The effect of heat treatments on pure and potassium doped drawn tungsten wires: Part I - Microstructural characterization

Abstract

Advanced tungsten fibre-reinforced composites (Wf/W), showing pseudo ductile behaviour even at room temperature, are a promising option for future fusion reactors as the intrinsic brittleness of tungsten can be mitigated effectively. The drawn tungsten wires used as reinforcements are the key component of the composites, thus their mechanical properties and thermal stability define the allowed operation / fabrication temperature of the composite material itself. In this work, a comprehensive characterization of the pure and potassium doped tungsten wires was performed, focusing on the influence of various heat treatments on different microstructural features (nature of grain boundaries, grain shape and size, texture analyses) and mechanical properties. Annealing in the temperature range from 900-1600°C enables the investigation of the microstructural stability of the two materials and arising annealing phenomenarecovery, recrystallization and grain growth. The results demonstrate that the pure tungsten recrystallizes fully in the temperature range 1300-1500°C accompanied with tremendous coarsening and a complete loss of the initial fibrous, elongated grain structure. In contrast to this, potassium doped wire shows superior high temperature properties, where performed heat treatments cause only milder microstructural changes, consequently suppressing recrystallization and grain growth to temperatures well above the investigated ones. Furthermore, hardness measurements and observed softening complement the discussion of the grain morphology evolution.

Micromechanical and microstructural properties of tungsten fibers in the as-produced and annealed state: Assessment of the potassium doping effect

“…One can see a very strong texture in <110> direction i.e. parallel to the Drawing Axis (DA) for both studied conditions, which is in a good correlation to the analyses performed in [27]. Strong <110> texture is indeed expected for the cold-drawn materials [37], regardless of the annealing temperature.…”

“…Usually, the most common deformation processes that are applied to polycrystalline metals (i. e. rolling and forging) are responsible for introducing LAGBs in the microstructure to store energy (of externally applied work) that becomes the driving force for recrystallization and grain growth under subsequent annealing [36]. The observation of the high fraction of HAGBs in the as-received and 1300°C annealed conditions (being around 50%) is not common for a highly deformed materials but was also observed for highly deformed W wires before [27]. In that study, the fraction of LAGBs was seen to growth under annealing up to 1100°C, while above 1300°C HAGBs were seen as dominant types of interfaces.…”

“…After annealing at 1627°C grains became equiaxed and limited microstructural coarsening was reported, which also ascribed to the recrystallization. Furthermore, Nikolić et al performed microstructural characterization of both pure and potassium doped tungsten wires, annealed for 1h in the temperature range from 900 to 1600°C [27]. The full recrystallization was observed above 1300°C for the undoped material, while the potassium doped wires showed only milder microstructural changes, consequently suppressing recrystallization and grain growth to temperatures above 1600°C.…”

Tungsten–potassium: a promising plasma-facing material

“…The superior properties at high temperatures and suppressing recrystallization due to K-doping were also demonstrated by Nikolić et al [56]. The grain growth and full recrystallization in wires were successfully enhanced to temperatures higher than 1600 °C by doping K. In their views, the utilization of K-doped wires (W f /W composite) was highly recommended to serve as the structural materials for future fusion facilities [55,56].…”

“…Recently, it is generally recognized that WK is a metal matrix composite because the non-alloyable constituent K, is almost insoluble in W. The K bubble in W is recognized as the strongest pinning phase at high temperatures against the migrations of GBs and dislocations [51]. The excellent performances of the WK filaments at high temperatures make people associate them with the possible applications in nuclear fusion installations [16, 19, 24, 25, 31, 32, 38, [55][56][57][58][59][60][61][62][63]. On the basis of summarizing the previous research work, this paper briefly reviews WK performances including mechanical properties, the resistance to thermal shock, and irradiation tolerance.…”