  Controlling microstructure in fusion-based metal additive manufacturing (AM)
remains a challenge due to numerous parameters directly impacting
solidification conditions. Multiprincipal element alloys (MPEAs) offer a vast
compositional design space for microstructural engineering due to their
chemical complexity and exceptional properties. Here, we establish a novel
alloy design paradigm in MPEAs for AM using the FeMnCoCr system. By exploiting
the decreasing phase stability with increasing Mn content, we achieve notable
grain refinement and breakdown of columnar grain growth. We combine
thermodynamic modeling, operando synchrotron X-ray diffraction, multiscale
microstructural characterization, and mechanical testing to gain insight into
the solidification physics and its ramifications on the resulting
microstructure. This work paves way for tailoring grain sizes through targeted
manipulation of phase stability, thereby advancing microstructure control in
AM.
