一种刻画纳米晶材料中应力诱导晶粒长大的多尺度本构模型

Stress-driven nano-grain growth (NGG) plays a significant role in improving the synergy of strength and ductility in nanocrystalline materials, which provides a new route for fabricating light materials with high strength and ductility. Extensive investigations have shown that there are two main modes of NGG, i.e., shear-coupled migration of grain boundaries and nano-grain rotation. To date the mechanism of shear-coupled migration of grain boundaries has been clarified while that of nano-grain rotation remains unclear due to the difficulty of its occurrence in bi-crystals. In this project, the applicant will try to reveal the mechanism of NGG, based on which a multiscale constitutive model will be developed in order to quantitatively predict the effect of NGG on the mechanical behavior of nanocrystalline materials. First, molecular dynamics (MD) simulations will be conducted to study the mechanism of nano-grain rotation and its coupling with shear-coupled migration of grain boundaries. Secondly, mechanistic models will then be developed to investigate the energy change during the process of NGG and the critical stress that is needed to initiate NGG by applying the theory of defects in solids. Finally, based on the above MD and theoretical results, the phase field micro-elasticity model will be modified to describe the evolving process of stress-driven NGG in mesoscale. The modified phase field model will be then introduced into a macroscale visco-plastic constitutive model to establish a multiscale constitutive model. This model could provide a convenient and feasible theoretical guide for designing nanocrystalline materials with optimal balance between strength and ductility.

应力诱导晶粒长大能有效提高纳米晶材料的强韧综合性能，从而为制备轻质、高强、高韧的材料开辟了一条新的途径。已有研究表明，晶粒在应力诱导下主要有两种长大模式——晶界迁移和晶粒旋转。目前晶界迁移的机理已较为清楚，而晶粒旋转由于较少发生于双晶结构，其机理尚缺乏深入研究。本项目在揭示晶粒长大机理的基础上，旨在构建一种多尺度本构模型，定量表征晶粒长大对纳米晶材料强韧性能的影响。首先，利用分子动力学模拟研究晶粒旋转及其与晶界迁移相耦合的机理；其次，采用固体缺陷理论建立相应的力学模型，考察晶粒长大过程的能量变化及临界驱动应力，进一步阐明应力诱导晶粒长大的机理；最后，在介观尺度建立模拟应力诱导晶粒长大过程的相场微弹性模型，并将该介观模型引入至宏观的粘塑性本构模型中，构建刻画晶粒长大的多尺度本构模型，从而为设计具有最优强韧综合性能的纳米晶材料提供方便可行的理论指导。

应力诱导晶粒长大能有效提高纳米晶材料的强韧综合性能，从而为制备轻质、高强、高韧的材料开辟了一条新的途径。已有研究表明，晶粒在应力诱导下主要有两种长大模式——晶界迁移和晶粒旋转。目前晶界迁移的机理已较为清楚，而晶粒旋转由于较少发生于双晶结构，其机理尚缺乏深入研究。本项目在揭示晶粒长大机理的基础上，旨在构建一种定量的细观力学模型，定量表征晶粒长大两种模式协同耦合作用对纳米晶材料强韧性能的影响。首先，建立基于位错的力学模型研究晶粒旋转及其与晶界迁移相耦合的机理；其次，采用固体缺陷理论研究上述耦合过程对纳米晶材料裂纹钝化及断裂韧性的影响；最后，研究了一种特殊的晶界迁移过程——多层级纳米孪晶结构中第二级孪晶的退孪晶化对材料裂纹钝化的影响，最终为设计具有最优强韧综合性能的纳米晶材料提供方便可行的理论指导。