Tungsten (W) plates modified by potassium (K) doping and rhenium alloying were irradiated in the BR2 reactor to about 1 dpa at 600 °C and 1100 °C, generating by transmutation ≈1.9 at.% Re and ≈0.2 at.% Os in pure W and K-doped W, and ≈1.4 at.% Re and ≈1.0 at.% Os in K-doped W–3%Re. K-doped W showed the smallest ductile-to-brittle transition temperature (DBTT) shift (≈235–255 °C), while K-doped W–3%Re — the best-performing grade in earlier low-fluence, low-transmutation irradiations — suffered a DBTT shift exceeding 595 °C at 600 °C but only ≈345 °C at 1100 °C. The unresolved problem, stated by the source paper, is threefold and jointly serves one qualification objective: (i) identify the microstructural mechanisms — formation, chemical evolution, and post-irradiation thermal stability of W–Re–Os clusters and precipitates, and their quantitative connection to fracture — that explain this alloy-specific, irradiation-temperature-dependent embrittlement; (ii) directly verify, or refute, the factors proposed to toughen these grades in the unirradiated state (grain refinement, low-angle grain-boundary density, K-bubble stabilization, Re-enhanced screw-dislocation mobility); and (iii) determine the long-term thermo-mechanical performance under extended neutron fluence over a wider irradiation-temperature range, sufficient to decide whether Re alloying remains viable for fusion plasma-facing components and to establish safe operating windows. Later work has characterized W–Re–Os precipitation in pure W and binary W–Re alloys but has not established the mechanism for these doped grades, the toughening-factor proofs, or the high-fluence behavior.

Background

Tungsten is the reference armor material for divertor plasma-facing components of ITER and future DEMO-class fusion reactors, chosen for its melting point, thermal conductivity, and sputtering resistance. Its main liability is brittle behavior at moderate temperature: in body-centered-cubic metals, plastic flow is controlled by screw dislocations whose motion is impeded by the high Peierls barrier (the lattice resistance to glide of a 1/2⟨111⟩1/2\langle 111\rangle screw dislocation), so W fails by cleavage below a ductile-to-brittle transition temperature (DBTT) — the temperature at which a bending specimen changes from brittle to ductile fracture. Two classical modifications lower the DBTT of wrought W. Potassium doping (the AKS process used for lamp filaments) leaves a dispersion of nanoscale K-filled bubbles that pin grain boundaries and dislocations, retarding recrystallization and refining the grain structure. Rhenium alloying of a few percent ductilizes W — the classical 'Re ductilizing effect' — and atomistic calculations show that Re changes the core structure of the screw dislocation in W and thereby its mobility (Romaner et al., 2010).

Neutron irradiation adds two coupled degradation channels. Displacement damage, quantified in displacements per atom (dpa — the fraction of atoms knocked permanently off their lattice sites), produces voids and dislocation loops that harden and embrittle the metal. In addition, transmutation converts W into neighboring elements — chiefly Re and, on further neutron capture, Os — through (n,γ)(n,\gamma) reactions with thermal neutrons followed by beta decay. Mixed-spectrum fission test reactors such as BR2 generate of order 1–2 at.% Re per dpa, whereas the 14 MeV-dominated spectrum of a fusion power plant produces far less transmutation per unit damage. Once local Re/Os levels are high enough, irradiation drives the precipitation of intermetallic σ\sigma- and χ\chi-type W–Re(–Os) phases — phases that in equilibrium require much higher solute contents — together with Re/Os-enriched 'clouds' around voids and dislocation loops, because the undersized transmutation solutes are dragged to defect sinks (documented for pure W and binary W–Re in HFIR and HFR by Fukuda et al. 2014; Hasegawa et al. 2016; Hu et al. 2019).

