Tungsten-based bcc-superalloys: thermal stability and ageing behaviour

Tungsten is considered as a primary material for the divertor and first wall in many fusion reactor designs. There has been further interest in nano-structured multi-phase tungsten alloys and composites, such as oxide dispersion strengthened alloys, where interfaces may be harnessed as defect sinks to improve irradiation resilience, whilst also improving base mechanical strength, and potentially ductility. Here we further investigate the concept of tungsten-based ‘bcc-superalloys’ within the W-Ti-Fe ternary system, comprising W-TiFe, A2-B2, β-β’ nanostructures. Alloys were produced by arc melting and the microstructure controlled via thermal heat treatments, by solutionising at 1250 °C, followed by 750 °C ageing. The alloys were characterised using electron microscopy, including composition measurements, alongside hardness measurements. Building on our previous work, we have demonstrated that nano-scale B2 TiFe(W) forms within A2(W,Ti,Fe) in the W-Ti-Fe alloys, creating localised regions of the targeted A2-B2 (β-β’) precipitate reinforced structure. Further, here we evaluated ageing at 750 °C, where within the interdendritic domains decomposition consistent with B2TiFe(W) -> B2 + A2 and A2(Ti,Fe,W) -> A2 + A3 is proposed. An experimentally validated preliminary W-Ti-Fe ternary phase diagram has been produced, helping to understand the stable phases present and instructing onward optimisation of W-superalloys as a candidate material for fusion energy

An oxidation mechanism map for tungsten

A tungsten oxidation mechanism map is developed to clarify literature confusion about the dominant oxidation kinetic regime and to enable improved predictions in extreme environments. Thermogravimetry data is systematically extracted from 14 papers in the range 600–1600 °C and treated as three distinct kinetic regimes: parabolic oxidation, linear oxidation, and sublimation. A mechanism map is constructed that shows metal recession thickness contours for each regime in time-temperature space. The map enables consideration of a fusion reactor accident combining loss of coolant and vacuum, in which a tungsten first wall could reach ∼1200 °C for several weeks. Complete oxidation of the first wall and 2 mm of sublimation is predicted. Similar maps must be developed for accident tolerant tungsten alloys if reliable predictions are to be made about their performance

Melt-driven mechanochemical phase transformations in moderately exothermic powder mixtures

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

Tungsten boride shields in a spherical tokamak fusion power plant

The favourable properties of tungsten borides for shielding the central high temperature superconductor (HTS) core of a spherical tokamak fusion power plant are modelled using the MCNP code. The objectives are to minimize the power deposition into the cooled HTS core, and to keep HTS radiation damage to acceptable levels by limiting the neutron and gamma fluxes. The shield materials compared are W2B, WB, W2B5 and WB4 along with a reactively sintered boride B0.329C0.074Cr0.024Fe0.274W0.299, monolithic W and WC. Five shield thicknesses between 253 and 670 mm were considered, corresponding to plasma major radii between 1400 and 2200 mm. W2B5 gave the most favourable results with a factor of ∼10 or greater reduction in neutron flux and gamma energy deposition as compared to monolithic W. These results are compared with layered water-cooled shields, giving the result that the monolithic shields, with moderating boron, gave comparable neutron flux and power deposition, and (in the case of W2B5) even better performance. Good performance without water-coolant has advantages from a reactor safety perspective due to the risks associated with radio-activation of oxygen. 10B isotope concentrations between 0% and 100% are considered for the boride shields. The naturally occurring 20% fraction gave much lower energy depositions than the 0% fraction, but the improvement largely saturated beyond 40%. Thermophysical properties of the candidate materials are discussed, in particular the thermal strain. To our knowledge, the performance of W2B5 is unrivalled by other monolithic shielding materials. This is partly as its trigonal crystal structure gives it higher atomic density compared with other borides. It is also suggested that its high performance depends on it having just high enough 10B content to maintain a constant neutron energy spectrum across the shield

A high temperature W2B–W composite for fusion reactor shielding

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