Topologically Close-Packed μ Phase Precipitation in Creep-Exposed Inconel 617 Alloy

Two creep-exposed Inconel 617 alloy samples [923 K (650 °C) for 45,000 hours and 973 K (700 °C) for 4000 hours] have been studied using analytical electron microscopy and X-ray diffraction techniques. The thermodynamically predicted equilibrium phases in Inconel 617 alloy were compared with the phases observed which are molybdenum-enriched, topologically close-packed μ-phase, along with precipitates of gamma-prime (γ′), M₂₃C₆ and Ti(C,N). The μ-phase precipitates were in the size range 60 to 500 nm (with some larger agglomerates); they were situated both within the grains, along twin and grain boundaries, and near intra- and intergranular carbides. The stacking faults in the μ-phase were observed in high magnification electron microscopy. The precipitation of the μ-phase in these samples is significant for the potential use of this alloy in future generation steam power plants as the appearance of the μ-phase is associated with an increased tendency for cracks and voids to initiate. The μ-phase has not been previously identified in the literature relating to creep or thermal exposure of Inconel 617 alloy.