Strengthening mechanisms in selective laser-melted Inconel718 superalloy

The microstructures and mechanical properties of Inconel 718 (IN718) alloy fabricated by selective laser melting (SLM) are evaluated in as-built, direct aging (DA), and homogenization + aging (HA) conditions on considering the effect of applied loading direction. The results show that the microstructure of as-built and DA specimens consist of the intracrystalline fine degenerated dendritic structures (0.6–1 μm in spacing) with a high density of dislocations (10¹³–10¹⁴ m⁻²). Fine dendritic structures disappear after HA treatment, and the average geometrically necessary dislocation (GND) density decreases to 10¹² m⁻². A quantitative analysis of strengthening mechanisms is established by taking into consideration the grain boundary, solid solution, precipitation and dislocation strengthening. For the as-built and DA specimens, the contribution to the yield strength by dislocation strengthening is ~85 MPa in vertical deposition and ~170 MPa in horizontal deposition, respectively, and the Laves phases provide approximately 100–110 MPa. The contribution of precipitation strengthening to the strength increment in the DA and HA specimens is 590–600 and 830–850 MPa, respectively. The precipitation strengthening is mainly contributed by shearing mechanisms among which coherency strengthening plays dominant effect rather than order strengthening. For as-built, DA, and HA regimes, the specimens always exhibit higher strength along the horizontal direction than that along the vertical direction. The anisotropy of yield strength in the as-built and DA specimens is mainly attributed to the difference in dislocation density and effective grain size, and the anisotropy of yield strength in the HA specimens is mainly caused by the difference in effective grain size and Taylor factor.