均质结构金属的强度和拉伸塑性/断裂韧性/动态剪切韧性之间此消彼长的关系限制了其作为结构材料的实际应用。构筑异构已被证明可以获得优异的强度和拉伸塑性匹配，原因在于额外的非均匀变形诱导硬化提高了其应变硬化能力。而材料的断裂增韧/动态剪切增韧与其应变硬化能力密切相关，因此异构金属有望在保持高强度的同时获得优异的断裂韧性和动态剪切韧性。本文通过剧烈塑性变形和合适温度退火成功将晶粒尺寸异构引入到Ni和微带诱导塑性 (Microband-induced plasticity, MBIP) 钢中。针对异构Ni和MBIP钢的拉伸性能、断裂性能、动态剪切性能和变形机制开展研究，主要结论如下：

在晶粒尺寸异构Ni的断裂韧性研究中，异构Ni在保持高屈服强度的同时，具有优异的断裂韧性。相比于屈服程度相近的均质结构，其断裂韧性提高了~103%。该结果可归因于曲折的裂纹扩展路径、非均匀变形诱导硬化以及晶粒细化诱导硬化。额外的硬化机制提高了材料整体的应变硬化能力，有利于获得更大尺寸的裂尖塑性区和塑性区内更高程度的应变硬化，使得变形过程可以消耗更多能量。相比于均质结构的脆性断裂模式，异构Ni呈现出以微孔萌生、长大、聚集为主要特征的韧性断裂模式。

在异构MBIP钢的强韧化机理研究中，制备了同时具有双相和晶粒尺寸双峰分布的异构。相比于相近屈服强度的均质结构，其均匀延伸率提升了~340%。优异的性能匹配归因于非均匀变形诱导硬化和仅在奥氏体粗晶中存在的MBIP效应，奥氏体超细晶和铁素体相中均未发现微带的形成。这表明MBIP效应具有关于晶粒尺寸和相的依赖性，减小晶粒尺寸会抑制微带的形成。

在异构MBIP钢的断裂韧性的研究中，制备了一种单相晶粒尺寸异构。相比于屈服强度相近的均质结构，其均匀延伸率提高了~380%，断裂韧性提高了~15%。其强度和断裂韧性匹配优于已报道的绝大部分先进钢。优异的断裂韧性归因于二次裂纹的产生、韧窝尺寸的双峰分布、裂纹扩展路径周围的晶粒细化、高密度几何必需位错的产生以及粗晶内微带的形成。

在异构MBIP钢的动态剪切行为研究中，异构MBIP钢获得了优异动态力学性能的匹配。相比于动态剪切屈服强度相近的均质结构，其动态剪切韧性提高了~47%。该结果可归因于更高程度的动态晶粒细化，非均匀变形诱导硬化以及奥氏体超细晶显著的应变率硬化效应。同时发现，随着应变率的上升，MBIP效应在奥氏体超细晶生效的可能性增大。

The trade-off between strength and ductility/fracture toughness/dynamic shear toughness in metals with homogeneous structures limits their application as structural materials. It has been proven that excellent synergy of yield strength and ductility can be obtained for heterogeneous structures since extra hetero-deformation induced hardening would improve the strain hardening ability. It is found that the intrinsic toughening/dynamic shear toughening mechanisms of materials are closely related to the strain hardening ability. Thus, heterogeneous structures have the potential to obtain both high strength and excellent fracture toughness/dynamic shear toughness. In this study, heterogeneous grain structures were selected and successfully introduced into pure Ni and an advanced steel (Microband-induced plasticity steel, MBIP steel) through severe plastic deformation and critical temperature annealing processes. The tensile properties, fracture properties, dynamic shear properties and deformation mechanisms of pure Ni and the MBIP steel with heterogeneous grain structure were studied in detail. The main results are summarized as follows:

In the study of the fracture toughness of Ni with heterogeneous grain structure, the heterogeneous grain structure Ni has an excellent fracture toughness, which is improved by ~103% as compared to the homogeneous structure with similar yield strength. Its excellent mechanical properties can be attributed to the following aspects: curved crack path, hetero-deformation induced hardening and grain refinement induced hardening. These extra hardening mechanisms can induce a larger plastic zone and a higher strain hardening capacity in the plastic zone, which can dissipate more energy during deformation. Compared with the brittle fracture mode of the homogeneous structure, the heterogeneous structure Ni presents a ductile fracture mode characterized by void initiation, growth and aggregation.

In the study of the strengthening and toughening mechanisms of the MBIP steel, the heterogeneous structure with dual phase and bimodal distribution of grain size was designed. The heterogeneous structure has a remarkable uniform elongation, which is improved by ~340% as compared to the homogeneous structure with similar yield strength. The excellent tensile properties can be attributed to the hetero-deformation induced hardening and the MBIP effect which is only effective in the austenitic coarse grains. No microband is found in either the austenitic ultrafine grains or the ferritic phase. The results show that the MBIP effect is dependent on both grain size and phase. The formation of microband is inhibited by decreasing the grain size.

In the study of the fracture toughness of the MBIP steel, a single-phase heterogeneous grain structure was successfully designed. Compared with the homogeneous structure with nearly the same yield strength, the uniform elongation and the fracture toughness of single-phase heterogeneous grain structure are improved by ~380% and ~15%, respectively. The superior synergy of strength and toughness of the MBIP steel with heterogeneous grain structure is better than that of the other advanced steels. The remarkable fracture toughness can be attributed to the generation of secondary cracks, the dimples with various sizes, the grain refinement around the crack propagation path, the generation of high density of geometrically necessary dislocations, and the formation of microband in coarse grains.

In the study of dynamic shear behaviors of the MBIP steel, excellent dynamic mechanical properties were obtained in heterogeneous grain structures. The dynamic shear toughness of the heterogeneous grain structure is improved by ~47% as compared to the homogeneous structure with similar dynamic shear yield strength, which can be attributed to severer dynamic grain refinement, hetero-deformation induced hardening and remarkable strain-rate hardening ability in the austenitic ultrafine grains. It is also found that the occurrence possibility of the MBIP effect in austenitic ultrafine grains increases with increasing strain rate.