Abstract
The present study aimed to optimize the phase constituents and mechanical properties of the spark plasma sintered (SPS) Inconel 718 (IN718) alloy. A series of heat treatment routes were designed based on the phase relations in IN718 and performed for the optimization. The microstructure and phase compositions of the SPS IN718 alloys were examined by using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy disperse spectroscopy (EDS), and transmission electron microscopy (TEM). The mechanical properties of the samples were characterized at room temperature and at 650 °C. The results showed that large amounts of γ” (Ni3Nb) and γ’ (Ni3(Al, Ti)) strengthening phases precipitated in the IN718 alloy after direct aging (DA) of the as-fabricated sample. Moreover, the mechanical properties of the DA sample were comparable to that of the best one of the solution-treated and aged counterparts. The analysis showed that the rapid sintering and solid solution treatment of the IN718 alloy were achieved simultaneously by SPS. In the case of the SPS IN718 material, the direct aging regime had the same heat treatment effect as the conventional solid solution and aging treatment. This contributes toward improving the production efficiency and reduces manufacturing costs in the actual production process.
Highlights
As a precipitation strengthening alloy, the Inconel 718 (IN718) alloy has been widely used in the aircraft industry due to its good corrosion resistance and excellent mechanical properties at elevated temperatures [1,2,3]
The microstructure of the IN718 powder showed the dendritic grains, which was caused by the solidification partition coefficient difference of element compositions during the rapid cooling step of powder manufacturing
Direct aging promotes the full precipitation of γ”/γ’ phases and reduces the heat treatment steps, which is considered as the optimal heat treatment regime for the spark plasma sintered (SPS)-fabricated IN718 alloy
Summary
As a precipitation strengthening alloy, the Inconel 718 (IN718) alloy has been widely used in the aircraft industry due to its good corrosion resistance and excellent mechanical properties at elevated temperatures [1,2,3]. The γ’ phase has a cubic (fcc) structure and precipitates as a cube-to-cube relationship with the matrix [6]. The γ” phase is metastable and can transform into the δ (Ni3 Nb) phase at high temperatures [7,8,9]. Large amounts of δ phases will decrease the mechanical strength, since it consumes Nb atoms, leading to the loss of γ” phases in this alloy [13]. The relatively high Nb concentration results in the precipitation of carbide (NbC) and Laves ((Ni, Fe, Cr) (Nb, Mo, Ti))
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