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Crystal microstructure; Free energy; Gibbs free energy; Microstructural evolution; Nanoparticles; Phase equilibria; Single crystals; Superalloys; Thermodynamics, Characterization techniques; Chemical compositions; Interdendritic regions; Microstructural features; Single-crystal Ni-base superalloys; Temperature dependencies; Thermodynamic aspects; Thermodynamic calculations, Nickel
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. © 2016 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim
