Determining interphase boundary orientations from near-coincidence sites

A transmission electron microscope (TEM) study was made of the interphase boundary structure of delta plates precipitated from the gamma phase in alloy 718. A variety of interfacial defects were examined and identified. These results, together with available data obtained from bcc laths in fcc Ni-Cr alloys, were used to develop a method for predicting precipitate orientation relationships and boundary orientations. The method employs a geometric matching approach in three dimensions based upon the concept of near-coincidence sites. It is suggested that precipitates in a given system select an orientation relationship which produces the greatest areal density of near-coincidence sites and that the habit plane adopts an orientation that yields the greatest area of boundary containing contiguous near-coincidence sites. This article is based on a presentation made in the symposium “Kinetically Determined Particle Shapes and the Dynamics of Solid:Solid Interfaces,” presented at the October 1996 Fall meeting of TMS/ASM in Cincinnati, Ohio, under the auspices of the ASM Phase Transformations Committee.