Thermo-mechanical processing can substantially influence the phase transformation pathways in complex concentrated alloys (CCAs). These pathways lead to different combinations of phases, at multiple length scales, within these alloys. This presentation will focus on investigating and rationalizing how such transformation pathways can dramatically alter the microstructure and mechanical properties in face-centered cubic (FCC) based HEAs. Example systems to be considered include FCC-based 3d transition series alloys, such as Al0.3CoCrFeNi, Al0.5Co1.5CrFeNi1.5, and Al0.2Ti0.3Co1.5CrFeNi1.5. All these alloys inherently exhibit a competition between the precipitation of different ordered intermetallic phases, such as the L12, B2, and L21, within the FCC solid solution matrix. The thermodynamic rationale underlying such competition is the complex interplay between the driving force and the nucleation barrier associated with these phases. The resultant microstructural diversity within the same HEA can lead to dramatically different mechanical properties, as aspect which can be used for tuning their properties for various applications.