许多具有微纳米结构的材料能够表现出相对于宏观传统材料更加优异的力学性能，且这些力学性能往往具有明显的尺寸效应。众多研究表明，这些微纳 米结构材料内部分布着大量的表/界面，而随着尺寸的减小，材料的比表/界面面 积不断增大。这些表/界面所引起的应变梯度效应与表/界面效应是产生这种力学 性能差异的重要原因。本文针对两种材料在微纳米尺度下结合所产生的表/界面， 通过分子动力学模拟与跨尺度力学理论，探究了材料在表/界面附近的力学行为 及其对材料整体的力学性能产生的影响，主要取得了以下研究成果： （1）建立了由 Al 和 Ni 组成的双材料纳米杆的分子动力学拉伸模型，提出 了分子动力学与有限元方法相结合以表征模型中的连续应力应变场的方法 （MD-FEA 方法），并与传统的通过离散的变形梯度法计算的原子应变以及原 子的维里应力相比较，验证了其准确性。这种方法同时解决了原子应变的计算 依赖于截断半径，以及原子应变，维里应力在表界面附近计算不准确的缺点。 运用这种新的方法，计算不同尺寸的双材料杆拉伸模型界面附近的应变场，揭 示了在均一载荷下，材料的应变在界面附近有明显的变化：在最靠近界面的原 子层处，材料由于原子错配产生规则的应变波动；而在更大的范围内，相同拉 伸载荷下，高应变材料 Al 在界面附近的应变降低，低应变材料 Ni 的应变则升 高。研究显示，界面影响区域的大小与界面面积正相关，且模型中心区域受到 界面的影响更为明显。同时，通过金刚石压头，Al 基体的纳米压痕模拟，对比 了不同方法计算的基体的应力应变场，进一步验证了 MD-FEA 方法的有效性。 （2）建立了考虑应变梯度效应的一维双材料杆拉伸模型，引入应变梯度特 征尺度以表征材料力学性能的尺寸相关性，探讨了新理论下边界条件的选取问 题，并通过界面无滑移和界面附近的应变能守恒的关系，引入新的边界条件进 行求解。结果显示，应变梯度特征尺度主要在界面附近对材料的力学响应产生 影响，界面附近的应变与远离界面的材料体内应变有明显的差异，距离界面越 近，应变的差异越大。应变发生显著变化的范围和应变梯度特征尺度相当。在 微米尺度上，随着材料特征尺度的增大，界面的影响范围也随之增大，从而对 材料整体力学性能的影响更为明显。而在远离界面处，材料的力学响应和传统 宏观材料一致，尺度效应基本不产生影响。通过引入等效模量和材料在界面区 域的应变能差值，作为衡量应变梯度效应大小的参量。研究结论表明，应变梯 度效应对模型的等效模量影响较小，但对界面能量差的影响明显。应变梯度特 征尺度越大，应变梯度效应越明显。 （3）协同考虑材料的应变梯度效应和表/界面效应，建立双材料杆拉伸的 跨尺度力学模型。引入材料界面附近应变能之和与传统理论下的差值，作为材 料在变形过程中产生的表/界面能，并引入表/界面能密度，探究其对微纳米材料的力学性能的影响。分析了表面效应和界面效应在变形过程中对整体表/界面能 产生的不同影响，并与应变梯度效应相结合，求解了界面附近区域的应变场。 分析了跨尺度理论下应变梯度效应和表/界面效应对模型等效模量与界面能量差 的影响。研究结果显示，在纳米尺度上，材料的表/界面能密度会显著影响材料 的等效模量和界面能量差。材料的界面能密度越高，材料尺寸越小，对材料性 能的影响越明显。而在微米尺度上，表/界面效应产生的影响远低于应变梯度效 应，应变梯度特征尺度越大，材料尺寸越小，对材料性能的影响越明显。而在 宏观尺度下，两者产生的效应均不明显。由此可知，应变梯度效应在微米尺度 上开始对材料力学性能影响，表/界面效应则在纳米尺度上发挥明显作用，并超 过应变梯度效应的影响。

Materials with micro/nano structures often exhibit excellent mechanical properties compared to the traditional macroscopic ones, and these properties always have an obvious size effect. Numerous researches have shown that vast interfaces are distributed inside the materials, and when the material scale decreases, the ratio of the surface and interface area to the material volume gradually increases. The strain gradient effect and the surface/interface effect induced by the surface and interface are believed the most important reasons which lead to the differences of the material properties with different scales. In this thesis, an interface constructed by two different materials is studied through molecular dynamics simulation and trans-scale mechanical theory. The mechanical behavior near the interface and its influence on the entire material properties are discussed. The main work and the results are as follows: (1) Atomistic models of the stretching of bimaterial nanorods consist of materials Al and Ni with an interface are constructed. A new method combined with molecular dynamics (MD) and finite element analysis (FEA) is developed to describe the continuous stress and strain field in the discrete atomistic models. Stress and strain calculated by the new method are compared with the atom strain computed through the discrete deformation gradient and the virial stress, respectively, and the accuracy of the method is proved. The MD-FEA method avoids the deviation caused by the partly random choice of the cutoff radius when calculating the atom strain, and the errors of the atom strain and virial stress calculated on the surface and interface atoms. Using the new method, strain distributions on the surface and interface of different scales of the atomistic models are computed, the results show that under the uniform tensile force, the strain near the interface has obvious difference compared to the volume of the model: on the nearest layer from the interface, regular fluctuation of the strain caused by the atom mismatch is observed; in a larger range of the model, strain in high strain material Al is decreased and in low strain material Ni is increased near the interface. The range affected by the