Effect of thermomechanical treatment on the microstructure and mechanical properties of a nickel base superalloy heavily alloyed with substitution elements

摘要

The work has been devoted to a study of the microstructure and mechanical properties of a novel heavily alloyed nickel base superalloy in cast and heat-treated (HT) and thermomechanically treated (TMT) conditions. The cast condition subjected to HT, including homogenization annealing, solid solution treatment and ageing, was characterized by a coarse γ grain size and uniformly distributed γ' precipitates with a size dγ'?=?0.1–?0.25?μm. TMT included unidirectional two-step forging in a thick-walled can made of stainless steel, which provided high quasi-hydrostatic pressure during forging, and intermediate recrystallization annealing at subsolvus temperatures. The TMT condition was aged. TMT led to the uniform development of recrystallization processes and the formation of predominantly recrystallized microstructure with a γ grain size dγ?=?2?–?50?μm. Non-recrystallized areas with up to 100?μm in size were also observed. After TMT and ageing, the microstructure contained mostly dispersed γ' precipitates with a size of dγ'?=?0.1–?0.3?μm. Tensile tests revealed that the strength properties in the TMT condition were by 20?–?70?% higher than in the cast and HT condition. The TMT condition also showed an appreciably higher ductility than the HT condition. The following tensile properties were obtained at room temperature: σUTS?/?σ0.2?=1533?/?1083?MPa, δ?=11?% after TMT and ageing, and σUTS?/?σ0.2?=1015?–1030?/?900?MPa, δ?=?4.5?–?5.2?% after HT. Higher tensile properties after TMT are explained by the γ grain refinement, the high γ' phase content, and the solid solution strengthening due to heavy alloying with substitution elements. At the same time, the presence of topologically close-packed phases probably reduced the mechanical properties.