Processing-structure characterization of rheocast IN-100 superalloy

The rheocasting solidification process has been applied in the production of IN-100 nickel base superalloy. A high vacuum furnace for rheocasting superalloys was used to rheocast ingots under different processing conditions. Processing variables which were evaluated include stirring speed, isothermal stirring time, and volume fraction solid during isothermal stirring. Ingots, furnace cooled at the same rate but without stirring, were also examined for comparison with the rheocast ingots. A detailed microstructural examination was made of the resultant microstructure both on furnace cooling after stirring and on reheating to the isothermal stirring temperature followed by water quenching. Rheocasting yielded fine-grained structures, where the extent of microsegregatiori, the variation in macrostructure, and the solidification-induced porosity were found to be reduced in comparison to the unstirred ingot. The grain size and nonuniformity in the as-cast ingot were reduced by increasing the stirring speed, isothermal stirring time, or the volume fraction solid during stirring. The degree of the microsegregation decreased significantly with increasing volume fraction solid. Grain boundaries, both with and without solute enrichment, were found in the rosette-like solid particles after rheocasting, lending support to the Vogel-Cantor-Doherty model of rheocasting based on the formation of grain boundaries by strain-induced recrystallization and by sintering. It is clear from these results that the microstructure of this superalloy was significantly improved by rheocasting. Improved mechanical properties were also found and will be reported separately.