日益严苛的服役环境对金属丝材的强韧性提出了愈来愈高的要求，尤其是在低温等极端条件下的服役安全。然而，传统金属丝材却不可避免地受到强度与塑性倒置关系的限制。幸运的是，近几年兴起的中高熵合金，为我们提供了解决传统丝材中棘手问题的新思路。本论文通过设计优化丝材制备工艺研制了高强韧CoCrNi中熵合金丝材和超高强AlCoCrFeNi_2.1共晶高熵合金丝材，系统研究了每种丝材的微观结构特征、力学性能和变形机理等，取得的主要创新成果如下：

（1）采用玻璃包覆法首次成功制备出尺寸在微米量级的CoCrNi中熵合金丝材；采用拉拔法并结合热锻和热轧工艺，成功制备出直径2 mm的CoCrNi中熵合金丝材；采用拉拔法并结合浇注和热轧工艺，成功制备出直径0.5 mm的AlCoCrFeNi_2.1共晶高熵合金丝材；

（2）CoCrNi中熵合金微米丝展现出优异的强度塑性组合。细致的微结构表征表明，其优异的力学性能主要源于层错、Lomer-Cottrell位错锁和纳米变形孪晶多种变形机制的协同作用。出乎意料的是，这些微米丝在拉伸时表现出了反常的尺寸效应，即直径40 μm的丝材展现出了更高的强度和塑性，这与传统单主元合金微米丝材拉伸时可忽略的尺寸效应形成了鲜明的对比。分析发现，直径40 μm的丝材变形更不均匀且存在更高的几何必需位错密度，所导致的高应变梯度与多级变形孪晶共同促进了高强度和高塑性的结合。

（3）CoCrNi中熵合金毫米丝的初始结构由单一的FCC相组成，晶粒内存在高密度的位错和大量纳米孪晶。该毫米丝表现出优异的室温和低温力学性能，其屈服强度、断裂强度和延伸率在室温（293 K）下分别为1.1 GPa、1.3 GPa和24.5%，并在液氮温度（77 K）下分别被提升至1.5 GPa、1.8 GPa 和37.4%。微结构表征表明丝材在低温下的优异性能源于位错、高密度的变形孪晶和FCC-HCP相变多种变形机制的共同作用。

（4）通过精心设计多种机械热处理工艺，我们成功在AlCoCrFeNi_2.1共晶高熵合金毫米丝中引入了梯度异构片层结构，即较硬的B2相片层呈梯度地分布在较软的FCC相基体中。该共晶高熵毫米丝也在室温和低温下展现出出众的强度塑性组合水平，它不仅在293 K下具有1.85 GPa的高抗拉强度和12%的均匀伸长率，而且在77 K下甚至具有2.52 GPa的超高抗拉强度和14%的均匀伸长率。 深入的微观组织表征表明，在丝材的拉伸过程中，独特的梯度异构片层结构促进丝材内几何必需位错沿径向呈梯度分布，即几何必需位错密度从表面区域向中心区域逐渐减小，几何必需位错的存在可以诱导产生明显的应变梯度强化效应，从而有利于丝材力学性能的提升。有趣的是，在77 K下加载时，我们首次在共晶高熵合金毫米丝的B2相中观察到了稠密的交滑移，而且这些交滑移可以引起强烈的动态微结构细化效应。因此，稠密交滑移的激活在为材料提供充足塑性的同时产生了明显的动态Hall-Patch效应，成为丝材极佳低温力学性能背后最有效的变形机制之一。

Metallic wires with higher strength-ductility have been urgently required for the harsh service circumstance, especially for the service safety under extreme conditions such as cryogenic temperature. However, traditional metallic wires are tortured inevitably by strength-ductility trade-off dilemma. Fortunately, the recent emergence of high/medium entropy alloys broaden our horizons to solve many intractable problems existing in traditional metallic wires. In this dissertation, superior strength-ductility CoCrNi medium-entropy alloy (MEA) wires and ultra-strong AlCoCrFeNi_2.1 eutectic high-entropy alloy (EHEA) wires were successfully fabricated by designing and optimizing the preparation processes of wires. Furthermore, their microstructural features, mechanical properties and deformation mechanisms were systematically investigated, and the main innovative developments are as follows:

(1) Micron-sized CoCrNi MEA wires are successfully fabricated by glass-coated method for the first time; CoCrNi MEA wire with diameter of 2 millimeters are successfully fabricated by drawing method combined with hot forging and hot rolling; AlCoCrFeNi_2.1 EHEA wire with diameter of 0.5 millimeters are successfully fabricated by drawing method combined with pouring and hot rolling.

(2) The CoCrNi MEA microwires exhibit an excellent combination of tensile strength and ductility. In-depth microstructure characterization indicates the superior mechanical properties stem from the synergy of stacking faults, Lomer-Cottrell locks and deformation nano-twinning. Surprisingly, an anomalous size effect is presented in the tension of these microwires, i.e., the much higher tension strength and ductility are observed in the 40 micron-wire, in sharp contrast to conventional single-principal element alloys only showing negligibly minor tension size effect. It is found that, much higher density of geometrically necessary dislocations (GNDs) accompanying heterogeneous deformation exist in 40 micron-wire, leading to a high strain gradient, which is in turn joined with multiple deformation twins giving rise to high strength and ductility in 40 micron-wire.

(3) The CoCrNi MEA millimeter wire is initially composed of single FCC phase, and there are high density of dislocations and a large number of nano-twins in the grains. The CoCrNi MEA millimeter wire exhibits superior mechanical properties at both room and cryogenic temperatures, whose yield strength, ultimate tensile strength and elongation could reach 1.1 GPa, 1.3 GPa and 24.5% at ambient temperature (293 K), and are further enhanced to 1.5 GPa, 1.8 GPa and 37.4% at liquid-nitrogen temperature (77 K), respectively. In-depth microstructure characterization indicates the superior strength-ductility at cryogenic temperature derives from the synergy of dislocations, high-density deformation twins and clear FCC-HCP phase transition.

(4) A gradient heterogeneous lamella structure, characterized with hard gradient-distributed B2 lamellae embedded in soft FCC lamellae matrix, is successfully introduced into AlCoCrFeNi_2.1 EHEA millimeter wire by well-designed multiple-stage thermomechanical processes. The EHEA millimeter wire also achieves an outstanding strength-ductility synergy at both room and cryogenic temperatures, which exhibits not only high tensile strength of 1.85 GPa and sufficient uniform elongation of ~12% at 293 K, but also ultra-high tensile strength of 2.52 GPa and even slightly elevated uniform elongation of ~14% at 77 K. In-depth microstructure characterization indicates that the unique gradient heterogeneous lamella structure facilitates a radial gradient distribution of GND during tension, i.e., the GND density decreases gradually from the surface region to the central region of EHEA millimeter wire, which induces pronounced strain gradient strengthening effect and thus benefits the mechanical properties. Intriguingly, dense cross-slip which gives rise to intensively dynamic microstructure refinement is firstly observed in the B2 phase of EHEA millimeter wire at 77 K. Therefore, the activation of dense cross-slip provides sufficient ductility while inducing evidently dynamic Hall-Petch effect, becoming one of the most effective deformation mechanisms contributing to the unprecedented cryogenic tension properties.