Microalloying of Cr-Si alloys to bind nitrogen and oxygen

• Applicants: Privatdozent Dr.-Ing. Mathias Galetz; Professorin Dr.-Ing. Anke Silvia Ulrich
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins ; Metallurgical, Thermal and Thermomechanical Treatment of Materials
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 545140561

Chromium-rich chromium-silicon alloys are promising candidates for achieving higher working temperatures in high-temperature processes. Their melting point is in the range of 1700 - 1900 °C. Therefore, they can withstand potentially higher temperatures than the currently used Ni-based superalloys by simultaneously providing a lower density. The biggest challenges to date in the development of Cr-based alloys are: i) A brittle-ductile transition temperature range, which is highly dependent on impurity levels such as nitrogen and oxygen and is above room temperature. ii) The nitridation of the Cr solid solution matrix when exposed to air at high temperatures and the formation of brittle Cr nitride scales. iii) The oxidation resistance at T > 1000 °C, as there are increased stresses in the oxide layer. In this project, the focus is on binding the embrittling elements nitrogen and oxygen as well as reducing oxide and nitride formation through microalloying. In previous studies and projects it was shown that small amounts of the reactive elements yttrium and zirconium reduce the formation of cracks in the oxide scale at T > 1000 °C in pure chromium and thus increase the oxidation and nitration resistance. Both elements are strong oxide formers and therefore bind oxygen in the material. To reduce embrittlement due to Cr2N formation or to bind nitrogen in the material, small amounts of the antiperovskite phase formers platinum and rhodium are added. So far it has been shown that by alloying with platinum instead of a brittle Cr2N layer, thermodynamically more stable Cr3PtN antiperovskite precipitates are formed and nitrogen can be bound locally, which reduces the materials hardness. Therefore, in this project, Cr-Si alloys with approx. 0.3 at. % yttrium or zirconium are alloyed as oxygen-binders and platinum or rhodium as nitrogen-binders. The alloys are examined regarding their brittle-ductile transition temperature, their creep resistance, oxidation and nitridation resistance with and without external loading (creep experiments). The microstructure development is determined experimentally and by phase-field modelling. A further development of the solution annealing process contributes to the reduction of nitriding at phase and grain boundaries and thus to the reduction of crack initiation. Finally, a Cr-Si alloy with an nitrogen- and an oxygen-binding additive is produced and examined.

• DFG Programme: Research Grants