<div>This research aims at investigating the direct in-situ synthesis of high-performance multi-component alloys such as high entropy alloys, bulk metallic glasses, and metal matrix composites using the directed energy deposition (DED) process, and modeling the entire solidification and microstructure evolution of these alloys via a novel three-dimensional cellular automata-phase field (3D CA-PF) model. These alloys are currently the focus of significant attention in the materials and engineering communities due to their superior material properties. In the 3D CA-PF model, the growth kinetics including the growth velocity and solute partition at the local solid/liquid interface is calculated by the multi-phase and multi-component PF component, and the 3D CA component uses the growth kinetics as inputs to calculate the dendrite morphology variation and composition redistribution for the entire domain, which could save the computational cost more than five orders of magnitude compared to the PF modeling that can only be applied to small domains due to its heavy computational requirements. Coupled with the temporal and spatial temperature history predicted by the experimentally validated DED model, this computation-efficient 3D CA-PF model can predict the microstructure evolution within the entire macro-scale depositions, which is known to be nonuniform due to the particular nature of additive manufacturing (AM) processes. </div><div>To achieve the final goal of direct in-situ synthesis of five-component CoCrFeCuNi high entropy alloys (HEA), and modeling of the solidification and microstructure evolution during the DED process, the proposed research is carried out in progressive stages with the increasing complexity of alloy systems. First, a simple binary material system of Ti-TiC composite was studied. The thermodynamically-consistent binary PF model is used to simulate the formation mechanism of detrimental resolidified dendritic TiCx. To capture the polycrystalline solidification, a grain index is introduced to link different crystallographic orientations for each grain. This PF model simulates the microstructure evolution of TiCx in different zones in the molten pool by combining the temperature history predicted by the DED model. The simulated results provide the solution of limiting the free carbon content in the melt, according to which, the formation of TiCx dendrites is successfully avoided by experimentally controlling the melting degree of premixed TiC particulates.</div><div>Second, the solidification, grain structure evolution, and phase transformation in the DED-built ternary Ti6Al4V alloy under the influences of thermal history are systematically simulated using the established simulation framework and a phase prediction model. The thermal history in a three-track deposition is simulated by the DED model. With such thermal information, the 3D CA model simulates the grain structure evolution on the macro-scale. The thermodynamically-consistent PF model predicts the local grain structure and concentration distributions of solutes Al and V on the micro-scale. The meso-scale CA-PF model captures the sub-grain microstructure evolution and concentration distributions of solutes within the entire molten pool. The dendritic morphology is captured within the large β grains. When the temperature drops below the β-transus temperature, the solid-state phase transformation of β→α/ is studied by the phase prediction model. Based on the predicted volume fractions of and α, the microhardness is also successfully assessed using rules of mixtures. </div><div>Third, the material system is expanded to a four-component ZrAlNiCu bulk metallic glass composite, whose raw composition is prepared by premixing the four pure elemental metals. The DED model is employed to obtain the temperature field and heating/cooling rates in single-track ZrAlNiCu bulk metallic glass composite, which provides insights for microstructure evolution. By delicate control of the material composition and utilization of the thermal history of the DED process, an amorphous-crystalline periodic structure is produced with in-situ formed crystalline particulates embedded in the amorphous matrix. This crack-free microstructure is successfully maintained within bulk parts, where a high fraction of the amorphous phase and crystalline phases are produced in the fusion zone and heat-affected zone, respectively. The large volume percentage of the amorphous phase contributed to the hardness, strength, and elastic modulus of the composite while the various soft crystalline phases improve the ductility by more than three times compared to monolithic metallic glasses. Nanoindentation tests are also performed to study the deformation behavior on the micron/sub-micron length scale. </div><div>Fourth, the material system is expanded to a five-component CoCrFeNiTi HEA alloy. Three CoCrFeNiTi HEA alloys with different compositions are designed and synthesized from premixed elemental powders via the DED process. Through a delicate design of composition and powder preparation, different microstructures are formed. H3-Co24.4Cr17.4Fe17.5Ni24.2Ti16.5 is mainly composed of a soft face-centered cubic (FCC)-γ phase while σ-FeCr, δ-NiTi2, and a small amount of Ni3Ti2 are precipitated and uniformed distributed in the FCC matrix for H1-Co22.2Cr16.1Fe19Ni21.8Ti20.9 and H2-Co25.9Cr15Fe17Ni20.8Ti21.3. With a large percent of the secondary phases, H1 exhibits a hardness value of about 853 HV0.5. These HEA alloys display a high oxidation resistance comparable to Inconel 625 superalloy. A detailed evaluation of the hardness, oxidation resistance, and wear resistance of these HEAs are conducted as compared with those of a reference HEA and two popular anti-wear steels.</div><div>Finally, a novel 3D Cellular Automata-Phase Field (CA-PF) model that can accurately predict the dendrite formation in a large domain, which combines a 3D CA model with a 1D PF component, is developed. In this integrated model, the PF component reformulated in a spherical coordinate is employed to accurately calculate the local growth kinetics including the growth velocity and solute partition at the solidification front while the 3D CA component uses the growth kinetics as inputs to update the dendritic morphology variation and composition redistribution throughout the entire domain. Taking advantage of the high efficiency of the CA model and the high fidelity of the PF model, the 3D CA-PF model saves the computational cost more than five orders of magnitude compared to the 3D PF models without losing much accuracy. By coupling the thermodynamic and kinetic calculations into the PF component, the CA-PF model is capable of handling the microstructure evolution of any complex multi-component alloys. Al-Cu binary alloys with 2 wt.% and 4 wt.% Cu are first used to validate the 3D CA-PF model against the Lipton-Glicksman-Kurz analytical model and a 3D PF model. Then, the 3D CA-PF model is applied to predicting the dendrite growth during large-scale solidification processes of directional solidification of Al-30wt.%Cu and laser welding of Al-Cu-Mg and Al-Si-Mg alloys. </div>