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

Abstract

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 γ′‐volume fractions. Using thermodynamic calculations one can explain, why γ‐nanoparticles are observed in the central regions of large cuboidal γ′‐particles and why tertiary γ′‐nanoparticles form in the γ‐channels. The chemical compositions of the γ‐channels and of the newly formed γ‐particles differ because of the Gibbs–Thomson pressure which acts on the small particles. With increasing size of secondary γ′‐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 L12 (γ′) phase are considered. The importance of cooling rate after homogenization is discussed.