Prediction and characterization of variant electron diffraction patterns for gamma '' and delta precipitates in an INCONEL 718 alloy

A universal theoretical method has been developed in this article to predic t and characterize electron diffraction patterns (EDPs) which contain vario us variants of precipitate(s) with a matrix. The plane and direction transi tion matrices for three gamma "-phase variants and 12 delta-phase variants precipitated from a gamma matrix in the INCONEL 718 alloy were deduced, fro m which the EDPs for seven low-index zones of gamma matrix containing gamma " precipitates were predicted by plotting and were found to be consistent with transmission electron microscopy (TEM) observations, showing that some of the results reported by Quist et al. should be corrected. Meanwhile, th ree variants of delta phase, precipitated from any one of four {111} matric es in 12 possible orientational variants, were also predicted and confirmed by EDPs. Different from Paulonis' conclusion, our theoretical calculations indicated that the {1/2 1 0}-type superlattice reflections in the [100] zo ne of the gamma matrix permitted detection of both gamma "- and delta-phase precipitates, but not of gamma "-phase precipitates. Therefore, the precip itates shown in dark-field images using these superlattice reflections cann ot be unambiguously determined to be gamma " phase. A unique approach for i dentification of gamma " precipitates in the alloy has been proposed.