The effects of heat treatment temperature, time, deformation amounts, and initial mean grain size on the abnormal grain growth behavior of nickel-base superalloy, René 88, are studied. In a as-extruded specimen (initially 1-2 ㎛ mean grain size) without additional deformation, abnormal grain growth occurs after tens of hours heat treatment at 1150℃ which is above the γ′ solvus temperature (1107℃), while it occurs within a few second at 1200℃. Low deformation shortens incubation time which is the time to onset of abnormal grain growth. For example, 4 % compressive deformation induces abnormal grain growth in a as-extruded specimen after annealing at 1150℃ for 1 h. The term "critical strain" is introduced to describe the minimum amount of room-temperature deformation necessary to induce abnormal grain growth after isochronal annealing. The grain size is largest for the critical strain, and decreases at higher strains. The critical strain decreases as the isothermal annealing time increases and finally reaches 0 % strain after tens of hours heat treatment at 1150℃. When annealing temperature increases to 1200℃ or initial mean grain size increases to 14 ㎛, critical strain also decreases. When the specimens of initially 14 ㎛ mean grain size are annealed for 1 h between 1150℃ and 1200℃, two critical strains appear with strain. It is suggested that abnormal grain growth enhanced by low deformation occurs at the lower critical strain and recystallization occurs at the higher critical strain. Abnormal grain growth induced by low deformation can be suppressed by pre-annealing below gamma' solvus temperature before final annealing above γ′ solvus temperature. These grain growth behaviors are hard to be explained by decrease of pinning force in the particle coarsening theory but can be explained by non-linear relationship between growth rate and driving force in the solute drag theory or the step growth hypothesis.
