The components operate in the creep range where they have to withstand mechanical loads at elevated temperatures. Creep is characterized by a continuous evolution of strain with time and a strong dependence of creep rate on stress and temperature. It is well known that creep (as well as other high temperature mechanical properties) strongly depends on the microstructure of the material. Most importantly, under high temperature creep conditions microstructures evolve: atoms segregate to interfaces, new phases form, precipitates and grains coarsen and dislocation substructures evolve. A combination of elementary microstructural processes governs creep deformation. Moreover, high temperature materials typically have non-equilibrium microstructures which evolve during high temperature service. The availability of advanced high-resolution characterization tools (like in-situ TEM, in-situ SEM, HR TEM and 3D APT) allows to tackle open questions which could not be answered in the past. This is the background of the research activities in MPIE’s high temperature materials group, which has two main scientific objectives:
First, we use advanced mechanical and microstructural characterization methods to identify new elementary processes which govern high temperature strength. As an example, we have discovered the mechanism of the formation of Laves phase particles in tempered martensite ferritic steels under long term creep [1,2] at about 600°C temperature range. At much higher temperatures, above 1000°C, we studied the formation of TCP phases in Ni-base superalloys .
Second, we use a mechanism-based approach to assist in the development of new high temperature materials and to improve processing technologies by studying microstructural key events that occur during casting, thermo mechanical heat treatments. Thus we investigate the role of in-grown dislocations in the development of the gamma / gamma prime interfaces of Ni-base superalloys . Or we investigate the microstructural processes which govern solidification and heat treatment .
Presently, a strong focus of the group’s activities is on co-ordinating research activities between contributions from RUB, MPIE and other partners within the collaborative research center SFB/TR 103, funded by the German research association. A number of joint publications from this field appeared so far [6-9].