Imperial College London

DrMarkWenman

Faculty of EngineeringDepartment of Materials

Senior Lecturer
 
 
 
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Contact

 

+44 (0)20 7594 6763m.wenman

 
 
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Location

 

B301aRoyal School of MinesSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

68 results found

Whiting TM, King DJM, Wenman MR, 2020, On the formation and structure of Mn-Ni-Si Γ2 precipitates in steels, Journal of Nuclear Materials, Vol: 542, Pages: 1-7, ISSN: 0022-3115

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.

Journal article

Vecchiato FL, de Winton H, Hooper PA, Wenman MRet al., 2020, Melt pool microstructure and morphology from single exposures in laser powder bed fusion of 316L stainless steel, Additive Manufacturing, Vol: 36, Pages: 101401-101401, ISSN: 2214-8604

Journal article

Fallah AS, Giannakeas IN, Mella R, Wenman MR, Safa Y, Bahai Het al., 2020, On the computational derivation of bond-based peridynamic stress tensor, Journal of Peridynamics and Nonlocal Modeling, Vol: 2, Pages: 352-378, ISSN: 2522-896X

The concept of ‘contact stress’, as introduced by Cauchy, is a special case of a nonlocal stress tensor. In this work, the nonlocal stress tensor is derived through implementation of the bond-based formulation of peridynamics that uses an idealised model of interaction between points as bonds. The method is sufficiently general and can be implemented to study stress states in problems containing stress concentration, singularity, or discontinuities. Two case studies are presented, to study stress concentration around a circular hole in a square plate and conventionally singular stress fields in the vicinity of a sharp crack tip. The peridynamic stress tensor is compared with finite element approximations and available analytical solutions. It is shown that peridynamics is capable of capturing both shear and direct stresses and the results obtained correlate well with those obtained using analytical solutions and finite element approximations. A built-in MATLAB code is developed and used to construct a 2D peridynamic grid and subsequently approximate the solution of the peridynamic equation of motion. The stress tensor is then obtained using the tensorial product of bond force projections for bonds that geometrically pass through the point. To evaluate the accuracy of the predicted stresses near a crack tip, the J-integral value is computed using both a direct contour approximation and the equivalent domain integral method. In the formulation of the contour approximation, bond forces are used directly while the proposed peridynamic stress tensor is used for the domain method. The J-integral values computed are compared with those obtained by the commercial finite element package Abaqus 2018. The comparison provides an indication on the accurate prediction of the state of stress near the crack tip.

Journal article

Fogarty R, Smutna J, Wenman M, Horsfield Aet al., 2020, Beyond two-center tight binding: Models for Mg and Zr, Physical Review Materials, Vol: 4, ISSN: 2475-9953

We describe a systematic approach to building ab initio tight-binding models and apply this to hexagonal metals Mg and Zr. Our models contain three approximations to plane-wave density functional theory (DFT): (i) we use a small basis set, (ii) we approximate self-consistency, and (iii) we approximate many-center exchange and correlation effects. We test a range of approximations for many-center exchange-correlation and self-consistency to gauge the accuracy of each in isolation. This systematic approach also allows us to combine multiple approximations in the optimal manner for our final tight-binding models. Furthermore, the breakdown of errors into those from individual approximations is expected to be a useful guide for which approximations to include in other tight-binding models. We attempt to correct any remaining errors in our models by fitting a pair potential. Our final tight-binding model for Mg shows excellent agreement with plane-wave results for a wide range of properties (e.g., errors below 10% for self-interstitial formation energies and below 3% for equilibrium volumes) and is expected to be highly transferable due to the minimal amount of fitting. Calculations with our Zr model also show good agreement with plane-wave results (e.g., errors below 2% for equilibrium volumes) except for properties where self-consistency is important, such as self-interstitial formation energies. However, for these properties we are able to generate a tight-binding model which shows excellent agreement with non-self-consistent DFT with a small basis set (i.e., many-center effects are captured accurately by our approximations). As we understand the source of remaining errors in our Zr model we are able to outline the methods required to build upon it to describe the remaining properties with greater accuracy.

