Monotonic tension and creep response of electron-beam powder bed fusion processed IN718 superalloy: Role of orthorhombic Ni3Nb δ-phase at grain boundaries

This investigation focuses on examining the deformation behavior of the Inconel 718 (IN718) superalloy, manufactured via electron-beam powder bed fusion (E-PBF), under both ambient and high-temperature conditions. Two specific heat treatment routes were applied to the As Printed (AP) material: one aimed at precipitating the δ phase at the grain boundaries (HT1) and the other at keeping the grain boundaries free of the δ phase (HT2), while retaining the γ' and γ'' phases in the matrix. The optimized heat treatment resulted in a granular and discontinuous δ phase at the grain boundaries. The HT2 specimens demonstrated higher yield strength and strain hardening rates compared to the HT1 and AP specimens at both room and high temperatures. The stress-strain curves at room temperature were free from serrations, whereas serrations appeared at high temperatures. Although all specimens exhibited temperature-dependent stress drops, this effect was minimal in the HT2 specimens. Post-deformation analysis revealed that the HT1 specimens did not accommodate strain at the grain boundaries, whereas the HT2 specimens showed more homogeneous deformation. In the HT1 specimens, the δ phase exhibited a strong orientation relationship with neighboring grains at the grain boundaries. Additionally, HT1 specimens displayed superior creep resistance compared to HT2, with a two-order magnitude difference in the steady-state creep rate. Electron backscattered diffraction (EBSD) analysis, coupled with Crystal Plasticity Fast Fourier Transform (CPFFT) simulations of the monotonic stress-strain curves, confirmed that HT2 specimens exhibited higher initial resistance to slip and higher hardening rates than HT1 and AP specimens. Creep studies further substantiated that HT1 specimens outperformed HT2 and AP specimens in high-temperature deformation response.