The thermal stability of dispersion-strengthened tungsten as plasma-facing materials: a short review
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One key challenge for the development of fusion energy is plasma-facing materials. Tungsten-based materials are promising candidates for plasma-facing components (PFCs) in the magnetic confinement nuclear fusion reactors because of their high melt temperature, high-thermal conductivity, high-thermal load resistance, low tritium retention, and low sputtering yield. In fusion reactors, PFCs are exposed to high-thermal flux, because there are some transient events such as plasma disruptions, edge-localized modes, and vertical displacement events (VDEs). Especially, in VDEs, a heat flux of 10–100 MW m−2 with duration of milliseconds-to-several seconds can induce recrystallization and then change the microstructure of tungsten-based plasma-facing materials, leading to instability of microstructures. Then, a significant degradation of material properties is caused such as a reduction of mechanical strength and fracture toughness, a rise in the ductile-to-brittle-transition temperature well, and decrease of irradiation/high-thermal load resistance. Therefore, many efforts were devoted to improve the thermal stability of tungsten-based materials as high as possible, such as oxide dispersion strengthening, carbide dispersion strengthening, and K bubbles dispersion strengthening. Here, the thermal stabilities of various dispersion-strengthened tungsten materials are reviewed by evaluating their recrystallization temperature and the corresponding hardness evolutions. In addition, the possible development trends are proposed.
KeywordsTungsten Plasma-facing materials Thermal stability Dispersion strengthening
Tungsten is a promising candidate for plasma-facing materials (PFMs) in future fusion reactors because of its excellent thermal diffusivity, low sputtering yield, high stability, and high hardness/strength, which all together can result in a long lifetime of plasma-facing components (PFCs) [1, 2, 3, 4, 5]. In spite of these advantages, the challenge of tungsten materials as PFMs is also noticeable, because PFMs are directly exposed to extreme conditions in the fusion plasma, such as high-energy neutron irradiation, high-thermal flux (0.1–20 MW m−2) [5, 6, 7], sputtering erosion induced by high flux plasma with low energy, blistering, and exfoliation, transient events such as plasma disruptions, edge-localized modes (ELMs), and vertical displacement events (VDEs). During these transient events, it is expected that PFCs are exposed to high-thermal shocks. Especially, in VDEs, a heat flux of 10–100 MW m−2 with duration of milliseconds-to-several seconds is predicted [8, 9, 10, 11, 12]. The transient high energy acting on W materials leads to simultaneously high stress and high surface temperatures, destroying the original microstructure of tungsten materials by recrystallization and surface melting. The grain growth induced by recrystallizaiton can lead to the resegregation of impurities such as O, N, and P on the grain boundaries (GBs), decreasing the cohesion of GBs and inducing GB embrittlement. More importantly, recrystallization in W materials will result in producing new GBs with disordered orientations . These unstable high-energy state random GBs are favorable sites for crack formation and thus easy to fracture as referred as recrystallization embrittlement which causes a significant degradation of properties such as the loss of mechanical strength, reduction of fracture toughness, rise of the ductile-to-brittle-transition temperature (DBTT), and weakness of irradiation/thermal load resistances [14, 15, 16]. Therefore, increasing the thermal stability is desirable for PFMs.
The thermal stability can be characterized by recrystallization temperature (RCT), at which the new grains begin to form and grow up after annealing. It is well known that dispersion strengthening is an effective method to improve the performance of tungsten, especially to raise RCT. The nanoparticles pin and hinder the migration of GBs and dislocations in tungsten, which enhance the strength and creep resistance as well as the thermal stability by raising RCT. In addition, the dispersion of nanoscale particles produces a great number of phase interfaces that could act as sinks for irradiation-induced point defects, and thus could improve the irradiation resistance. For examples, oxides (e.g., La2O3 and Y2O3) or carbides (e.g., TiC, ZrC, TaC, and HfC) or nanosized K bubbles were introduced into W matrix to form the oxide or carbide dispersion-strengthened (ODS or CDS) and K-doped W materials [6, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29]. And recently, a series of ODS-W or CDS-W materials with enhanced thermal stabilities were developed. For example, the strength, RCT, and thermal shock resistances of W–La2O3 and W–Y2O3 were enhanced compared with pure W [6, 17, 24, 27, 30]. The carbides such as SiC, TiC, ZrC, TaC, and HfC, having higher melting temperatures and better compatibility with tungsten, were used to develop CDS-W which may lead to excellent thermal stability [20, 21, 22, 23, 25, 28, 31, 32, 33]. To give a comprehensive insight, in this paper, the thermal stabilities of these newly developed ODS-W, CDS-W, and K-doped W materials are reviewed.
2 Thermal stability of pure W, K-doped, and oxide dispersion-strengthened W alloys
3 Thermal stability of carbide dispersion-strengthened tungsten alloys
4 Conclusion and outlook
The thermal stability is very important for tungsten-based materials as PFCs, which determines the servicing performance at high temperatures. RCT, as a parameter to characterize the thermal stability of materials, is determined by the evolutions of grain sizes and Vickers hardness with annealing temperatures. This paper reviews the effects of different dispersion-strengthening phases on the RCT of W materials. The second-phase particles could pin the GB and hinder its migration, which significantly raise RCTs of W materials. For different kinds of particles such as oxides, K bubbles, and carbides, there is no obvious influence of the category of the second-phase particles on thermal stabilities of tungsten-based materials. On the other hand, RCT may depend on the original microstructure much. The particle size and the number density of strengthening particles could influence the RCT, because the higher the number density of finer particles is, the stronger the pinning ability is, which leads to a higher RCT. Therefore, for certain dispersion-strengthening W materials, refining-strengthening particles could further improve the RCT, because for the same content of the second-phase particles, the smaller the size is, the larger the number density is, resulting in stronger pinning effects on migration of grain boundaries. Meanwhile, the fabrication technology pronouncedly influences on the RCT by changing the microstructures. For example, the hot-rolled W has a higher RCT than that of SPSed W. Moreover, the different deformation degrees can produce different textures, for example, the different proportion of high angle grain boundary and low angle grain boundary, and distributions of the second-phase particles, which should have a significant effect on RCT. However, there are no specific data about the effect of deformation degrees on RCT. In this work, the hot-rolled W–ZrC/HfC/TaC/TiC plates have the same deformation degree: reduction of ~ 70%, but the deformation degrees of W–K and W–K–Re plates are unknown. Therefore, optimizing the technology can further improve the thermal stability of tungsten materials.
This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51771184, 11735015, 11575241, 51801203 and 11575231), the Natural Science Foundation of Anhui Province (Grant No. 1808085QE132) and the Open Project of State Key Laboratory of Environment Friendly Energy Materials(Grant No. 18kfhg02).
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