Journal article

Haynes TA, Shepherd D, Wenman MR, 2020, Preliminary modelling of crack nucleation and propagation in SiC/SiC accident-tolerant fuel during routine operational transients using peridynamics, Journal of Nuclear Materials, Vol: 540, Pages: 152369-152369, ISSN: 0022-3115

Journal article

Than YR, Wenman MR, Grimes RW, 2020, Cu and Sb in tetragonal ZrO2 on fuel cladding, Journal of Applied Physics, Vol: 128, Pages: 135101-135101, ISSN: 0021-8979

Atomic scale simulations were used to predict defect formation in tetragonal ZrO2 doped with Cu and Sb. Both dopants form strong associations with oxygen vacancies impeding oxygen progression through the oxide. Sb suppresses the free oxygen vacancy population though Cu increases the concentration. Thus, while the addition of Sb is predicted to be beneficial against corrosion, Cu will show a more complex behavior. Previous simulations showed that Ni0 promotes molecular hydrogen dissociation. Neither Cu nor Sb exhibit this behavior despite Cu+ having the same electronic configuration as Ni0. Both Cu and Sb show a favorable response to applied local space charges.

Journal article

Dong P, Scatigno GG, Wenman MR, 2020, Effect of Salt Composition and Microstructure on Stress Corrosion Cracking of 316L Austenitic Stainless Steel for Dry Storage Canisters, Journal of Nuclear Materials, Pages: 152572-152572, ISSN: 0022-3115

Journal article

Harrison RW, Gasparrini C, Worth RN, Buckley J, Wenman MR, Abram Tet al., 2020, On the oxidation mechanism of U3Si2 accident tolerant nuclear fuel, CORROSION SCIENCE, Vol: 174, ISSN: 0010-938X

Journal article

Williams RJ, Vecchiato F, Kelleher J, Wenman MR, Hooper PA, Davies CMet al., 2020, Effects of heat treatment on residual stresses in the laser powder bed fusion of 316L stainless steel: Finite element predictions and neutron diffraction measurements, Journal of Manufacturing Processes, Vol: 57, Pages: 641-653, ISSN: 1526-6125

Heat treatments are used in laser powder bed fusion (LPBF) to reduce residual stress and improve service life. In order to qualify components for service, the degree of stress relaxation under heat treatment must be known. In this work, the effect of heat treatment on residual stress (RS) in LPBF 316L stainless steel was studied. Finite element (FE) models were developed to predict the RS distribution in specimens in the as-built state and subjected to heat treatment. The models simulated the thermo-mechanical LPBF build process, sample removal from the build plate and creep stress relaxation effects from a 2 h heat treatment at 700 C. The predictions were validated by neutron diffraction measurements in as-built and heat treated samples, in both build orientations. Large tensile RS of around 450 MPa were predicted at the vertical sample's outer gauge surfaces, balanced by high compressive stresses of similar magnitude at the centre. The residual stresses in the horizontal sample were significantly lower, by around 40%. The influence of sample removal from the base plate on the RS distribution was found to be strongly dependent on the sample orientation and geometry. The heat treatment preserved the unique microstructure of the LPBF process and reduced the peak RS by around 10% in the vertical sample and 40% in the horizontal sample. The FE model predictions were found in good agreement with the experimental measurements, thus providing an effective tool for RS predictions in LPBF components and proving the effectiveness of the heat treatment on RS relaxation.

Journal article

Pavlov T, lestak, Wenman M, Vlahovic, Robba, Cambriani, Staicu, Dahms, Ernstberger, Brown, Bradford M, Konings R, Grimes Ret al., 2020, Examining the thermal properties of unirradiated nuclear grade graphite between 750 and 2500 K, Journal of Nuclear Materials, Vol: 538, Pages: 1-11, ISSN: 0022-3115

This study presents the first high temperature measurements (between 750 K and 2500 K) of thermal conductivity, thermal diffusivity, specific heat and spectral emissivity of virgin graphite samples (type IM1-24) from advanced gas-cooled reactor (AGR) fuel assembly bricks. Scanning electron microscope (SEM) and X-ray computed tomography (XRT) techniques were used to verify the presence of Gilsocarbon filler particles (a characteristic microstructural feature of IM1-24 graphite). All thermal properties were investigated in two orthogonal directions, which showed the effective macroscopic thermal conductivity to be the same (to within experimental error). This can be linked to the morphology of the filler particles that consist of concentrically aligned graphitic platelets. The resulting spherical symmetry allows for heat to flow in the same manner in both macroscopic directions. The current thermal conductivity results were compared to other isotropic grade graphite materials. The significant discrepancies between the thermal conductivities of the individual grades are likely the result of different manufacturing processes yielding variations in the microstructure of the final product. Differences were identified in the filler particle size and structure, and possibly the degree of graphitization compared to other reported nuclear graphites.

