金属的强度和塑性是结构材料最重要的两个力学性能指标，而强度和塑性往往呈现倒置关系，这在很大程度上限制了金属材料的应用。近年来材料研究者开发出了一系列异质结构材料，通过在金属材料中构筑异质结构，以实现强度和塑性的协同作用。异质结构材料优异的力学性能主要得益于在塑性变形过程中，由于异质区域间的塑性不兼容引发的塑性应变梯度的产生、几何必需位错的堆积和背应力强化，使得异质结构金属材料同时具有较高的强度和良好的延展性，这是传统均质结构材料无法实现的。本文以梯度纳米结构和异质层状结构这两类典型的异质结构材料为研究对象，通过晶体塑性有限元方法以及晶体塑性本构模型，探究异质结构金属材料的微观塑性变形机制以及宏观力学性能，并揭示其结构与性能之间的关系，为进一步提高其力学性能以及研发新的、性能更加优异的异质结构金属材料提供理论指导和技术支持。论文的主要研究内容及结论如下：

（1）采用基于位错密度演化的晶体塑性有限元方法，研究梯度纳米结构镍分别在0°、45°和90°角单轴拉伸下的力学行为，探讨其微观结构（如晶粒尺寸梯度、多晶织构）对材料宏观塑性各向异性的影响及其微观机理。结果表明，梯度纳米结构镍表现出显著的塑性各向异性。这种塑性各向异性起源于晶粒尺寸梯度和多晶织构的共同作用。晶粒尺寸梯度的存在导致在90°加载条件下的试样（即平行于晶粒尺寸梯度方向）的塑性变形主要由底部塑性较“软”的粗晶提供，使该材料在此加载条件下呈现出较低的屈服强度和流变应力，从而形成较强的塑性各向异性；而多晶织构则通过Schmid效应对塑性各向异性产生影响，即平均Schmid因子最低的加载方向表现出最高的屈服强度。

（2）通过基于多种变形机制的晶体塑性本构模型来探索铜—黄铜异质层状与梯度结构试样中异质界面和晶粒尺寸梯度的耦合效应。研究发现，不同晶粒尺寸梯度的引入会对铜—黄铜异质层状材料的界面效应产生影响，进而改变材料的变形机制和力学行为。在纯铜层中引入单调晶粒尺寸梯度后，材料的屈服强度升高，但由于界面效应减弱导致其背应力降低，造成其延展性下降；在黄铜层中引入对称的晶粒尺寸梯度会导致材料的延展性增加，但强度下降；只有将对称的晶粒尺寸梯度引入纯铜层，更强的背应力和晶界强化才能同时提高材料的强度和延展性。

Strength and ductility are among the most important mechanical properties of metallic materials for structural applications. However, for traditional metallic materials with homogeneous microstructures, an increase in strength will inevitably incur a loss in ductility and vice versa, which is termed the “strength-ductility tradeoff”. Such a tradeoff has been the Achilles’ heel of homogeneous materials, which largely limits their engineering applications. In recent years, materials researchers have developed a series of heterostructured materials to achieve a combination of high strength and extraordinary ductility by introducing heterogeneity into their microstructures. The superior mechanical properties of heterostructured materials are mainly attributed to the generation of plastic strain gradients, the pileup of geometrically necessary dislocations, and back stress strengthening triggered by plastic incompatibility between heterogeneous domains during plastic deformation, rendering heterostructured metallic materials a combination of high strength and excellent plasticity, which cannot be realized in traditional homogeneous structured materials. This thesis focuses on gradient nanostructured nickel and copper-bronze heterogeneous laminate and gradient structures, which are two types of heterostructured materials, and investigates their plastic deformation mechanisms and mechanical properties via crystal plasticity finite element method as well as constitutive modeling and simulation. This work aims to reveal the fundamental relationship between their structures and properties, and gain insight into the mechanisms underlying their plastic deformation, which it will provide theoretical guidance and technical support for improving their properties and performance, as well as the development of novel heterostructured metallic materials with superior mechanical properties. The main contents and conclusion are as follows:

(1) A dislocation density-based crystal plasticity finite element model was used to study the mechanical behaviors of gradient nanostructured nickel (GNS Ni) under uniaxial tension at angles of 0°, 45° and 90°, and investigate the effects of microstructural characteristics of GNS Ni, i.e. grain size gradient and texture, on their plastic anisotropy. It is found that GNS Ni demonstrates significant plastic anisotropy, which is attributed to both the grain size gradient and the polycrystalline texture. The presence of the grain size gradient leads to the plastic deformation of the material in the direction parallel to the grain size gradient being mostly accommodated by the plastically softer coarse grains at the bottom of the specimen, resulting in diminished yield and flow stresses and consequently significant plastic anisotropy. The texture, on the other hand, induces plastic anisotropy via the Schmid effect, where the loading direction with the lowest average Schmid factor demonstrates the highest yield stress.

(2) The coupled effects of interfaces and grain size gradients in copper-bronze heterogeneous laminate and gradient samples are explored via multiple mechanism-based constitutive modeling. It is revealed that different grain size gradients have profound influence on the interface effects, which in turn changes the deformation mechanisms and mechanical behaviors of the materials: a monotonic grain size gradient in the copper layers strengthens the material yet reduces its ductility as a result of weakened interface effects; a symmetric grain size gradient in the bronze layers leads to an increase in ductility but a decrease in strength of the material; only by introducing a symmetric grain size gradient into the copper layers can both the strength and ductility of the materials be improved simultaneously, due to enhanced back stress and grain boundary strengthening.