In this study the composition of a Ni-Cr-Co based superalloy was modified with silicon additions in order to observe its effects on the microstructure. The evolution of the microstructure was investigated by electron microscopy of as-cast samples containing various amounts of Si ranging from 0.48 to 5.43 wt.%. The precipitated phases were identified through electron diffraction to be sigma (sigma), chi (chi), and M23C6. The sigma precipitates of blocky morphology appeared to enclose the precipitates of X phase. Lamellar regions consisting of sigma and M23C6 were observed to form along the boundaries between the sigma particles and the neighboring phases. These lamellar cells were observed to have grown into the intermetallic phases sigma and chi. A crystallographic relationship was also found to exist between the sigma and M23C6. (c) 2006 Published by Elsevier Inc.