Microstructural Characterization of Cast 718

Abstract

A detailed investigation of cast IN 718 was conducted on phases formed during solidification, phases precipitated on cooling and phase stability. Clearly cast 718 is a composite structure. Extensive as-cast segregation is evident which shows a low Cb content, about 2%, in the dendritic matrix, and a high Cb content, about 10% in the interdendritic region. Homogenization studies show that Laves solutioning does not uniformly redistribute the Cb. This work also indicates that the standard GE homogenization heat treatment of 2OOO'F/l hour dissolves most of the delta and a portion of the Laves. Depending on the Cb content in the alloy, total Laves solutioning requires at least 3 hours at 21OOF, but a more uniform Cb distribution requires 100 hours or more beyond the standard heating cycle. The results of this investigation may serve as a guide in microstructural interpretation and can offer a viable explanation for the variability of mechanical properties on bars extracted from castings. Superalloy 718-Metallurgy and Applications Edited by E.A. Loria The Minerals, Metals & Materials Society, 1989