Abstract
The solidification and micro- and macro-segregation behaviors of as-cast INCONEL 718 (IN718) alloy at different temperatures under a slow cooling rate (5 °C/min) were investigated in this study. The results indicate that the solid-liquid interface grows into reticulation of hexagons during solidification. The variation trend of the solid fraction and transition rate of the solid phase with solidification time can be well characterized by the Boltzmann and Gaussian distribution, respectively. The order of segregation degree of negative segregation elements is: Fe > Cr > Al. Nb is the most principal positive segregation element, which is abundant in the long-term unsolidified remaining liquid. At the terminal stage of solidification, the increasing tendencies of the Nb and Mo contents in the liquid and the residual liquid density with decreasing temperature reverse due to the formation of the Laves phase. The freckles are most likely to form in the early stages of solidification, at which the liquid fraction is between 0.3 and 0.2, and the temperature range is about 1320 °C to 1310 °C. The information produced is expected to characterize the solidification and segregation behaviors of IN718 alloy when cooled at a slow rate characteristic of larger ingots typical of those required for industrial gas turbines and aircraft engines.
Highlights
Nickel-based superalloy, INCONEL 718 (IN718), is widely used as raw materials to manufacture components for the aerospace, nuclear, and land-based turbine industry [1,2]
The IN718 samples used in this study were extracted from the center of a 21 kg ingot produced by vacuum-induction melting (VIM)
Combining these results with the above conclusions indicates that Nb is the most principal segregation element in IN718 alloy during solidification under slow-cooled conditions, which is abundant in the remaining liquid at a late stage of solidification and involved in promoting the generation and growth of NbC carbides and the Laves phase
Summary
Nickel-based superalloy, INCONEL 718 (IN718), is widely used as raw materials to manufacture components for the aerospace, nuclear, and land-based turbine industry [1,2] It is a precipitation strengthened alloy containing a large amount of strengthening elements, such as Nb, Mo, Al, Ti, and Cr, which can prominently improve the corrosion resistance, weldability, creep strength, rupture strength, and high temperature structural stability of the alloy in the temperature range of −253 ◦ C to 650 ◦ C [3,4,5]. The NbC carbide and Laves phase, which are considered to be the main microstructural segregation constituents, will be precipitated in the solute-rich interdendritic liquids at different stages of solidification Their formations depend on the local non-equilibrium conditions of solidification and corresponding degree of segregation of elements [15]. The purpose of this work is to conduct fundamental research on the solidification and micro- and macro-segregation behaviors of IN718 alloy under a slow cooling rate in the laboratory, and to provide basic experimental data and forecasts to guide the control of micro- and macro-structures in the casting applications
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