金属和共价键材料是两类很重要的结构材料，由于其具有优良的力学性能而被广泛的应用于航空航天、交通运输以及工程建筑等领域。工程设计要求结构材料必须具有高的强度以及高的断裂韧性。因此提高金属以及共价键材料的强度已成为推动国民经济和人民生活进步的重要动力。传统强化材料的方法为细晶强化、固溶强化、第二相强化和加工硬化。这四种传统强化方法虽然能提高材料的强度，但是降低了材料的塑性。最近，实验合成的纳米孪晶金刚石和纳米孪晶铜的力学性质表明在结构中引入纳米孪晶界能够同时提高材料的强度以及断裂韧性，解决了材料领域中强度与韧性相互排斥的问题。但是纳米孪晶金刚石的硬化机理、纳米孪晶铜软化机理还存在疑问，解决这些问题对于理解纳米孪晶金刚石和纳米孪晶铜的力学行为以及设计新材料具有重要的指导意义。为此，本文以纳米孪晶金刚石以及纳米孪晶铜为研究对象，通过分子动力学和理论推导的方法，分别研究了纳米孪晶金刚石硬化机理，纳米孪晶铜强化-软化机理以及纳米孪晶材料中最佳孪晶厚度的起源。 以纳米孪晶金刚石为对象研究了纳米孪晶共价键材料的硬化机理。通过分析纳米孪晶金刚石中位错线和扭折与孪晶面的取向关系，将纳米孪晶金刚石中位错的滑移分为三种模式：塞积穿透模式、受限滑移模式和平行于孪晶面滑移模式。通过分子动力学模拟了这三种模式中位错与孪晶界的相互作用，得到了位错反应的反应热、激活能和障碍强度，并建立了不同滑移模式的临界分切应力表达式。根据位错滑移模式的临界分切应力和Sachs模型，计算得到了不同孪晶厚度下纳米孪晶金刚石的硬度，且与实验数据吻合。研究结果表明，造成纳米金刚石高硬度的因数有两个：一个是金刚石中的高晶格摩擦应力；另一个是霍尔-佩奇效应引起的高非热激活应力。这两个因素都源于位错在孪晶平面内形核和运动的激活体积低以及激活能高。该研究为纳米金刚石的硬化机理提供了新的见解，对今后开发新型超硬材料具有重要的指导意义。 以纳米孪晶铜为对象研究了纳米孪晶金属屈服强度的软化机理。通过分析纳米孪晶铜中位错滑移面和伯格斯矢量与孪晶界的取向关系，将纳米孪晶铜中位错的滑移分为四种模式：塞积穿透模式、受限滑移模式、部分位错平行孪晶界滑移模式和非共格孪晶界迁移模式。通过分子动力学研究了这四种滑移模式与孪晶界的相互作用，建立了纳米孪晶铜的位错滑移模式临界分切应力的表达式。根据位错滑移模式的临界分切应力和Sachs模型，得到了纳米孪晶铜的屈服强度与孪晶厚度的关系，与实验数据吻合。研究结果表明，纳米孪晶铜的屈服强度先随着孪晶厚度的降低而增加，在孪晶厚度为15纳米时达到最大值940 MPa。当进一步减小孪晶厚度时，纳米孪晶铜的屈服强度会发生软化。纳米孪晶铜中的这种软化行为是由于软化模式从部分位错沿着孪晶界滑移转变为非共格孪晶界沿着孪晶界迁移。这种转变的临界孪晶厚度是由孪晶界的能量以及剪切模量决定的。低孪晶界能量和高剪切模量的纳米孪晶材料具有低的临界孪晶厚度。这项工作揭示了纳米孪晶金属的强化／软化机制，为设计更好性能的材料提供提供了重要线索。 以纳米孪晶面心立方金属和纳米孪晶共价键材料位错行为为基础，研究了纳米孪晶材料的最佳孪晶厚度的物理起源。根据纳米孪晶材料的软化机理，并在位错理论的基础上，建立了纳米孪晶金属和共价材料的最佳孪晶厚度理论模型，并研究了材料本征参数对最佳孪晶厚度的影响。研究结果表明，纳米孪晶金属和纳米孪晶共价键材料临界孪晶厚度的物理起源是相同的，是由于孪晶界能量驱动的去孪晶和晶格摩擦力的阻碍的竞争决定的。材料的孪晶界的能量是本征层错能的一半，晶格摩擦力正比于剪切模量。因此低层错能和高剪切模量的纳米孪晶材料具有低的最佳孪晶厚度。对于纳米孪晶共价键材料，它比金属具有更高的扭折迁移能垒，因此它们的晶格摩擦力大于金属位错滑移的晶格摩擦力，这导致纳米孪晶共价键材料的最佳孪晶厚度小于纳米孪晶金属的最佳孪晶厚度。这一工作为纳米孪晶材料的力学性质提供了新的认识，对今后设计性能更好的新型纳米孪晶材料具有一定的指导意义。 以交织纳米孪晶金刚石为对象研究了提出了超强纳米孪晶共价键材料的设计思路。通过在晶粒中引入不同取向的孪晶界建立了交织纳米孪晶晶体结构，并使用分子动力学方法计算了交织纳米孪晶金刚石、孪晶金刚石和多晶金刚石的屈服强度。结果表明，交织纳米孪晶金刚石、孪晶金刚石和多晶金刚石的的屈服强度分别为161， 150和140 GPa。因此，纳米孪晶共价键材料的硬度可以通过引入不同取向的孪晶界来进一步提高。这一工作对今后设计高硬度的材料具有一定的指导意义。 关键词：纳米孪晶金刚石；纳米孪晶铜；位错；强度；分子动力学模拟；

 Metals and Covalent materials are two great important structural materials. Because of their excellent mechanical properties, they are widely used in aerospace, transportation and engineering construction. Engineering design requires that structural materials must have high strength and high fracture toughness. Therefore, improving the strength of metals and covalent materials has become an important driving force to promote the national economy and the progress of people's lives. The traditional methods of strengthening materials are fine-grained strengthening, solid solution strengthening, second phase strengthening and work hardening. These four traditional strengthening methods can improve the strength of materials, but reduce the plasticity of materials. Recently, the mechanical properties of nanotwinned diamond and nanotwinned copper synthesized experimentally shown that the introduction of nano-twin into the structure can simultaneously improve the strength and fracture toughness of materials, and solve the problem of mutual exclusion between strength and toughness in the material field. But there are still some doubts about the hardening mechanism of nanotwinned diamond and the softening mechanism of nanotwinned copper. It is important to solve these problems for understanding the mechanical behavior of nanotwinned diamond and nanotwinned copper and designing new materials. In this paper, the nanotwined diamond and nanotwinned copper were set as object to study the mechanical properties of nanotwinned materials. By using molecular dynamics and theoretical methods, the hardening mechanism of nanotwinned diamond, the strengthening-softening mechanism of nanotwinned copper and the physical origin of optimum twin thickness in nanotwinned materials were studied. Nanotwinned diamond has been used to study the hardening mechanism of nanotwinned covalent materials. By analyzing the orientation relationship of dislocation line and kink respected to twin boundary in nanotwinned diamond, the slip of dislocation in nanotwinned diamond can be divided into three modes: slip transfer mode, confined layer slip mode and parallel to twin plane slip mode. The interaction between dislocations and twin boundaries in these three modes is studied by using molecular dynamics simulations. The reaction heat, activation energy and barrier strength of dislocation reaction are obtained, and the critical shear stress for different slip modes are obtained. Based on the critical resolved shear stress and Sachs model, twin