Multimodal γ′ precipitation in Inconel-738 Ni-based superalloy during electron-beam powder bed fusion additive manufacturing

Additive manufacturing is a promising alternative method for fabricating components of Ni-based superalloys which are difficult to cast, form and join. However, typical thermal cycles associated with laser powder bed-fusion techniques suppress the formation of desirable microstructures containing γ′ particles, necessitating long-time post-process heat treatments. Here we report in-situ precipitation of γ′ (L12-ordered) particles and carbides during electron-beam powder bed-fusion of Inconel-738. The γ′ particles are homogenously distributed across the build and exhibit a multimodal size distribution. Based on atom-probe microscopy, we propose a eutectic reaction and multiple nucleation, growth, coarsening and dissolution bursts during thermal cycling as formation mechanism.