SOME ASPECTS OF THE PRECIPITATION OF METASTABLE INTERMETALLIC PHASES IN INCONEL-718

Some aspects of the precipitation of the metastable intermetallic phases-gamma" and gamma'-in the commercial nickel base superalloy, INCONEL 718, have been investigated over a wide range of aging temperatures. It has been confirmed that the spherical gamma' particles and the ellipsoidal gamma" particles evolve predominantly through homogeneous nucleation. Precipitation of the former does not appear to precede that of the latter in this alloy. The tetragonal distortion associated with the gamma" particles has been found to increase with increasing precipitate size. It has been observed that at certain temperatures, physical association between precipitates of the two types occurs frequently, leading to the development of different composite precipitate morphologies. During coarsening, the precipitate size has been found to depend linearly on the cube root of the aging time for gamma' as well as gamma" particles.