22 results found
Vreeswijk M, Kot A, Giuliani F, et al., 2021, Abnormal WC crystal growth from liquid Co flux occurs via eta phase decomposition, International Journal of Refractory Metals and Hard Materials, Vol: 99, Pages: 1-6, ISSN: 0263-4368
The growth mechanism of large WC crystals from a liquid Co-based flux is identified. This is achieved by systematically varying the growth temperature and Co content from 1200 to 1400 °C and 70–83 at.% respectively. Crystal growth was characterised using metallography and X-ray diffraction. The WC grains were bimodally distributed, consisting of a smaller (10–20 μm) population of grains, which nucleated homogenously from the liquid, and a secondary population of abnormally large grains, several millimetres in size. The abnormal grains nucleated on the (W,Co)6C eta phase, and subsequently consumed it through a carburisation reaction. The size of abnormal grains was enhanced by adopting the eutectic composition, such that the first solid phase to form was the eta phase, whilst at the same time cooling slowly through the eta➔WC transformation temperature, at ~1300 °C. This growth mechanism could be exploited for a variety of metal carbides with similarly sluggish diffusion rates.
Applications from nuclear energy to rockets and jet engines are underpinned by advanced high temperature materials. Whilst state of the art, the performance of current nickel-based superalloys is fundamentally limited to Ni’s melting point, T. Here, we develop an analogous superalloy concept but with superior high temperature capability by transitioning to a bcc tungsten base, T. This strategy involves reinforcing bcc -W by TiFe intermetallic compound, which results in impressive high temperature compressive strengths of 500 MPa at. This bcc-superalloy design approach has wider applicability to other bcc alloy bases, including Mo, Ta, and Nb, as well as to refractory-metal high entropy alloys (RHEAs). By investigation of the underlying phase equilibria, thermodynamic modelling, characterisation and mechanical properties, we demonstrate the capability of ternary W-Ti-Fe tungsten-based bcc-superalloys as a new class of high temperature materials.
Humphry-Baker S, Vandeperre L, 2021, Creep deformation of WC hardmetals with iron-based binders, International Journal of Refractory Metals and Hard Materials, Vol: 95, Pages: 1-8, ISSN: 0263-4368
Iron is a candidate to replace cobalt in WC hardmetals, due to its lower cost and toxicity. A WC-FeCrhardmetal was compression tested at 900-1200 °C. Particular attention is paid to the steady-state creeprates and stress-exponents (n) during isostress treatments. Three regimes of n are observed. Two ofthese were previously reported for WC-Co: power law creep (n»3) at stresses below ~100MPa; andgrain boundary sliding (n»1) at higher stresses. A previously unreported regime at very low stresses(<10MPa), with an exponent of n»2, is also observed. By combining electron microscopy with X-raydiffraction texture measurements, the low stress regime is attributed to viscous flow of the binder,which is accommodated by diffusional creep in the WC skeleton. The mechanism may be applicableto other hardmetals. Compared to analogous WC-Co materials, WC-FeCr shows improved creepresistance below 1000 °C, which can be explained by its lower self-diffusivity, and a lower solubilityfor WC than Co. However, at temperatures corresponding to liquid eutectic formation (~1140 °C), itscreep resistance becomes inferior. These results indicate FeCr may be a suitable replacement for Coprovided the eutectic temperature is not exceeded.
