High-entropy alloys (HEAs) open a new field to explore and develop high-performance materials among the center area of multi-component phase diagrams. The chemical composition selection affects the microstructure, and a proper selection can bring unexpected property optimization than the rule of mixtures. This thesis is to study the influence of Al on the microstructure evolution of CoCrFeNi HEA at different temperatures and to design an AlCoCrFeNi-based HEA with enhanced mechanical properties through microstructure adjustment. The first part focuses on the reactive diffusion behavior of the Al/CoCrFeNi diffusion couple under isothermal and isochronal annealing conditions. At temperatures below 1373 K, the major products of the reaction are intermetallic compounds as the process is enthalpy-dominated. Formation of the phases with lower energy than the disordered solid solution state causes the decomposition of the HEA. At and above 1373 K, entropy contribution to the system energy plays a more important role. Al diffuses toward the HEA matrix to form a more disordered state. The strong bonding tendency of Al and Ni indirectly induces new phases and causes the uphill diffusion of Ni toward the Al side in the fully disordered state. The second part studies the response of AlCoCrFeNi-based eutectic HEA (EHEA) to high-pressure torsion (HPT). Introducing numerous defects causes the fragmentation of the lamellar structure through dislocation multiplication and arrangement. Fragmentation initiates close to the interface due to stress concentration and gradually evolves into the grain boundary. The soft face-centered cubic (Fcc) phase develops misorientation faster than the ordered body-centered cubic (B2) phase. With the increase of shear strain, a high number of nanograins form with the disordering of the B2 phase to Bcc phase, and the remaining Bcc lamellae evolve into vortex clusters. The structural change leads to a significant strengthening of the material. Both the yield strength and ultimate strength are enhanced by a factor of around two with certain ductility remaining. By utilizing a wide thermal stability range of Al,Ni-rich B2 phase and Co,Cr,Fe-rich Fcc phase during reactive diffusion, the final part studies the structural optimization of an EHEA to enhance mechanical properties. The lamellar microstructure of an EHEA is adjusted to an equiaxed grain structure through a combination of HPT and annealing. As a result, the yield strength is improved from 703 MPa to 1199 MPa without reducing ductility. Detailed characterization of the deformation process shows that it has a Lüders-type deformation followed by a work hardening stage. High-angle grain boundaries and phase boundaries act as sources for dislocations and block the dislocation transfer. The dual-phase structure avoids the necking in the early stage while the soft Fcc phase blocks the expansion of microcracks for the B2 phase. This provides a guideline for designing alloys with enhanced mechanical properties without dependence on loading direction.