Effect of thermal exposure on precipitation behavior and hardness of alloy 718

The precipitation and grain growth behavior during isothermal treatment of hot-extruded alloy 718 was investigated. It is found that the hardness of the hot-extruded alloy 718 increases with holding time at 600-700°C, but slightly decreases at 800°C. When the exposure temperature reaches above 900°C, a rapid decrease in hardness of alloy 718 occurs before the holding time of 0.3 h. Above the δ solvus temperature, γ′ and γ″ phases are dissolved directly into the matrix without δ precipitation, resulting in a rapid growth.