INONEL 718 alloy is a high Nb age-strengthened Ni-Cr-Fe based deformation high temperature alloy with excellent high temperature performance and mechanical properties, which is widely used in aerospace and other fields. However, when the service temperature of INONEL 718 alloy exceeds 650°C, the γ” phase will rapidly grow up and transform into δ phase, and the strength of the alloy will decrease rapidly. In order to improve the service temperature of INONEL 718 alloy, a lot of research has been carried out at home and abroad. Among them, the U.S. development of INONEL 718 Plus alloy service temperature to 704 ℃, the composition of the INONEL 718 alloy has been greatly adjusted, which Co and Al content changes on the performance of the alloy is particularly important.
Co is an important strengthening element that increases the creep resistance and high temperature strength of the alloy. However, the standard composition of INONEL 718 alloy specifies a Co content of <1.0 wt%, so the role of Co in INONEL 718 alloys has not yet been fully grasped. Studies have shown that a moderate amount of Co content can improve the high temperature strength and creep resistance of the alloy, but when the Co content is too high, it can lead to a reduction in the cold work hardening ability of the alloy.
Al is a commonly used γ”-phase inhibiting element, which can inhibit the precipitation of γ”-phase and improve the high-temperature strength and creep resistance of alloys. However, the increase of Al content will not only inhibit the precipitation of γ” phase, but also promote the precipitation of harmful phases such as Laves and σ. Therefore, the control of Al content is very critical, and it is necessary to find an equilibrium point so that the γ” phase can be effectively suppressed and the precipitation of harmful phases can be controlled.
In order to investigate the role of Co and Al in INONEL 718-based alloys, a series of experimental studies were carried out in this paper. The results show that the appropriate amount of Co and Al can significantly improve the high temperature strength and creep resistance of the alloy. When the Co content is 1.5 wt% and Al content is 1.2 wt%, the high temperature strength and creep resistance of the alloy are maximised. At this time, the γ’ phase is mainly precipitated in the alloy within the crystal, and the main strengthening phase at the grain boundary is the η phase, while the precipitation of the γ” and δ phases are effectively suppressed. In addition, the plasticity of the alloy decreases with the increase of Co and Al content, while the hardness and strength are enhanced.
In conclusion, Co and Al have significant effects on the organisation and mechanical properties of INONEL 718-based alloys. Appropriate amounts of Co and Al can improve the high-temperature strength and creep resistance of the alloy, but attention needs to be paid to the plasticity reduction of the alloy, and the precipitation of harmful phases needs to be controlled as well.
Post time: Jul-22-2023
