无钴双相型高熵合金的结构调控与强韧化机理研究

Generally speaking, high entropy alloys show high compressive strength, high hardness and resistance to tempering softening, and great wear resistant, leading to potential application in high performance structures performed under complicated stress conditions. However, most of the high entropy alloys show undesirable brittleness and low tensile strength. Research on the strengthening and toughening mechanisms of dual phase high entropy alloys helps to guide the production of high entropy alloys with high strength and toughness. Most of the reported high entropy alloys contain Co. Designing and developing Co free high entropy alloys helps to save Co resources, which meet the requirements of sustainable development of resources. This project intends to start from the optimization of the empirical prediction method for predicting microstructure of high entropy alloys by calculating the thermodynamic parameters, the eutectic point, the Creq and Nieq of the multi component alloys. Our aims are to make the prediction methods more accurate and realize the accurate control of the microstructure of high entropy alloys. Take this as the basis, a series of bi-phase high entropy alloys within Al-Cu-Mn-Ni-Cr-Fe system are expected. By proper heat treating, the solid solution type high entropy alloys are induced to precipitate the second phase particles. The microstructures of this kind of high entropy alloys are then controlled by changing the heat treatment process and parameters. Both macro and nano mechanical properties of the bi-phase high entropy alloys with different phase ratio will be tested and analyzed in detail so as to clarify the relationship between the macro and nano mechanical properties of this kind of alloys. By analyzing the micro damage mode of the constituent phase separately using the nano indentation technique, we'll finally reveal the strengthening and toughening mechanisms of bi-phase high entropy alloys.

高熵合金具有高强度、高硬度、耐回火软化、耐磨等特性，有望应用于复杂受力条件下的高性能结构件。但大部分高熵合金力学性能的局限性在于其脆性较大，抗拉强度偏低。研究双相高熵合金的强韧化机理有利于指导生产高强韧性高熵合金。现有高熵合金体系多为含钴体系，设计开发无钴型高熵合金可节约钴资源，符合资源可持续发展的要求。本项目以Al-Cu-Mn-Ni-Cr-Fe合金系为主要研究对象，拟通过热力学参数、多元合金共晶点和Creq、Nieq计算，对现有高熵合金结构预测方法进行优化，实现无钴双相型高熵合金凝固组织的准确设计和调控。通过适当的热处理工艺，诱发固溶体型高熵合金析出硬质第二相颗粒，通过控制热处理工艺方法和参数对高熵合金组织进行调控。利用纳米压痕技术，对双相高熵合金的各组成相在加载过程中的微损伤模式进行研究，将各组成相的纳米力学性能与宏观力学性能进行关联，揭示双相型高熵合金的强化、韧化机理。

双相高熵合金可兼顾软、硬单相高熵合金的优势，获得相较于单相高熵合金更优的综合力学性能。本项目系统研究了双相高熵合金的成分设计、组织调控及其力学性能的特点。结果表明：在高熵合金凝固组织设计时，可以在保证多元合金系无主元的前提下，保持合金系δ≤5.5约束条件，增加合金系的Ni含量同时降低Al含量可增加合金中FCC相的相对量，直至获得单FCC相结构的高熵合金；增加Cr当量，或降低Cu含量同时增加Al含量，可增加合金中BCC相的相对量，直至获得单BCC相结构的高熵合金。通过热处理或塑性变形+热处理的方式，可以调整高熵合金的结构从而改善高熵合金的强韧性。热处理温度对高熵合金组织的影响程度比保温时间的影响程度更大。从强化角度来看，将纯固溶体（BCC或FCC）结构高熵合金组织转变为固溶体+弥散分布的硬质颗粒组织后，强化效果显著。通过热处理工艺等促使高熵合金中的FCC相中析出BCC相或其它硬质相时，不能必然带来合金强硬度的提升，只有控制工艺条件使析出的新BCC相数量足够，且呈分散、均匀分布时才具有明显改善效果。改变工艺条件，促使高熵合金中的BCC相中析出软相FCC相时，不必然降低合金的强硬度。如果新析出的FCC相数量适当，且为交叉短棒形式分布时，则可在改善BCC型高熵合金塑性的同时，提升合金强硬度，实现强韧化。双相高熵合金在外加载荷作用下发生变形至断裂的过程中，两相的变形不同步。部分FCC型高熵合金在常规热处理过程中不易诱发相结构转变，即FCC相不析出BCC相或其它硬质相。对于此类高熵合金或以FCC固溶体相为主的双相高熵合金，采用适当变形量的塑性变形+再结晶退火处理可有效细化合金组织，提高合金强韧性。上述研究成果对于双相高熵合金的成分设计及强韧化方向提供了一定的指导，对推动高熵合金尽早进入实际应用领域有重要意义。