Enhanced High-Temperature Oxidation Resistance of Refractory High-Entropy Alloys Al-Cr-Mo-Ta-Ti by Aluminizing Using Pack Cementation

Refractory high-entropy alloys (RHEAs) show promising properties for applications as structural materials in high-temperature applications, such as high solidus temperature and high strength. Improving their density, oxidation resistance, and room temperature ductility are still the aims of research in alloy development. In this study, Al-rich diffusion coatings by pack cementation are developed for three different alloys in the system Al-Cr-Mo-Ta-Ti in order to improve their high-temperature oxidation resistance. Equimolar AlCrMoTaTi, Al-rich Al3CrMoTaTi, and Ti-rich AlCrMoTaTi3 are synthesized by vacuum arc melting with subsequent milling to powder, consolidation to bulk material by field-assisted sintering technology/spark plasma sintering (FAST/SPS), and homogenization heat treatment. The applied aluminizing coatings are investigated by gravimetry, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). Experimental analyses are supplemented by CALPHAD simulations. Compact, uniform, and adhesive Al-rich diffusion coatings are produced on all three substrate RHEAs and exhibit single-layered D022 Al3(Cr,Mo,Ta,Ti) intermetallic compound analogous to Al3Ti in the binary Al-Ti system. Isothermal oxidation at 1000 °C for 48 h in ambient air results in the formation of 1–2 µm thin protective single-layered alumina scale—in contrast to multi-layered oxide scales in uncoated condition—and mass gains as low as binary Al3Ti and Ni-based superalloys.