Influence of γ′ precipitates on the critical strain and localized deformation of serrated flow in Ni-based superalloys

Although γ′ precipitates enhance the strength of Ni-based superalloys, they can also promote an undesirable phenomenon called the Portevin-Le Chatelier (PLC) effect. Alloys designed and fabricated with various γ′ contents were subjected to uniaxial tensile tests at different temperatures (523-923 K) and strain rates (1 × 10−4-3 × 10−3 s−1). It was found that the γ′ precipitates facilitated a transition of the PLC effect from normal to inverse behavior. From the viewpoint of the critical strain, the temperature transition region shifted to low temperatures with increasing γ′ content. Digital image correlation was applied to the observations of the localization band in relation to the γ′ content at 773 K and 1 × 10−4 s−1. Surface strain mapping indicated that severe localized deformation was accompanied by high-amplitude serrations. Microscopic observations from transmission electron microscopy indicated that the deformation mode switched from dislocation motion to stacking fault (SF) formation with increasing γ′ content. With the aid of Suzuki segregation and SF intersections of different slip planes, a large number of SFs might induce high-amplitude serrations, especially those associated with the inverse PLC effect.