TEMPERATURE-DEPENDENCE OF THE LATTICE MISMATCH OF THE AM1 SUPERALLOY - INFLUENCE OF THE GAMMA' PRECIPITATES MORPHOLOGY

The lattice mismatch between the gamma and gamma' phases of single-cry stal specimens of the nickel-base superalloy AM1 has been measured as a function of temperature by a high-resolution neutron diffraction tec hnique for various treatments and after high-temperature creep deforma tion. It was observed that the misfit depends on the precipitate morph ology. This effect is strongly enhanced for the 'rafted microstructure ' typical of the crept sample where the value, the sign and the temper ature evolution of the misfit measured along and perpendicular to the [001] stress axis are different. Therefore it appears that the relevan t structural parameter which characterizes a superalloy family is not the one-dimensional (1D) misfit parameter delta but a three-dimensiona l (3D) parameter such as the relative difference in unit cell volumes of the gamma and gamma' phases. The large influence of plastic relaxat ions is evidenced and a mechanism is proposed to describe the variatio ns with temperature of the various delta values measured along the thr ee crystallographic directions [001] on the crept sample. For the heat -treated sample, measurements suggest a qualitative explanation for th e temperature evolution of the misfit and of the distortion of the cub ic cell.