Dependence of creep strength on cooling rate after subsolvus solution treatment in wrought alloy 718

Abstract

It was recently found that the creep strength and creep life of wrought alloy 718 samples showed an interesting dependence on the cooling rate in the standard subsolvus solution temperature: the time to 0.2% creep strain varies from 5 to 400 h depending on the cooling rate between 1 and 199 °C/min with the maximum value at an intermediate cooling rate of 51 °C/min under a creep condition of 621 °C/724 MPa. In the present paper, microstructures were observed for samples after creep deformation to 0.2% creep strain to understand the dependence of the creep strength on the cooling rate. The microstructural observations using a field emission type scanning electron microscope and transmission electron microscope showed that the size of coherent γ″/γ′ precipitates in grains decreased and the number density increased with increasing the cooling rate but they saturated around the cooling rate at which the creep strength was maximized. A careful observation on grain boundaries revealed that the precipitation of fine γ″ phase precipitates was enhanced along grain boundaries and coherent twin boundaries at intermediate cooling rates and that of 8 phase platelets was pronounced along grain boundaries and incoherent twin boundaries at slower cooling rates. These microstructural features suggest that the maximized creep strength is related to an enhanced precipitation of γʺ/γ′ particles along grain/twin boundaries as well as a high number density (fine size) of fine γ″/γ′ particles in grain interiors.