辐照条件下层状微纳米结构金属材料力学性能研究

Mechanical properties under the extreme conditions such as high temperature and strong irradiation are crucial to the design of future fission/fusion reactors. Besides the great performance of high strength and toughness, multilayered metallic micro/nanostructures offer huge volume of interface which may act as obstacles to slip and sinks for the radiation-induced defects in the form of dislocation loops and voids, which provide a great prospect of potential applications in nuclear engineering. In this proposal, based on molecular dynamic simulation (MDs) results and a series of experiments at microscopic HRTEM experiments and macroscopic mechanical experiments, the interface of the multilayer structures, the irradiation-induced defects and the mechanical performances will be characterized and the basic database for the concerned irradiated materials will be established. Based on the understanding of interaction among dislocations, interfaces and irradiation-induced defects obtained by MDs and discrete dislocation dynamics (DDD), the micro- mechanisms of irradiation effects including irradiation hardening and irradiation embrittlement will be explored. Based on those micro-mechanisms, a meso-mechanics model with the assist of crystal plasticity will be provided. The proposed model will be performed by means of crystal plasticity finite element method (CPFEM) and elastic visco-plasticity self-consistent method (EVPSC) to get the macroscopic material performance. Therefore, a multi-scale framework to connect the micro-mechanism to macroscopic properties is established. Based on previous methods, we will explore the effect of layer thickness, defect density, grain size, residual stress, interface strength, crystal orientation relationship and misfit dislocations on the macroscopic mechanical properties to achieve the optimal design of new type of multilayered metallic material with high irradiation damage tolerance. The goal of this project is to develop a theoretical framework to analyze and evaluate the influence of interfaces and irradiation-induced defects on the mechanical properties, investigate the correlation between atomic-level mechanism and macroscopic material properties, and provide a feasible tool for the optimal design of multilayered structures.

材料在极端环境下的力学性能是新型核反应堆设计的核心问题，本项目结合理论与实验，研究层状微纳米金属材料在强辐照环境下的力学性能，为开发新型抗辐照材料提供理论基础。项目基于实验与分子模拟的结果，获得材料的界面特征、辐照缺陷类型、界面强度、残余应力、宏观力学性能等信息，建立层状金属材料抗辐照性能数据库；采用分子模拟和位错动力学等方法，重点考察位错、界面、辐照缺陷间的相互作用，探索辐照硬化、脆化的微观机理；建立基于微观机理、计及残余应力和界面损伤效应的晶体塑性细观力学模型，采用晶体塑性有限元法、弹粘塑性自洽方法预测层状微纳米辐照材料的力学性能，实现微观结构、变形机理与宏观性能的跨尺度关联，进而实现新型层状抗辐照材料的优化设计。本项目拟采用的理论与方法，可为层状辐照材料的研究提供基础性的框架；所揭示的微观变形机理与发展建立的细观力学模型，可为极端环境下不同材料的抗辐照性能的评估提供有益的参考。

辐照导致材料的硬化、脆化、肿胀等，严重危害材料的力学性能和核能工程的安全。层状微纳米材料具有高强高韧的特点，同时高的界面比可以有效吸收辐照缺陷，因此开展基于微观变形机理的层状材料抗辐照力学性能研究具有重要意义。本项目针对层状金属材料结构特征、层状金属材料微观变形机理、辐照缺陷生长机理及其相互作用、辐照缺陷对界面强度和对层状材料力学行为的影响等核心问题进行了系统性地研究。创新性的成果包括：. 基于弹性理论和分子模拟，研究了纳米孔洞和氦泡的表面能效应，首次提出氦泡平衡内压和等效表面能的概念，从弹性场和原子构型角度厘清了纳米孔洞表面能与常规表面能的区别，得到了计算表面能诱导应力的表达式。对六种材料的分子模拟结果表明，BCC材料的等效表面能不具有尺寸效应，而对FCC材料铜和镍，当孔径小于5nm时具有较为明显的尺度效应。. 针对一般多面体纳米晶体表面能诱发应力场问题，首次提出了一种“拉伸-释放”策略，利用叠加原理将原问题分解为拉伸子问题和释放子问题，前者可用解析法求解，后者可用有限元方法求解。计算表明大块单晶中往往存在不可忽略的表面效应。. 通过分子模拟方法，模拟分析了辐照氦泡对层状材料界面剪切强度的影响及其相应的变形机理，提出界面均匀滑移与氦泡诱导界面局部滑移扩展的竞争机制，结合界面两侧原子相对滑移的分析，合理解释了氦泡对强剪切方向和弱剪切方向力学行为的不同影响，同时结果表明界面剪切强度具有明显的各向异性。. 发展了适用于层状材料的晶体塑性本构模型，通过ABAQUS二次开发平台编制了计及位错密度、辐照缺陷密度、尺度等效应的计算程序，实现了FCC/BCC层状材料的显式动力学分析，建立了微结构参数与宏观力学性能的跨尺度关联。研究发现塑性变形局域化是层状材料压缩失效的主要形式，而界面可以阻碍塑性变形扩展，并改变其扩展路径，从而提高了材料的强度和韧性。. 上述成果可为抗辐照材料设计提供理论指导。