近些年，中高熵合金因其组元之间接近无限互溶的组份范围，原子尺度的短程有序结构及优异的力学性能备受关注。其中，具有单相面心立方结构的CrCoNi中熵合金，力学性能和低温性能都很突出，是目前最具代表性的低层错能单相多组元合金。传统的奥氏体不锈钢同样具有单相面心立方结构，力学性能也很优异，典型的316L奥氏体不锈钢还可看作是高浓度的Fe-Cr-Ni体系。然而中高熵合金与传统不锈钢之间的性能对比，尤其是在高应变率和低温下应变硬化机制的差异却鲜有报道。而且目前报道的CrCoNi中熵合金强度水平依然很低，在高强度下提高断裂韧性仍是面临的一大难题。基于以上研究背景，本文比较分析了低强度下 (屈服强度，σ_y<500 MPa) CrCoNi中熵合金与奥氏体不锈钢在不同温度下的力学性能和变形机理；然后，研究了两种CrCoNi基中熵合金在高强度 (σ_y>1.0 GPa) 下的断裂韧性，旨在提高材料的强韧性匹配。

在CrCoNi中熵合金和奥氏体不锈钢 (316L和316LN) 的力学行为和微观机理的对比研究中，在373-4.2 K的温度范围进行了准静态拉伸和动态冲击韧性测试，在298 K和77 K进行了断裂韧性测试。研究表明：1) 拉伸测试中，在298 K三种合金具有相近的强度与塑性，但随温度降低，316L和316LN不锈钢的均匀延伸率逐渐降低，强度和塑性出现此消彼长，而CrCoNi中熵合金的强度和塑性单调增加，低温下具有更优异的强塑性匹配；2) 冲击测试中，CrCoNi合金在不同测试温度下均具有最优异的强度-冲击韧性的匹配；3) 断裂测试中，在298 K和77 K，316L和316LN不锈钢均表现出优于CoCrNi合金的强韧性匹配。但不同于CoCrNi合金随温度降低断裂韧性不断增加的趋势，316L和316LN的断裂韧性逐渐降低。因此，在极低温 (4.2 K) 环境和高应变速率载荷下，CrCoNi合金的力学性能更加优异。对三种合金的硬化机理研究表明：随温度降低，CrCoNi合金通过高密度的变形孪晶提供硬化，而316L和316LN不锈钢发生了不同程度的马氏体相变。结合微结构表征，详细讨论了三种合金在不同温度和加载条件的微观变形机制。

在中熵合金CrCoNi-1.75 at.%N的高强韧性研究中，通过冷轧和不完全再结晶退火获得了两种不同强度下的异质结构。对于部分再结晶的层片异构，屈服强度 σ_y~ 1.3 GPa，断裂韧性K_IC约为93 MPa∙m^1/2；而对于完全再结晶的三级晶粒尺寸异质结构 (晶粒尺寸从纳米跨越至微米量级)，σ_y~ 1.0 GPa， K_IC约为168 MPa∙m^1/2。结合相应的微结构，发现断裂样品裂纹尖端的软区界面处存在大量几何必需位错，这提供了额外的异质变形诱导硬化机制。而在裂纹附近还发生了显著的晶粒细化，这也一定程度上增强了裂尖硬化水平。另外，在结构中，变形前后均存在亚纳米尺度的化学短程有序结构。相应的几何相位分析表明这些短程有序结构增加了基体晶格畸变程度，在变形中还可以阻碍位错运动，有效提高了材料硬化水平。此外，该研究中还通过裂尖的显微硬度数据定义了一个特征参数J_Hv，并发现J_Hv和裂纹萌生时的J积分值 (J_IC) 存在线性相关性，创新性地建立了两者的定量关系。

以CrCoNi-9.5 at.%Al中熵合金为对象的高强韧性研究中，通过冷轧和不完全再结晶退火，获得了一种含有少量B2相的复合层片异质结构(HS₂)。在该结构中，由再结晶晶粒和拉长的变形晶粒组成晶粒间的层片结构；在变形晶粒内，又由高密度相同取向的小角晶界、孪晶界以及沿孪晶界分布的再结晶晶粒组成晶粒内的层片结构。两种层片延伸方向相互垂直，使得沿轧制方向 (RD) 加载的HS₂-RD品和沿截面方向 (TD) 加载的HS₂-TD样品获得了相近的强度，塑性和断裂韧性。结合相应的微结构表征，发现该结构的层片异质界面附近具有高密度的几何必需位错，这提供了显著的异质变形诱导硬化，变形晶粒内的层片结构也显著改善了硬相中的塑性变形能力。另外，在该合金中还发现了化学短程有序和中程有序的存在，这些结构促进了位错的平面滑移，增强了裂纹尖端的加工硬化水平。裂纹扩展阶段，晶粒间的层片对HS₂-RD样品的裂纹扩展影响显著，而晶粒内的层片结构也可以阻碍HS₂-TD样品的裂纹扩展。因此，这种复合层片结构改善了TD方向加载的力学性能，显著降低了各相异性的影响。该研究详细地阐述了裂纹尖端的硬化机制以及复合层片异构中沿不同方向加载的裂纹演化规律。