interface expands with the increasing of the area of the interface, and the influence is more obvious in the center of the model than near the surfaces. A nanoindentation model with a diamond tip and Al substrate is also constructed and the strain and stress in the substrate are calculated to verify the validity of the MD-FEA method. (2) A stretching model of a one-dimensional bimaterial rod is established with the strain gradient effect considered. The strain gradient characteristic length scale parameter which represents the size dependency of the material properties is introduced to the constitutive equations. The choices of the boundary conditions in the new model are discussed. Through the no-gliding condition and the conservation of the strain energy condition on the interface, the strain distribution of the model is determined. The results show that the strain gradient length scale mainly affects the material properties near the interface, leading to the difference in the strain between the interface area and the volume. The closer it is to the interface, the more obvious the strain difference shows. The sizes of the areas where strain makes differences are comparable to the strain gradient characteristic length scales. On the micron scale, the sizes of the affected areas near the interface enlarge with the increasing of the length scale parameters and make a larger contribution to the overall material properties. In the material volume away from the interface, the strain is consistent with the result of conventional theory and the length scale effect is negligible in each material. The effective module of the model and the difference of the strain energy near the interface between the two materials are introduced to measure the size effects. The results show that size effects have little influence on the effective module of the model but dramatically affect the interface energy difference. The strain gradient effect is more obvious when the length scale gets larger. (3) A trans-scale mechanical model of bimaterial stretching rod by introducing both the strain gradient effect and the surface/interface effect is established. The difference of the strain energy near the interface computed between the trans-scale theory and conventional theory is considered as the surface/interface excess energy, then the surface/interface energy densities are introduced in the theory to investigate their effects on the overall mechanical properties in micro/nano scale materials. The difference between the surface effect and the interface effect that contributed to the deformation of the model is analyzed. The strain field near the interface of the model is determined by combining the surface/interface effect and the strain gradient effect. The effects of the strain gradient and the surface/interface to the effective module and the interface energy difference are investigated and the results show that on the nanoscale, the surface/interface energy densities dramatically affect the effective module and the interface energy difference of the model, the higher the interface energy or the smaller the material size is, the more obvious the effects are. And on the micron scale, the surface and interface have much lower effects compared to the strain gradient effect, the larger the characteristic length scale or the smaller the material size is, the more obvious the effect on the material properties. On the macro scale, both the strain gradient effect and the surface/interface effects are negligible. It means that the strain gradient affects the material properties on the micron scale while the surface/interface effects on the nanoscale, and their effects exceed the strain gradient effect on the nanoscale.