近年来，多主元合金因其优异的综合力学性能和丰富的微结构特征，成为材料科学领域的研究热点。然而，强度与塑性和韧性之间存在此消彼长的制约关系，这不仅是传统金属材料长期以来的困境，在多主元合金中也不例外。传统强化手段虽然能有效提升强度，但会牺牲材料的加工硬化能力，从而导致塑性和韧性大幅降低。特别是在具有超高屈服强度的合金中，塑性失稳会提前发生，从而产生不可逆转的应变局域化，即颈缩。这是由于从变形开始，位错的产生和储存就会变得越来越困难，导致传统的林硬化不足以抵抗进一步的塑性变形。因此，在高强度纳米结构金属中，如何形成并储存位错是实现其加工硬化的难题，更是挑战。此外，由于裂纹尖端与单轴拉伸的应力状态和变形条件不同，即使材料优异的加工硬化能力能有效增加裂纹扩展阻力，但良好的强塑性匹配并不一定意味着优异的断裂韧性。本文通过控制热机械加工路线，制备了具有不同屈服强度的多主元VCoNi合金，系统研究了超细晶组织在准静态拉伸加载下的塑性失稳和加工硬化，以及均质/异质结构在室/低温下的裂纹扩展行为和断裂韧性，主要结果如下：

(1)通过冷轧和高温短时退火获得了一种由fcc等轴超细晶、L1₂ 金属间化合物和局部化学有序LCOs组成的双重异质结构。该结构具有十分优异的强塑性匹配：室温下的屈服强度高达2.0 GPa，均匀塑性为16%；在液氮和液氦温度下，屈服强度和均匀塑性同时提升，分别为2.2 GPa和20%，优于已报道的其他合金。此外，样品在拉伸过程中表现为吕德斯带式非均匀变形，带前端存在宏观的塑性失稳。吕德斯带传播过程中，带前端形成了持续的局部颈缩，从而产生三轴应力和应变梯度。三轴应力和提升的von Mises应力，促进了带前端高密度位错的快速产生，其中位错密度增量为9.3×10¹⁴ m^-2，位错增殖速度为4.6×10¹³ m^-2·s^-1。这些位错引起了林位错加工硬化和异质变形诱导硬化，后者是几何必需位错与化学短程有序的应变场之间交互作用的结果。双重加工硬化相结合，反向抑制和稳定早期颈缩并促进均匀变形。当带前端的加工硬化足以稳定塑性失稳时，吕德斯应变随晶粒尺寸减小而不断增大。因此，可以通过控制提前失稳获得协同加工硬化，使得具有超高屈服强度的超细晶或纳米晶恢复塑性。拉伸变形后，fcc晶粒内部存在大量的位错缠结，而L12 内部的塑性变形程度很小，仅通过晶格旋转来协调变形。

(2)通过高温热轧获得了单相粗晶组织，其屈服强度和断裂韧性从室温下的~704 MPa和~256 MPa×m^1/2同时提高到液氮温度下的~944 MPa和~290 MPa×m^1/2，在室温和低温下都具有优异的强韧性匹配。裂纹尖端前方塑性区内发生显著的加工硬化，形成了典型的韧性断裂模式：明显的裂尖钝化和微孔形核与聚集。此外，在晶界或孪晶界附近，甚至是晶粒内部，都存在明显的几何必需位错以协调塑性区内的应变梯度。室温下，塑性区内的变形机制为单一的位错介导塑性，晶粒内存在高密度滑移带和明显位错缠结，有效促进了位错交互作用与增殖。低温下，位错和层错主导的塑性变形同时存在，滑移模式由室温下的波状滑移转变为平面滑移。多个{111}面上的层错和位错之间的交互作用形成了间距更小的泰勒晶格，从而更有效地阻碍位错运动。低温下更优异的断裂韧性归因于额外的裂纹偏折、更大的塑性区以及更高程度的加工硬化。

(3)通过减小冷轧变形量和不完全再结晶退火，获得六种具有不同屈服强度的异质结构。随退火温度的升高，异质结构从部分再结晶组织逐渐转变为晶粒尺寸呈双峰分布的完全再结晶组织。虽然屈服强度从1.67 GPa降低至1.08 GPa，但断裂韧性从70.4 MPa×m^1/2升高至181.8 MPa×m^1/2，在高强度水平下获得优异的断裂韧性。通过显微硬度测试，建立了塑性区加工硬化程度与基于J 积分和临界裂纹张开位移测得的起裂韧性之间的线性关系。通过该线性关系，修正了由于样品厚度不足而被高估的临界起裂韧性。结果表明，裂纹尖端塑性区内的加工硬化是优异断裂韧性的主要来源。结合相应的微结构表征，发现在异质界面附近存在几何必需位错的堆积，提供了额外的异质变形诱导加工硬化。受变形程度和晶粒尺寸的影响，异质结构表现为单一的位错介导塑性，具有典型的平面位错滑移特征。

