Evolution of Microstructure and Mechanical Properties of a Ni Base Superalloy during Thermal Exposure

The microstructural evolution of a cast Ni base superalloy, IN738LC, has been investigated after long term exposure at several temperatures. Most of the fine secondary γ ' particles resolved after 2000 hour exposure at 816 o C. At higher temperatures of 871 o C and 927 o C, secondary γ ' resolved after 1000 hours of exposure, and cuboidal primary γ ' grew with exposure time. During the thermal exposure, σ phase formed at all tested temperatures, and η phase was observed around interdendritic regions due to carbide degeneration. The influence of microstructural evolution during thermal exposure on the mechanical properties has been analyzed. The effects of γ ' particle growth are more pronounced on the high temperature creep properties than on the room temperature tensile properties.