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 gamma/gamma' partitioning ratios determined by energy dispersive spectrometer attached to a transmission electron microscope showed that the addition of Mo enhanced the partitioning of Re, Wand Cr in the gamma matrix while decreased the concentration of Ta in the matrix. Synchrotron radiation diffraction was adopted to measure the gamma/gamma' lattice misfit at room temperature. The results indicated that Mo addition changed the gamma/gamma' lattice misfit towards larger negative as well as increased the tetragonal distortion of the gamma lattice. Additionally, Mo addition led to microstructural instability and altered the precipitation behavior of topologically close-packed phases during 1100 degrees C exposure. Instead of precipitating directly from the matrix, the m phase was observed to be converted from the sigma phase which precipitated preferentially as a metastable intermediate in the alloy with high Mo content.