Humphry-Baker SA, Ramanujam P, Smith GDW, et al., 2020, Ablation resistance of tungsten carbide cermets under extreme conditions, International Journal of Refractory Metals and Hard Materials, Vol: 93, ISSN: 0263-4368
A cobalt-free tungsten carbide cermet (WC-FeNi) has been subjected to oxyacetylene flame tests to simulate extreme operating conditions such as a worst-case fusion reactor accident. In such an accident, air-ingress to the reactor may impinge on components operating at surface temperatures in excess of 1000 °C, leading to tungsten oxide formation and its subsequent hazardous volatilisation. Here, the most challenging accident stage has been simulated, where the initial air-ingress could lead to extremely rapid air-flow rates. These conditions were simulated using an oxidising oxyacetylene flame. The separation between flame nozzle and sample was varied to permit peak surface temperatures of ~950–1400 °C. When the peak temperature was below 1300 °C, the cermet gained mass due to the dominance of oxide scale formation. Above 1300 °C, the samples transitioned into a mass loss regime. The mass loss regime was dominated by ablation of the scale rather than its volatilisation, which was confirmed by performing a systematic thermogravimetric kinetic analysis. The result was unexpected as in other candidate shielding materials, e.g. metallic tungsten, volatilisation is considered the primary dispersion mechanism. The unusual behaviour of the cermet scale is explained by its relatively low melting point and by the lower volatility of its FeWO4 scale compared to tungsten's WO3 scale. The substantially lower volatility of the WC cermet scale compared to metallic W indicates it may have a superior accident tolerance.
Athanasakis M, Ivanov E, del Rio E, et al., 2020, A high temperature W2B–W composite for fusion reactor shielding, Journal of Nuclear Materials, Vol: 532, Pages: 1-10, ISSN: 0022-3115
We have developed a new material for neutron shielding applications where space is restricted. W2B is an excellent attenuator of neutrons and gamma-rays, due to the combined gamma attenuation of W and neutron absorption of B. However, its low fracture toughness (3–4 MPa m1/2) and high melting point (2670 °C) prevent the fabrication of large fully-dense parts with adequate mechanical properties. Here we meet these challenges by combining W2B with a minor fraction (43 vol%) of metallic W. The material was produced by reaction sintering W and BN powders. The mechanical properties under flexural and compressive loading were determined up to 1900 °C. The presence of the ductile metallic W phase enabled a peak flexural strength of ∼850 MPa at 1100 °C, which is a factor of 2–3 higher than typical monolithic transition-metal borides. It also enabled a ductile-brittle transition temperature of ∼1000 °C, which is not observed in monolithic borides. Compression tests showed hardening below ∼1500 °C and significant elongation of the phase domains, which suggest that by forging or rolling, further improvements in ductility may be possible. These results have implications for W2B–W shield design; neutronics performance will likely improve with increased boron content, however this study suggests mechanical properties and manufacturability will degrade.
Humphry-Baker S, Smith GDW, Pintsuk G, 2019, Thermal shock of tungsten carbide in plasma-facing conditions, Journal of Nuclear Materials, Vol: 524, Pages: 239-246, ISSN: 0022-3115
Tungsten carbide (WC) has been found to have higher resistance to plasma-induced thermal shock compared to rolled tungsten. The electron beam device JUDITH 1 was used to simulate likely thermal shock conditions induced by edge localised modes and plasma disruptions. Loading conditions of 100–1000 cycles, heat fluxes of 0.19–1.13 GW/m2 and base temperatures of 400–1000 °C were employed on two candidate WC-based materials: a monolithic WC ceramic, and a WC-FeCr composite. Surprisingly, the monolith outperformed the composite under all conditions. This was unexpected, particularly at 400 °C, based on the calculated thermal shock resistance parameters. The result was explained by preferential melting of the metallic FeCr binder. Compared to available data collected under identical conditions on rolled tungsten plate, monolithic WC had lower surface roughness from thermal shock damage, particularly when tested at 400 °C. This shows promise for its use as a plasma facing material. Strategies for further improving performance are discussed.
King DJM, Cheung STY, Humphry-Baker SA, et al., 2019, High temperature, low neutron cross-section high-entropy alloys in the Nb-Ti-V-Zr system, Acta Materialia, Vol: 166, Pages: 435-446, ISSN: 1359-6454
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 HEAs 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. HEAs 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 °C. However, the beta (BCC) matrix decomposed following aging at 700 °C, into a combination of nano-scale beta, alpha (HCP) and C15 Laves phases.
