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

Abstract

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.

Keywords

Refractory high-entropy alloys / AlCrMoTaTi / Oxidation resistance / Diffusion coating / Aluminizing / Pack cementation

Acknowledgments

This research project was supported by the European Union and the Free State of Saxony under EFRE which is gratefully acknowledged.

Author Contributions

Conceptualization, F.H., U.G., T.D. and T.W.; Methodology, F.H., U.G. and T.D.; Validation, F.H.,E.R. and T.D.; Formal Analysis, F.H.; Investigation, F.H., E.R. and W.P.; Resources, F.H., E.R., W.P. and T.D.; Data Curation, F.H.; Writing—Original Draft Preparation, F.H.; Writing—Review & Editing, F.H., T.D. and U.G.; Visualization, F.H.; Supervision, T.D., U.G. and T.W.; Project Administration, F.H., T.D. and T.W.; Funding Acquisition, F.H., T.D. and T.W.

Ethics Statement

Not applicable.

Informed Consent Statement

Not applicable.

5Data Availability Statement

The datasets generated and analyzed during the current study are available from the corresponding author on reasonable request.

Data Availability Statement

The datasets generated and analyzed during the current study are available from the corresponding author on reasonable request.

Funding

F.H. gratefully acknowledges financial support for this research project by KMM-VIN (European Virtual Institute on Knowledge-based Multifunctional Materials AISBL) with Prof. Appendino Research Fellowship 2024 (16th Call).

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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