Effect of solution heat treatment on microstructure and electrochemical behavior of electron beam smelted Inconel 718 superalloy

Effect of solution heat treatment on microstructure and electrochemical behavior was investigated for the electron beam smelted (EBS) 718 superalloy. The results imply that the solution heat treatment has a great influence on the precipitation behavior of EBS 718 superalloys and thus affects the electrochemical behavior. The increase of solution temperature gives rise to the solution of δ phase for both solution treated and aged EBS 718 superalloys. The 980 °C solution treated and aged EBS 718 superalloy exhibits the maximum content of the volume fraction of γʹʹ phase, in parallel with the highest lattice misfits. Further increase of solution temperature results in the precipitation of dispersively distributed γʹ particles. The pitting potential for the solution treated superalloys increases gradually with increase of solution temperature, and a similar behavior is observed for the aged superalloys with an exception of the alloy processed at 980 °C. The effect of solution temperature on the solution treated superalloys can be elucidated by considering the variation of the volume fraction and the size of δ precipitates. For the aged superalloys, the deterioration effect of Nb rich γʹʹ is obvious due to the tetragonal distortion and accompanying strain field in the nearby matrix. Higher solution temperature results in a uniform distribution of the nano-scale γʹ phase and smallest lattice misfits, with the highest apparent activation energy derived to be 52.86 kJ/mol.