纳米多孔金属力学性能表征与变形机理研究

Based on numerical simulation, continuum mechanics model and experimental characterization, a systematic investigation is carried out on both mechanical behavior and deformation mechanism of porous metals at nano sacle. First, fundamental materials constants, such as Young's modulus and yield strength, will be calculated to clarify the currently controversial opinions in this field. The validation and possible modification of the classical Gibson-Ashby foam scaling equations will also be discussed to make contribution to the issue whether the statement 'smaller is stronger' can hold on micro- and nano-scale. These conclusions are instructive to some basic concepts in modern materials science and also useful for characterization, fabrication and industrial applications of nano porous metals. Secondly, both dislocation movement and stress localization will be investigated to characterize the unique plastic deformation mechanism in nano porous metals. Concerns will be addressed on the pressure-sensitive plastic behavior which conflicts with the well-known statement that plastic deformation is incompressible in continuum mechanics, as well as the dislocation nucleation and egression through the ligaments. Findings are expected to supplement new insights on people's common sense about slip and dislocation which are considered as the main sources of plastic flow in materials. Thirdly, due to the large ratio of surface area to volume in nano porous metals, focus will also be emphasized on the influence of both surface stress and surface energy on the mechanical behaviors such as actuation and plastic deformation. The manipulation of surface stress is expected to serve as a useful tool to tailor the mechanical property of nano porous metals and shed light on potential nano porous metal actuators and man-made metallic muscles. Finally, because of the complex geometrical shape and non-uniform stress, it's challenging to build proper phenomenological constitutive laws and failure criteria for nano porous metals. Simulation results and stress analysis in this project will make contribution to this point. Research results in this project will not only establish a good starting point for both further theoretical investigations and potential industrial applications of nano porous metals, but also illumine the study in related fields such as the exploration of strength and deformation mechanisms in individual nanoscale objects (e.g. metal nanowires) and of nanoporous biological systems.

基于数值模拟、连续介质力学模型和实验表征技术，对纳米尺度下多孔金属的力学行为和变形机理进行系统性研究。首先，运用实验表征和数值模拟，获取杨氏模量和屈服强度等基本材料参数，澄清目前围绕Gibson-Ashby尺度律的分歧观点，提出修正的理论模型；其次，针对纳米多孔金属可能与静水压力相关的独特塑性变形行为，研究系带中特殊的位错运动、滑移和应力集中，建立微结构演化与宏观变形之间的对接机制；接下来，基于孔隙带来的超大表体比，考察表面能与表面应力对纳米多孔金属塑性变形和作动行为的影响，探讨其在作动器及人造肌肉等领域的应用前景；最后，克服几何形状复杂和应力状态非均匀带来的影响，建立纳米多孔金属的连续介质力学本构模型和断裂破坏准则。本课题研究成果将为纳米多孔金属进一步的理论研究和工程应用提供科学依据和指导，并为相关领域的研究，如新型功能纳米器件的强度和变形机制、纳米多孔生物系统等，提供启发与参考。

本项目使用计算机模拟方法，主要围绕纳米多孔金属、黑磷、金刚石碳管等新型功能材料的力学行为和变形机理，开展计算力学研究，取得的主要进展如下：.(1) 纳米多孔金属模型构建方面，构建了包含规则球形孔、椭圆孔和柱形孔的纳米多孔金属模型，并可调节其孔隙率和相对密度。.(2) 纳米多孔金属基本材料参数表征与尺度律方程方面，研究了经典的Gibson-Ashby尺度律方程在纳米多孔铜变形行为中的应用，并对其进行了修正；考察了孔隙率、孔洞尺寸、相对密度、系带尺寸等因素对变形机制的影响。.(3) 变形过程中位错微结构演化及其对材料力学性能影响方面，一方面考察了孪晶界等微结构对纳米多孔金属中位错运动的阻碍作用与强化机制；另一方面，考察了夹杂界面对位错形式与运动过程的影响。.(4) 温度和应变率对纳米多孔金属力学行为影响方面，考察了纳米多孔铜在不同环境温度和应变率条件下的力学行为，并与宏观尺度下金属材料的超塑性行为进行比较，揭示了纳米多孔金属的低温塑性行为。.(5) 孔表面对纳米多孔金属力学性能影响方面，一方面考察了形貌对钯纳米环催化性能的影响，分析了自由驰豫阶段的构型特征和循环稳定性，考察了比表面积和活性位点的变化。另一方面，考察了孔表面在位错萌生、起源和增殖过程中的作用。另外，还考察了孔洞形状和取向对材料力学行为的影响。.(6) 黑磷、金刚石碳管等新型功能材料力学性能研究方面，一方面研究了多层黑磷材料在拉伸载荷作用下的脆-韧转换行为及在惰性气体保护环境下黑磷片层的力学行为；另一方面，研究了在层间引入sp3共价键的新型金刚石石墨烯材料，考察了由其卷曲而成的金刚石碳管，在轴向压缩和扭转载荷作用下的屈曲稳定性、轴向拉伸载荷作用下的软化-硬化转变行为、及其在温度升高时独特的热缩行为和热稳定性，分析了层间断键对整体构型的关键影响。