Negative creep and microstructure of long-term exposed gas turbine alloys

The phenomenology as well as the microstructural background of the negative creep, i.e. contractive specimen length change under tensile creep test conditions (respectively increase of the residual stress under relaxation test conditions) is surveyed. The measured negative creep of typical superalloys results from the superposition of two opposite processes, namely common plastic creep on the one hand, and simultaneous volume contraction due to the thermally unstable phase microstructure on the other. In the particular case of the nickel-base cast alloy IN 738 LC the measured significant specimen contractions cannot be attributed to the observed massive changes of the morphology of the γ' (Ni3Al/Ti) major phase which governs the creep strength. Rather, it is demonstrated that the indirect effects of a 'latent' reaction between the carbide minor phases are strictly controlling. The specimen contractions modelled by using measured microstructural parameters describe the measured trend satisfactorily.