High‐entropy alloys (HEAs) with their almost limitless number of possible compositions have raised widespread attention in material science. Next to wear and corrosion resistive coatings, their application as tunable electrocatalysts has recently moved into the focus. On the other hand, fundamental properties of HEA surfaces like atomic and electronic structure, surface segregation and diffusion as well as adsorption on HEA surfaces are barely explored. The lack of research is caused by the limited availability of single‐crystalline samples. In the present work, the epitaxial growth of face centered cubic (fcc) CoCrFeNi films on MgO(100) is reported. Their characterization by X‐ray diffraction (XRD), energy dispersive X‐ray spectroscopy (EDX), and transmission electron microscopy (TEM) demonstrates that the layers with a homogeneous and close to equimolar elemental composition are oriented in [100] direction and aligned with the substrate to which they form an abrupt interface. X‐ray photoelectron spectroscopy (XPS), low‐energy electron diffraction (LEED), and angle‐resolved photoelectron spectroscopy are employed to study chemical composition and atomic and electronic structure of CoCrFeNi(100). It is demonstrated that epitaxially grown HEA films have the potential to fill the sample gap, allowing for fundamental studies of properties of and processes on well‐defined HEA surfaces over the full compositional space.

The microstructure and early-stage oxidation behavior of the equiatomic CoCrCuFeMnNi high-entropy alloy (HEA) and its six sub-alloys, obtained by omitting one element each, were investigated. Alloys were prepared using induction levitation melting, cold rolled, and oxidized for 1 h at 800°C in air. The Ni-free and Co-free HEAs showed an inhomogeneous microstructure associated with liquid phase separation. The other alloys were either single-phase (Cu-free HEA) or contained two face-centered cubic phases, one Cu-rich and one Cu-poor. The Cu and Mn-containing two-phase alloys showed preferential oxidation of the Cu/Mn-rich phase, leading to Mn-rich oxides that are prone to spallation. The Mn-free alloy exhibited a thicker oxide (~ 5 µm) on the Cu-rich phase, whereas the Cu-poor phase was covered by a thin base oxide (< 1 µm). The single-phase Cu-free (‘Cantor’) alloy formed an approximately 1-µm-thick oxide of the crystal structure types of Mn₃O₄, Mn₂O₃, MnCr₂O₄, and Cr₂O₃. For prospective high-temperature applications, reducing the Cu and Mn content and thus avoiding formation of a second Cu-rich phase is a promising route to facilitate formation of a protective oxide.

With the rise and development of aerospace, communications, electronics, medical, transportation and other fields, magnesium (Mg) and its alloys have attracted much attention for their high specific strength and stiffness, good electromagnetic shielding properties, excellent damping properties and other advantages. However, magnesium has a high affinity for oxygen, producing magnesium oxide (MgO), and MgO’s Pilling–Bedworth ratio (PBR) of 0.81 is not protective. The occurrence of catastrophic oxidation is unavoidable with the increase of oxidation time and temperature. A promising approach is to perform an appropriate pretreatment in conjunction with alloying to obtain a dense and compact composite protective film. In this work, the effect of a preheating treatment on the oxidation resistance (OR) of Mg-xCa (x = 1, 3 and 5 wt. %) was investigated. The preheating was carried out in an Ar atmosphere at 400 °C for 8 h. Upon it, a dense and compact MgO/CaO composite protective film was formed on the surface, which is CaO-rich especially in the vicinity to the surface. The alloys’ oxidation resistance was strongly increased due to the composite protective film formed during the preheating treatment, in particular for Mg-3Ca. Relative to the Mg-hcp phase, the OR of the Mg₂Ca phase was significantly raised.

High-harmonic generation (HHG) is used as a source for various imaging applications in the extreme ultraviolet spectral range. It offers spatially coherent radiation and unique elemental contrast with the potential for attosecond time resolution. The unfavorable efficiency scaling to higher photon energies prevented the imaging application in the soft X-ray range so far. In this work we demonstrate the feasibility of using harmonics for imaging in the water window spectral region (284 eV to 532 eV). We achieve nondestructive depth profile imaging in a heterostructure by utilizing a broadband and noise-resistant technique called soft X-ray Coherence Tomography (SXCT) at a high-flux lab-scale HHG source. SXCT is derived from Optical Coherence Tomography, a Fourier based technique that can use the full bandwidth of the source to reach an axial resolution of 12 nm in this demonstration. The employed source covers the entire water window, with a photon flux exceeding 10 ⁶ photons/eV/s at a photon energy of 500 eV. We show local cross sections of a sample consisting of Aluminium oxide and Platinum layers of varying thickness on a Zinc oxide substrate. We validate the findings with scanning and transmission electron microscopy after preparation with focused ion beam milling.