Refractory high-entropy alloys (RHEAs) with high melting points and low neutron absorption cross-section are sought for generation-IV fission and fusion reactors. A high throughput computational screening tool, Alloy Search and Predict (ASAP), was used to identify promising RHEA candidates from over 1 million four-element equimolar combinations. The selected VNbCrMo RHEA was further studied by CALPHAD to predict phase for-mation, which was compared to an experimentally produced ingot aged at 1200 degrees C. The VNbCrMo RHEA was found to constitute a majority bcc phase, with a 6% area fraction of C15-Laves formed at interdendritic regions, in contrast to the predictions of single-phase. The prediction of the yield strength by a model based upon edge dislocation mechanisms indicated 2.1 GPa at room temperature and 850 MPa at 1000 degrees C for the equimolar single bcc phase. The hardness of the alloy with C15-Laves was 748 HV (yield strength-2.4 GPa). Finally, the macroscopic neutron absorption cross-section was modelled for a wide range of energies. Displacements per atom per year and activation calculations, up to 1000 years after 2 years of continuous operation, in typical fusion and fission reactor scenarios were also performed using the inventory code FISPACT-II. This work gives new insight into the phase stability and performance of the VNbCrMo RHEA, which is compared with a similar design concept alloy, to assess the potential of novel RHEAs for use in advanced nuclear applications.

Molecular dynamics simulations of single crystal zirconium fracture were performed to study the deformation mechanisms active on the basal and prismatic planes. The effects of temperature (0 to 300 K) and strain rate (108–1010 s−1) were investigated. Crack tip orientation was found to strongly affect the fracture behaviour. On the basal plane twinning ({112̄1}<11̄26>) and emission of type dislocations that then dissociated into partial dislocations around pyramidal I2 stacking faults were seen to occur during fracture. At higher strain rates (109 and 1010 s−1), twinning occurred. The emission of edge dislocations (13<12̄10> type) was prevalent on the prismatic plane and were found to be strongly affected by temperature. At higher temperature (150 and 300 K), the dislocation density increased. The crack grew further at 150–300 K than at 0 K and the shielding effect of dislocations was limited due to their movement away from the crack tip. The addition of iodine at basal I2, pyramidal I1 and I2 stacking faults was seen to decrease the energy of its formation whereas for the prismatic stacking fault it was found to increase it. The iodine also changed the order of favourability of the stacking faults with basal I2 and pyramidal I1 stacking faults becoming much more favourable and prismatic going from most to least favourable.

In water reactors, iodine stress corrosion cracking is considered the cause of pellet-cladding interaction failures, but the mechanism and chemistry are debated and the protective effect of oxygen is not understood. Density functional theory calculations were used to investigate the interaction of iodine and oxygen with bulk and surface Zr under applied hydrostatic strain (-2% to +3%) to simulate crack tip conditions in Zr to ZrO2, using a variety of intermediate suboxides (Zr6O, Zr3O, Zr2O and ZrO). The formation energy of an iodine octahedral interstitial in Zr was found to decrease with increasing hydrostatic strain, whilst the energy of an iodine substitutional defect was found to be relatively insensitive to strain. As the oxygen content increased, the formation energy of an iodine interstitial increased from 1.03 eV to 8.61 eV supporting the idea that oxygen has a protective effect. At the same time, a +3% tensile hydrostatic strain caused the iodine interstitial formation energy to decrease more in structures with higher oxygen content: 4.56 eV decrease in Zr0 compared to 1.47 eV decrease for pure Zr. Comparison of the substitutional and interstitial energies of iodine, to the adsorption energy of iodine, in the presence of oxygen, shows the substitutional energy of iodine onto a Zr site is more favourable for all strains and even interstitial iodine is favourable between strains of +1-5%. Although substitutional defects are preferred to octahedral interstitial defects, in the ordered suboxides, a 3% tensile strain significantly narrows the energy gap and higher strains could cause interstitial defects to form. (C) 2021 Elsevier B.V. All rights reserved.