In recent years, multi-principal element alloys (MPEAs) have emerged as a hot topic of research in materials science, due to their exceptional synergistic mechanical properties and abundant microstructural characteristics. However, the inherent trade-off between strength and ductility or toughness, has been a long-standing dilemma not only in traditional metallic materials but also in MPEAs. The conventional strengthening methods can effectively enhance strength, while often come at the expense of the  work hardening ability, resulting in a dramatic reduction in ductility and toughness. Particularly, premature plastic instability will occur in alloys with ultra-high yield strength (UHYS, usually around 2 GPa), leading to irreversible strain localization, i.e., necking. This is due to the growing difficulty in the production and accumulation of dislocations from the very beginning of tensile deformation that renders the conventional forest hardening insufficient to resist further plastic deformation. Consequently, in ultra-strong nanostructured metals, how to produce and accumulate dislocations for achieving work hardening is a difficult and challenging problem. Moreover, although the work-hardening ability of material can significantly enhance crack extension resistance, the attainment of an excellent strength-ductility balance does not inherently imply a corresponding superior fracture toughness. This is attributed to the distinct stress states and deformation conditions at the crack tip compared with uniaxial tensile deformation. In this study, the VCoNi MPEAs with varying yield strengths were fabricated by controlling thermo-mechanical processing routes, and the plastic instability of ultra-fine-grained (UFGs) structure under quasi-static tensile deformation, as well as the crack extension behaviors of homo/hetero-geneous structures under ambient and cryogenic temperatures were investigated systematically. The main results are as follows:

(1) A dual heterostructure composed of equiaxed face-centered-cubic (fcc)-structured UFGs, L12  intermetallic compounds, and local chemical orders (LCO) regions was obtained by cold rolling and annealing at high-temperature with short-time. The structure exhibits an exceptional strength-ductility balances with yield strength of 2.0 GPa and uniform ductility of 16% at room temperature. Remarkably, at liquid nitrogen and liquid helium temperatures, both the yield strength and uniform ductility are further enhanced to 2.2 GPa and 20%, which have exceeded other reported alloys. Furthermore, the Lüders band (LB) typed non-uniform deformation have been observed during tensile deformation, with macroscopic plastic instability at the band front. During the LB propagation, ongoing localized necking occurs at the band front to induce triaxial stress and strain gradient.

The triaxial stresses and elevated von Mises stresses will facilitate to trigger the rapid dislocation multiplication at the band front, with the dislocation density increment of 9.3×10¹⁴ m^-2 and dislocation multiplication rate 4.6×10¹³ m^-2·s^-1. These dislocations induce forest dislocation work hardening and hetero-deformation-induced (HDI) hardening, the latter is the result of the strain-field interaction between geometrically necessary dislocations (GNDs) and LCO region. The dual work hardening combines to restrain and stabilize the premature necking in reverse and to facilitate uniform deformation. When the work-hardening at band front is sufficient to stabilize the plastic instability, the Lüders strain increases continuously with decreasing grain size. Thus, harnessing the premature instability is practical for synergistic work hardening to regain ductility at UFGs and nanostructures with UHYS. After tensile deformation, numerous dislocation entanglements are observed inside the fcc grains, while L12  accommodates the plastic strains mainly by lattice rotation with limited deformation.

(2) A single-phase coarse-grained structure (CG) was obtained by high-temperature hot rolling. The excellent strength-toughness balances are achieved at both ambient and low temperatures, with an increase in both yield strength and fracture toughness from 704 MPa and 256 MPa·m^1/2 at room temperature to 944 MPa and 290 MPa·m^1/2 at liquid nitrogen temperature, respectively. Notably, significant work-hardening within the plastic zone ahead of the crack tip led to a typical ductile fracture, characterized by pronounced crack-tip blunting and the micro-void initiation and coalescence. Moreover, obvious GNDs were observed near the grain or twin boundaries and even within the grains, facilitating the accommodation of strain gradient within the plastic zone. At room temperature, deformation mechanism within the plastic zone is controlled by single dislocation-mediated plasticity, with high density of slip bands and evident dislocation tangles within the grains, which effectively promote dislocation interaction and accumulation. However, at lower temperature, dislocation-mediated and stacking faults (SFs)-dominated plasticity have been observed simultaneously, with a transition of slip pattern from wavy slip at room temperature to planar slip. Interaction of activated SFs and dislocation along multiple {111} planes promote the formation of Taylor lattice with smaller spacing, impeding the dislocation gliding effectively. Consequently, the superior fracture toughness at low temperatures can be attributed to extra crack deflection, a larger plastic zone, and more facilitated work hardening.

(3) Six heterostructures with varying yield strength were obtained by reducing the deformation of cold rolling and incomplete recrystallization annealing. With the increasing annealing temperature, the heterostructures transformed from partially recrystallization to fully recrystallization with a bimodal distribution of grain sizes. Although the yield strength decreased from 1.67 GPa to 1.08 GPa, the fracture toughness increased from 182 MPa·m^1/2 to 70 MPa·m^1/2, demonstrating excellent fracture toughness at high strengths. The linear correlations between the degree of work-hardening inside the plastic zone and the critical initiation toughness based on the J-integral and critical crack opening displacement are established by microhardness tests. Furthermore, the overestimated critical crack initiation toughness due to insufficient sample thickness is revised based on this linear relationship. This indicates that the work-hardening inside the plastic zone at the crack tip is the primary origin of excellent fracture toughness. Combined with the corresponding microstructural characterization, the pile-ups of GNDs were observed to be formed at the heterogeneous interfaces to accommodate strain gradient at the plastic zone of the crack tip, offering extra HDI hardening. Depending on the deformation degree and grain size, the heterostructures exhibit single dislocation-mediated plasticity with typical planar dislocation slip characteristics.