亚微米级晶粒尺寸下金属的塑性在某一临界晶粒尺寸显著降低是制约细晶强化应用的瓶颈。目前，对此共性问题的物理本质以及造成塑性降低的临界晶粒尺寸的影响因素与计算模型研究尚不充分并存在疑义。并且，针对应用广泛的亚微米晶金属的增塑研究也不充分。本项目旨在探明晶粒细化导致塑性降低及其影响因素的科学问题，建立临界晶粒尺寸计算模型；探索在临界晶粒尺寸以下利用第二相粒子实现增强增塑的条件，解决亚微米晶金属低塑性的瓶颈问题。项目以位错动力学为基础，分析晶内位错数量及分布、晶界特征等与晶粒尺寸的关系，研究晶粒尺寸对位错形核-增殖-运动-湮灭过程的影响，获得晶粒尺寸与位错动力学行为之间的关系规律。研究第二相粒子与晶粒尺寸的有效耦合及对位错动力学行为的影响，揭示亚微米晶粒条件下第二相粒子作为晶内附加位错源实现增强增塑的条件和机制。项目的开展不仅可以丰富完善金属塑性变形理论，还可为金属的高性能化提供新的技术思想。

The drastic reduction of ductility in ultrafine grained metallic materials with submicrometer grain at a critical grain size is the roadblock problem for the grain refining strengthening. Up to now, the researches on the intrinsic mechanism of this common phenomenon and the factors affecting the critical grain size which causes the ductility reduction and the calculation model of the critical grain size are still insufficient and doubtful. In addition, the works on the enhancement of ductility in the widely application metallic materials with submicrometer grain are still less. Based on the above challenges, the purpose of this project, on one hand, is to find out the scientific problems of the ductility reduction induced by grain refinement and its influencing factors, and to establish the calculation model of critical grain size. On the other hand, this project aims at solving the bottleneck problem of low plasticity in metallic materials with submicrometer grain by using secondary precipitated particles and discussing the condition for the simultaneous enhancement of ductility and strength. In this project, the relationship between dislocation sources, the characterization of grain boundaries and grain size will be discussed too, the effect of grain size on the process of nucleation, multiplication, movement and annihilation of dislocations will be investigated based on the principle of dislocation movement. Thereby, the law of relationship between grain size and dislocation dynamics is obtained. Further, the effective coupling between secondary precipitated particles and grain size and the effect of the coupling on the dynamics of dislocation movement will be studied. The intrinsic mechanism and condition of the enhancement of strength and ductility by using secondary precipitated particles in metallic materials with the submicrometer grain size will be elaborated on the basis of introducing dislocation sources by secondary precipitates. The implement of the proposed research will not only enrich the theory of plastic deformation in ultrafine grained materials at submicrometer range, but also provide a new scientific principle for developing metallic materials with high performance.