Additive manufacturing (AM) produces microstructure heterogeneities across multiple length scales due to the interplay between alloy’s chemistry and complex thermal history of the process. These heterogeneities, often considered detrimental for property optimization, can instead be utilized to engineer microstructure and improve properties through careful control of solute segregation and defect interactions. In this study, the evolution of microstructural heterogeneities arising from both liquid-solid and solid-solid phase transformations in AM-processed alloys were investigated. A central focus was placed on understanding how solute segregation can be harnessed to interact with lattice defects and control microstructure evolution in a beneficial manner during post-AM heat treatments. To this end, two alloy systems were selected. The first is a computationally designed single-phase face-centered cubic (fcc) Al10.5Co25Fe39.5Ni25 multi-principal element alloy (MPEA), in which heterogeneities predominantly originate from liquid-solid transformations intrinsic to the AM process. The second is a commercial-grade DP600-like low-alloy steel, where microstructural heterogeneity emerged from a combination of solidification and solid-state phase transformations. Both systems were subjected to two heat treatments: (i) solution annealing or full austenitization followed by aging or intercritical annealing, and (ii) direct treatments such as direct aging or intercritical annealing. The latter were specifically designed to activate AM-induced heterogeneities and exploit solute-decorated defects as nucleation sites for phase evolution. A comprehensive computational-experimental framework supported the understanding behind the alloy-specific microstructure evolution mechanisms. CALPHAD-based thermodynamic modeling and microstructure-resolved multi-phase field (MPF) simulations were employed to guide composition and process selection for the MPEA, as well as to understand segregation behavior and phase transformation pathways for both alloys. These insights were coupled with multiscale experimental characterization to assess the evolution of microstructure and mechanical properties. This work proposes a shift in microstructure control for AM: rather than eliminating, embracing the intrinsic heterogeneities of the AM process makes it possible to transform these features into functional microstructural assets. The combination of computationally guided alloy development, microstructural modifications, and strategically designed heat treatments offers a powerful pathway for developing next generation AM alloys.
Ahmet Turnali (Thu,) studied this question.