This review considers current Zr alloys and opportunities for advanced zirconium alloys to meet the de-mands of a structural material in fusion reactors. Zr based materials in the breeder blanket offer the potential to increase the tritium breeding ratio above that of Fe, Si and V based materials. Current com-mercial Zr alloys might be considered as a material in water-cooled breeder blanket designs, due to the similar operating temperature to fission power plants. For breeder blankets designed to operate at higher temperatures, current commercial Zr alloys will not meet the high temperature strength and thermal creep requirements. Hence, Zr alloys with an operational temperature capability beyond that of current commercial fission alloys have been reviewed, specifically: binary Zr alloy systems Zr-Al, Zr-Be, Zr-Cr, Zr-Nb Zr-Ti, Zr-Si, Zr-Sn, Zr-V and Zr-W; as well as higher order Zr alloys Zr-Mo-Ti, Zr-Nb-Ti, Zr-Ti-Al-V and Zr-Mo-Sn. It is concluded that, with further work, higher order Zr alloys could achieve the required high temperature strength, alongside ductility, while maintaining a low thermal neutron cross-section. However, there is limited data and uncertainty regarding the structural performance and microstructural stability of the majority of advanced Zr alloys for temperatures 50 0-70 0 degrees C, at which they would be expected to operate for helium-and liquid metal-cooled breeder blanket designs.Crown Copyright (c) 2021 Published by Elsevier B.V. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )

A series of G-phase strengthened ferritic stainless steels Fe-20Cr-3Ni-3Si-X (X = 2Mn, 1Mn-2Ti, 1Mn-2Nb and 1Mn-2Ta) are characterized after aging using experimental (microhardness, TEM and APT) and theoretical (DFT) techniques. The results indicate that the Ni16Mn6Si7 G-phase shows sluggish precipitation during aging treatment. This was attributed to the small difference in the enthalpy of formation between the Ni16Mn6Si7 G- and BCC phase and the requirement of high Ni:Fe ratio. A superfine Ni16Mn6Si G-phase was found to precipitate as a core accompanied with an "envelope" of Fe2TiSi-L2(1) Heusler phase during early aging (<= 24 h) in the Ti containing alloy. This morphology is predicted to occur due to early Ni clustering in ferrite and a negative Ni concentration gradient away from the cluster that favors Fe2TiSi formation. The G-phases show only particle coarsening without obvious chemical composition evolution for further aging up to 96 h. A prominent hardness increase of 100-275 HV was also observed during aging. These findings provide valuable insight into methods for precipitating low lattice mismatch silicide phases for the development of future high strength steels. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The Γ2-phase is postulated to form from solute clusters in neutron irradiated low-alloy steels. Density functional theory calculations were used to identify the ground state Γ2 structures with variation in Si and Ni contents. It was found that favourability of the Γ2-phase was proportional to Si content, however Si-Si first nearest neighbours reduced favourability. The substitutional enthalpies of Al, Cr, Cu, Fe, Ge, Hf, Mo, Nb, P, Ta, Ti, V, Zr, and vacancies into the ground state Γ2-phase structure from a ferrite matrix were calculated. It was found that Zr, Hf, Nb, Cu, Ti and Ta favourably substitute onto the Mn sites, Cu and P onto the Si sites and Cu onto the Ni sites and vacancies will substitute favourably onto all atomic sites. Finally, it is shown that, for ≤20 at% Fe concentrations, the Γ2-phase becomes more favourable than the bcc structure therefore it is plausible that the Γ2-phase could be thermodynamically stabilised provided these conditions are met.

Cr-Mo-V-W high-entropy alloy (HEA) is studied, with 2553 K equilibrium solidus and high Cr content to promote protective oxide scale formation, suggesting potential applications in hot, oxidising environments. Alloy Search and Predict (ASAP) and phase diagram calculations found a single phase, body-centred cubic (BCC) solid solution at elevated temperatures, across the range of compositions present within the system - uncommon for a HEA of refractory and transition metals. Density functional theory identified solubility of 22 at.% Cr at solidus temperature, with composition-dependent drive for segregation during cooling. An as-cast, BCC single-phase with the composition 31.3Cr-23.6Mo-26.4 V-18.7 W exhibiting dendritic microsegregation was verified. Crown Copyright (C) 2018 Published by Elsevier Ltd on behalf of Acta Materialia Inc. All rights reserved.

