Mechanisms of high temperature creep of nickel-base superalloys under low applied stress

[001] single-crystal (SC) specimens of the superalloys CMSX-4 and CMSX-10 were tested for creep at 1100 degrees C under tensile stresses between 105 and 135 MPa. For these testing conditions superalloys show creep curves with the classical shape: primary creep is followed by a long period with low, nearly constant creep rate. It was found that porosity significantly increases during this steady creep. The central point of the investigation was to understand the deformation mechanism of steady creep and its connection with porosity growth. Specimens deformed up to certain strain levels were analyzed by density measurements, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) which supplied information about porosity growth, evolution of the gamma/gamma' microstructure as well as the mobility and reactions of dislocations during the creep process. It was found that the non-conservative movement (glide/climb) of the a/2 < 011 > dislocations deposited in the gamma/gamma' interface during primary creep produces most of the plastic strain during the following steady creep. Interfacial climb creates vacancies, which diffuse either to pores increasing porosity or to interfacial a < 100 > edge dislocations, assisting their climb through the gamma' phase. Under the testing conditions applied the recovery mechanism is gamma' climb, which has the following two effects: it reduces the constraining of the plastically deformed gamma phase by the elastically deformed gamma' phase, thus allowing further dislocation glide in the matrix channels and it decreases the osmotic force, which suppresses interfacial glide/climb. Creep deformation accelerates when the gamma/gamma' microstructure coarsens and the gamma' rafts form junctions, enabling a higher mobility of the gamma' dislocations.