The untraditional nature of metal additive manufacturing (AM) derived microstructures calls for a fresh perspective for materials design. During AM, alloys undergo distinct thermal histories associated with the repeated melting-solidification and heating-cooling cycles alongside with steep temperature gradients and very high cooling rates. This potentially leads to various liquid-solid and solid-solid phase transformations, whereas the spatially varying thermal profiles trigger the formation of locally dependent microstructures that are heterogeneous on multiple length-scales. Such hierarchically heterogeneous microstructures were accounted to facilitate unique material properties that, in some cases, cannot be achieved by their conventionally processed counterparts.

In particular, cell structures stand out as important features within the hierarchically heterogeneous microstructures generated by AM. Cell structures predominantly appear as a combination of solute segregation, dislocation networks or existence of precipitates at cell-boundaries, all of which occur during solidification and intrinsic heat treatments throughout the AM process. Consequently, cell structures encompass regions exhibiting compositional and structural gradients, which can be refined to sub-micron scales based on the utilized AM technology. This presents a distinctive window into the local variations of phase stability, which has a significant influence on the material’s local phase transformation behavior during post-AM heat-treatments to facilitate the formation of secondary phases and precipitates. Therefore, profound understanding of the influence and evolution of the microstructural heterogeneities on phase transformation kinetics and pathways during post-AM heat-treatments are essential to actively make use of this possibility.

In this study, we aim to investigate the effect of laser powder bed fusion (L-PBF) inherent microstructural heterogeneities on phase transformations in an Al_10.5Co₂₅Ni₂₅Fe_39.5 multi-principal element alloy (MPEA) upon post-AM heat-treatments. To this end, Al_10.5Co₂₅Ni₂₅Fe_39.5 MPEA was manufactured by L-PBF and subsequently heat-treated by two different annealing strategies. In the first strategy, as-built samples were directly aged to initiate the formation of B2-type (B2-NiAl and B2-CoFe) precipitates. In the second strategy, the as-built microstructure was solution annealed to induce partial recrystallization and to reduce the microstructural heterogeneities (e.g., elemental micro-segregations, dislocation substructures) inherited from the L-PBF process. Subsequently, the solution annealed state was also aged and utilized as a reference state to investigate the effect of L-PBF-specific microstructural heterogeneities on solid-state phase transformation pathways and kinetics during precipitation of B2-type particles. Multi-scale microstructure characterizations including scanning electron microscopy (SEM) with electron backscatter diffraction (EBSD), synchrotron X-ray diffraction (SYXRD) and scanning transmission electron microscopy (STEM) were accompanied with thermodynamic calculations to gain fundamental insights on the evolution of secondary phases governed by L-PBF specific microstructural heterogeneities.