Microstructural analysis of a partially recrystallized nickel-based superalloy undergoing delta-processing

Abstract

Large volume fractions of stable δ-phase in nickel-based superalloys, such as in Inconel 718 (IN718), are being avoided in the forging industry because of their deleterious effect in causing brittleness. However, it has been found that a proper morphology of this phase when located at the grain boundaries provides a desirable pinning effect to prevent grain growth during hot deformation. To verify this phenomenon, partially recrystallized samples of IN718 were taken from an industrially deformed workpiece. The specimens were subjected to delta-processing (DP718) in order to reach a δ-phase saturation and, subsequently, were also subjected to hot deformation above and below the δ-solvus temperature (960 °C and 1020 °C, respectively) at three different strain rates (0.001 s−1, 0.01 s−1, and 1 s−1). The corresponding microstructural evolution was evaluated using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The mechanical properties were analyzed in terms of microhardness (Vickers). It was found that an intergranular needle-like δ-phase provides a pinning effect on grain growth during deformation, especially when placed at the grain boundaries while keeping an orientation relationship [011]γ//[11−2]δ.