The surface hardening of single-face-centred cubic (fcc)-phase CrMnFeCoNi and the manganese-free CrFeCoNi alloy was conducted using low-temperature nitrocarburisation. The microstructural investigations reveal the successful formation of a homogeneous diffusion layer with a thickness of approximately 16 µm. The interstitial solution of carbon and nitrogen causes an anisotropic lattice expansion. The increase in microhardness is in accordance to the graded concentration profile of the interstitial elements. Wear tests show a significantly enhanced resistance at different loads. The electrochemical tests reveal no deterioration in the corrosion resistance. The absence of precipitates is proven by microstructural investigations. The results prove the applicability of the concept of solution hardening by the formation of supersaturated solutions for the material group of high-entropy alloys. Hence, an increase of entropy with the consideration of lattice interstices provides new development approaches.

Plasma nitriding is widely used in various industrial applications to improve surface hardness and wear properties. Especially for tool steels, it is also used to improve the support and adhesion of diamond-like carbon (DLC) coatings. The properties of the nitrided zone produced by plasma nitriding are influenced by the applied process parameter, in particular temperature and time. However, for high-alloy tool steels, a deeper understanding of the underlying diffusion processes of the nitrogen and the interaction with the existing microstructure, as well as the effects on the case depth is still lacking. Therefore, in this study, specimens of high-alloy tool steel X153CrMoV12 were plasma nitrided at varying temperatures (480 °C, 520 °C, 560 °C) and treatment times (2 h, 4 h, 16 h). The resulting nitrided zones were investigated by optical and scanning electron microscopy (OM and SEM), depth-dependent glow discharge optical emission spectroscopy (GDOES), X-ray diffraction (XRD), and hardness measurements to characterize their microstructure, chemical composition, and hardness depending on the process parameters. The distribution of carbides (M₇C₃), e.g., chromium carbides, affects the diffusion of the nitrogen and the layer growth. An increase of temperature and duration leads to an increased layer thickness. The composition of the compound layer is, e.g., influenced by the process parameters: $\mathrm{\epsilon }$ nitrides (Fe_2–3N) occurred preferentially at lower temperatures, while ${\mathrm{\gamma }}^{\prime }$ nitrides (Fe₄N) appeared mostly at higher temperatures. In order to investigate the influence of the carbides of the high-alloy tool steel on the nitriding process, a new methodology was developed by means of finite element analysis (FE), which makes it possible to analyze this influence on the development of the nitrogen concentration profile. This methodology makes it possible for the first time to map the heterogeneous nitrogen evolution and distribution.

Single-phase, face-centered cubic (FCC) high-entropy alloys (HEA) are promising materials for future applications. In order to improve the mechanical properties, especially the tensile strength of these materials, this study focuses on the combination of spark plasma sintering (SPS) and equal-channel angular pressing (ECAP). The initial fine-grained microstructure produced by SPS is further refined by ECAP in a 90°-die. Optical microscopy and electron backscatter diffraction (EBSD) confirm this considerable grain refinement, leads to a grain size below 1 µm after 1 ECAP pass. An alternating arrangement of fine-grained areas and much coarser regions, aligned under an angle of approximately 27°, is found. Moreover, a first microstructural investigation of the twin structure is conducted. The mechanical behavior was investigated by hardness measurements and tensile testing. Both the hardness and tensile strength are remarkably increased after ECAP. In contrast, the uniform elongation and elongation at fracture are significantly reduced due to the strengthening mechanisms of strain hardening and grain refinement. It is concluded that the combination of SPS and ECAP is an attractive approach for designing (ultra)fine-grained HEAs with superior properties. The investigated techniques could be applied to understand the underlying microstructural mechanisms.

The processing of high-entropy alloys (HEAs) via laser metal deposition (LMD) is well known. However, it is still difficult to avoid chemical intermixing of the elements between the coating and the substrate. Therefore, the produced coatings do not have the same chemical composition as the HEA feedstock material. Single-layer CrFeCoNi and AlCrFeCoNi HEA coatings were deposited using high-speed laser metal deposition (HS-LMD). Elemental mapping confirmed a good agreement with the chemical composition of the powder feedstock material, and revealed that chemical intermixing was confined to the immediate substrate interface. The coatings are characterized by a homogeneous structure with good substrate bonding. The machining of these coatings via turning is possible. Subsequent diamond smoothing results in a strong decrease in the surface roughness. This study presents a complete manufacturing chain for the production of high-quality HS-LMD HEA coatings.

Herein, the strain‐rate sensitive deformation behavior of a sintered Al‐ and Mo‐modified CrFeCoNi high‐entropy alloy (HEA) is focused. The sintered microstructure is investigated using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), energy‐dispersive X‐ray spectroscopy (EDS), and X‐ray diffraction. It was found that after sintering, the microstructure is of very low porosity (<1%) and consists of an ultrafine Mo‐ and Cr‐rich σ phase embedded in a fine‐grained face‐centered cubic (FCC) matrix. Electron microscopy reveals another secondary phase in which Al and Ni are concentrated. For a detailed discussion of the deformation behavior under tension and compression, tensile and compression tests were conducted at strain rates of 10⁻³, 10, and 10² s⁻¹. Strain‐rate sensitive deformation behavior in terms of strength is observed. The strain hardening rates show no significant strain rate sensitivity (SRS) up to strain rates of 10 s⁻¹. However, at higher strain rates and under compressive loading, a distinctly increased strain hardening rate is found. Tension/compression asymmetry is determined since the SRS value m is twice as high under compression than under tension.