DETERMINING INTERPHASE BOUNDARY ORIENTATIONS FROM NEAR-COINCIDENCE SITES

A transmission electron microscope (TEM) study was made of the interph ase boundary structure of delta plates precipitated from the gamma pha se in alloy 718. A variety of interfacial defects were examined and id entified. These results, together with available data obtained from bc c laths in fee Ni-Cr alloys, were used to develop a method for predict ing precipitate orientation relationships and boundary orientations. T he method employs a geometric matching approach in three dimensions ba sed upon the concept of near-coincidence sites. It is suggested that p recipitates in a given system select an orientation relationship which produces the greatest areal density of near-coincidence sites and tha t the habit plane adopts an orientation that yields the greatest area of boundary containing contiguous near-coincidence sites.