Neutron irradiation of ITER-specification stress-relieved tungsten bar to ~1 dpa at 800 °C (BR2 reactor, thermal-neutron shielding limiting transmutation to ~2 at.% Re and 0.2 at.% Os) shifts its ductile-to-brittle transition temperature (DBTT) by 600–625 °C and produces mixed cleavage/intergranular fracture with grain-boundary pores on intergranular facets. The source study closed with five unresolved issues that jointly define one objective — establishing the physical mechanisms behind the measured embrittlement and the reliability of its measurement. The follow-up literature (2020–2026) reports partial progress on each issue: cavities are now directly observed to form during irradiation, and Re/Os grain-boundary segregation has been directly measured by electron microscopy; however, the mechanistic origin of the qualitatively diverse high-temperature fracture modes remains unexplained, the mechanical weakening role of transmutation segregation remains undemonstrated (first-principles calculations even predict rhenium strengthens tungsten grain boundaries), the effect of post-irradiation thermal exposure on measured mechanical properties is quantified only at the defect-kinetics level, and the true (EBSD-measured) grain size of the irradiated bar is unpublished. The problem remains open.

Background

Tungsten (W) is the plasma-facing material chosen for the divertor of ITER, the international tokamak under construction, and is the reference armour material for future fusion reactors: its extremely high melting point, high thermal conductivity, and low sputtering yield make it suitable for the reactor wall components exposed to the hot plasma. In a fusion (or fission-test) neutron environment, tungsten accumulates lattice damage measured in displacements per atom (dpa — the average number of times each atom is knocked from its lattice site) and, because neutron capture transmutes tungsten isotopes, it also accumulates rhenium (Re) and osmium (Os) atoms in solid solution. A central materials-qualification problem is that neutron irradiation embrittles tungsten: the ductile-to-brittle transition temperature (DBTT — the temperature below which the material fails in a brittle, non-ductile manner) rises by hundreds of degrees after irradiation at 600–800 °C to doses below 1 dpa, threatening the integrity of divertor components. The reference measurements are quasi-static fracture-mechanics tests following ASTM E399 on miniaturized three-point-bend (3PB) or disc-shaped compact-tension specimens, determining the plane-strain fracture toughness KIcK_{Ic}, combined with fractography (microscopic examination of fracture surfaces) of unirradiated and irradiated material. Gaganidze, Chauhan, Schneider, Terentyev, Borghmans and Aktaa (J. Nucl. Mater. 547, 2021, 152761) reported such tests on a stress-relieved ITER-specification tungsten bar irradiated in the fuel channel of the BR2 material-test reactor at 800 °C to ~1 dpa, with thick-wall stainless-steel capsules shielding thermal neutrons so that transmutation stayed near fusion-relevant levels (~2 at.% Re, 0.2 at.% Os). They found severe embrittlement (DBTT shift 600–625 °C, so the irradiated DBTT lies at or above the irradiation temperature of 800 °C), deformation-free brittle fracture at 800 °C with mixed transgranular cleavage and intergranular (grain-boundary) fracture, a transition to ductile behaviour above 1000 °C, notch-tip blunting with complete suppression of failure in some specimens, and occasional stable ductile crack growth with dimple formation. Grain-boundary pores were visible on the intergranular fracture surfaces. Companion studies of the same campaign family exist at 600 °C (Yin et al., Phys. Scr. T171, 2020, 014052, fracture toughness of two ITER-specification products irradiated to 0.08–0.67 dpa) and at 400, 600 and 1000 °C to ~1 dpa (Gaganidze et al., J. Nucl. Mater. 556, 2021, 153200, DBTT shifts up to ~650 °C or higher), and a group review (Terentyev et al., Tungsten 3, 2021, 415–433) concluded that ITER-specification plate and bar grades embrittle similarly, with a considerable DBTT shift already at 0.2–0.5 dpa in the 600–800 °C range. In its conclusions, the source paper listed five unresolved issues — the physical origin of the diverse fracture modes at high test temperature, the formation stage of the grain-boundary pores, the role of Re/Os grain-boundary segregation in boundary weakening, the possible annealing/coarsening artifacts when testing above the irradiation temperature, and the lack of a true (EBSD-based) grain-size measurement after irradiation (EBSD, electron backscatter diffraction, is a scanning-electron-microscopy technique mapping crystal orientations and hence grain structure). These five issues are mechanically linked: together they ask what microstructural mechanism produces intergranular embrittlement and fracture-mode diversity in this material, and how far the measured response is distorted by the measurement itself.

Problem Statement

Determine the physical mechanisms responsible for the fracture response measured on neutron-irradiated ITER-specification (stress-relieved bar) tungsten — ~1 dpa at 800 °C in BR2 with fusion-relevant Re (~2 at.%)/Os (~0.2 at.%) transmutation levels — and for the reliability of that measurement. Concretely, resolve the five unresolved issues raised by the source study: (i) identify the physical origin of the qualitatively diverse fracture behaviour observed in high-temperature three-point-bend tests of the irradiated material — deformation-free brittle fracture at Ttest = Tirr = 800 °C, transition to ductile behaviour above 1000 °C with stable ductile crack growth and characteristic dimple formation, and complete suppression of failure by notch-tip blunting at severe deflection — including whether the distribution of grain-boundary strengths and notch-tip triaxiality (stress-state constraint) control which mode occurs; (ii) establish at what stage the grain-boundary pores seen on intergranular fracture surfaces of the irradiated material form — during irradiation or during mechanical testing — and correlate them with the irradiation-induced cavity population now known to exist in the as-irradiated bulk; (iii) directly demonstrate, or refute, the hypothesized mechanism that segregation and diffusion of the transmutation elements Re and Os to grain boundaries weaken grain-boundary cohesion in neutron-irradiated tungsten, given that measured segregation exists but first-principles results indicate Re can strengthen tungsten grain boundaries; (iv) quantify systematically how post-irradiation thermal exposure during high-temperature mechanical testing (necessary because the irradiated DBTT lies at or above the 800 °C irradiation temperature) anneals or coarsens irradiation defects and thereby biases the measured fracture toughness and DBTT; and (v) measure the true grain size of the irradiated material (e.g. by EBSD) to determine whether real irradiation-enhanced grain growth occurs or the apparent coarsening seen on intergranular fracture surfaces is purely morphological. These five issues are jointly one problem: a coherent answer identifies the microstructural mechanisms (defects, cavities, transmutation solutes and precipitates, boundary structure and strength statistics) that produce the measured embrittlement and fracture-mode behaviour, and separates genuine irradiation effects from measurement artifacts. A complete answer must address each of (i)–(v): parts (i), (iii) and (iv) call for a mechanistic explanation or a demonstration-or-refutation, while (ii) and (v) call for empirical determinations. Because the quantities required by (ii) and (v) are not established in the published literature, an answer may resolve those parts by specifying the measurement or characterization that would settle each and predicting its outcome.

References

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