Optimization of Microstructure and Mechanical Properties of Hipped Inconel 718 by Various Heat Treatment Processes

The influence of heat treatment processes on hardening and harmful phases in microstructure and mechanical properties of hipped Inconel 718 (IN718) alloy was investigated. In comparison with conventional solid solution treatment, modified solid solution one entirely eradicates δ phase from the microstructure of hipped IN718 alloy. However, a thin film of δ phase precipitates at the grain boundaries of γ matrix. Aging at 750 °C for 4 h, another morphology of needle-like δ phase appeared as small colonies in the microstructure of IN718 alloy. Prolonging the aging duration time to 50 h, these colonies enlarge in size and spread in the whole matrix. X-ray diffraction and transmission electron microscope observations were used to identify different phases such as γ, γ′, γ″, and δ in the microstructure of IN718. Hardness property was measured representing the mechanical properties of investigated heat-treated IN718 alloy. Conventional solutionized specimens have higher rate in hardness increasing with aging time compared to modified solutionized ones. The latter specimens need an incubation period of about 24 h to ascent suddenly achieving the same hardness level of the other corresponding specimens. These heat treatment conditions are considered as the best compromise to achieve the optimum mechanical properties.