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 {\epsilon} phase. In absence of Al, FeBe_5 is predicted to form at equilibrium above ~ 1250 K, while the {\epsilon} phase is stable only below ~ 1650 K, and FeBe_2 is stable at all temperatures below melting. Small additions of Al are found to stabilise FeBe_5 over FeBe_2 and {\epsilon}, while at high Al content, AlFeBe_4 is predicted to form. Deviations from stoichiometric compositions are also considered and found to be important in the case of FeBe_5 and {\epsilon}. The propensity for disordered vs ordered structures is also important for AlFeBe_4 (which exhibits complete Al-Fe disordered at all temperatures) and FeBe_5 (which exhibits an order-disorder transition at ~ 950 K).Comment: 14 pages, 10 figures, accepted for publication in J. Alloy Compd. on 14 March 201

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

312 MAX Phases: Elastic Properties and Lithiation

Interest in the Mn+1AXn phases (M = early transition metal; A = group 13–16 elements, and X = C or N) is driven by their ceramic and metallic properties, which make them attractive candidates for numerous applications. In the present study, we use the density functional theory to calculate the elastic properties and the incorporation of lithium atoms in the 312 MAX phases. It is shown that the energy to incorporate one Li atom in Mo3SiC2, Hf3AlC2, Zr3AlC2, and Zr3SiC2 is particularly low, and thus, theoretically, these materials should be considered for battery applications

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

Peroxide as a mechanism to accommodate excess oxygen

Atomic scale simulations are used to predict how excess oxygen is accommodated across three distinct groups of oxides: group II monoxides [1], fluorite di-oxides [2] and zirconate perovsites [3]. In addition to the crystallographic position and orientation of the peroxide molecule, transport of the species is also considered. For each group, three different cations are considered in order to determine how stability and structure change as a function of cation size. Peroxide molecular vibrational frequencies are also predicted to facilitate experimental investigation of the various structural models. For all simulations, the density functional code VASP was employed. Starting with the 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 molecular orientation changes from \u3c100\u3e to \u3c110\u3e to \u3c111\u3e with increasing 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. Calculations of the BO2 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 a surprisingly small oxygen deficiency. For the di-oxides, accommodation of hyperstoichiometry is considered in CeO2, ThO2 and UO2. Calculations indicate a preference for the peroxide species over an isolated interstitial in CeO2 and ThO2 but not in UO2. Frenkel pair defects are investigated to understand if the interstitial component could assume a peroxide like configuration in the vicinity of the vacancy. While it is expected that this would not be the case for UO2 since peroxide was not stable, it is also not found to be the case for CeO2 and ThO2 with the peroxide disassociating into a lattice species and a separate interstitial ion. For the zirconate perovskites, again group II cations are the variable: BaZrO3, SrZrO3 and CaZrO3. While facilitated by peroxide, in contrast to the monoxide, the solution energy of O2 is predicted to be positive (though close to zero) for BaZrO3. Similar to the monoxides, the peroxide molecule is less favourably accommodated in SrZrO3 or CaZrO3. This trend is tested experimentally by exposing SrZrO3 and BaZrO3 to hydrogen peroxide solution and carrying out Raman spectroscopy measurements to look for a peak indicative of peroxide ions. A peak was observed at 1000 cm-1 in both compositions, suggesting the theoretically predicted peroxide ion is present. The transport of the excess oxygen through the perovskite lattice was predicted to proceed with activation energies of less than 1 eV in each of the systems. 1. Middleburgh S. C., Lagerlof K. P. D. & Grimes R. W. “Accommodation of Excess Oxygen in Group II Monoxides” J. Am. Ceram. Soc. 96, 308 (2013). 2. Middleburg S. C., Lumpkin G. R. & Grimes R. W. “Accommodation of excess oxygen in fluorite dioxides” Solid State Ionics, 253, 119 (2013). 3. Middleburgh S. C., Karatchevtseva I., Kennedy B. J., Burr P. A., Zhang Z., Reynolds E., Grimes R. W. & Lumpkin G. R. “Peroxide defect formation in zirconate perovskites” J. Mater. Chem. A, 2, 15883 (2014)

Defect processes of M3AlC2 (M = V, Zr, Ta, Ti) MAX phases

The interest on the Mn+1AXn phases (M = early transition metal; A = group 13–16 element and X = C and/or N) stems from their combination of advantageous metallic and ceramic properties. Aluminium containing 312 MAX phases in particular are deemed to enhance high-temperature oxidation resistance. In the present study, we use density functional theory calculations to study the intrinsic defect processes of M3AlC2 MAX phases (M = V, Zr, Ta, Ti). The calculations reveal that Ti3AlC2 is the more radiation tolerant 312 MAX phase considered here. In Ti3AlC2 the carbon Frenkel reaction is the lowest energy defect process with 3.17 eV. Results are discussed in view of recent experimental and theoretical results of related systems.Publisher Statement: NOTICE: this is the author’s version of a work that was accepted for publication in Solid State Communications. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Solid State Communications, [(in press), (2017)] DOI: 10.1016/j.ssc.2017.06.001© 2017, Elsevier. Licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International http://creativecommons.org/licenses/by-nc-nd/4.0