Ni-base superalloys are an important class of high temperature structural materials that are used in the hot section of aircraft gas turbine engines since they possess the inherent capability to retain strength and resistance to creep, fatigue, and oxidation at elevated temperature. These components are subjected to elevated temperatures and complex stress states where time dependent creep deformation is of utmost concern and needs to be accounted for from a design criteria point of view. In this study, the creep deformation mechanisms of a newer generation turbine disk alloy, Rene 104, was investigated using transmission electron microscopy characterization techniques. The creep deformation behavior and underling creep deformation mechanisms were found to be highly dependent upon stress, temperature and microstructure.
Microtwinning was found to be the dominant deformation mechanism following creep at an intermediate temperature and stress regime. Microtwins form by the motion of paired a/6<112> Shockley partial dislocations that shear both the γ matrix and γ' precipitates. The rate limiting process in this mechanism is diffusion mediated atomic reordering that occurs in the wake of the shearing, twinning partial dislocations in order to maintain the ordered L12 structure of the γ' precipitates. To determine the salient microstructural features that lead to microtwinning specimens with varying γ' precipitate size scale, volume fraction and γ channel width spacing were crept at the same temperature and stress (677°C and 724MPa). The most creep resistant microstructure consisted of a bimodal distribution of γ' precipitates with a finer secondary γ' precipitate size, low volume fraction of γ' and narrow γ channel width spacing. Due to the combined effects of narrow γ channel width spacing, stacking fault energy, and resolved shear stress the a/2<110> dislocations dissociate into leading and trailing a/6<112> Shockley partial dislocations at low strain, which was determined to be a necessary precursor microtwinning at higher strains. Based on post mortem and in-situ TEM straining characterization a model was proposed which describes how a/6<112> Shockley partial dislocations can layer atop one another on adjacent {111} glide planes which could then cooperatively shear both γ matrix and γ' precipitates thereby creating a microtwin.