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There are still debates regarding the mechanisms that lead to hot cracking inparts build by additive manufacturing (AM) of non-weldable Ni-basedsuperalloys. This lack of in-depth understanding of the root causes of hotcracking is an impediment to designing engineering parts for safety-criticalapplications. Here, we deploy a near-atomic-scale approach to investigate thedetails of the compositional decoration of grain boundaries in thecoarse-grained, columnar microstructure in parts built from a non-weldableNi-based superalloy by selective electron-beam melting. The progressiveenrichment in Cr, Mo and B at grain boundaries over the course of theAM-typical successive solidification and remelting events, accompanied bysolid-state diffusion, causes grain boundary segregation induced liquation.This observation is consistent with thermodynamic calculations. We demonstratethat by adjusting build parameters to obtain a fine-grained equiaxed or acolumnar microstructure with grain width smaller than 100 $\mu$m enables toavoid cracking, despite strong grain boundary segregation. We find that thespread of critical solutes to a higher total interfacial area, combined withlower thermal stresses, helps to suppress interfacial liquation.

Atomic-scale grain boundary engineering to overcome hot-cracking in additively-manufactured superalloys