面心立方 (FCC) 多主元合金，凭借优异的拉伸塑性和断裂韧性受到广泛关注，但与典型工程合金材料，如奥氏体不锈钢相比，其屈服强度不足，以及强化后韧塑性偏低，乃是该类合金材料主要的力学性能短板。对此，如何有效提高强化后合金的加工硬化率，是需要考虑的首要科学问题。近来，异构构筑及其在后续变形过程中展现出的异构变形诱导 (HDI) 力学效应，为解决上述材料强韧化问题提供了新的力学思路。HDI效应很大程度上取决于异构界面设计构筑和优化调控。异构界面实为一种力学上的约束性协调界面，其在材料变形和断裂过程中主要功用：一是促进及诱发应变协调和HDI硬化；二是引入及强化裂纹偏折、分层、分叉和钝化等外禀韧化机制从而实现止裂增韧。
基于此，为进一步证实异构可能诱发的额外增塑、韧化效应，本研究有针对性地，分别研制出单相 (CrCoNi)99.8P0.02和沉淀硬化型Al0.5FeCoCrNi1.5等两种FCC基多主元合金 (MPEAs)，并在其中可控构筑出层状、P元素偏聚等两种异构界面，通过显微硬度、准静态拉伸、加卸载、断裂韧性等跨尺度力学测试，结合OM、SEM、EBSD、TEM、HRTEM和3DAPT等多层级微结构表征，探索两种异构FCC基MPEAs的优异力学性能，尝试建立相应结构-性能关系。主要结论如下：
(1) 设计构筑了一种晶界偏聚异构(CrCoNi)99.8P0.02 MPEA，揭示其晶粒长大规律和Hall-Petch关系。研究表明，经铸造及不同温度固溶退火后，该合金均具有完全FCC结构。P元素合金化后，分别固溶于晶粒内部，及偏聚于晶界处。对于1273 K退火样品，晶内固溶和晶界偏聚浓度，分别达约0.09 at.%和0.86 at.%。与传统CrCoNi MPEAs相比，晶内固溶P元素降低了合金的晶粒长大指数，增加了晶格畸变程度，将晶格摩擦力由参比合金的218 MPa 提升至该合金的242 MPa；晶界偏聚P元素提高了晶界热稳定性，将晶界迁移激活能由251 kJ/mol提升至339 kJ/mol；同时，P元素添加还显著提高了晶界强化贡献量，将Hall-Petch系数由265 MPaμm1/2提升至460 MPaμm1/2。在低温 (77 K) 下，该合金的晶格摩擦力和Hall-Petch系数得到了进一步提高，分别达507 MPa和636 MPaμm1/2，促进了在塑性应变早期出现了由变形层错取代典型位错平面滑移的特征。
(2) 揭示晶界偏聚异构(CrCoNi)99.8P0.02 MPEA的超大塑性和层错增塑机制。研究发现，与经典CrCoNi MPEAs相比，(CrCoNi)99.8P0.02 MPEA具有更低的的晶格常数和层错能，以及更高的屈服强度和均匀伸长率。在相同屈服强度水平下，其室温拉伸均匀塑性高达90%，远优于CrCoNi、以及其他元素掺杂的CrCoNi-(C, N) MPEAs。晶界偏聚异构MPEA屈服强度的增强，可以归因于晶格畸变加剧导致的置换固溶强化效应增强，以及P元素在晶界偏聚所产生的晶界强化效应。在拉伸变形过程中，由于较低的平均层错能甚至更低的原子尺度局部层错能，导致发达的变形层错墙分割网络形成、以及丰富变形纳米层错充斥分割网格之间，形成了边界间距约数至数十纳米的平行六面体畴，使初始微结构发生显著动态分割及细化。这种层错诱导的微结构动态分割和细化，一方面阻碍位错滑移，提高流变应力，另一方面在层错界和畴内大量缠结和积累位错，导致HDI应力和加工硬化的显著提高，从而实现了高强度-超大塑性匹配。
(3) 设计构筑一种层状异构Al0.5FeCoCrNi1.5 MPEA，揭示层状异构形成和演化机制，阐明γ′-Ni3Al和B2-NiAl双析出沉淀硬化效应及其时效动力学。研究表明，层状异构由严重拉长粗晶与部分再结晶超细晶区域交替排列构成，其内部含有大量晶界B2-NiAl沉淀相、以及晶内γ′-Ni3Al纳米共格沉淀相。其中，B2-NiAl晶界相的Zener界面钉扎效应，是提高层状异构界面热稳定性，以及层状异构形成的主控微观机制；而晶内密实的纳米尺度γ′-Ni3Al共格L12有序相是该合金的主要强化相，经773-973 K时效后，该层状异构MPEAs实现沉淀硬化增量可达1.0 GPa；相应地，在过时效阶段，γ′-Ni3Al相回溶/分解和B2-NiAl相粗化是其主要的软化机制。此外，层状异构的在拉伸变形过程中，展示出显著的HDI强化及加工硬化，实现了屈服强度与均匀伸长率的协同提高。
(4) 探明层状异构Al0.5FeCoCrNi1.5 MPEA的优异断裂韧性，揭示层状异构抑制裂纹萌生及扩展的丰富韧化行为，并提出异构界面止裂机制。研究表明，该层状异构MPEA的断裂韧性则来自于内禀和外禀增韧两方面。其中，林位错硬化和HDI硬化共同促使层状异构裂纹尖端具有大的塑性区尺寸和加工硬化能力，在裂纹萌生能量消耗，使其主要的内禀韧化机制；当裂纹在层状异构内扩展时，由于层状异构界面两侧具有显著力学行为响应差异、以及不同应力状态和时效结构，从而诱导扩展裂纹发生偏折、分层、分叉和钝化等丰富止裂行为，在高强度下实现充分能量耗散和增韧。此外，在层状异构MPEA中建立了裂纹尖端塑性区硬化水平与断裂韧性之间的正比关系方程，为异构材料断裂韧性和裂尖微区能量耗散等的高效定量评价提供依据及参考。

Face-centered cubic (FCC) multi-principal element alloys (MPEAs) have attracted widespread attention due to their excellent tensile ductility and fracture toughness. However, compared to typical engineering alloys like austenitic stainless steel, these alloys often suffer from insufficient yield strength and reduced ductility after strengthening, which are major mechanical shortcomings. Effectively increasing the work hardening rate of strengthened alloys is a primary scientific challenge. Recently, heterogeneous construction and the heterogeneous deformation-induced (HDI) mechanical effects observed during subsequent deformation processes have provided new mechanical insights for addressing the strengthening and toughening of these materials. The HDI effect largely depends on the design and optimization of heterogeneous interfaces. These interfaces act as mechanically constrained coordination interfaces, with primary functions during material deformation and fracture: firstly, to promote and induce strain coordination and HDI hardening; secondly, to introduce and enhance extrinsic toughening mechanisms such as crack deflection, delamination, branching, and blunting, thereby achieving crack arrest and toughness enhancement.
To further confirm the additional plasticity and toughening effects that heterogeneity may induce, this study specifically develops two types of FCC-based MPEAs: the single-phase (CrCoNi)99.8P0.02 and the precipitation-hardened Al0.5FeCoCrNi1.5. These alloys incorporate two types of controlled heterogeneous interfaces: lamellar and phosphorus (P)-enriched. Through mechanical tests spanning different scales, including microhardness, quasi-static tensile, loading-unloading-reloading, and fracture toughness, complemented by multi-scale microstructural characterization using optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and three-dimensional atom probe tomography (3DAPT), the study explores the superior mechanical properties of these heterogeneous FCC-based MPEAs and attempts to establish corresponding structure-property relationships. The main conclusions are as follows:
(1) Heterogeneous grain boundary segregation in (CrCoNi)99.8P0.02 MPEA: This study reveals the grain growth behavior and Hall-Petch relationship. After casting and solid-solution annealing at various temperatures, the alloy maintains a complete FCC structure. Upon P alloying, P is in solid solution within the grains and segregates at the grain boundaries. In samples annealed at 1273 K, the concentrations of P in the grains and at the grain boundaries are approximately 0.09 at.% and 0.86 at.%, respectively. Compared to traditional CrCoNi MPEAs, the solid solution of P reduces the grain growth exponent and increases lattice distortion, enhancing lattice friction stress from 218 MPa to 242 MPa. The segregation of P at grain boundaries improves their thermal stability, increasing grain boundary migration activation energy from 251 kJ/mol to 339 kJ/mol. Additionally, P significantly enhances grain boundary strengthening, raising the Hall-Petch coefficient from 265 MPa·μm1/2 to 460 MPa·μm1/2. At low temperatures (77 K), the lattice friction stress and Hall-Petch coefficient further increase to 507 MPa and 636 MPa·μm1/2, promoting early deformation stacking faults instead of typical dislocation glide planes during plastic strain.