Numerous experimental studies have found the presence of (Cu)-Ni-Mn-Si clusters in neutron irradiated reactor pressure vessel steels, prompting concerns that these clusters could lead to larger than expected increases in hardening, especially at high fluences late in life. The mechanics governing clustering for the Fe-Mn-Ni-Si system are not well-known; state-of-the-art methods use kinetic Monte Carlo (KMC) parameterized by density functional theory (DFT) and thermodynamic data to model the time evolution of clusters. However, DFT-based KMC studies have so far been limited to only pairwise interactions due to lack of DFT data. Here, we explicitly calculate the binding energy of triplet clusters of Mn, Ni, Cu, Si, and vacancies in bcc Fe using DFT to show that the presence of vacancies, Si, or Cu stabilizes cluster formation, as clusters containing exclusively Mn and/or Ni are not energetically stable in the absence of interstitials. We further identify which clusters may be reasonably approximated as a sum of pairwise interactions and which instead require an explicit treatment of the three-body interaction, showing that the three-body term can account for as much as 0.3 eV, especially for clusters containing vacancies.

The data presented here is complementary to the publication entitled "High temperature, low neutron cross-section high-entropy alloys in the Nb-Ti-V-Zr system" [1]. A homogenization methodology with slower cooling rate (∼2 °C/min) was performed. X-ray diffraction and scanning electron microscopy (backscattered electron and energy dispersive spectroscopy) data pertaining to annealed high-entropy alloy composition NbTiVZr is presented.

Recently, Matsukawa et al. [1] published a paper investigating the nucleation and growth of Mn6Ni16Si7 G-phase precipitates in duplex stainless steel using experimental and theoretical techniques. The G-phase cubic unit cells simulated by the authors for the theoretical analysis are non-physical, with lattice parameters of ≤0.550 nm, leading to a partially erroneous conclusion regarding the structure of the observed precipitates. In this comment we use density functional theory results from our previous study of the G-phase [2] to offer an alternative explanation to the experimental observations made by Matsukawa et al., viz, the Mn-Ni-Si precipitate is in an intermediate P1 structure, resulting from an energy minimum, in the transformation from BCC packing to G-phase.

Density functional theory (DFT) calculations were used to model G-phase precipitates of formula X6M16Si7 where X is Cr, Hf, Mn, Mo, Nb, Ta, Ti, V, W and Zr and M is either Fe or Ni. It was found that the occupancy of the d-orbital is correlated to the formation enthalpies of each structure. Past thermal expansion coefficient data was used to predict the lattice misfit between each G-phase and body centred cubic (BCC) Fe. All except Hf and Zr containing G-phases were predicted to have zero misfit between 581-843 K. Of the Ni containing G-phases, Mn6Ni16Si7 was predicted to have the most similar elastic properties to BCC Fe. DFT calculations of the substitution energies of Al, Cr Cu, Fe, Ge, Hf, Mo, Nb, P, Ta, Ti, V, Zr, and vacancies onto the Mn6Ni16Si7 G-phase from BCC Fe were performed. It was predicted that Cu, P and vacancies favour G-phase substitution. Suppression of the G-phase is predicted when Si content is reduced by half, at which point the BCC phase is favoured. It is hypothesised that including Zr to form a (Mn,Zr)(6)Ni16Si7 precipitate will allow for higher ageing temperature and expediate nucleation in an Fe alloy. Thermocalc was used to predict that a mixture of FebalCr9Ni4Si2(Mn0.6Zr0.4)(1.2) (at.%) will produce a G-phase strengthened Fe alloy with potential for a good balance of strength, ductility and oxidation/corrosion resistance at room temperature. This alloy composition was experimentally determined to precipitate the G-phase in <= 24 h with cube-on-cube orientation to the BCC Fe matrix. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

High-entropy alloys (HEAs) with high melting points and low thermal neutron cross-section are promising new cladding materials for generation III+ and IV power reactors. In this study a recently developed high throughput computational screening tool Alloy Search and Predict (ASAP) has been used to identify the most likely candidate single-phase HEM with low thermal neutron cross-section, from over a million four-element equimolar combinations. The selected NbTiVZr HEA was further studied by density functional theory (DFT) for moduli and lattice parameter, and by CALPHAD to predict phase formation with temperature. HEM of NbTiVZrx (x = 0.5, 1, 2) were produced experimentally, with Zr varied as the dominant cross-section modifier. Contrary to previous experimental work, these HEAs were demonstrated to constitute a single-phase HEA system; a result obtained using a faster cooling rate following annealing at 1200 degrees C. However, the beta (BCC) matrix decomposed following aging at 700 degrees C, into a combination of nano-scale beta, alpha (HCP) and C15 Laves phases. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Orientation relationships (ORs) can form during solidification by a variety of mechanisms that are often difficult to distinguish after solidification. Here we study three ORs formed by the nucleation of Al3Ti on TiB2, and by the pushing and engulfment of TiB2 by growing Al3Ti facets in hyperperitectic Al-rich melts. The nucleation OR is identified by growing a relatively large TiB2 crystal, solidifying multiple small Al3Ti crystals on one (0001) facet of TiB2, and measuring the resulting OR by electron backscatter diffraction (EBSD). Pushing and engulfment ORs are investigated by statistical analysis of EBSD measurements, density functional theory (DFT) calculations of interface energies, and imaging of cross-sections of TiB2 particles being pushed and engulfed by Al3Ti facets. It is shown that the lowest energy OR is formed by nucleation as well as by pushing/engulfment. The higher energy ORs, formed by pushing and engulfment, correspond to local interfacial energy minima and can be explained by rotation of TiB2 particles on Al3Ti facets during pushing.

