It could be due to the precipitation hardening. The phenomenon of precipitation hardening, or age hardening, provides one of the most widely used mechanisms for the strengthening of metal alloys. As a result, the ductility decreases with precipitation hardening.
Can you share the complete history. Is it hot ductility or room temperature ductility. Are you sure that you have 18% precipitate in your alloy? This is abnormally high.
I agree with V. Kumar. 18vol% would require an amazing high carbon content which appears unreal. Ni-base superalloys are typically materials with a gamma/gamma' microstructure. Gamma' already works as precipitation hardening (see K.J. Habib) so that I also guess that a formation of carbides would work as additional hardening, especially if they are that small (looks like 1/5..1/10 of gamma'). Which technique did you use to determine the volume fraction of M23C6?
The gamma prime vol% is 18 not the carbides. The ductility decreases with increase in temperature. But it decreases abruptly at 760C. The ductility at room temperature is 50% where as at 760C it is 6%. The question is why there is a abrupt change at 760C?