Effects of solution heat treatment on the microstructure and hardness of Inconel 740 superalloy prepared by electron beam smelting

• Electron beam smelting, a new method, was used to prepare the Inconel 740 superalloy.
• The effect of solution heat treatment on electron beam smelting 740 was investigated.
• TiNiSi phase’s content changes exponentially with solution temperature increasing.
• γ′ precipitates coarsening occurs at 1150 °C, higher temperature cause their solution.
• γ″ phase transformed into Nb0.03Ni3Ti0.97 phase at 1210 °C.

Electron beam smelting (EBS) was used to prepare the Inconel 740 superalloy. The microstructure and hardness of Inconel 740 were investigated both under hot working condition and followed by solution heat treatment under high temperature ranging from 1120 to 1210 °C for 30 min. The results show that fine γ′ nucleus and primary MC carbides exist in the hot working superalloy. With increasing temperature of solution treatment, TiNiSi phase is gradually solutionized into the matrix, whose volume fraction decreases exponentially. γ′ precipitates coarsening occurs at 1150 °C, while higher temperature solution treatment leads to the solution of γ′ precipitates. γ″ phase has been found keeping a stringent coherent relationship with Nb0.03Ni3Ti0.97 phase at 1180 °C, and utterly transformed into Nb0.03Ni3Ti0.97 phase at 1210 °C. The hardness of the solution-treated superalloy is lower than that of hot working, the value decreases at first and then increases with increasing of solution temperature, which is attributed to dislocations interaction and localized shear stress caused by γ′ precipitates and local stress fields.