Tungsten, the primary material under consideration as the divertor material in magnetic-confinement nuclear fusion reactors, has been known for the last decade to form ‘fuzz’—a layer of microscopic, high-void-fraction features on the surface—after only a few hours of exposure to helium plasma. Fuzz has also been observed in molybdenum, tantalum, and several other metals. Helium bubbles in tungsten and other metals are also known to change the hardness of the surface, accumulate at grain boundaries and dislocations, and increase hydrogen isotope retention. This article reviews helium- and hydrogen-induced surface evolution, including fuzz formation, in tungsten and other plasma-facing materials, as well as modeling and experimental efforts that have been undertaken to understand the mechanisms of fuzz formation, helium and hydrogen transport in plasma-facing materials, and relevant atomic-scale and electronic effects relevant to plasma-facing materials.

https://iopscience.iop.org/article/10.1088/2053-1591/aa8c22/pdf

Helium, hydrogen, and fuzz in plasma-facing materials

Study on effect of double austenitization treatment on fracture morphology tensile tested nuclear grade P92 steel

The effect of double quenching and tempering (DQT) treatment as well as conventional high temperature quenching and tempering (CQT) treatment on the microstructures and mechanical properties of low carbon 5Cr martensitic as cast steel produced by electroslag casting was investigated. The microstructure changes were characterized by optical microscope (OM), scanning electron microscope (SEM), electron back scatter diffraction (EBSD) and transmission electron microscopy (TEM). The characteristics of carbides precipitated during tempering were analyzed on both carbon extraction replica and thin foil samples by TEM. The mechanical performance was evaluated by Vickers hardness test, tensile test, and Charpy V-notch impact test at ambient temperature. The results of microstructure study indicated that DQT treatment led to a finer microstructure than that of CQT. The carbides of the tempered samples were identified as M 7 C 3 . The carbides along the prior austenite grain boundaries nucleated directly while those within the laths should be transformed from cementite which formed at the early tempering stage. Compared with CQT condition, yield strength slightly increased after DQT treatment, and impact toughness improved a lot. The strengthening mechanisms were analyzed and it was found that grain refining and precipitation strengthening were mainly responsible for the increase of strength. The superior toughness of DQT condition was attributed to the finer microstructure resulting in more frequent deflections of the cleavage crack and the smaller size of carbides along the prior austenite boundaries. EBSD analysis showed that both martensitic block and packet of low carbon 5Cr tempered martensitic steel could hinder crack propagation, while the latter was more effective.

Effect of double quenching and tempering heat treatment on the microstructure and mechanical properties of a novel 5Cr steel processed by electro-slag casting

The significance of Ti in promoting the nucleation of MX-type carbides and further refine the MX carbides is currently unknown. Thus, we study here by high resolution transmission electron microscopy (HRTEM) and electron back scattered diffraction (EBSD), the determining role of titanium on the orientation of grains, texture, deformation and fracture of low activation martensitic steel with different Ti-content and determine the phase diagram. Strengthening mechanisms played by grain boundary and nanosized precipitates are discussed in detail. As the Ti content increased from 0.02 to 0.10 wt%, grain size of the LAM steels decreased significantly from 12 μm to 7 μm, accompanied by an increase in the volume fraction of MX carbide from 0.2 to 0.3% and decrease in the volume fraction of M23C6 carbide from 3.5 to 1.2%. Yield strength (σy) decreased with increasing Ti-addition. At room temperature, σy decreased slightly from 630 MPa to 590 MPa while it decreased from 315 MPa to 260 MPa at 600 °C for 0.02 and 0.10 wt % LAM steels. In contrast, elongation significantly increased from 37% to 47% at 600 °C with increase of Ti content. Boundary strengthening and MX-type carbide strengthening increased while M23C6 strengthening decreased greatly with increasing Ti-addition, which promoted the precipitation of fine MX-type and depressed M23C6. Excessive Ti-addition retarded the precipitation of large M23C6 carbides, which was responsible for significantly increasing elongation of 0.10 wt % Ti LAM steel, especially at 600 °C.

Strengthening a fine-grained low activation martensitic steel by nanosized carbides

This review summarizes the strengthening mechanisms of reduced activation ferritic/martensitic (RAFM) steels High-angle grain boundaries, subgrain boundaries, nano-sized M23C6, and MX carbide precipitates effectively hinder dislocation motion and increase high-temperature strength M23C6 carbides are easily coarsened under high temperatures, thereby weakening their ability to block dislocations Creep properties are improved through the reduction of M23C6 carbides Thus, the loss of strength must be compensated by other strengthening mechanisms This review also outlines the recent progress in the development of RAFM steels Oxide dispersion-strengthened steels prevent M23C6 precipitation by reducing C content to increase creep life and introduce a high density of nano-sized oxide precipitates to offset the reduced strength Severe plastic deformation methods can substantially refine subgrains and MX carbides in the steel The thermal deformation strengthening of RAFM steels mainly relies on thermo-mechanical treatment to increase the MX carbide and subgrain boundaries This procedure increases the creep life of TMT(thermo-mechanical treatment) 9Cr-1W-006Ta steel by ∼20 times compared with those of F82H and Eurofer 97 steels under 550°C/260 MPa

Strengthening mechanisms of reduced activation ferritic/martensitic steels: A review

Effect of twice quenching and tempering on the mechanical properties and microstructures of SCRAM steel for fusion application

Low-activated steel structure material for fusion reactor

Effects of Ti element on the microstructural stability of 9Cr–WVTiN reduced activation martensitic steel under ion irradiation

The invention provides a method for evaluating mechanical property of a material subjected to ion irradiation. The method comprises the following steps of: machining a specified irradiation sample, and measuring an actual thickness D1 from the bottom of a U-shaped notch to the lower surface of the irradiation sample and a width D2 of the sample; performing double-sided irradiation on the sample which is put into an ion implanter, performing a tensile test and an impact test on the irradiated sample, thus acquiring experimental data; and finally, evaluating the mechanical property of the material according to the acquired data, wherein the toughness of the material is evaluated by impact work acquired by the impact test, and the strength of the material is obtained by the formula that Rm=F/(D1*D2), wherein Rm is the tensile strength of the material, and F is maximum force determined from a tensile curve in the experimental process. The method solve the problem that the mechanical property of the material subjected to ion irradiation is difficult to evaluate, and has an important scientific research value.

Method for testing mechanical property of material subjected to ion irradiation

Microstructural evolution of super-clean reduced-activation martensitic steels irradiated with single-beam (Fe + ) and sequential-beam (Fe + plus He + ) at 350 °C and 550 °C was studied. Sequential-beam irradiation induced smaller size and larger number density of precipitates compared to single-beam irradiation at 350 °C. The largest size of cavities was observed after sequential-beam irradiation at 550 °C. The segregation of Cr and W and depletion of Fe in carbides were observed, and the maximum depletion of Fe and enrichment of Cr occurred under irradiation at 350 °C.

Microstructural evolution of reduced-activation martensitic steel under single and sequential ion irradiations

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