Effect of two-step solid solution on microstructure and δ phase precipitation of Inconel 718 alloy

Inconel 718 is the most popular nickel-based superalloy, extensively used in aerospace, automotive and energy industries owing to its extraordinary thermomechanical properties. The effects of different two-step solid solution treatments on microstructure and δ phase precipitation of Inconel 718 alloy were studied, and the transformation mechanism from γ" metastable phase to δ phase was clarified. The precipitates were statistically analysed by X-ray diffractometer test results. The results show that the content of δ phase increased with the increase in second step solid solution temperature. The changes of microstructure and δ phase were studied by scanning electron microscopy and transmission electron microscopy. Intragranular δ phase formed in Inconel 718 alloy with a second step solid solution temperature of 925°C, and its orientation relationship with γ matrix was determined as [1(—)00]δ//[011(—)]γ and (010)δ//(111)γ. Furthermore, Vickers hardness of different heat treatment samples was measured, and hardness of sample treated by second step solid solution treatment at 1010°C reached the maximum value of 446.84 HV.