Temperature dependence of the lattice mismatch of the Am1 superalloy influence of the γ ‘precipitates’ morphology

The lattice mismatch between the γ and γ′ phases of single-crystal specimens of the nickel-base superalloy AM1 has been measured as a function of temperature by a high-resolution neutron diffraction technique for various treatments and after high-temperature creep deformation. It was observed that the misfit depends on the precipitate morphology. This effect is strongly enhanced for the ‘rafted microstructure’ typical of the crept sample where the value, the sign and the temperature evolution of the misfit measured along and perpendicular to the (001) stress axis are different. Therefore it appears that the relevant structural parameter which characterizes a superalloy family is not the one-dimensional (ID) misfit parameter b but a three-dimensional (3D) parameter such as the relative difference in unit cell volumes of the γ and γ′ phases. The large influence of plastic relaxations is evidenced and a mechanism is proposed to describe the variations with temperature of the various δ values measured along the three crystallographic directions (001) on the crept sample. For the heat-treated sample, measurements suggest a qualitative explanation for the temperature evolution of the misfit and of the distortion of the cubic cell.