# 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.

## Key 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.

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Source: https://tomesphere.com/paper/1905.09537