Strength-ductility paradox in a directionally solidified nickel base superalloy

In contrast to other structural alloys, yield strength of nickel base superalloys either remains constant or increases with temperature up to 700-800 °C making them more suitable for high temperature structural applications. However, the increase in yield strength is usually accompanied by a decrease in ductility. The results of the present investigation describe the substurural evolution in a directionally solidified nickel base superalloy CM 247 DS LC during tensile deformation, where ductility is not impaired even when yield strength is highest (at 750 °C). Extensive TEM studies offer plausible dislocation based mechanisms for retention of high ductility at 750 °C. The highest yield strength is attributed to the presence of intersecting faults along with partials bounded by anti-phase boundary. However, high ductility is due to the formation of SISF inside γ′ precipitates. These mechanisms provide necessary impetus for further alloy development.