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

TL;DR

This study investigates hot cracking in additively manufactured Ni-based superalloys by examining grain boundary chemistry at near-atomic scale, revealing how microstructure control can prevent cracking despite segregation.

Contribution

It introduces a near-atomic-scale analysis of grain boundary segregation and demonstrates how microstructure engineering can mitigate hot cracking in AM superalloys.

Findings

Grain boundary segregation of Cr, Mo, B causes liquation.

Finer microstructures (<100 μm) prevent hot cracking.

Adjusting build parameters reduces thermal stresses.

Abstract

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