Recently, high/medium entropy alloys (H/MEAs) have attracted much attention due to their nearly infinite component range, short range ordered structure at atomic scale and excellent mechanical properties. Among them, CrCoNi MEA with single-phase face-centered cubic structure (FCC) have more outstanding mechanical properties and low temperature performance, making it the most representative low SFE single-phase multi-component alloy. The traditional austenitic stainless steels have a single-phase FCC structure and excellent mechanical properties, as well. The typical 316L ss can also be regarded as high concentrated Fe-Cr-Ni system. While few reports have been reported on the comparation of the mechanical properties, especially microstructure differences between H/MEAs and conventional alloys at high strain rates and low temperatures. In addition, the strength level of CrCoNi MEA reported so far is still very low, and improving the fracture toughness at high strength is still a major problem. Based on the above background, the mechanical properties and deformation mechanism of CrCoNi and 316L at different temperatures were compared at lower strength levels (yield strength, ). The fracture toughness of two CrCoNi-based MEAs at high strength ( > 1.0 GPa) was further studied, in order to improve the strength and toughness synergy.

In the study of comparative analysis of mechanical behavior and microscopic mechanism of CrCoNi and austenitic stainless steels (316L and 316LN), quasi-static tensile tests, Charpy-impact tests were carried out at the temperature range from 373 K to 4.2 K, and fracture toughness tests were carried out at 298 K and 77 K, respectively. The results show, 1) In tensile tests, all three alloys exhibit similarly strength and plasticity at 298 K, but with the decrease of temperature, CrCoNi increases its tensile uniform elongation while uniform elongation of 316L and 316LN ss increases firstly and decreases soon afterwards; 2) In impact tests, CrCoNi MEA shows a higher impact absorbed energy than 316L and 316LN at different temperature range; 3) In fracture toughness tests, 316L and 316LN have a higher fracture toughness than CrCoNi. However, the fracture toughness of 316L and 316LN ss decreases monotonously, which is different from that of CrCoNi. Therefore, CrCoNi MEA has best mechanical properties only at very low temperature (4.2 K) and high strain rate loading. To explain the differences in macroscopic mechanical properties of the three alloys,microstructural hardening mechanisms were surveyed. CrCoNi MEA relies on abundant mechanical twinning, while 316L and 316LN undergo martensitic phase transition in different degrees at low temperatures. In this study, the microstructure of the three alloys under different temperatures and loading conditions were analyzed in detail through the microstructure characterization of samples after tensile, impact and fracture toughness tests.

In the study of high strength and fracture toughness for heterogeneous CrCoNi-1.75 at.%N MEA, two kinds of heterogeneous structures with different strength were obtained by cold rolling and incomplete recrystallization annealing. For heterogeneous lamella structure (HS₁) with partial recrystallization, the fracture toughness (K_IC) is 93 MPa∙m^1/2 at a yield strength (σ_y) of nearly 1.3 GPa; For completely recrystallized three-level heterogeneous grain structure (HS₂) with grain sizes spanning from the nanometer to micrometer, is 168 MPa∙m^1/2 at a yield strength of 1.0 GPa. With microstructure characterization, it was found that geometrically necessary dislocations (GNDs) of high density piles-up near soft domain boundaries, which provides an additional mechanism for heterogenous deformation induced hardening. Significant grain refinement also happens near the crack, which enhances the hardening degree of crack tip. In addition, the sub-nanometer scale chemical short-range orders were also found before and after deformation. The corresponding analysis shows that these short-range orders can cause severe lattice distortion and hinder the dislocation movement during the deformation, providing strain hardening effectively. Besides, a characteristic parameter was defined based on the microhardness data of crack tip, and there is a linear correlation between J_Hv and the J-integral value (J_IC) at crack initiation, which establishes a quantitative relationship between them innovatively.

In the study of high strength and fracture toughness for heterogeneous CrCoNi-9.5 at.% Al MEA, a complex lamellar heterostructure (HS₂) containing a few B2 phase was obtained by cold rolling and incomplete recrystallization annealing. In this structure, the lamellae among grains are composed of recrystallized grains and elongated deformed grains, while in the deformed grains, high-density low angle grain or twin boundaries with the same orientation, and recrystallized grains along the twin boundaries consist of the lamellae within the grains.These two lamellas extend orthogonality which makes samples loaded along the rolling direction (RD) (HS₂-RD) and samples loaded along transverse direction (TD) (HS₂-TD) obtain similar strength, plasticity, and fracture toughness, and both have excellent matching of strength andtoughness. Combined with the corresponding microstructure characterization, it was found that plenty GNDs near the lamellar heterogeneous interface in the structure provides significant hetero-deformation induced (HDI) hardening, and the lamellar structure in the deformed grains also significantly improves the plastic deformation ability in the hard region. In addition, the chemical medium/short-range orders in FCC matrix promote the plane slip of dislocations and increase the work hardening near crack tip. In the crack growth stage, the lamellae among grains have a significant effect on the crack growth of HS₂-RD samples, while the lamellae in deformed grains also effectively hinders the crack growth of HS₂-TD sample. Therefore, this complex lamellar heterostructure improves the mechanical properties of TD direction loading and significantly reduces the effect of anistropy. In this work, the hardening mechanism of crack tip and the crack evolution of samples loaded along different directions in the complex lamellar structure were researched in detail.