Earlier low-fluence data supported Re alloying: in the Japan–US PHENIX campaign, irradiation of pure W, K-doped W, W–3%Re, and K-doped W–3%Re to 0.74 dpa at 600–800 °C, with little transmutation, left the K-doped W–3%Re plate ductile while pure W lost its ductility (Miyazawa et al., 2020). Nogami et al. (2021) then irradiated the Tohoku plate products — ITER-specification pure W (IGP), K-doped W, and K-doped W–3%Re (developed in Nogami et al., 2020) — in the Belgian BR2 reactor inside a thermal-neutron-shielded rig to 1.135 dpa at 600 °C and 1.040 dpa at 1100 °C, producing ≈1.9 at.% Re and ≈0.2 at.% Os in pure W and K-doped W (1.87 at.%-Re/dpa and 0.192 at.%-Os/dpa) and ≈1.4 at.% Re and ≈1.0 at.% Os in K-doped W–3%Re (1.44 at.%-Re/dpa and 0.96 at.%-Os/dpa), and measured the DBTT by miniaturized three-point bending (DBTT defined as the test temperature at which the maximum flexural strain reaches 5%; L–T orientation). Unirradiated DBTT values were 145 °C (IGP W), 65 °C (K-doped W), and −15 °C (K-doped W–3%Re). After irradiation at 600 °C, the DBTT shift Δ\DeltaDBTT exceeded 435 °C for IGP W, was ≈235 °C for K-doped W, and exceeded 595 °C for K-doped W–3%Re; after irradiation at 1100 °C it was below 435 °C for IGP W, ≈255 °C for K-doped W, and ≈345 °C for K-doped W–3%Re. The Re-alloyed grade thus lost the advantage it had shown at low fluence: once transmutation is significant, Re-free K-doped W becomes the most irradiation-tolerant grade, and the strong temperature dependence of the K-doped W–3%Re embrittlement was attributed to W–Re–Os cluster formation at 600 °C and to their partial dissolution or coarsening at 1100 °C — an attribution resting at the time on analogies with pure-W and binary W–Re studies rather than on microstructural observations of these doped grades. The paper closes by stating that the microstructural mechanisms underlying the embrittlement remain unresolved, that the factors credited for the low unirradiated DBTT (grain refinement, increased low-angle grain-boundary density, stabilization by K bubbles, and enhanced screw-dislocation mobility due to Re) are unproven hypotheses requiring dedicated microstructural investigation, and that the long-term thermo-mechanical performance at higher fluence and wider irradiation-temperature range — needed to judge the viability of Re alloying and to set safe operating windows for fusion components — is unknown.

Problem Statement

For the neutron-irradiated tungsten grades of Nogami et al. (2021) — ITER-specification pure W, K-doped W, and K-doped W–3%Re, irradiated in BR2 to about 1 dpa at 600 °C and 1100 °C, with transmutation generating about 1.9 at.% Re and 0.2 at.% Os in pure W and K-doped W and about 1.4 at.% Re and 1.0 at.% Os in K-doped W–3%Re — establish, at the microstructural-mechanism level, why the embrittlement behaves as observed, and use that understanding to qualify these grades for fusion service. Concretely, three linked deliverables jointly serve this single objective: (i) determine the formation, chemical evolution, and post-irradiation thermal stability of W–Re–Os clusters and precipitates in these specific doped grades — including their interaction with K bubbles, grain-boundary structure, and irradiation defects — and connect them quantitatively to fracture, so as to explain why K-doped W–3%Re suffers a DBTT shift exceeding about 595 °C after irradiation at 600 °C but only about 345 °C at 1100 °C, while Re-free K-doped W shifts by only about 235–255 °C, and why the low-fluence advantage of Re alloying reverses once transmutation becomes significant; (ii) provide direct experimental verification or refutation of the factors proposed to explain the low unirradiated DBTT of these grades — grain refinement, increased low-angle grain-boundary density, stabilization by potassium bubbles, and enhanced screw-dislocation mobility due to rhenium; and (iii) determine the long-term thermo-mechanical performance of these materials under extended neutron fluence across a wider range of irradiation temperatures, sufficient to assess whether Re alloying remains a viable strategy for fusion plasma-facing components and to establish safe operating temperature and fluence windows for them. All three deliverables are mandatory, and none alone resolves the problem. Within (ii), each of the four listed factors must be separately confirmed or refuted, so that resolving only a subset does not complete (ii). Resolution requires a direct, grade-specific determination rather than inference by analogy to pure W or binary W–Re, and for (iii) 'sufficient' means a quantified dependence of the embrittlement on both fluence (extending beyond the ≈1 dpa of the present data) and irradiation temperature, detailed enough to decide the viability of Re alloying and to state the safe operating temperature and fluence windows; a qualitative restatement of the existing ≈1 dpa results does not satisfy (iii).

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