Effect of high temperature exposure on the microstructure of Udimet-500 super alloy

Udimet-500 alloy is a high-strength nickel-bases super alloy used for turbine blades applications. It is primarily strengthened by very fine precipitates called gamma prime (γ′) which coarsened with time when exposed to high temperature. In this study, Udimet-500 alloy in the standard aged condition was exposed at 850–1100 °C for 25–100 h in air. The change in the size of γ′ with respect to the high temperature exposure was characterized using scanning electron microscope. A systematic coarsening of the γ′ was observed with the change in the exposure temperature/time. The γ′ size in the virgin sample was 0.1 μm while after high temperature exposures it coarsened to 1 μm. The Larson-Miller parameter (LMP) of the high temperature exposure was also measured. It was found that the LMP had power relation with the γ′ size. In addition, degradation of the primary as well as secondary carbides was also observed which can be used as add-on microstructural information in high temperature exposure. Elevated temperature exposure (≥1000 °C) also lead to (a) near surface de-alloying, (b) precipitation of needle-like Ti-rich phase and (c) depletion of γ′. It may contribute in the accelerated corrosion and loss of strength of alloy.