Hochtemperaturstabilität zweiphasiger Chrom-Germanium-Silizium-Legierungen
Zusammenfassung der Projektergebnisse
This work presents a study of the oxidation behavior and the evolution of the substrate alloy microstructure of alloys in the binary Cr-Si system. Nitridation was systematically studied in this system for the first time. It was found that the A15 silicide shows superior oxidation resistance and high temperature stability in air. In the two-phase Cr-Cr3Si system the silicide phase forms during oxidation under the oxide scale and protects the substrate’s microstructure against nitridation. Based on the binary system, the chromium-rich Cr-Si-Ge ternary system was characterized. The chromium corner of the ternary phase diagram was investigated and the effect of Ge-Si substitution on the crystallography (lattice parameter), microstructural evolution, and oxidation-nitridation behavior of the Cr-Si-Ge was determined. It was found that all alloys with 84 at% Cr lay in the two phase Cr-Cr3(Ge,Si) domain and Ge/Si are miscible and can substitute each other in both the Crss and the A15 phase. The fact that Ge promotes the formation of A15 in the Cr-Ge system was used to modify the Cr-Cr3Si structure and to improve its high temperature oxidation-nitridation resistance. It was found that an amount of 1-2 at% Ge is highly beneficial against nitridation and oxidation of Cr-Cr3(Si,Ge). Ge-alloying reduced the dissolution rate of the Crss phase (which is the preferred Cr supply for chromia formation) and accordingly influenced the oxidation kinetics. In addition, Ge promoted the formation of an A15 phase matrix with a disconnected, fine network of Crss phase (sensitive to nitridation), and it was shown that with this microstructural design using Ge alloying the nitridation of Cr at high temperatures can be significantly suppressed. This material has a proven resistance against nitridation and oxidation for at least (tested) 1000 hours of oxidation at 1200°C in air. As nitridation was the biggest obstacle for the use of chromium alloys at temperatures above 1000°C, we are encouraged to develop the studied alloy system further to improve the mechanical properties of alloys at this temperature range. Therefore a proposal is being prepared to be submitted to DFG to continue the work on alloys which have the required amount of Si and Ge for oxidation and nitridation protection. However the amount of such alloying elements should be restricted to heat treatable alloys in this system which allow the precipitation of about 50% intermetallic phase. Additionally, selected further alloying elements to toughen the chromium solid solution phase as well as the A15 phase should be investigated in order to further increase the mechanical and oxidative properties of the system and to push the system towards industrially interesting alloys less dependent on microstructural evolution during solidification casting. The results of this work have already attracted strong interest from industry, including the offer to produce the optimized alloys via powder metallurgy on a larger scale. This would allow us to compare different alloy making methods and to characterize the mechanical properties of alloys such as elastic modulus (E) and creep resistance in a follow-up project.
Projektbezogene Publikationen (Auswahl)
- Cr-Ge-Si Alloys for High-Temperature Structural Applications: Microstructural Evolution, Metallurgical and Materials Transactions A 45 (3), 2014, 1639
A. Soleimani-Dorcheh, M.C. Galetz
(Siehe online unter https://doi.org/10.1007/s11661-013-2091-2) - On ultra‐high temperature oxidation of Cr–Cr3Si alloys: Effect of germanium, Materials and Corrosion 65 (12), 2014, 1143
A. Soleimani‐Dorcheh, W. Donner, M.C. Galetz
(Siehe online unter https://doi.org/10.1002/maco.201307423) - Oxidation and Nitridation Behavior of Cr–Si Alloys in Air at 1473 K, Oxidation of Metals, 2015
A. Soleimani-Dorcheh, M.C. Galetz
(Siehe online unter https://doi.org/10.1007/s11085-015-9544-5)
