Abstract
The morphology of phase and grain of superalloy IN718 are important for the properties of final components as well as the forming process with repeated heating. Phase transformation and grain growth behaviors of IN718 at 920℃-1080℃ were studied by XRD and SEM. A model of volume fraction and size evolution of phases δ, γ′ and γ″ was established and used to predict the effect of phase transformation on the concentration of solute elements in the matrix. Based on the phase transformation model, a grain growth model of IN718 was established with consideration of solute drag and phase pinning effects. It was found that the drag effect of Mo and Nb is greater than that of the phase δ due to the 1.7 % phase δ content in the as-received alloy. The effect of phase δ content on grain growth was then studied by increasing the volume fraction of phase δ without change of grains. It was found that the needle-like phase δ has greater hindrance than short rod-like phase δ on grain growth and needs a correction coefficient for it in the grain growth model. The research provides a good understanding and develops a reliable method to design heat treatment process.
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