An additively manufactured IN718 strengthened by CSL boundaries with high-temperature tensile and short-term creep resistance up to 800°C

Emerging aerospace applications such as reusable rockets, ramjets, and next-generation turbine engines require non-cracking additive manufactured (AM) alloys with short-term creep resistance beyond the usual 650°C service temperature of the IN718 superalloy. Integrating previous phenomena reported in multi-step heat treatment processes (designed as RHSA), IN718 produced by laser beam powder bed fusion (PBF-LB) was subjected to an optimized scheme with stress relief (SR), hot isostatic pressing (HIP) and solution (SS) all above the supersolvus temperatures to minimize brittle phase formation. The high-temperature tensile and creep behaviors were investigated at up to 800°C. The supersolvus RHSA process produced a densified, low-texture microstructure with a high fraction of coincident site lattice (CSL) boundary (64 %). The low-Σ CSL boundaries resisted grain boundary (GB) cracking at high temperatures. GB strain measured by EBSD and a creep stress exponent of 10.9 indicates primarily power law creep at 700°C, with increasing contribution by grain boundary sliding (GBS) at 800°C. Stress-assisted coarsening of γ'' and δ precipitations was also observed at 800°C with preferential alignment to the maximum shear stress directions. The particle coarsening rates exceeded the CALPHAD-based calculations with only diffusion contributions while more in line with the stress-assisted rate (R_δ) equation determined experimentally from another study. The creep resistances in terms of the Larson Miller parameter (LMP) were comparable to hot-rolled IN718 while higher than cold-rolled or AM IN718 as well as IN625 previously reported. The relatively high creep properties are attributed to the high proportion of the CSL boundary that simultaneously limits GB cracking at high temperatures and intragranular deformation.