The microstructure and properties of hot-rolled W-0.5%Hf alloy

【Objective】 Tungsten (W) is considered one of the most promising candidates for plasma-facing materials (PFMs) in fusion reactors due to its high melting point, excellent sputtering resistance, low deuterium-tritium retention, and high thermal conductivity. However, several drawbacks of tungsten, including low-temperature brittleness, irradiation embrittlement, and recrystallization embrittlement, severely restrict its engineering applications. This study aims to fabricate a fine-grained W alloy by powder metallurgy and hot rolling with 0.5wt.% Hf addition, investigate its microstructure, high-temperature mechanical properties, and thermal stability, and reveal the strengthening mechanism of Hf in W matrix.
【Method】 Pure W powder and HfH₂ powder were used as raw materials to prepare pure W (PW) and W-0.5% Hf alloy. The mixed powders were ball-milled in argon atmosphere for 4 h with a ball-to-powder ratio of 8:1 and a rotating speed of 300 r/min. The green compacts were obtained by cold isostatic pressing at 200 MPa for 5 min, followed by sintering at 2 300 ℃ for 7 h in hydrogen atmosphere. The sintered bulks were rolled with five passes at 1 400-1 600 ℃ with a total deformation of ~68%, and then annealed at 1 100 ℃ for 60 min to release residual stress. The recrystallization temperature was evaluated by isochronal annealing at 1 200-1 600 ℃ for 1 h. The density was measured by the Archimedes method, and microhardness was tested by Vickers indentation. High-temperature tensile tests were carried out at 100-1 000 ℃ in vacuum. Microstructure was characterized by optical microscope (OM), scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS), and transmission electron microscope (TEM).
【Result】 The relative densities of pure tungsten (PW) and W-0.5%Hf alloy are both approximately 99.4%, and Hf addition has no obvious effect on densification. Compared with pure tungsten, the W-0.5 Hf alloy possesses finer fibrous grains and higher Vickers hardness. Uniformly dispersed nanoscale second-phase particles are observed in the alloy, which are identified as monoclinic hafnium oxide (HfO₂) by EDS and selected area electron diffraction (SAED). High-temperature tensile results demonstrate that W-0.5% Hf exhibits superior strength and plasticity over pure tungsten at all tested temperatures. At 100 ℃, the ultimate tensile strength (UTS) reaches (1 008±7.09) MPa, which is 14.5% higher than that of pure tungsten. At 200 ℃, the total elongation (TE) is 9.15%±0.77%, with an increase of 80.8%. At 1 000 ℃, the UTS still remains at (466.64±7.63) MPa. Fracture morphology reveals that the alloy is dominated by transgranular cleavage fracture at low temperatures and transforms into typical ductile fracture at high temperatures, with HfO₂ particles distributed at the center of dimples. The recrystallization temperature of pure tungsten is about 1 200 ℃, while that of the W-0.5% Hf alloy increases to 1 400 ℃, and HfO₂ particles effectively pin grain boundaries and inhibit grain growth during annealing.
【Conclusion】 Hf introduced by HfH₂ reacts with impurity oxygen at W grain boundaries to form thermally stable HfO₂ nanoparticles, which purifies and strengthens grain boundaries. The nano HfO₂ particles pin dislocations and grain boundaries, leading to grain refinement and dispersion strengthening. As a result, W-0.5 Hf alloy presents significantly improved tensile strength and ductility compared with pure W, especially the elongation is greatly enhanced at medium temperatures. Meanwhile, the pinned grain boundaries effectively inhibit recrystallization and grain growth, raising the recrystallization temperature by about 200 ℃ and improving high-temperature stability. This hot-rolled W-0.5%Hf alloy with fine grains, high strength, good ductility, and elevated recrystallization temperature provides a promising candidate for high-performance plasma-facing materials in nuclear fusion reactors.