Microstructure and Solidification Sequence of the Interdendritic Region in a Third Generation Single-Crystal Nickel-Base Superalloy

An analysis of the microstructure of the interdendritic regions in an as-cast third generation single-crystal nickel-base superalloy CMSX-10K has been carried out using automated serial sectioning with sequential focused ion beam (FIB) milling and image capture of the secondary electron image in a combined FIB-scanning electron microscope (SEM). Two interdendritic morphologies with vastly different gamma/gamma' spacing were identified; these are (1) fine gamma/gamma' morphology and (2) coarse cellular gamma' separated by narrow gamma channels. The FIB results indicate that following primary solidification (L -> gamma), solidification of the interdendritic region commences in areas that show the fine gamma/gamma' morphology, (1) on the gamma dendrite lobe. This is followed by a gradual transition to growth of the coarse cellular gamma/gamma' channel structure, (2) which occur up to the termination of the solidification process.