Humphry-Baker S, Smith GDW, 2019, Shielding materials in the compact spherical tokamak, Philosophical Transactions A: Mathematical, Physical and Engineering Sciences, Vol: 377, ISSN: 1364-503X
Neutron shielding materials are a critical area of development for nuclear fusion technology. In the compact spherical tokamak, shielding efficiency improvements are particularly needed because of severe space constraints. The most spatially restricted component is the central column shield. It must protect the superconducting magnets from excessive radiation-induced degradation, but also from associated heating, so that energy consumption of the cryogenic systems is kept to an acceptable level. Recent simulations show that tungsten carbide and its composites form an attractive class of neutron-attenuating materials. In this paper, the key structure–property relationships of these materials are assessed, as they relate to generic materials challenges for plasma-facing materials. We first consider some fundamental materials properties of monolithic tungsten carbide including thermal transport, mechanical properties and plasma interaction. WC is found to have generally favourable properties compared to metallic tungsten shields. We then report progress on the development of a new candidate cermet material, WC-FeCr. Recent results on its accident safety, thermo-mechanical properties, and irradiation behaviour are presented. This review also highlights the need for further study, particularly in the areas of irradiation damage and hydrogen trapping.
Hsieh Y-H, Humphry-Baker SA, Horlait D, et al., 2018, Durability of hot uniaxially pressed Synroc derivative wasteform for EURO-GANEX wastes, JOURNAL OF NUCLEAR MATERIALS, Vol: 509, Pages: 43-53, ISSN: 0022-3115
A new candidate fusion engineering material, WC-FeCr, has been irradiated with He ions at 25 and 500 °C. Ions were injected at 6 keV to a dose of ~15 dpa and 50 at. % He, simulating direct helium injection from the plasma. The microstructural evolution was continuously characterised in situ using transmission electron microscopy. In the FeCr phase, a coarse array of 3–6 nm bubbles formed. In the WC, bubbles were less prominent and smaller (~2 nm). Spherical-cap bubbles formed at hetero-phase interfaces of tertiary precipitates, indicating that enhanced processing routes to minimise precipitation could further improve irradiation tolerance.
Humphry-Baker S, Marshall J, 2018, Structure and properties of high-hardness silicide coatings on cemented carbides for high temperature applications, Coatings, Vol: 8, ISSN: 2079-6412
Cemented tungsten carbides (cWCs) are routinely used in mining and manufacturing but are also candidate materials for compact radiation shielding in fusion power generation. In both applications, there is a need for oxidation to be minimized at operating temperatures. In a recent study, Si-based coatings deposited by pack cementation were demonstrated to improve the oxidation resistance of cWCs by up to a factor of 1000. In this work, these coatings are further characterized, with the focus on growth kinetics, phase composition, and hardness. By combining quantitative X-ray diffraction, electron microscopy, and instrumented micro-indentation, it is shown that the coating layer has a 20% higher hardness than the substrate, which is explained by the presence of a previously-unknown distribution of very hard SiC laths. To interpret the coating stability, a coating growth map is developed. The map shows that the structure is stable under a broad range of processing temperatures and cWC compositions, demonstrating the wide-ranging applicability of these coatings.
Humphry-Baker S, Schuh CA, 2017, Spontaneous solid-state foaming of nanocrystalline thermoelectric compounds at elevated temperatures, Nano Energy, Vol: 36, Pages: 223-232, ISSN: 2211-2855
Nanocrystalline thermoelectric materials have improved properties, but are difficult to process to full density. During routine thermal processing operations such as powder consolidation and annealing, such compounds can spontaneously form pores, thus degrading their thermoelectric and mechanical properties. We systematically investigate pore formation during heat treatment of cold-pressed compacts of nanocrystalline Bi2Te3, combining dilatometry and electron microscopy to quantify pore morphology and the rate of pore growth. Pores are found to nucleate on Te-rich precipitates, which are ultimately attributable to a non-equilibrium solubility shift associated with defects in the nanostructured compound, and they grow by diffusional creep under the driving force of Te vapor pressure. This mechanistic insight reveals that, ironically, the same nonequilibrium processing and nanostructure desirable for improved thermoelectric performance also encourage foaming and challenge the formation of stable high density material. With an improved mechanistic understanding, however, we are also able to suggest strategies for improved materials design and processing.
