Microstructural characteristics of sigma phase and P phase in Ru-containing single crystal superalloys

摘要

Microstructural instability caused by topologically close-packed (TCP) phase precipitation restricts the useful compositional range of advanced Ni-base single crystal superalloys in industrial applications. Limited systematic investigations of TCP formers (Cr and Mo) additions on microstructural evolution of both the a phase and the P phase in Ru-containing single crystal superalloys have been reported. In this study, the microstructural characteristics of sigma phase and P phase were investigated in three Ru-containing superalloys with different levels of Cr and Mo additions at 950 degrees C and 1100 degrees C by using phase extraction, X-ray diffraction, scanning electron microscope and high resolution transmission electron microscopy. The experimental results indicated that the high level additions of Cr and Mo promoted the formation of sigma phase and P phase, respectively. The amount of a phase was much higher than that of P phase after long term exposure at 950 degrees C and 1100 degrees C. The sheet-like sigma phase existed in the alloy with higher Cr addition after thermal exposure at 950 degrees C and 1100 degrees C for 1000 h, while the needle-like P phase precipitated in high Mo content alloy after thermal exposure at 1100 degrees C for 1000 h and the intergrowth of sigma phase and P phase was observed after thermal exposure at 950 degrees C for 500 h. Both the sigma phase and P phase were enriched in Re, W, Cr and Mo, but the sigma phase contained more Re and Cr while the P phase contained more Mo and Ni, and Ru was found in both phases. The nucleation of sigma phase was much easier than P phase due to the more ledge steps in the interfacial structure between sigma phase and matrix, as well as the higher partitioning ratios of Re, Cr and Mo. This study is helpful to understand the microstructural evolution of sigma phase and P phase, and to optimize the alloy design in Ru-containing superalloys. (C) 2016 Elsevier Inc. All rights reserved.