Journal article

Gasparrini C, Xu A, Short K, Wei T, Davis J, Palmer T, Bhattacharyya D, Edwards L, MRWenmanet al., 2020, Micromechanical testing of unirradiated and helium ion irradiated SA508 reactor pressure vessel steels: Nanoindentation vs in-situ microtensile testing, Materials Science and Engineering: A, ISSN: 0921-5093

In this paper, microtensile testing is demonstrated to be a viable technique for measuring irradiation hardening and reduction of ductility of ion irradiated hot isostatic pressed SA508 ferritic/bainitic steel. Ion irradiation with He2+ was used as a surrogate for neutron irradiation to reach a damage level of 0.6 dpa (Kinchin-Pease). The mechanical properties of four unirradiated microtensile steel specimens were measured and compared to the bulk properties: when averaged the 0.2% proof stress was 501.6 ± 56.0 MPa, in good agreement with the macrotensile 0.2% proof stress of 456.2 ± 1.7 MPa. On the basis of the agreement between microtensile and standard tensile 0.2% proof stress in the unirradiated material, it was possible to directly measure irradiation induced hardening from ion irradiation performed with He2+ ions to a dose of 0.6 dpa. Microtensile testing of the ion irradiated steel revealed an increase in 0.2% proof stress of approximately 730 MPa. The irradiation hardening measured by nanoindentation was 3.22 ± 0.29 GPa. Irradiation hardening was higher than that previously observed in neutron irradiated low alloy steels exposed to similar doses at low temperatures (<100 °C). The reason for the higher hardening was related to the presence of fine helium bubbles implanted in the irradiated layer that, alone, was calculated to produce a 707 ± 99 MPa increase in yield stress.

Journal article

Jones LD, Vandeperre LJ, Haynes TA, Wenman MRet al., 2020, Theory and application of Weibull distributions to 1D peridynamics for brittle solids, Computer Methods in Applied Mechanics and Engineering, Vol: 363, Pages: 1-11, ISSN: 0045-7825

Peridynamics is a continuum mechanics modelling method, which is emerging as a solution for – in particular – the modelling of brittle fracture. The inherent variability of brittle fracture is captured well by the Weibull distribution, which describes the probability of fracture of a given material at a given stress. Recreating a Weibull distribution in peridynamics involves adjusting for the fact that the body is made up of a large number of bonds, and the distribution of strengths associated with these bonds must be different to the distribution of strengths associated with the peridynamic body. In the local case, where the horizon ratio, m=1 is used, Weibull’s original simple size scaling gives exact results, but the overlapping nature of non-local bonds that occurs in higher m cases, typically used in the peridynamics literature (such as m=3), causes a significant distortion of Weibull distributions. The cause of these distortions is spurious toughening and partial component failures as a result of the reduced localisation associated with larger horizon ratios. In order to remove these distortions, appropriate size scaling is used for the bonds, and a methodology that is capable of reflecting the heterogeneity of the material in the model, is proposed. The methodology described means Weibull parameters measured at specimen or component level can be reproduced for higher values of m.

Journal article

Smutna J, Fogarty RM, Wenman MR, Horsfield APet al., 2020, Systematic development of ab initio tight-binding models for hexagonal metals, Physical Review Materials, Vol: 4, Pages: 043801-1-043801-18, ISSN: 2475-9953

A systematic method for building an extensible tight-binding model from ab initio calculations has been developed and tested on two hexagonal metals: Zr and Mg. The errors introduced at each level of approximation are discussed and quantified. For bulk materials, using a limited basis set of spd orbitals is shown to be sufficient to reproduce with high accuracy bulk energy versus volume curves for fcc, bcc, and hcp lattice structures, as well as the electronic density of states. However, the two-center approximation introduces errors of several tenths of eV in the pair potential, crystal-field terms, and hopping integrals. Environmentally dependent corrections to the former two have been implemented, significantly improving the accuracy. Two-center hopping integrals were corrected by taking many-center hopping integrals for a set of structures of interest, rotating them into the bond reference frame, and then fitting a smooth function through these values. Finally, a pair potential was fitted to correct remaining errors. However, this procedure is not sufficient to ensure transferability of the model, especially when point defects are introduced. In particular, it is shown to be problematic when interstitial elements are added to the model, as demonstrated in the case of octahedral self-interstitial atoms.

