In situ study of thermal shock damage to high-temperature ceramics

New generations of fusion devices need alternative plasma-facing materials. The currently approved material composition for the first wall and divertor of the ITER tokamak has a number of disadvantages: insufficient resistance to thermal shock, sputtering of microparticles into plasma and high atomic number Z of the armor material. A promising but largely untested idea is the proposal to use high-temperature ceramics as armor materials for the most heat-loaded plasma-facing components of new-generation fusion devices. Among the advantages of ceramics are the low Z and high enough resistance to intense heating. More research is needed that would help to understand how the material withstands high heat fluxes during transient plasma events. This work is devoted to the description of an experimental method that makes it possible to estimate the critical temperature at which the damage of ceramics begins as a result of a thermal shock of submillisecond duration. As a demonstration of the efficiency of the method, the critical temperature for hot pressed B4C under thermal shock was determined: its value was about 1200–1400 K

In-situ study of the processes of damage to the tungsten surface under transient heat loads possible in ITER

Experiments on the effect of fast heat loads on the surface of tungsten were carried out on the BETA facility at the Budker Institute. Tungsten samples were uniformly heated by an electron beam with a heat flux factor below the melting threshold. During and shortly after exposure, the 2D surface temperature distribution was measured, as well as the temperature history on selected surface areas. Active diagnostics using the scattering of CW laser light on a surface exposed by the electron beam allowed us to monitor the damage dynamics. At the heating stage, an increase in the surface roughness occurred, caused by inhomogeneous elastic and plastic deformations of the heated layer. As the cooling progressed, the residual plastic deformations remained. Simultaneously with the modification of the surface, bending of samples with a thickness of 3-4 mm occurred. The bending dynamics of the sample was measured by the intensity of a converging laser beam reflected from the back surface of the sample, polished to a mirror state. The residual sag due to bending increases with the heat load similarly as residual roughness of the front surface of the sample. These data, together with simultaneously measured temperature dynamics and the spatial heating profile, can provide an experimental basis for the numerical calculation of the residual stresses in the sample. The data obtained in situ were compared with those measured outside the vacuum chamber with X-ray diffraction, optical profiler, and optical interferometer. At the stage of cooling, after a sufficient intensity of heating, the second stage of damage took place — the cracking of the surface layer. The time before the start of this relatively fast process usually exceeded the time to achieve a DBTT by 1–4 orders of magnitude. © 2020 Elsevier B.V

Microstructure and lattice parameters of suction-cast Ti–Nb alloys in a wide range of Nb concentrations

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A novel operando approach to analyze the structural evolution of metallic materials during friction with application of synchrotron radiation

International audienceIn this study, we describe an experimental setup and a new approach for operando investigation of structural evolution of materials during wear and friction. The setup is particularly suited for testing various friction pairs, including those in which both rubbing bodies are made of metals. The developed device allows circumventing the problems related to significant scattering of X-rays produced by metals and makes it possible using “real samples” in synchrotron beamlines operating in reflection mode. To demonstrate the capabilities of the device and the proposed new approach, an iron-based massive sample was subjected to thousands of friction cycles using a cemented carbide pin. The material was probed with synchrotron X-ray radiation within a few milliseconds after leaving the friction zone. The results of the microstructural and structural analysis, as well as results obtained from diverse mathematical models, allowed us to evaluate several features, including gradual accumulation of defects, microstructural refinement, dislocation density changes, surface layer oxidation, as well as several other phenomena caused by the dry sliding friction process. Mainly, it was possible to conclude that the process of wear occurred due to the cooperative action of oxidation and plastic deformation, which began during the first cycle of frictional interaction and was manifested in increasing the dislocation density, whose type was changed gradually during testing. The number of defects quickly reached a threshold value and subsequently fluctuated around it due to periodically repeated processes of defect accumulation and stress relaxation resulting from material wear. It was also observed that friction led to the quick formation of a mechanically mixed layer, consisting of the sample material and a mixture of two types of iron oxide – hematite and magnetite. The delamination of this layer was probably the primary wear mechanism