Effect of heterogenization on the microstructure evolution and high temperature deformation behaviour of a nickel base superalloy

Medium alloyed nickel-based superalloy DMR SN 742 is widely used as high-pressure compressor (HPC) and turbine (HPT) rotors in various aeroengines. Establishing innovative processing schemes is essential to improve the workability of this difficult-to-deform material. A combination of heterogenization and high temperature deformation has been employed in this study to address this issue. Heterogenization is intended to coarsen the gamma prime (gamma ') phase present in the material while the deformation is intended to refine the size of gamma matrix. A combination of the aforementioned processes is expected to assist in producing fine grained equiaxed duplex microstructure comprising of gamma prime (gamma ') and gamma (gamma) that exhibits better workability. In order to identify and establish suitable heterogenization scheme for DMR SN 742, sub-solvus and super-solvus heterogenization was carried out by imposing a controlled cooling rate of 10 degrees C/hr after high temperature exposure. Detailed microstructural evaluation carried out on material subjected to heterogenization revealed multimodal distribution of gamma ' precipitate within the parent gamma matrix. The coarsening of gamma ' precipitates was observed to be significantly higher for super-solvus heterogenization when compared to sub-solvus heterogenization. Further, the high temperature deformation behaviour of sub and super solvus heterogenized material was investigated by carrying out isothermal hot compression test at 1000 degrees C with a constant true strain rate of 10(-3) and 10(-1)/s. Heterogenized material was found to exhibit better workability when compared to the as-received material. Further, super solvus heterogenized material exhibited a relatively less flow stress, higher flow softening and higher discontinuous dynamic recrystallized grain fraction. A combination of super solvus heterogenization followed by deformation at 1000 degrees C with a strain rate of 10(-1)/sec can be used to coarsen the gamma ' precipitate (primary: 1.2 +/- 0.4 mu m, secondary: 0.66 +/- 0.19 mu m) and refine the size of gamma matrix (1-1.5 mu m) in nickel based superalloy DMR SN 742.