Effect of Mo Addition on Microstructural Characteristics in a Re-containing Single Crystal Superalloy

The effect of Mo addition on microstructural characteristics of a nickel-base single crystal superalloy containing 4 wt% Re was investigated. The γ/γ′ partitioning ratios determined by energy dispersive spectrometer attached to a transmission electron microscope showed that the addition of Mo enhanced the partitioning of Re, W and Cr in the γ matrix while decreased the concentration of Ta in the matrix. Synchrotron radiation diffraction was adopted to measure the γ/γ′ lattice misfit at room temperature. The results indicated that Mo addition changed the γ/γ′ lattice misfit towards larger negative as well as increased the tetragonal distortion of the γ lattice. Additionally, Mo addition led to microstructural instability and altered the precipitation behavior of topologically close-packed phases during 1100 °C exposure. Instead of precipitating directly from the matrix, the μ phase was observed to be converted from the σ phase which precipitated preferentially as a metastable intermediate in the alloy with high Mo content.