Refractory high-entropy alloys (RHEAs) are considered to be one of the most promising candidates for high temperature applications beyond the temperature range of Ni- and Co-based superalloys [1]. However, the oxidation resistance of many RHEA is still insufficient because of the formation of non-protective and non-stable oxide scales [1]. Gorr et al. successfully developed a new strategy to design intrinsically oxidation resistant RHEAs in the Al-Cr-Mo-Ta-Ti alloy system [2-4]. Despite of this progress there remains the huge challenge of poor manufacturability that results from strong room temperature embrittlement and limits the synthesis of these alloys to a few grams in the lab-scale, exclusively.

In previous work the authors have demonstrated that laser metal deposition (LMD) utilizing in situ alloying of different pre-alloyed or elemental powders is a very reliable experimental tool for the high-throughput screening, design and fabrication of high-entropy alloys (HEAs) and compositionally complex alloys (CCAs) [5]. Utilizing this approach, in the current work, the synthesis of the AlXCrYMoTaTi RHEA system by LMD is for the first time explored covering a wide compositional range from 0 to 15 at% Al and Cr content. Gradient and single composition wall structures were produced by in-situ mixing of MoTaTi, Al and Cr powders using an in-house developed cladding system. The changes in microstructure with increasing Al and/or Cr content was analysed in the as-built as well as the homogenized condition by means of scanning electron microscopy (SEM) including X-ray spectroscopy (EDS) and electron backscattered diffraction (EBSD). Additionally, the evolution of sample micro-hardness with increasing Al and Cr content was determined. 

The obtained results show that LMD is a suitable synthesis method for the processing of relatively brittle AlXCrYMoTaTi RHEA system that allows crack free processing up to a hardness of 600 HV. Generally, in the as built condition, a dendritic structure consisting of Ta and Mo-rich dendrites and Ti-, Al- and/or Cr-rich interdendritic regions is formed. With increasing Al and Cr content and depending on the homogenization treatment a transition from the disordered bcc A2 phase to a mixed structure consisting of disordered A2 and ordered B2 phase is observed. This change in phase composition is accompanied by a substantial increase in hardness. 

References

[1] O.N. Senkov, D.B. Miracle, K.J.Chaput, J. Mater. Res., 2018, 33, 3092–3128

[2] B. Gorr, F. Müller, M. Azim, H.-J. Christ, T. Müller, H. Chen, A. Kauffmann, M. Heilmaier, Oxid Met, 2017, 88, 339–349

[3] B. Gorr, F. Müller, S. Schellert, H.-J. Christ, H. Chen, A. Kauffmann, M. Heilmaier, Corrosion Science, 2020, 166, 108475

[4] S. Laube, A. Kauffmann. S. Schellert, S. Seils, A. S. Tirunilai, C. Greiner, Y. Eggeler, B. Gorr, H.-J. Christ, M. Heilmaier,
Science and Technology of Advanced Materials, 2022, 23, 692-706

[5] M. Kuczyk, L. Kotte, J. Kaspar, M. Zimmermann, C. Leyens, Frontiers in Materials, 2020, 7, A242