增材制造的中高熵合金结合了新型制造技术与新型合金设计理念，具有高强度和高塑性等优异的力学性能，极具工业应用前景。探究中高熵合金的增材制造技术及其变形行为，并研究其中的力学问题是固体力学和材料科学的前沿热点之一，对于中高熵合金的设计和制备具有重要的科学意义和应用价值。本文采用数值模拟、实验测试和理论分析相结合的方法，研究了中高熵合金的增材制造工艺、微结构、变形机制和力学性能及其之间的关联。基于二维元胞自动机方法研究了梯度温度场下多晶粒的生长过程，揭示了组织形貌的择优取向与温度梯度方向相关，并将基于连续体假设的热传导模型与元胞自动机方法相结合，发展了增材制造中组织结构演化的数值模拟方法，模拟获得了增材制造的In718镍基合金、316L不锈钢和CoCrFeNiMn高熵合金的组织结构，模拟结果与实验结果相吻合。此外，将该数值模拟方法拓展到三维尺度的模拟，研究了增材制造中组织结构在三维空间中的演化过程，并研发了增材制造模拟软件。采用增材制造模拟软件，预测了Al0.3CoCrFeNi高熵合金的增材制造工艺参数相图，指导了Al0.3CoCrFeNi高熵合金在增材制造过程中的工艺参数选择，并采用选区激光熔化技术制备了Al0.3CoCrFeNi高熵合金样品。单轴拉伸测试结果表明，其屈服强度为555 MPa，拉伸强度为711 MPa，均匀延伸率为15.6%，表现出良好的强塑性匹配。通过微结构表征和理论计算，揭示了其强化机理源于晶界强化、位错强化、固溶强化以及第二相强化。此外，退火处理引起的残余应力释放和局部再结晶进一步提高了材料强度。采用增材制造模拟软件，指导了增材制造的VCoNi中熵合金的工艺参数选择，并采用选区激光熔化技术制备了VCoNi中熵合金样品。单轴拉伸结果表明，其屈服强度为758 MPa，拉伸强度为1077 MPa，延伸率为37%，展现出优异的强韧协同。通过微结构表征与分析，揭示了其高强度与原子尺度的晶格畸变和短程有序结构、纳米尺度的高密度位错、纳米孪晶和析出相、以及微米尺度的晶界所组成的复杂多层级微结构有关，而其良好的塑性源于短程有序结构、相互缠结的高密度位错以及纳米孪晶与位错的相互作用，提升了材料的应变硬化能力。此外，高周疲劳测试结果表明，增材制造的VCoNi中熵合金的疲劳极限为650 MPa，表现出优异的抗疲劳性能。
The medium and high entropy alloys fabricated by additive manufacturing processes exhibit excellent mechanical properties (such as high strength and good ductility) and hence will have very promising applications in the industrial field. It is of great significance to study the additive manufacturing processes of medium and high entropy alloys and their mechanical behaviors/properties for future design and fabrication, which is one of the advanced research topics in the fields of solid mechanics and material science. In this dissertation, we systematically studied the additive manufacturing processes of medium and high entropy alloys, and microstructures, deformation mechanisms and mechanical properties of these alloys by combining the simulations, experiments and theoretical analyses.We investigated the effect of temperature gradient on grain growth using the two-dimensional cellular automaton method. The results indicated that the preferred growth direction of grains coincides with the temperature gradient. Furthermore, we developed a numerical method for additive manufacturing, which incorporates the heat conduction model with the cellular automaton method. Then, we used this method to predict the microstructures of In718 nickel-based superalloy, 316L stainless steel and CoCrFeNiMn high entropy alloy (HEA) prepared by additive manufacturing processes. The predictions from numerical simulations agree well with the corresponding experimental results. We also used this numerical method to simulate the evolution of three-dimensional microstructure during additive manufacturing. We further developed an additive manufacturing simulation software tool based on this numerical method, which guides our subsequent studies.We predicted an effective parameter window of Al0.3CoCrFeNi HEA by using the additive manufacturing simulation software we developed. We then fabricated Al0.3CoCrFeNi HEA by selective laser melting (SLM) according to the predicted parameters. The uniaxial tensile tests showed that Al0.3CoCrFeNi HEA exhibits a combination of high strength and good ductility, with a yield strength of 555 MPa, a tensile strength of 711 MPa and a uniform elongation of 15.6%. A combination of microstructure characterizations and theoretical analyses revealed that high yield strength is attributed to multiple strengthening mechanisms, including grain boundary strengthening, dislocation strengthening, solid solution strengthening and dispersion strengthening. Furthermore, an annealing treatment can further improve the yield strength of Al0.3CoCrFeNi HEA due to relaxation of residual stress and grain refinement induced by the local recrystallization.Guided by the results from additive manufacturing simulations, we fabricated VCoNi medium entropy alloy (MEA) via SLM. Uniaxial tensile tests showed that VCoNi MEA exhibits an exceptional strength-ductility synergy, with a yield strength of 758 MPa, a tensile strength of 1077 MPa and an elongation of 37%. Microstructure characterizations of VCoNi MEA indicated that high strength is related to the hierarchically heterogeneous microstructures consisting of atomic-scale severe lattice distortion and short-range order structures, nanoscale dislocations, nanotwins and precipitations, and microscale grain boundaries, while high elongation is attributed to the steady and progressive work-hardening mechanisms regulated by the short-range order structure, intertwined dislocations and nanotwins. Moreover, high-cycle fatigue tests showed that VCoNi MEA exhibits a high fatigue limit of 650 MPa.
