Normal and abnormal grain growth in a FGH96 superalloy during thermomechanical treatment

In order to investigate the grain growth behavior of a FGH96 superalloy during thermomechanical treatment, isothermal compression experiments of the superalloy were carried out on a Gleeble thermal simulator at the temperatures from 1020 to 1110 °C with strain rates from 10-3 to 1 s-1. Uniform heat treatment was performed on the deformed samples with typical Zener-Hollomon (Z) parameters after hot compression. Optical microscope (OM), electron backscatter diffraction (EBSD), transmission electron microscope (TEM) and finite element (FE) simulation were employed to explore the relationship between deformation conditions and microstructure evolution during thermomechanical treatment of the superalloy. The results show that with the decrease of Z parameter, the deformed microstructure in the center of the cross-section of the specimen transforms from a mixed state to a fully dynamic recrystallization (DRX) state. Under the conditions of high and medium Z parameters, obvious abnormal grain growth (AGG) emerges in the cross-section of the heat-treated samples, and inclines to form at higher strains with the decreasing Z parameter. Combined with the results of FE simulation, deformation conditions (Z parameter and strain) are suggested to be the main factors that control the normal grain growth (NGG) and AGG behaviors during thermomechanical treatment of the alloy, by influencing the deformation energy storage, the degree of DRX and γ′ precipitates. Stored energy in the special range of 1.25-1.38 J/mol, incomplete DRX and partial dissolution of γ′ precipitates can stimulate the occurrence of AGG. Under the stored energy conditions to ensure the occurrence of AGG, less DRX grains will favor individual formation of abnormally large grains (ALGs) with larger size after heat treatment. The microstructure with local misorientation (LM) between 0.25°-0.8° plays an important role in the later stage of ALGs growth. A variable β described as β = e·lnZ was proposed to define the formation condition of ALGs during thermomechanical treatment of the superalloy, and its corresponding mechanisms were also elucidated.