(2) Exceptional ductility and stacking fault-induced plasticity in (CrCoNi)99.8P0.02 MPEA: The study finds that this MPEA has a lower lattice constant and stacking fault energy, along with higher yield strength and uniform elongation compared to conventional CrCoNi MPEAs. At the same yield strength level, its room temperature tensile uniform plasticity reaches up to 90%, significantly superior to CrCoNi and other element-doped CrCoNi-(C, N) MPEAs. The enhanced yield strength is attributed to increased substitutional solid solution strengthening from intensified lattice distortion and grain boundary strengthening from P segregation. During tensile deformation, lower average and atomic-scale stacking fault energies lead to a well-developed deformation stacking fault wall network and abundant deformed nano-scale stacking faults, forming parallelepiped domains with boundary spacing of several to tens of nanometers. This dynamic segmentation and refinement of the initial microstructure hinder dislocation slip, increasing flow stress, while extensive dislocation tangling and accumulation at stacking fault boundaries and within domains significantly increase HDI stress and work hardening, achieving high strength-super plasticity matching.
(3) Heterogeneous lamellar structure in Al0.5FeCoCrNi1.5 MPEA: This study reveals the formation and evolution mechanisms of the heterogeneous lamellar structure and elucidates the precipitation hardening effects of γ′-Ni3Al and B2-NiAl, along with their aging kinetics. The heterogeneous lamellar structure consists of severely elongated coarse grains alternately arranged with partially recrystallized ultrafine grains, containing numerous B2-NiAl precipitates at grain boundaries and γ′-Ni3Al nanoscale coherent precipitates within grains. The Zener pinning effect of the B2-NiAl boundary phase is the main mechanism controlling thermal stability and lamellar formation. The dense nanoscale γ′-Ni3Al coherent L12 ordered phase is the primary strengthening phase. After aging at 773-973 K, the precipitation hardening increment reaches up to 1.0 GPa. During over-aging, the dissolution or decomposition of γ′-Ni3Al and coarsening of B2-NiAl are the main softening mechanisms. Additionally, the heterogeneous lamellar structure demonstrates significant HDI strengthening and work hardening during tensile deformation, achieving a synergistic increase in yield strength and uniform elongation.
(4) Fracture toughness mechanism in heterogeneous lamellar structure Al0.5FeCoCrNi1.5 MPEA: The mechanism for the excellent fracture toughness is elucidated, which also reveals the rich toughening behaviors that inhibit crack initiation and propagation. These toughening behaviors provide a crack-arrest mechanism at heterogeneous interfaces. The fracture toughness originates from both intrinsic and extrinsic toughening. Forest dislocation hardening and HDI hardening create a large plastic zone size and work hardening capability at the crack tip, serving as the primary intrinsic toughening mechanisms by consuming energy during crack initiation. When a crack propagates within the heterogeneous lamellar structure, differences in mechanical response, stress states, and aged structures on either side of the interfaces induce crack-stopping behaviors such as deflection, delamination, branching, and blunting, achieving substantial energy dissipation and toughening at high strength. A proportional relationship between the hardening level of the plastic zone at the crack tip and fracture toughness is established, providing a basis for efficient quantitative evaluation of fracture toughness and energy dissipation in heterogeneous materials.