Solidification Mechanisms in High Entropy Alloys: Influence of the Four Core Effects and Pathways to Strengthening Mechanisms—A Comprehensive Review

High-entropy alloys (HEAs) represent a distinctive class of multicomponent materials with promising properties, yet the mechanisms linking solidification to microstructural evolution and performance remain incompletely understood. This review establishes a mechanistic framework connecting thermal gradient, growth rate and cooling rate with phase selection, grain morphology, dendritic evolution, elemental segregation and defect formation. It highlights that the four core effects of HEAs, particularly lattice distortion and sluggish diffusion, are composition- and process-dependent phenomena influenced by non-equilibrium solidification, multicomponent diffusion and chemical short-range order (CSRO). A comparative assessment of FCC-based HEAs, refractory HEAs and eutectic HEAs reveals distinct thermodynamic and kinetic pathways that govern phase stability, microstructural heterogeneity and defect susceptibility. The effects of conventional casting, directional and assisted solidification, and additive manufacturing are examined in relation to microstructural refinement, phase selection and defect control. The review further evaluates the complementary capabilities and limitations of CALPHAD, phase-field modelling and molecular dynamics in predicting solidification behaviour, emphasizing database uncertainty, computational demands and restricted simulation scales. Despite advances in processing and modelling, accurately predicting the coupled effects of segregation, CSRO, lattice distortion and multicomponent transport remains challenging. An integrated process–structure–property framework is therefore proposed to guide mechanism-driven alloy design, microstructure control and the development of HEAs with tailored mechanical and functional performance.