Fine grains forming process, mechanism of fine grain formation and properties of superalloy 718

The mechanical properties of Inconel 718 superalloy are determined primarily by its microstructure and grain size. The grain structure of Inconel 718 is traditionally refined by aging treatment, and a high volume fraction of acicular δ phase precipitates before the structure forms. During the following static or dynamic recrystallization process, the existing δ phase inhibits recrystallized grain growth and acquires a fine grain structure. In the proposed approach, the Inconel 718 specimens are re-solution heat treated at a temperature higher than the δ solvus temperature to ensure thorough dissolution of the precipitated δ phase into the austenite matrix and produce a niobium oversaturated matrix. The specimens are then cold compressed to produce a dislocation saturated matrix and are finally recrystallized at 950°C to induce the precipitation of fine δ phase. The δ phase precipitates exert a strong grain-boundary pinning effect, and thus a fine grain structure is obtained despite the high recrystallization temperature. The average grain size in the refined microstructure is found to be 2-3 μm, which is around half that of the grain size in the specimens prepared using the conventional process. Hardness testing and tensile testing at 25 and 650°C revealed its superior mechanical properties.