Influence of oxidation on near-surface γ′ fraction and resulting creep behaviour of single crystal Ni-base superalloy M247LC SX

Abstract

Literature results show that creep properties differ significantly with specimen thickness. This behaviour can be attributed to the influence of oxidation. In the present work the effect of oxidation on creep of single crystal nickel-based superalloy M247LC SX samples with varying thickness was studied. Results reveal that aluminium depletion during oxidation results in a modified matrix/γ′-microstructure in the near-surface regions. The differences from nominal alloy composition were measured as a function of distance from the oxidised surface. From these measurements five compositions were chosen representing different positions within the oxidised specimen. All of these alloys were cast as single crystals with different γ′ fractions.Creep experiments were carried out at 980°C in vacuum to determine the influence of the varying γ′ fraction on secondary creep. The dependence of stress level on creep rate was approximated by a Norton creep law. The stress exponent n plotted against the γ′ fraction shows a S-shaped behaviour almost doubling at 50% volume fraction. This behaviour suggests a change in the dominant creep mechanism. Using precipitate hardening theories this progression can be attributed to the transition of dislocation movement from cutting ordered γ′ precipitates to Orowan bowing mechanism with decreasing γ′ fraction. These results enable a correlation of oxidation-affected γ′ fraction and secondary creep, which is essential for the design and the improved prediction of creep deformation of thin sections of single crystal turbine blades.