Microstructure evolution of Ni-Cr-Mo superalloy fabricated by microwave-assisted combustion synthesis

In this study, the microwave-assisted self-propagation high-temperature synthesis (SHS) technique was utilized to successfully synthesize the Nistelle super C commercial alloy with the chemical composition of Ni59Cr23Mo18. The precursors used for this purpose were NiO, Cr2O3, and MoO3 oxide compounds. The thermodynamicall calculations were carried out by the Factsage software. The synthesized alloys' chemical composition, phase, and microstructure were evaluated by induction-coupled plasma, X-ray diffraction, optical microscopy, and scanning electron microscopy. The findings revealed considerable residual silicon in the final product due to using Si as the reducing agent. The presence of residual Si in the chemical composition led to forming a eutectic Ni gamma-FCC+ Mo2Ni3Si microstructure in the alloys produced by the alumino/silicothermic method. The high silicon content in the excess Al-containing sample led to forming of proeutectic Mo2Ni3Si phases with a petaloid morphology. Besides, this alloy displayed small islands of the interdendritic nickel silicide phase. Conversely, the aluminothermic sample showed a nearly single-phase structure with low amounts of the precipitates of chromium and molybdenum-rich phases. The phase diagram plotted for this sample indicated that the precipitates were probably TCP phases like sigma and P. In all the samples, the segregation of Mo occurred significantly, while Cr was distributed almost uniformly in all regions. The formation of the Mo2Ni3Si intermetallic phase resulted in the significant segregation of Mo and depletion of the Ni gamma-FCC matrix from this element. However, the segregation of Mo was also observed near the grain boundaries in the single-phase sample. The results showed that the dissolution of the alloying elements, especially molybdenum, caused severe offset in some peaks of the Ni gamma-FCC XRD pattern.