal conductivity is predicted in (UₓTh₁-ₓ)O₂ and (UₓPu₁-ₓ)O₂ as compositions deviate from the pure end members: UO₂, PuO₂ and ThO₂. The reduction in thermal conductivity is most apparent at low temperatures where phonon-defect scattering dominates over phonon–phonon interactions. The effect is greater for (UₓTh₁-ₓ)O₂ than for (UₓPu₁-ₓ)O₂ due to the greater mismatch in cation size and mass. Parameters for analytical expressions have been developed that describe the predicted thermal conductivities over the full temperature and compositional ranges. These expressions may be used in higher level fuel performance codes