Alloy Design and Microstructural Evolution During Heat Treatment of Newly Developed Cast and Wrought Ni-Base Superalloy M647 for Turbine Disk Application

Abstract

A new cast and wrought Ni-base superalloy, M647, has been developed for turbine disk application with high mechanical properties at elevated temperatures and reasonable hot deformability. Conventional Ni-base superalloys are known to be strengthened by primary, secondary, and tertiary γ′, but optimal heat treatment is dependent upon specific chemical composition. For optimal mechanical properties, microstructural evolutions including austenitic γ grain size and γ′ morphology were identified for M647. To evaluate these properties, a full-scale low-pressure turbine disk was manufactured. This was subjected to a sub-solvus solution heat treatment, such that primary γ′ remained to prevent grain growth, enabling required mechanical properties, especially proof stress and low-cycle fatigue, to be achieved. As for secondary γ′, precipitation behavior was controlled intentionally by changing cooling rate after solution treatment, and this also resulted in modifying the precipitation behavior of tertiary γ′ during aging. In this study, microstructural evolution and tensile properties at 650 °C were investigated to clarify the relationship between them and to identify the strengthening mechanisms. From the result of SEM observation, size distributions of secondary γ′ had a clear relationship with cooling rate. Moreover, the mean diameter of secondary γ′ was unchanged during aging and this suggested that only tertiary γ′ was precipitated during aging. Finally, critical resolved shear stress was calculated using both weakly and strongly coupled dislocation models, and it was clear that M647 was strengthened by a high volume fraction of secondary γ′ and a small volume fraction of fine tertiary γ′ precipitates.