Grain growth activation during supersolidus liquid phase sintering in a metal injection molded nickel-base superalloy

Abstract State-of-the-art sintering practices for metal injection molded (MIM) nickel superalloys (NiSAs) lead to fine-grain microstructures, reducing creep resistance compared to cast microstructures. MIM uses fine pre-alloyed powders which can increase prior particle boundary (PPB) carbide and oxide formation, restricting grain growth efforts. In the present work, the ability to activate grain growth in a MIM NiSA during supersolidus liquid phase sintering (SLPS) is evaluated using differential scanning calorimetry (DSC). A comparative DSC analysis technique was developed to quantify the SLPS liquid fraction with time and temperature and the grain growth behaviour of the MIM NiSA during SLPS was determined from optical microscopy. SEM-EDS of the sample microstructures revealed that the formation of refractory metal (RM)-Ta mixed MC carbides at the PPB restricts grain growth in the MIM NiSA. Time above the solidus temperature was found to dissolve the RM-Ta mixed MC carbides, enacting grain growth after a short incubation period. The results underscore the difficulty of accessing grain growth under the challenging conditions of MIM while demonstrating that SLPS can increase the grain size in the MIM NiSA.