A Demonstration (DEMO) thermonuclear reactor is the next step after the International Thermonuclear Experimental Reactor (ITER). Designs for a DEMO divertor and the First Wall require the joining of tungsten to steel; this is a difficult task, however, because of the metals’ physical properties and necessary operating conditions. Brazing is a prospective technology that could be used to solve this problem. This work examines a state-of-the-art solution to the problem of joining tungsten to steel by brazing, in order to summarize best practices, identify shortcomings, and clarify mechanical property requirements. Here, we outline the ways in which brazing technology can be developed to join tungsten to steel for use in a DEMO application.

For the manufacture of thin-walled precision structures with complex geometry, the method of brazing is used. It allows to obtain joints with a given structural-phase state, high mechanical characteristics and resistance to corrosion. This overview examines alloys based on copper, precious metals, nickel and iron, which are actively used for joining steels. The data on the operational characteristics of the joints obtained by brazing using nickel-based filler metals are presented and structured. Special attention is paid to nickel braze alloys based on Ni – B, Ni – Si – B, Ni – Cr – B, Ni – Cr – Si, Ni – Cr – Si – B, Ni – Mn, Ni – Cr – P, Ni – Si – Be systems. Such alloys are the most widespread and applicable throughout the world. The overview describes the main phases that are formed in the seams and the heat-affected zones of joints. The dependences of the technological parameters of the process, such as the temperature-time mode of brazing and the size of the gap on the microstructure of the joints and their mechanical characteristics, are shown. The corrosion resistance of steel joints brazed by various nickel-based filler metals is considered. The overview is based on the extensive experience of MEPhI in the production and development of rapidly solidified amorphous-crystalline foils based on nickel. In addition to the world experience, the overview considers works of the research team of the MEPhI laboratory, which has been working on this topic since 1995. This overview will help technologists and designers of energy-intensive equipment to determine the trajectory for the selection of the filler metal composition to obtain the optimal technological parameters, mechanical characteristics and corrosion resistance of the joints applicable to a specific design problem. The reported study was funded by RFBR, project number 20-12-50106.

The knowledge of alloy–process–structure–property relationships is of particular interest for several safety-critical brazed components and requires a detailed characterization. Thus, three different nickel-based brazing filler metals were produced with varying chromium and molybdenum content and were used to braze butt joints of the austenitic stainless steel AISI 304L under vacuum. Two holding times were used to evaluate diffusion-related differences, resulting in six specimen variations. Significant microstructural changes due to the formation and location of borides and silicides were demonstrated. Using X-ray diffraction, alloy-dependent residual stress gradients from the brazing seam to the base material were determined and the thermal-induced residual stresses were shown through simulations. For mechanical characterization, impact tests were carried out to determine the impact toughness, as well as tensile tests at low and high strain rates to evaluate the strain-rate-dependent tensile strength of the brazed joints. Further thermal, electrical, and magnetic measurements enabled an understanding of the deformation mechanisms. The negative influence of brittle phases in the seam center could be quantified and showed the most significant effects under impact loading. Fractographic investigations subsequently enabled an enhanced understanding of the fracture mechanisms.

This article gives a brief overview of the development of vanadium alloys as structural materials for fast and fusion reactors in future. The most important properties of vanadium alloys are considered with respect to the properties of austenitic and ferritic-martensitic steels. Vanadium is sufficiently spread in nature metal, and series alloys have good processability. Short-term strength properties of vanadium alloys remain at a high level up to the temperature of 970 K. In the high-temperature strength properties (thermal creep, long-term strength) vanadium alloys significantly exceed similar characteristics of austenitic and ferritic-martensitic reactor steels. Vanadium-base alloys are characterized by higher melting point and lower density than steels, and have a lower linear thermal expansion and higher thermal conductivity. At the same time, vanadium alloys are able to withstand higher heat loads due to better thermal and physical properties than steels. Analysis of the nuclear physical properties shows that the helium production rate is 2–3 times lower in vanadium alloys than the one in austenitic and ferritic-martensitic steels (as well as hydrogen production). At the same time nuclear heating rates are lower for the vanadium alloys compared to both steels. These effects provide a significant advantage to vanadium alloys as compared with steels. Alloys V – (4–5) Cr – (4–10) Ti have the highest potential in long-term radioactivity and show the advantages over low activation steels. From the point of view of radiation embrittlement, it is concluded that the temperature of 700 K should be taken as the lower limit of the operating temperature for vanadium alloys when they are used as a reactor structural materials. The data on radiation creep of vanadium alloys are extremely limited and contradictory. The ways are noted of solving the existing problems: helium and hydrogen by corresponding alloying, the sensitivity to impurities using high-purity vanadium and/or special alloying, compatibility with liquid sodium using the composite material “steel – vanadium alloy – steel”.

Hydrogen retention in vanadium and its binary alloys with titanium are investigated by means of hydrogen thermal desorption spectrometry. The samples are saturated with hydrogen for 2000 h at a temperature of 620 K in an autoclave, where hydrogen is fed at a pressure of 16.8 MPa. It is shown that in V-Ti alloys there are two main types of traps for hydrogen and from which the hydrogen release occurs in the temperature range of 780–790 (peak I) and 870–880 K (peak II) The position of peak I on the temperature scale does not depend on the alloy composition, and the intensity and position of peak II on the temperature scale are determined by the alloy composition. Titanium non-monotonically affects the amount of hydrogen retained: an addition of 0.5% Ti decreases, 1 and 5% Ti increases, and 10% Ti again decreases the amount of hydrogen in the alloy. A physical explanation of the appearance of two peaks of hydrogen thermal desorption and nonmonotonic influence of titanium on the amount of hydrogen in vanadium is proposed.

The formation of nanocrystal layers on the surface of various steels is experimentally studied; this procedure is performed by cladding of the surfaces with rapidly quenched metal filler alloys...