The influence of microstructure on the measurement of γ-γ′ lattice mismatch in single-crystal Ni-base superalloys

Lattice mismatch in multicomponent high refractory single-crystalline Ni-base superalloys has been measured in situ by hot-stage X-ray diffraction. Prior to X-ray examination, all samples were subjected to long-term aging treatments at 1120 °C to relieve coherency stresses. The resolution of the individual γ and γ′ peaks at high Bragg angles in the X-ray spectra and the magnitude of the misfit was found to be sensitive to the microstructure of the material. When the precipitation of coherent γ′ during cooling from the aging temperature could largely be suppressed, the corresponding matrix peaks were narrower and of higher intensity as compared with samples where cooling γ′ was present. Also, a slightly larger misfit, 0.04%, was measured in the microstructures where the cooling γ′ was not present. Procedures for deconvoluting X-ray data are outlined in detail, and the experimental results are discussed in terms of changes in phase compositions and misfit strains produced by the cooling γ′.