Segregation-guided alloy design via tailored solidification behavior

This study presents an alloy design perspective guided by elemental segregation during solidification to determine the site-specific chemistry and related local thermodynamic properties of dendritic microstructures. This was accomplished via manipulation of the microsegregation behavior by means of nominal alloy composition and thermal conditions of the solidification processes, including modified cooling rates spanning over six orders of magnitudes using ingot casting, directed energy deposition (DED-LB/M) additive manufacturing (AM) and laser powder bed fusion (PBF-LB/M) AM processes. Our approach was demonstrated by computationally designing a novel AlxCo25Fe(50-x)Ni25 multi-principal element alloy (MPEA) as a model system, employing a combination of CALPHAD, Scheil, and multiphase-field simulations, and by experimentally validating the resulting microstructure evolution. The lower Al content (x = 10.5) was designated to generate a supersaturated single-phase fcc matrix suitable for heat-treatments to trigger local phase transformations. The higher Al content (x = 14.5) was selected to define the size and morphology of dual-phase microstructures by controlling phase nucleation and growth through segregation during solidification. Our results showcased how selective enrichment of the desired elements in interdendritic regions can be employed to induce local phase transformations during solidification or post heat-treatments, while their size can be flexibly controlled by the degree of undercooling during solidification. The suggested segregation-guided design approach can be transferred to other alloy systems, enabling effective tuning of local functional, structural, kinetic, and, as shown in this study, thermodynamic properties of dendritic microstructures by predetermining the nature of the alloy matrix through tailored solidification behavior.