Entwicklung und Charakterisierung neuartiger refraktärer Hochentropielegierungen für Hochtemperaturanwendungen

The aim of this work was, among others, to extend the previously available thermodynamic database for the multicomponent Mo-Cr-Ti-Al system by Ta and Nb and to validate it with various experimental methods of microstructure analysis. Furthermore, the oxidation behavior of various ternary, quaternary, quinary refractory multicomponent alloys in a temperature range of 900–1500 °C in air was investigated in detail and oxidation mechanisms were derived from the studies.
Published data on binary and ternary subsystems were essentially used to extend the thermodynamic database to include Nb and Ta. Phase formation and phase stability under equilibrium conditions were studied experimentally on quinary alloys. For this purpose, samples were aged under inert gas (Ar) for 80 h at 700 °C, 300 h at 800 °C, 100 h at 1000 °C, and 24 h at 1300 °C, and the forming phases and microstructure were studied and compared with the thermodynamic equilibrium calculations.
Based on the thermodynamic calculations, it could be shown that the formation of a single-phase bcc microstructure can be expected in the Ta/Nb Mo Ti system in a very wide concentration range. However, the addition of Cr and Al to alloys of the Ta/Nb Mo Ti system favors the formation of (i) Laves phases (Cr2Nb and Cr2Ta, respectively), (ii) an A15 phase (AlMo3 type), and (iii) an ordered B2 phase (CsCl type).
In addition to the extension of the thermodynamic database and the extensive microstructural investigations, the high-temperature corrosion behavior of the complex multicomponent alloys from the Ta-Nb-Mo-Cr-Ti-Al system was investigated in this work, essentially at 900–1100 °C and in some cases even up to 1500 °C in air. On the one hand, the macroalloying influence of various base elements was analyzed using several model alloys. On the other hand, a microalloying effect due to the addition of small amounts of Si or Y was investigated using the two high-entropy alloys NbMoCrTiAl and TaMoCrTiAl as examples.
For the Ta and Cr containing alloys (TaMoCrAl, TaMoCrTiAl), the formation of a complex protective oxide layer of CrTaO4 was observed. The formation of CrTaO4 in these alloys occurred after a short incubation period and appears to reduce metal outward diffusion. Therefore, oxygen inward diffusion was found to be rate-determining in these alloys and the growth of the CrTaO4 layer occurred at the metal-oxide interface following a parabolic rate law. For the Nb-containing alloys (NbMoCrAl, NbMoCrTiAl), the formation of Nb2O5 near the metal-oxide interface caused the formation of a poorly adherent and non-protective oxide layer due to the high anisotropic thermal expansion.
Microalloying NbMoCrTiAl with 1 at. % Si improved the oxidation resistance at 1000 °C and 1100 °C in air, at least temporarily; however, no significant effect was observed at 900 °C. Unlike NbMoCrTiAl, the Si-containing alloy formed predominantly compact and protective Cr2O3- and Al2O3-rich oxide layers, which appear to be largely responsible for the higher oxidation resistance of this alloy.
The addition of 1 at. % Si to TaMoCrTiAl had a slightly detrimental effect on the oxidation resistance of the alloy in air at 900–1100 °C. The Si addition to TaMoCrTiAl resulted in a significant increase in the volume content and grain size of the Cr2Ta Laves phase, increasing internal corrosion attack within and along the Laves phase grains, and significantly greater precipitation depths of Cr-/Ti-nitrides and Al2O3.
Finally, the effect of Y additions of 0.5 at. % and 1 at. % on the high temperature oxidation resistance of the equimolar alloy NbMoCrTiAl at 1000 °C in air was investigated. It was found that the additions of Y lead to the precipitation of Al2Y phase along the grain boundaries, which during the stationary phase of oxidation, causes local stresses leading to cracking and subsequent increase in oxidation rates.