Creep behavior of thin-cast single crystalline structures

• Applicant: Professor Dr.-Ing. Uwe Glatzel
• Subject Area: Metallurgical, Thermal and Thermomechanical Treatment of Materials
• Term: from 2017 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 374400892

To increase the efficiency of turbines further, one is anxious to increase the gas inlet temperature and to design turbine components as light as possible. By inserting cooling channels and the design of filigree structures it is possible to fulfill these aims. With these two options however, regions in the part are generated with thin sections.Own investigations and others found in literature show that the creep resistance of nickel-based superalloys decreases with reduced section size. Generally investigations were performed with creep specimens with machined surface, although in real applications, surfaces are rarely machined or can not be machined (hollow structures).To forecast the durability of thin nickel-based structures under creep deformation, it is essential to know, if the results attained from thin cast non-machined structures are comparable with results attained from specimens with machined surface. The submitted proposal which covers the topic extensively leads to three central questions.a) How big is the influence of thin section sizes on the creep resistance for as cast specimens compared to machined specimens? Which are the most important influencing factors? b) Is there a difference in the casting structure of reduced section sizes? Is there a difference in the dendrite structure in the surface near regions compared to regions inside the cast part?c) Is there an enrichment of certain elements during the solidification of the single crystal thin structures in the surface near regions, which could influence the oxidation behavior and thereby the creep resistance positively or negatively?To answer these questions, single crystal parts are cast with alloy MAR M247 in the vacuum induction furnace at the chair of Metals and Alloys with a minimal wall thickness of 0.3 mm. Creep specimens are made from the cast parts, and tested under air and Ar H2 (reduction of the influence of the atmosphere). The attained results, are compared with results of specimens with machined surface. Microstructural investigations, specifically from the surface near regions shall reveal explanations for observed differences. At this juncture the dendrite spacing is of particular interest, as the typical arm spacing of 0.2 - 0.5 mm is in the same magnitude as the desired specimen thickness.Castings with varying solidification velocities, leading to varying dendrite spacings, shall reveal whether the casting respectively dendrite structure is changing in the surface near region, or if occurring segregations have consequences for the formation of the matrix/gamma prime microstructure and the oxidation behavior.

• DFG Programme: Research Grants
• Major Instrumentation: Wärmeleitfähigkeitsmessgerät
• Instrumentation Group: 8690 Sonstige Meßgeräte für thermische Größen (außer 860-868)