On Local Phase Equilibria and the Appearance of Nanoparticles in the Microstructure of Single-Crystal Ni-Base Superalloys

High-resolution characterization techniques are combined with thermodynamic calculations (CALPHAD) to rationalize microstructural features of single crystal Ni-base superalloys. Considering the chemical compositions of dendritic and interdendritic regions one can explain differences in gamma-volume fractions. Using thermodynamic calculations one can explain, why gamma-nanoparticles are observed in the central regions of large cuboidal gamma-particles and why tertiary gamma-nanoparticles form in the gamma-channels. The chemical compositions of the v-channels and of the newly formed gamma-particles differ because of the Gibbs-Thomson pressure which acts on the small particles. With increasing size of secondary gamma-particles, their shape changes from spherical to cuboidal. Some general thermodynamic aspects including the temperature dependencies of the Gibbs free energy G, the enthalpy H, and the entropy S and site occupancies in the ordered L1(2) (gamma) phase are considered. The importance of cooling rate after homogenization is discussed.