Effect of Phosphorus Additions on the Microstructure and Creep Properties of a Wrought Ni-Base Superalloy

A wrought Ni-base superalloy, Waspaloy, with four nominal levels of P additions (0, 0.01, 0.02, and 0.04 wt pct) was studied in order to better understand how P influences the grain boundary microstructures as well as creep resistance. Results from this investigation revealed that P additions promoted the precipitation of M 23 C 6 carbides at grain boundaries. The effect of P additions on the creep properties was assessed by both creep rupture and interrupted creep tests at 816 °C/180 MPa and 650 °C/760 MPa. Changes in the fraction and morphology of the M 23 C 6 carbides at grain boundaries due to P additions were found to result in a notable reduction of the creep ductility when tested at 650 °C/760 MPa where the creep strain was localized along the grain boundaries. However, the additions of P and associated changes in the M 23 C 6 carbides did not affect the creep behavior of samples tested at 816 °C/180 MPa where the creep strain was largely transgranular. This study reveals that P affects the creep resistance of the polycrystalline Ni-base superalloys by accelerating the nucleation and growth kinetics of M 23 C 6 along the grain boundaries.