Harnessing additive manufacturing-induced microstructure and solute heterogeneities for the design of precipitation-strengthened alloys
Ahmet Turnali, Avinash Hariharan, E. Polatidis, Nicolas J. Peter, Jaqueline Gehlmann, Christos Sofras, Zoltán Hegedues, Lennart Sayk, Tarek Allam, Johannes Henrich Schleifenbaum, Christian Haase
Abstract
Solute enrichment at lattice defects is a well-established phenomenon for promoting phase transformations. Metal additive manufacturing (AM) inherently enables this by promoting cellular structures during solidification and thermal cycling. Cellular structures exhibit compositional and lattice defect density variations between cell cores and boundaries, leading to site-specific phase-transformation (e.g., precipitation) behavior that can be selectively activated by post-AM heat treatments. Despite this potential, cellular structures have largely been treated as byproducts rather than intentionally exploited alloy design features. Guided by these insights, we designed a model Al 10.5 Co 25 Fe 39.5 Ni 25 multi-principal element alloy to intentionally control composition and thus, precipitation driving forces across cellular structures. The alloy composition was computationally selected to promote segregation of a fast-diffusing, precipitate-forming element into the interdendritic regions during solidification in the laser powder bed fusion (PBF-LB/M) process. This segregation aligned with dislocation walls at cell boundaries, creating a “pre-conditioned” state with enhanced chemical driving force and reduced nucleation barrier for precipitation. This targeted design enabled site-specific nucleation and growth of precipitates at cell boundaries during aging. Comprehensive multiscale characterization complemented by in situ synchrotron X-ray diffraction confirmed that cellular structures accelerated precipitation, increased precipitate volume fraction and refined the precipitate size compared to the reference state where cellular structures were removed via solution annealing before aging. As a result, the alloy achieved enhanced yield strength (122.2 % increase), and improved tensile properties compared to the reference state. These findings demonstrate the potential of harnessing cellular structures as functional components to control microstructure evolution in precipitation strengthened AM alloys.