Einfluss der Zusammensetzung auf das Hochtemperaturoxidationsverhalten im refraktären Hochentropielegierungssystem Ta-Mo-Cr-Ti-Al

Refractory High Entropy Alloys (RHEAs) or Refractory Compositionally Complex Alloys (RCCAs), by contrast with conventional alloys, are composed of a large number of base elements in equimolar or near- equimolar ratios. The alloy class of RHEAs/RCCAs is regarded to be promising candidates for high-temperature applications due to their high melting temperature and outstanding high-temperature strength. However, a serious disadvantage of most RHEAs/RCCAs is the insufficient oxidation resistance attributed to the high refractory metal content. In previous studies on equimolar TaMoCrTiAl, it was revealed that an almost unexplored, continuous oxide layer of rutile-type Cr-Ta oxides provides reliable protection against oxidation. The work presented aims at a sound understanding of the oxidation mechanism of TaMoCrTiAl alloys as well as a mechanistic comprehension of the effects of the elements on the oxidation behavior. Another emphasis lies in the characterization of the micro-structure in the Ta-Mo-Cr-Ti-Al system. To achieve these objectives, several series of alloys derived from the reference alloy TaMoCrTiAl, within which the concentrations of a single base element were varied, were investigated before and after the oxidation experiments carried out at 1200°C in air. X-ray diffraction and cross-scale electron micro-scopy studies in combination with various analytical methods including electron energy loss spectroscopy were applied for characterization. Microstructural investigations in conjunction with thermodynamic calculations reveal a decrease of Ta or Cr to suppresses the formation of the intermetallic Laves phase Cr2Ta (C14-type), whereas a reduction of the Ti or Mo concentration results in higher volume fractions of Laves phase. By reducing the Al content, the disordered A2 phase (W-type) is stabilized. It is demonstrated that the equimolar TaMoCrTiAl alloy possesses the highest oxidation resistance, which is related to a complex interaction of all base elements. The oxidation resistance is attributed to a non-stoichiometric rutile (Cr,Ta,Ti)O2 oxide layer formed at the oxide/substrate interface, which grows inward as a result of oxygen inward diffusion. The experimental results lead to the conclusion that the Ti4+ cations reduce the oxygen vacancy concentration. Reduction of the Ta concentration causes the formation of thicker and multi-phase oxide layers, which do not prevent evaporation. Quantitative assessments indicate significantly increasing Mo volatilization with lower Ta concentrations. A positive effect of Al exists above the content of 15 at.% that is explained by the nucleation effect, the getter effect of Al and an accelerated Cr diffusion due to Al2O3 particle formation. Decreasing the Cr concentration below 15 at.% leads to the formation of non-protective multi-phase buckling oxide layers consisting of (Al,Cr,Ta,Ti)O2 and Ta2O5. The removal of the element Mo causes an enhanced internal corrosion attack due to additional phase boundaries. In addition, it is concluded that the oxidation of Ti nitrides leads to the formation of pores and consequently to a degradation of the oxidation resistance.