An unexplained discrepancy exists between the experimentally measured and theoretically calculated magnetic moments of Mn in alpha-Fe. In this study, we use density functional theory to suggest that this discrepancy is likely due to the local strain environment of a Mn atom in the Fe structure. The ferromagnetic coupling, found by experiment, was shown to be metastable and could be stabilized by a 2% hydrostatic compressive strain. The effects of Mn concentration, vacancies, and interstitial defects on the magnetic moment of Mn are also discussed. It was found that the ground-state, antiferromagnetic (AFM) coupling of Mn to Fe requires long-range tensile relaxations of the neighboring atoms along < 111 > which is hindered in the presence of other Mn atoms. Vacancies and Fe interstitial defects stabilize the AFM coupling but are not expected to have a large effect on the average measured magnetic moment.

The metallic phase fission product containing Mo-Pd-Rh-Ru-Tc can be described as a hexagonal high-entropy alloy (HEA) and is thus investigated using atomic scale simulation techniques relevant to HEAs. Contrary to previous assumptions, the removal of Tc from the system to form the Mo-Pd-Rh-Ru analog is predicted to reduce the stability of the solid solution to the point that sigma-Mo5Ru3 may precipitate out at typical fuel operating temperatures. The drive for segregation is attributed to the increased stability of the solid solution with the ejection of Mo and Ru. When Tc is included in the system, a single phase hexagonal solid solution is expected to form for a wider range of compositions. Furthermore, when cooled below 700 degrees C, this single phase solid solution is predicted to transition to a partially ordered structure. Future studies using the Tc-absent analogue will need to take these structural and chemical deliberations into consideration. (C) 2017 Elsevier B.V. All rights reserved.

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.

The crystalline and amorphous structures of U3Si have been investigated using density functional theory techniques for the first time. The effects of disorder and the impact of fission products has been separated to understand the swelling characteristics of U3Si in both crystalline and amorphous U3Si. Initially, the stability of the three experimentally observed polymorphs of U3Si were explored. Subsequently, we modelled the amorphous U3Si system and conclude that initial increase in volume observed experimentally at low temperature corresponds well with the volume change that occurs with the observed amorphisation of the material. The solubility of Xe and Zr into both the crystalline and amorphous systems was subsequently investigated.

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. Crown Copyright (C) 2014 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. All rights reserved.

We have employed a variety of computational methods to understand the behaviour of Pu, generated by neutron capture reactions in UO2 fuel, with Cr2O3 and Al2O3, two common UO2 fuel dopants. Structure search calculations using density functional theory and empirical potentials show that PuCrO3 and PuAlO3 are likely to form in these systems. The lowest energy structure adopted by both compounds is predicted to be the orthorhombic (Pnma) GdFeO3-type perovskite structure. Relative to UO2, the thermal conductivity of PuCrO3 was calculated to be approximately three times smaller over the explored temperature range and therefore the presence of this phase will impact the microstructure, fission product distribution and gas release properties of UO2-based fuels. In contrast, the PuAlO3 phase had a similar thermal conductivity to UO2. Calculated defect energies suggest that defects in both PuCrO3 and PuAlO3 will be dominated by antisite defects and that their radiation tolerances are similar to that of UO2. Calculation of the solution and partition energies of a range of trivalent cations indicate that minor actinides are likely to substitute for Pu in the perovskite structure having an impact on the in-reactor behaviour of Cr-containing fuels and subsequently the waste reprocessing route.