Multi-element systems with defined entropy (HEA—high entropy alloy or MEA—medium entropy alloy) are rather new material concepts that are becoming increasingly important in materials research and development. Some HEA systems show significantly improved properties or combinations of properties, e.g., the overcoming of the trade-off between high strength and ductility. Thus, the synthesis, the resulting microstructures, and properties of HEA have been primarily investigated so far. In addition, processing is crucial to achieve a transfer of potential HEA/MEA materials to real applications, e.g., highly stressed components. Since fusion welding is the most important joining process for metals, it is of vital importance to investigate the weldability of these materials. However, this has rarely been the subject of research to date. For that reason, in this work, the weldability depending on the surface preparation of a CoCrFeMnNi HEA and a CoCrNi MEA for TIG welding is investigated. The fusion welding of longer plates is described here for the first time for the CoCrNi alloy. The welds of both materials showed distinct formation of cracks in the heat affected zone (HAZ). Optical and scanning electron microscopy analysis clearly confirmed an intergranular fracture topography. However, based on the results, the crack mechanism cannot be conclusively identified as either a liquid metal embrittlement (LME) or hot cracking-like liquid film separation.

In order to improve the application-related properties, as fracture toughness, creep response and oxidation behavior of Mo-Si-B alloys, certain alloying strategies can be applied. Promising alloying partners like Zr and La2O3 may help to enhance the compressive and flexural strength of Mo-Si-B materials. Accordingly, a combination of 1 at.% Zr and 0.5 wt.% La2O3 was chosen in order to improve the characteristic properties of a powder metallurgical (PM) Mo-6Si-5B alloy. The influence of Zr and La2O3 on the phase distribution and the mechanical properties will be discussed by means of three-point-flexure at room temperature as well as uniaxial com-pressive creep tests at elevated temperatures (> 1000 °C). Compared with other PM Mo-Si-B alloys the present Zr-La2O3-strengthened alloy shows highly improved fracture toughness at room temperature.