Microstructural evolution in a Ni-Cr-Co based superalloy during cooling from the melt

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 σ (sigma), χ (chi), and M23C6. The σ precipitates of blocky morphology appeared to enclose the precipitates of χ phase. Lamellar regions consisting of σ and M23C6 were observed to form along the boundaries between the σ particles and the neighboring phases. These lamellar cells were observed to have grown into the intermetallic phases σ and χ. A crystallographic relationship was also found to exist between the σ and M23C6.