Humphry-Baker S, Lee WE, Peng K, 2017, Oxidation resistant tungsten carbide hardmetals, International Journal of Refractory Metals and Hard Materials, Vol: 66, Pages: 135-143, ISSN: 0958-0611
We present a new method for retarding the oxidation rate of hardmetals. By diffusion impregnating a WC-FeCr hardmetal with silicon, we manufacture two-layered silicide coatings consisting of an FeSix outer crust and WSi2 beneath. The structure results from a preferential reaction between silicon and the metallic binder. The FeSix outer layer is crucial to providing oxidation resistance as when exposed to oxygen it passivates, forming a protective SiO2 surface film – while simultaneously preventing exposure of the underlying WSi2, which is known to oxidise in an active manner. Our analysis shows the coating method is applicable to various hardmetals structures.
Humphry-Baker SA, Garroni S, Delogu F, et al., 2016, Melt-driven mechanochemical phase transformations in moderately exothermic powder mixtures, Nature Materials, Vol: 15, Pages: 1280-1286, ISSN: 1476-1122
Usually, mechanochemical reactions between solid phases are either gradual (by deformation-induced mixing), or self-propagating (by exothermic chemical reaction). Here, by means of a systematic kinetic analysis of the Bi–Te system reacting to Bi2Te3, we establish a third possibility: if one or more of the powder reactants has a low melting point and low thermal effusivity, it is possible that local melting can occur from deformation-induced heating. The presence of hot liquid then triggers chemical mixing locally. The molten events are constrained to individual particles, making them distinct from self-propagating reactions, and occur much faster than conventional gradual reactions. We show that the mechanism is applicable to a broad variety of materials systems, many of which have important functional properties. This mechanistic picture offers a new perspective as compared to conventional, gradual mechanochemical synthesis, where thermal effects are generally ignored.
Humphry-Baker S, Lee WE, 2016, Tungsten carbide is more oxidation resistant than tungsten when processed to full density, Scripta Materialia, Vol: 116, Pages: 67-70, ISSN: 1872-8456
Previous studies report that WC oxidises in air more readily than W.However, systematic thermogravimetric studies reveal considerably sloweroxidation kinetics in WC samples, which outperform previous measurements by1-2 orders of magnitude. By combining X-ray diffraction and electronmicroscopy, the enhanced stability in WC is explained by a dense interlayer ofsub-stoichiometric WO3, approximately 10 microns in thickness, which formsadjacent to the substrate/oxide interface. The faster oxidation kinetics fromprevious studies are explained by the comparatively low densities of samplesused.
Humphry-Baker SA, Schuh CA, 2014, Suppression of grain growth in nanocrystalline Bi2Te3 through oxide particle dispersions, Journal of Applied Physics, Vol: 116, Pages: 173505-173505, ISSN: 0021-8979
Humphry-Baker SA, Schuh CA, 2014, Grain growth and structural relaxation of nanocrystalline Bi2Te3, Journal of Applied Physics, Vol: 116, Pages: 153502-153502, ISSN: 0021-8979
Humphry-Baker SA, Schuh CA, 2014, Anomalous grain refinement trends during mechanical milling of Bi2Te3, ACTA MATERIALIA, Vol: 75, Pages: 167-179, ISSN: 1359-6454
Humphry-Baker SA, Schuh CA, 2011, The nanocrystalline thermoelectric compound Bi2Te3 forms by a particle-wise explosive reaction during mechanical alloying, SCRIPTA MATERIALIA, Vol: 65, Pages: 516-519, ISSN: 1359-6462
Herbert W, Humphry-Baker S, 2009, Building up strength, MATERIALS WORLD, Vol: 17, Pages: 36-37, ISSN: 0967-8638
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