Cooling rate and solution temperature dependent γ' coarsening mechanism in DZ125 Ni-based superalloy

The effect of solution temperature and cooling rate on the microstructure and coarsening behavior of γ' precipitates, as well as the hardness of DZ125 Ni-based superalloy is investigated. Microstructural analysis reveals distinct differences in γ' precipitate morphology between the dendrite cores (DCs) and interdendritic regions (IRs). At lower solution temperatures (1220 °C), γ'-particles do not completely dissolve, leading to the co-existence of primary γ' (∼540 nm) and smaller spherical secondary γ' (∼50 nm) precipitates in IRs, while higher temperatures (1240–1260 °C) leads to coarser and more cuboidal secondary γ' particles. Water quenching, produces finer γ' particles (∼158 nm) near the surface whereas slower cooling methods like furnace cooling results in larger particles due to extended diffusion times. Both the γ' area fraction, A_γ’, and the hardness increases with increasing solution temperature and depth. While furnace cooled DZ125 exhibits the highest hardness of ∼470–480 HV, water-quenched samples have the lowest hardness of ∼430–440 HV and exhibit a gradient in the hardness along the cross-section. A clear correlation between A_γ’ and Vickers hardness is observed and the solution treatment at 1240 °C followed by furnace cooling leads to the highest enhancement in hardness. γ' coarsening mechanisms are discussed by considering the fits of Lifshitz-Slyozov-Wagner (LSW) model, also known as matrix-diffusion controlled model, and trans-interface diffusion-controlled (TIDC) model. Goodness-of-fit measures reveal that the coarsening kinetics undergoes a transition from LSW near the surface to TIDC in the bulk. These results help in developing heat treatment strategies for directionally solidified Ni-based superalloys.