Morphology evolution of γ′ precipitates in GTD-111 Ni-based superalloy with heat treatment parameters

GTD-111 is an Ni-based superalloy used in manufacturing of gas turbine blades. The superalloy attains appropriate high temperature strength through precipitation hardening. In this research the effect of isothermal aging on the morphology evolution and characteristics of precipitates was investigated. Two different heat treatment cycles were applied to the specimens. In cycle I all samples were solution-treated at 1 200 °C for 4 h and water quenched to obtain supersaturated solid solution. A microstructure containing fine γ′ precipitates was produced after heat treatment cycle I. The second heat treatment, cycle II, consisted of reheating of the specimens in the precipitate partial dissolution zone at 1 150 °C for 2 h to get coarse primary precipitates, and then water quenching to obtain fine secondary precipitates. The precipitate coarsening in the fine and duplex size precipitate distribution was studied by means of aging of the specimens for various times at 1 000 °C. The results showed that during long-term annealing of the single size distribution (Cycle Ι), precipitates line up to reduce interactive free energy and grow to fairly coarse sizes along a direction. In cycle ΙΙ, the precipitates were split into a group of eight small cuboids or pairs of parallel plates and a large number of γ′ particles were closely aligned along a direction.