Effect of Nb addition on the microstructure and mechanical properties of Inconel 718 fabricated by laser directed energy deposition

Abstract

To further optimize the microstructure and enhance the mechanical properties of Inconel 718 alloy fabricated via laser additive manufacturing, the Nb-reinforced Inconel 718 composites were fabricated by laser directed energy deposition (LDED) and subsequently subjected to full heat treatment and direct aging treatment. The microstructure and the mechanical properties of the samples with various Nb contents (4.9, 7.0, 8.0, 9.0, 11.0 wt%) under three treated conditions were investigated. The results showed that increasing Nb content led to a more advanced dendrite structure in the as-deposited sample, accompanying with the emergence of fine grains. After undergoing the full heat treatment, the advanced dendrite structure was broken and corresponding recrystallization behavior occurred. The dendrite structure in the directly aged samples was similar to that in the as-deposited sample. Augmenting the Nb content could significantly alter the morphology, the amount, and the size of the Laves phase, γ″ phase, and the δ phase in the three heat-treatment samples. Cracks appeared in the sample with 11.0 wt% Nb. With the increment of Nb, the tensile strength and the microhardness remarkably became enhanced while the elongation decreased in the three heat-treatment samples. The improvement effect of the direct aging on the tensile strength and the microhardness was superior to the full heat treatment. Grain fining and solid solution strengthening were mainly responsible for the enhancement of the strength and microhardness in the as-deposited samples. The variation of the γ″ phase and the δ phase in amount and size jointly improved the heat-treated strength and the microhardness. The 9.0 wt% Nb sample with an excellent tensile strength (1598.3 MPa) which was superior to the currently reported strength, and the microhardness (543.2 HV0.2) can be obtained after direct aging. The fracture morphologies of the samples with various Nb contents were also discussed.