Dynamic recrystallization in a heavily alloyed PM γ+γ' nickel-based superalloy

Effect of isothermal hot compression at temperatures below the γ' solvus temperature on the development of dynamic recrystallization was studied in a powder metallurgy (PM) γ+γ' nickel-based superalloy VV750P (ВВ750П, Ni-9.9(Al, Ti, Nb)-33.7(Co, Cr, Mo, W, Hf)-0.075(C, B) (wt.%)). This superalloy is heavily alloyed and has a high γ' solvus temperature (Ts =1190°C). In the initial HIPed condition, the average γ grain size was d ≈ 29 µm and volume fraction of γ' (Ni3(Al, Ti, Nb)) phase about 70 %. Small cylindrical samples were prepared from the HIPed material and subjected to isothermal single-stage compression in the temperature range of 1125 –1175°C ((Ts−65) – (Ts−15), where Ts is the γ' solvus temperature) with an initial strain rate έ =10−2 s−1 to an engineering strain ε = 75 %. The temperature / strain rate conditions were chosen on basis of previously performed studies on other PM nickel-based superalloys such as EP741NP (ЭП741НП) and VV751P (ВВ751П). In contrast to previously performed works, the present investigation aimed at achieving completely recrystallized and fine-grained structure without using annealing after HIP and intermediate recrystallization annealing. Hot compression at 1125 –1175°C led to development of dynamic recrystallization. However, recrystallization kinetics significantly depended on deformation temperature. The fastest kinetics of recrystallization was reached during compression at 1175°C (Ts−15) that is only slightly below the γ' solvus temperature. In contrast to compression at 1125 and 1150°C, hot compression at 1175°C resulted in a complete disappearance of coarse prior γ grains and formation of completely recrystallized and fine-grained structure. Thus, single-stage isothermal hot forging of HIPed PM superalloy at a slightly subsolvus temperature without additional annealing can be considered as an alternative processing route instead of two- or three-stage forging at subsolvus temperatures with intermediate annealing.

1. Russian Science Foundation - 25-29-00806