The Effect of a Grain Boundary Pinning B2 Phase on Polycrystalline Co-Based Superalloys with Reduced Density

The Al/W ratio was increased in a new polycrystalline γ/γ′ Co-based superalloy in order to introduce a grain boundary pinning phase while at the same time reducing the density and increasing the oxidation resistance. The grain boundary pinning phase was identified as β-phase (B2-(Ni,Co)Al) by high-energy X-ray diffraction. The grain boundary pinning effect was confirmed to be effective up to temperatures of 1050 °C by synchrotron radiation in-situ heating experiments. The precipitation of the β phase led to a reduced γ′ volume fraction thereby decreasing the high-temperature strength. The density was lowered to 8.52 g/cm³, so the specific strength was equivalent to the strength of the reference alloy up to 750 °C. Due to the low γ′ volume fraction and the low W content, the creep resistance was also reduced compared with the reference alloy. The oxidation resistance, however, was improved by the formation of a thin, continuous Al-oxide layer during oxidation at 900 °C for 50 hours.