Abstract
AbstractIncipient melting and oxidation of grain boundaries at elevated temperatures have been investigated in the investment cast billet of a two-phase (γ + γ′) nickel aluminide alloy (Ni74.48Al16.98Cr8.02Zr0.51B0.10), designated as IC-218. Due to enrichment of Zr, which can form eutectic alloys with Ni, the inter-dendritic regions and grain boundaries melted incipiently at 1150°C. During quenching the molten layers often opened up producing cracks at the boundaries. Annealing at 1200°C in air for 570 min produced elongated ZrO2 precipitates at the grain boundaries. There was Zr depletion inside the grains in the vicinity of the precipitates. Such precipitates were scarcely observed in the specimens annealed in air at 1100°C for 23 h. Thus, the molten Ni-Zr alloy at the boundaries promoted internal oxidation.
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