Study of the nucleation of MX- and mod. Z-phase particles

9-12% Cr steels are widely used for optimized caloric power plants with reduced CO 2 emission. These materials lose their initial creep strength after long service times. Mainly responsible for this decrease are changes in the microstructure, particularly the interaction between two precipitate groups: MX-phase and modified Z-phase. The interactions between these two phases are not only essential for this steel group, but also representative for a number of various alloys. For this reason, the alloy system Fe-Cr-V-N is investigated in this project, because here the interactions can be examined without any interference from other phases or side effects. MX- and mod. Z- phase are representative for two very different kinds of nucleation behavior. Whereas MX-precipitates nucleate very easily in the matrix and on dislocations, the opposite is true for mod. Z-phase: despite significantly higher driving force, any nucleation in the matrix has not been observed so far, even though this phase is the equilibrium V-containing phase. The reason for this behavior is presumably the strict ordering within the phase - the exact position of all V- and Cr-atoms in the crystal lattice makes the nucleation extremely unlikely. Instead, mod. Z- phase seems to form via a direct phase transition from MX-particles by diffusion of Cr into these particles. This kind of "nucleation" cannot be described by the classical nucleation theory. Because the formation of mod. Z-phase takes several years, the corresponding decrease of creep strength also occurs not before very long service times, and can not be excluded when starting with a material of ideal initial microstructure. Therefore, the aim of this work is to include the formation of MX- and mod. Z-phase particles into classical nucleation theory as comprehensively as possible. In order to achieve this goal, a theoretical treatment is required as well as experimental analyses. For the experimental part, Transmission Electron Microscopes with the highest spatial resolution are available at the Technical University of Denmark; which allow observation of small nuclei and precipitates in the first stages of growth. Many details can be investigated with this equipment: phase changes in very small nuclei, coherence effects at interfaces, mechanical strains in the matrix, the ordering of atoms within the mod. Z-phase, the evolution of this ordering during the transformation from MX- to mod. Z-phase are some examples. Parallel to this experimental work, theoretical tools will be developed for the implementation into the classical nucleation theory: modeling of nucleation regarding radii-dependent interphase energies, consideration of the ordering of mod. Z-phase during the nucleation, consideration of mechanical strains, phase changes in small nuclei etc. The results of this scientific project are directly going to effect the work of four different Austrian research groups which are working on related topics, both experimentally and theoretical. Thus, not only topics regarding this special alloy system will be treated, but the work will also strengthen the competence of Austrian research networks on a broad basis.