Journal article

Than YR, Grimes RW, Bell BDC, Wenman MRet al., 2020, Understanding the role of Fe, Cr and Ni in Zircaloy-2 with special focus on the role of Ni on hydrogen pickup, Journal of Nuclear Materials, Vol: 530, Pages: 151956-151956, ISSN: 0022-3115

Ni as an alloying addition in Zircaloy leads to an increase in hydrogen pick-up fraction. Atomic scale simulations of tetragonal ZrO2, based on density functional theory, are used to identify a possible mechanism for this observation. First, defect formation energies associated with Ni but also Fe and Cr are used to predict relative defect cluster and defect charge concentrations using Brouwer diagrams. At low oxygen partial pressures (), expected in the vicinity of the oxide metal interface, a cluster consisting of an oxygen vacancy adjacent to a charge neutral Ni0 atom is identified as the most populous cluster. Further simulations show that a hydrogen molecule will dissociate in the vicinity of this cluster. No other cluster is both sufficiently populous and acts in this way. This differentiates Ni from the other alloying elements.

Journal article

King DJM, Yang M, Whiting TM, Liu X, Wenman MRet al., 2020, G-phase strengthened iron alloys by design, Acta Materialia, Vol: 183, Pages: 350-361, ISSN: 1359-6454

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)6Ni16Si7 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.

Journal article

Reali L, Wenman MR, Sutton AP, Balint DSet al., 2020, Plasticity of zirconium hydrides: a coupled edge and screw discrete dislocation model, Journal of the Mechanics and Physics of Solids, ISSN: 0022-5096

© 2020 Understanding the plastic behaviour of thin zirconium hydrides is important for its implications on crack nucleation in the Zirconium alloy cladding used in fission reactors. Microvoids originate at fractured hydrides, and their coalescence may lead to the failure of the component. In this work, an innovative discrete dislocation framework is presented together with the preliminary results. The aim is to develop a model that is significantly faster than existing 3D formulations, to make it possible to run a statistical analysis on a simulated microstructure. This comes with limitations, which are discussed together with the planned developments. The model combines two planar and orthogonal simulations. In one only edge, in the other only screw dislocations are allowed, thereby describing all sides of a dislocation loop approximated as a rectangle. The two families of dislocations interact via their elastic stress, and this coupling proved to be important and significantly enhanced the dislocation density. The proposed model enables us to implement a 3D stress analysis of the hydrides. The simulations show that the most critical scenario is when neighbouring slip planes become populated with opposite-signed dislocations. This was observed both in the edge and in the screw case, and was reflected in the principal stress calculated by combining the two. It was also observed that the degree of permeability of the interface to dislocation crossing is inversely correlated to the stress inside the hydride and to the dislocation source activation.

Journal article

Reali L, El Chamaa S, Balint DS, Davies CM, Wenman MRet al., 2020, Deformation and fracture of zirconium hydrides during the plastic straining of Zr-4, MRS ADVANCES, Vol: 5, Pages: 559-567, ISSN: 2059-8521

Journal article

Scatigno GG, Dong P, Ryan MP, Wenman MRet al., 2019, The effect of salt loading on chloride-induced stress corrosion cracking of 304L austenitic stainless steel under atmospheric conditions, Materialia, Vol: 8, Pages: 1-11, ISSN: 2589-1529

The effect of salt loading on chloride-induced stress corrosion cracking in 304 L was studied at atmospheric pressure. Stress relieved samples were uniaxially pre-strained to 5% and were loaded with nine levels of MgCl2, investigating Cl− deposition levels from 1.7 × 10−4 to 3.1 x 10−2 g cm−2. Samples were subject to 60 MPa stress, 90 °C at 70% relative humidity, for 480 h. A direct correlation between chloride deposition and the density of cracking and corrosion was observed between 5.7 × 10−4 and 1.96 × 10−2 g cm−2. Crack propagation rates were constant between salt loadings of 5.7 × 10−4 and 2.4 × 10−2 g cm−2 at 1–2 µm h−1.

Journal article

Whiting T, Burr PA, King D, Wenman Met al., 2019, Understanding the importance of the energetics of Mn, Ni, Cu, Si and vacancy triplet clusters in bcc Fe, Journal of Applied Physics, Vol: 126, ISSN: 0021-8979

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) parameterised 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 stabilises 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.

