For high temperature applications, materials with a high phase and microstructure stability, good mechanical properties and reliable oxidation resistance are needed. So far, Ni-base alloys are the predominantly used materials in demanding high temperature applications. However, the thermal capability of these materials is limited by the relatively low melting point of Ni. A new class of refractory Compositionally Complex Alloys (CCAs) or High Entropy Alloys (HEAs) with a novel concept of alloy design offers the possibility to overcome this limit of the conventional high temperature materials.
In this study, the microstructure stability and high temperature oxidation resistance of two equiatomic quinary refractory high entropy alloys based on the Ta/Nb-Mo-Cr-Ti-Al alloy system and two equiatomic quaternary alloy Ta/Nb-Mo-Cr-Al were investigated. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) investigations revealed that all alloys show a BCC/B2 phase microstructure with various amounts of intermetallic compounds of mostly Cr2Nb/Cr2Ta Laves phase and AlMo3 (A15) phase. All alloys showed an increase of volume fraction of intermetallic compounds during exposure at 1000°C for up to 172h. Remarkably, the quaternary Ti-free alloys showed even higher volume fractions of the Laves phase and AlMo3 phase compared to the quinary alloys.
Various experimental techniques such as thermogravimetric analysis (TGA), XRD, scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) were used to characterise the oxidation behaviour of these alloys. The substitution of Nb by Ta was found to markedly increase the oxidation resistance due to the formation of a protective CrTaO4 oxide below the initially formed oxide layer consisting of Ti-, Al-, Cr-oxides. In the alloy Nb-Mo-Cr-Ti-Al, the formation of various Nb2O5-oxides resulted in pesting behaviour and led to the formation of thick, porous and quickly growing oxide layers. The Ti-free alloys showed a strong evaporation of MoO3-oxides and potentially deteriorated the oxidation resistance due to the increased Mo concentration in the alloy.
In our future work, the microstructure and the oxidation behaviour of the HEA Ta-Mo-Cr-Ti-Al with decreased Cr and/or Ta concentrations will be investigated. The modified alloy chemical compositions aim at the reduction of the brittle intermetallic phases in the Ta-containing alloys.