随着航天技术的快速发展，对性能更好的高温结构材料的需求越来越大。目前，镍基高温合金应用于工作温度可高达700℃的场合。然而，由于其熔点限制，镍基高温合金难以在更高的温度下工作。因此，寻找熔点更高、高温性能更好的材料具有重要意义。基于多主元高熵合金设计理念发展而来的难熔高熵合金材料由于其突破了传统合金设计理念中单一主元合金设计的框架，使在接近无限的相空间内进行合金设计具有了可能。因而，难熔高熵合金在航空航天工业等领域具有巨大的应用潜力。然而，绝大多数难熔高熵合金存在室温脆性的问题，这极大限制了其工业应用的能力。为提高难熔高熵合金的室温塑性，对其变形机制进行研究具有重要意义。本论文设计制备了一系列难熔高熵合金材料，并对其室温、高温力学性能和变形机制进行了系统的研究工作，取得了以下主要创新成果：

(1) 设计制备了高密度共格纳米强化难熔高熵合金材料(W₃₀Ta₅FeNi)，对其室温和高温准静态力学性能和微观组织结构进行了研究分析。W₃₀Ta₅FeNi在室温条件下准静态压缩强度可达2500 MPa以上，压缩应变约为30%。W₃₀Ta₅FeNi基体结构为面心立方(FCC)、体心立方(BCC)和μ相三相结构，在时效退火处理后，FCC基相中出现高密度弥散分布的共格纳米沉淀(D0₂₂)。多相结构协调强化机制使材料在室温压缩条件下达到超高强度。其中共格纳米沉淀强化机制为主要强化机制，时效强化处理后材料强度提高一倍。在高温准静态压缩实验中，材料在600 ℃、800 ℃和1000 ℃强度分别可达1500 MPa、1000 MPa和300 MPa。在高温变形过程中，纳米沉淀相发生了长大，沉淀强化机制从位错切过机制向绕过机制转变，这是材料高温强度下降的主要原因。经过和镍基高温合金力学性能对比，W₃₀Ta₅FeNi具有明显的高温强度优势。

(2) 为了进一步提高课题组前期发明的自锐钨高熵合金，本文采用μ相析出设计策略，对μ相的形貌和分布进行了调整，显著改善了钨高熵合金的拉伸性能。通过控制相变过程，使μ相由液固相变转变为固固析出相变，有效解决了μ相在晶界和相界偏析导致的脆性问题。此外，纳米μ相颗粒引起的Orowan效应有效地提高了拉伸强度(提高了~150%)，同时保证了拉伸塑性。该材料设计策略显著提高了合金的拉伸塑性，为解决类似的材料脆性问题提供了新的范式。

(3) 针对难熔高熵合金晶格畸变问题，使用基于同步辐射的X射线衍射(SR-XRD)、高分辨透射电子显微镜(HRTEM)和扩展X射线吸收精细结构谱(EXAFS)，对TiZrHfNbTa难熔高熵合金准静态拉伸实验前后的局部晶格畸变进行了详细分析。结果表明，在制备的TiZrHfNbTa难熔高熵合金中，锆原子中心存在明显的局部晶格畸变，铌原子中心存在中度畸变。拉伸试验后，这两种变形都更加明显。

因此，我们提出，因为其更高的局部晶格畸变，原子半径越大的原子在固溶强化中发挥更重要的作用。EXAFS拟合结果表明，在相同形变条件下，原子半径越大的锆和铪原子离中心原子的距离越远。在拉伸试验过程中，变大的晶格畸变可能导致材料塑性降低。

(4) 通过难熔合金元素设计，本文制备了一种新型(CoCrNi)_94.5W₃Ta_2.5合金材料。通过多个热力学过程，不仅能按照预期引入高密度均匀分布的γ’’ 纳米沉淀相，还能适当控制金属间化合物（即具有D0₂₄超晶格的η纳米沉淀相）的大小和分布，同时获得超细晶结构。因此，在室温和低温下都成功地实现了优越的强度-塑性协同性能。该合金材料不仅在室温下具有~1.5 GPa的高屈服强度和~23.8%的断裂延伸率，而且在低温下具有~2.0 GPa的超高屈服强度和~13.2%的断裂延伸率。通过微观组织结构分析发现，热处理过程中η和γ’’ 相相继出现。此外，两种类型的纳米沉淀相都发生位错切过强化作用。通过计算，γ’’ 纳米沉淀相在提高屈服强度方面发挥了最主要的作用，在不损失材料塑性的同时显著提高了强度。