Journal article

Weekes H, Dye D, Proctor JE, Smith DS, Simionescu C, Prior TJ, Wenman Met al., 2019, The effect of pressure on hydrogen solubility in Zircaloy-4, Journal of Nuclear Materials, Vol: 524, Pages: 256-262, ISSN: 0022-3115

The effect of pressure on the room temperature solubility of hydrogen inZircaloy-4 was examined using synchrotron X-ray diffraction on small groundflake samples in a diamond anvil cell at pressures up to 20.9 GPa. Differentcombinations of hydrogen level/state in the sample and of pressure transmittingmedium were examined; in all three cases examined, it could be concluded thatpressure resulted in the dissolution of d hydrides and that interstitialhydrogen retards the formation of w Zr. A pressure of around 9 GPa was requiredto halve the hydride fraction. These results imply that the effect of pressureis thermodynamically analogous to that of increasing temperature, but that theeffect is small. The results are consistent with the volume per Zr atom of thea, d and w phases, with the bulk moduli of a and d, and with previousmeasurements of the hydrogen site molar volumes in the a and d phases. Theresults are interpreted in terms of their implication for our understanding ofthe driving forces for hydride precipitation at crack tips, which are in aregion of hydrostatic tensile stress on the order of 1.5 GPa.

Journal article

Gasparrini C, Podor R, Fiquet O, Horlait D, May S, Wenman MR, Lee WEet al., 2019, Uranium carbide oxidation from 873 K to 1173 K, Corrosion Science, Vol: 151, Pages: 44-56, ISSN: 0010-938X

Oxidation of UC was studied from 873 to 1173 K in air and in 10 Pa oxygen using a High Temperature Environmental SEM (HT-ESEM). Conversion to U 3 O 8 improved when using 873 K as the oxide product was a fine powder. At higher temperatures (973 K to 1173 K) oxidation slowed due to a densification process with formation of coarse fragments. The oxide fragmentation at 973 K and 1073 K and oxide pulverisation at 873 K were observed in situ in a HT-ESEM. Cracking induced fragmentation and pulverisation was linked to stresses generated from the volumetric transformation from UC to U 3 O 8 .

Journal article

King DJM, Wenman MR, 2019, Comment on "The two-step nucleation of G-phase in ferrite", the authors: Y. Matsukawa et al. Acta Mater 2016; 116:104-133, Scripta Materialia, Vol: 163, Pages: 163-165, ISSN: 1359-6462

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.

Journal article

Haynes TA, Podgurschi V, Wenman MR, 2019, The impact of azimuthally asymmetric carbon deposition upon pellet-clad mechanical interaction in advanced gas reactor fuel, JOURNAL OF NUCLEAR MATERIALS, Vol: 513, Pages: 62-70, ISSN: 0022-3115

Journal article

Kenich A, Wenman MR, Grimes RW, 2018, Iodine defect energies and equilibria in ZrO2, JOURNAL OF NUCLEAR MATERIALS, Vol: 511, Pages: 390-395, ISSN: 0022-3115

Journal article

Than YR, Wenman MR, Bell BDC, Ortner SR, Swan H, Grimes RWet al., 2018, Modelling and experimental analysis of the effect of solute iron in thermally grown Zircaloy-4 oxides, Journal of Nuclear Materials, Vol: 509, Pages: 114-123, ISSN: 0022-3115

Simulations based on density functional theory (DFT) were used to investigate the behaviour of substitutional iron in both tetragonal and monoclinic ZrO2. Brouwer diagrams of predicted defect concentrations, as a function of oxygen partial pressure, suggest that iron behaves as a p-type dopant in monoclinic ZrO2 while it binds strongly to oxygen vacancies in tetragonal ZrO2. Analysis of defect relaxation volumes suggest that these results should hold true in thermally grown oxides on zirconium, which is under compressive stresses. X-ray absorption near edge structure (XANES) measurements, performed to determine the oxidation state of iron in Zircaloy-4 oxide samples, revealed that 3 + is the favourable oxidation state but with between a third and half of the iron, still in the metallic Fe0 state. The DFT calculations on bulk zirconia agree with the preferred oxidation state of iron if it is a substitutional species but do not predict the presence of metallic iron in the oxide. The implications of these results with respect to the corrosion and hydrogen pick-up of zirconium cladding are discussed.