thickness dependent of nanotwinned diamond hardness is evaluated which is in good agreement with the experimental data. The research results show that two factors contribute to the high hardness of nt-diamond: high lattice frictional stress by the nature of carbon bonding in diamond and high athermal stress due to the Hall-Petch effect. Both factors stem from the low activation volumes and high activation energy for dislocation nucleation and propagation in diamond twin planes. This work provides new insights into hardening mechanisms in nt-diamond, and will be helpful for developing new super-hard materials in the future. Nanotwinned copper has been used to study the softening mechanism of nanotwinned metals materials. By analyzing the orientation relationship of the dislocation slip plane and the Burgers vector respected to the twin boundary in nanotwinned copper, the dislocation slip modes in nanotwinned copper can be divided into four modes: slip transfer mode, confined layer slip mode, partial dislocation slip mode and incoherent twin boundary migration. By using the molecular dynamics simulations, the dislocation interaction with twin boundary in these four dislocation slip modes is studied. Based on the dislocation interaction with twin boundary, the critical shear stress expressions for four different slip modes are established. Based on the Sachs polycrystalline model, twin thickness dependent yield strengths are deduced that are in good agreement with experimental data. The research results indicated that the strength of nanotwinned copper increases with decreasing twin thickness, reaches a maximum 940 MPa at twin thickness of 15 nm, and then decreases with decreasing twin thickness. The strengthening-softening behavior is attributed to a transition from partial dislocations slipping in twin boundaries to detwinning induced by incoherent twin boundary migration. The critical twin thickness (λ〓) for this transition depends on the twin-fault energy (γ〓) and shear modulus (G). Nanomaterials with lower γ〓 and higher G have smaller λ〓. This work sheds light on the strengthening/softening mechanisms in nanotwinned metals, providing important clues to materials design for better performance. Based on the dislocation behavior of nanotwinned face-centered cubic metals and materials, the physical origin of the optimal twin thickness of nanotwinned materials was studied. According to the softening mechanism, a uniform optimum twin thickness model for both nanotwinned metal and covalent materials are constructed on basis of dislocation theory, and the effect of physical parameters on optimum twin thickness was obtained. The research results indicated that the optimum twin thickness of face-centered cubic metals and covalent materials shares same physical origin which is contributed to the competition between twin fault energy derived de-twinning process and lattice friction stress resistance. The twin boundary energy is about half of the intrinsic stacking fault energy, and the lattice friction stress is proportional to the shear modulu. For covalent materials, it has higher kink migration energy barriers than that of metals, hence their lattice friction stress resistance is usually larger than that of metals, this lead to a smaller optimum twin thickness than that of metals. This work provides a new understanding for nanotwinned materials’ mechanical properties, which will be helpful for designing novel nanotwinned materials with better performance in future. Intersectional nanotwinned diamond is used to study the strategy for improving the strength of nanotwinned covalent materials. The intersectional nanotwinned diamond was established by introducing twins with different orientations in the grains, and the yield strength of intersectional nanotwinned diamond, nanotwinned diamond and polycrystalline diamond was calculated by molecular dynamics method. The calculated results show that the yield strength of intersectional nanotwinned diamond, nanotwinned diamond and polycrystalline diamond are 161,150 and 140 GPa, respectively. Therefore, the strength of nanotwinned covalent bond materials can be further enhanced by introducing twin boundaries with different orientations in grains. This work will be helpful for designing novel nanotwinned materials with high hardness. Keywords: nanotwinned diamond; nanotwinned copper; strength; dislocation; molecular dynamics simulations.