With the rapid development of space technology, the demand for high temperature structural materials with better performance is increasing. At present, nickel-based superalloys are used at operating temperatures up to 700℃. However, due to its melting point, nickel-based superalloys cannot operate at higher temperatures. Therefore, it is of great significance to search for materials with higher melting point and better high-temperature performance. The refractory high entropy alloy material developed based on the design concept of multi-principal element high entropy alloy breaks through the framework of single principal element alloy design in the traditional design concept of alloy. It is possible to design alloys in nearly infinite phase space. Therefore, refractory high entropy alloys have great application potential in aerospace industry and other fields. However, most refractory high entropy alloys have the problem of room temperature brittleness, which greatly limits their ability for industrial applications. In order to improve the room temperature plasticity of refractory high entropy alloys, it is important to study the deformation mechanism. In this paper, a series of refractory high entropy alloy materials are designed and prepared. The mechanical properties and deformation mechanism at room temperature and high temperature were studied systematically. The following main innovation achievements have been achieved:

(1) High density common-reinforced refractory high entropy alloy (W30Ta5FeNi) was designed and fabricated. The quasi-static mechanical properties and microstructure of W30TA5Feni at room and high temperature were studied. The quasi-static compressive strength of W30Ta5FeNi can reach more than 2500 MPa at room temperature, and the compressive strain is about 30%. The matrix structure of W30Ta5FeNi is face-centered cubic (FCC), body-centered cubic (BCC) and μ-phase. After aging annealing treatment, high density dispersive copolymer nanocrystalline precipitates (D0₂₂) appeared in the FCC base phase. The polyphase structure coordination strengthening mechanism enables the material to achieve ultra-high strength under room temperature compression. The copolymer nanocrystalline precipitation strengthening mechanism is the main strengthening mechanism. After aging strengthening treatment, the strength of the material was doubled. In the high temperature quasi-static compression experiment, the strength of the material can reach 1500MPa, 1000MPa and 300MPa at 600℃, 800℃ and 100℃ respectively. In the process of high temperature deformation, the nano-precipitated phase grows. The precipitation strengthening mechanism changes from dislocation cutting mechanism to bypass mechanism, which is the main reason for the decrease of high temperature strength of materials. The mechanical properties of W30Ta5FeNi are compared with those of nickel-based superalloys.

(2) Here, by μ phase precipitation design strategy, the morphology and distribution of μ phase were tailored, which significantly improved the tension properties of the tungsten high-entropy alloy. Through controlling the phase transformation process, the μ phase changes from liquid-solid phase transformation to solid-solid precipitation phase transformation, which can effectively solve the problem of the brittleness caused by the μ phase segregation at the grain boundary and phase boundary. Moreover, the Orowan effect caused by nano-sized μ-phase particles improves the tensile strength effectively (enhancing ~150%) and ensure the ductility. This material design strategy significantly improves the tension ductility of the alloy and provides a new paradigm to solve the similar problem of material brittleness.

(3) The local lattice distortion of TiZrHfNbTa refractory high entropy alloy before and after quasi-static tensile test was analyzed by synchrotron radiation-based X-ray diffraction (SR-XRD), high resolution transmission electron microscopy (HRTEM) and extended X-ray absorption fine structure spectroscopy (EXAFS). The results show that the zirconium atomic center has obvious local lattice distortion and niobium atomic center has moderate distortion in the prepared TiZrHfNbTa refractory high entropy alloy. After tensile test, both kinds of deformation are more obvious. That is, atoms with larger atomic radius play a more important role in the hardening of the solid solution because of the higher degree of local lattice distortion. The EXAFS fitting results show that the zirconium and hafnium atoms with larger atomic radius are farther away from the central atom under the same deformation condition. During the tensile test, the larger lattice distortion may lead to the decrease of the ductility of the material.

(4) A new (CoCrNi)_94.5W₃Ta_2.5 alloy has been prepared by alloying refractory alloy elements. By conducting multiple thermomechanical processes, not only quantities of γ’’  nanoprecipitates are introduced as expected, but also the sizes and distributions of intermetallic compounds (i.e., η nanoprecipitates with a D0₂₄ superlattice) are proper controlled, meanwhile the ultrafine grains are achieved. Consequently, superior strength-ductility synergy is successfully achieved at both room and cryogenic temperatures. This MEA exhibits not only high yield strength of ~1.5 GPa and ultimate elongation of ~23.8% at room temperature, but also ultra-high yield strength of ~2.0 GPa and still sufficient ultimate elongation of ~13.2% at cryogenic temperature. In-depth microstructure characterization reveals that the η and γ’’  nanoprecipitates appear successively during heat treatments. Moreover, both two types of nanoprecipitates interact with dislocations and deformation twins by particle shearing mechanism. By calculation, the γ’’  nanoprecipitates play the most significant roles in enhancing yield strength and show extraordinary combined effects in the strength increase and elongation retention.