Journal article

Bell BDC, Murphy ST, Grimes RW, Wenman MRet al., 2018, The effect of Sn–VO defect clustering on Zr alloy corrosion, Corrosion Science, Vol: 141, Pages: 14-17, ISSN: 0010-938X

Density functional theory simulations were used to study Sn defect clusters in the oxide layer of Zr-alloys. Clustering was shown to play a key role in the accommodation of Sn in ZrO2, with the {SnZr:VO}×bound defect cluster dominant at all oxygen partial pressures below 10−20atm, above which SnZr×is preferred. {SnZr:VO}×is predicted to increase the tetragonal phase fraction in the oxide layer, due to the elevated oxygen vacancy concentration. As corrosion progresses, the transition to SnZr×, and resultant destabilisation of the tetragonal phase, is proposed as a possible explanation for the early first transition observed in Sn-containing Zr–Nb alloys.

Journal article

King DJM, Burr PA, Middleburgh SC, Whiting TM, Burke MG, Wenman MRet al., 2018, The formation and structure of Fe-Mn-Ni-Si solute clusters and G-phase precipitates in steels, Journal of Nuclear Materials, Vol: 505, Pages: 1-6, ISSN: 0022-3115

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.

Journal article

King DJM, Middleburgh SC, Burr PA, Whiting TM, Fossati PC, Wenman MRet al., 2018, Density functional theory study of the magnetic moment of solute Mn in bcc Fe, Physical Review B, Vol: 98, ISSN: 2469-9950

An unexplained discrepancy exists between the experimentally measured and theoretically calculated magnetic moments of Mn in α-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.

Journal article

Pavlov TR, Wangle T, Wenman MR, Tyrpekl V, Vlahovic L, Robba D, Van Uffelen P, Konings RJM, Grimes RWet al., 2018, High temperature measurements and condensed matter analysis of the thermo-physical properties of ThO2, SCIENTIFIC REPORTS, Vol: 8, ISSN: 2045-2322

Values are presented for thermal conductivity, specific heat, spectral and total hemispherical emissivity of ThO2 (a potential nuclear fuel material) in a temperature range representative of a nuclear accident - 2000 K to 3050 K. For the first time direct measurements of thermal conductivity have been carried out on ThO2 at such high temperatures, clearly showing the property does not decrease above 2000 K. This could be understood in terms of an electronic contribution (arising from defect induced donor/acceptor states) compensating the degradation of lattice thermal conductivity. The increase in total hemispherical emissivity and visible/near-infrared spectral emissivity is consistent with the formation of donor/acceptor states in the band gap of ThO2. The electronic population of these defect states increases with temperature and hence more incoming photons (in the visible and near-infrared wavelength range) can be absorbed. A solid state physics model is used to interpret the experimental results. Specific heat and thermal expansion coefficient increase at high temperatures due to the formation of defects, in particular oxygen Frenkel pairs. Prior to melting a gradual increase to a maximum value is predicted in both properties. These maxima mark the onset of saturation of oxygen interstitial sites.

Journal article

El Chamaa S, Patel M, Davies C, Wenman MRet al., 2018, The effect of grain boundaries and second-phase particles on hydride precipitation in zirconium alloys, MRS Advances, Vol: 3, Pages: 1749-1754, ISSN: 2059-8521

Understanding the precipitation of brittle hydride phases is crucial in establishing a failure criterion for various zirconium alloy nuclear fuel cladding. Accordingly, it is important to quantify the sensitivity of hydride precipitation to the component microstructure. This experimental investigation focuses on two microstructural characteristics and their role as hydride nucleation sites: The grain size and the alloy chemical composition. Samples of commercially pure zirconium (Zr-702) and Zircaloy-4, each with a wide range of grain sizes, were hydrided to 100 ppm and micrographs of the hydride distribution were optically analyzed for inter-granular and intra-granular precipitate sites. For most grain sizes, it was found that a significantly lower fraction of the precipitated hydrides nucleated at grain boundaries in Zircaloy-4 than in Zr-702, suggesting that a higher SPP content encourages the formation of intra-granular hydrides. Moreover, this effect became more prominent as the grain size increased; large-grain specimens contained a higher fraction of intra-granular hydrides than small-grain specimens of both Zr-702 and Zircaloy-4, highlighting the potency of grain boundaries as nucleation sites and how SPPs can influence the hydride distribution profile.

Journal article

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