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 and is 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 delta phase precipitation of Inconel 718 alloy were studied, and the transformation mechanism from gamma '' metastable phase to delta phase was clarified. The precipitates were statistically analyzed by X-ray diffractometry. The results show that the delta phase content firstly increased, and then decreased with the temperature of the second-step solid solution. The changes in microstructure and delta phase were studied by scanning electron microscopy and transmission electron microscopy. An intragranular delta phase formed in Inconel 718 alloy at the second-step solid solution temperature of 925 degrees C, and its orientation relationship with gamma matrix was determined as [1<overline>00]delta//[011<overline>]gamma\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$[\bar{1}00]_{\delta}//[01\bar{1}]_{\gamma}$$\end{document} and (010)delta//(111)gamma. Furthermore, the Vickers hardness of different heat treatment samples was measured, and the sample treated by second-step solid solution at 1010 degrees C reached the maximum hardness of HV 446.84.