分子动力学在镁及镁合金微观塑性变形中的应用进展

Progress in the Application of Molecular Dynamics in Microscopic Plastic Deformation of Magnesium and Its Alloys

镁合金作为最轻的金属结构材料,具有比强度和比刚度高等优点,被视为极具应用潜力的轻合金。然而,镁合金为密排六方结构,可启动的滑移系少,室温塑性变形能力较差,这成为限制其广泛应用的主要原因之一。因此深入研究镁及其合金的微观塑性变形机制能为合金设计、加工工艺优化提供思路。镁及镁合金的微观塑性变形行为和机制较复杂,仅通过试验方法较难厘清具体微观结构、变形条件与材料性质之间的关系。而分子动力学作为研究原子(或分子)微观尺度下的行为和性质的重要方法,近年来在镁及其合金中的应用受到越来越多的关注。分子动力学方法可用于计算各类热力学及动力学性质,同时还可以模拟特定加载条件下的原子运动情况。对于镁及镁合金的微观塑性变形机制,目前研究者多关注位错与滑移、孪晶、晶界等问题。镁及镁合金最基本的滑移系为基面滑移系,同时还有潜在的柱面滑移系和锥面滑移系,研究者一般利用分子动力学模拟拉伸、压缩等情况来探究镁及镁合金中各滑移系的开动情况,其中对〈 c+a 〉位错的滑移和分解的情况研究较多。同时,由于镁及镁合金滑移系较少,相比于面心立方和体心立方结构金属,孪生对镁及镁合金塑性变形能力的贡献更不容忽视,已有研究者利用设置初始缺陷、微结构、溶质原子等方法模拟孪晶的形核与长大过程,讨论孪晶的形核条件及产生的孪晶类型。对晶界的研究可与细晶强化、织构等多晶体塑性变形机制相联系,但目前分子动力学方法能达到的尺寸规模还难以模拟出微米级的多晶,因此研究者多关注双晶、纳米多晶的晶界迁移行为以及晶界与其他微结构的交互作用等问题。同时,分子动力学计算模拟的可靠性主要依赖于势函数的精确性,早期由于势函数的不足,分子动力学方法仅在纯镁中应用较多。近年来随着势函数的研究增多,特别是次近邻修正嵌入原子势的发展,可用于描述二元系镁合金原子间作用的势函数开始丰富,对于镁合金的分子动力学研究也逐渐增多。本文对分子动力学理论和方法进行了简介,重点综述了分子动力学方法对镁及其合金的微观塑性变形机理的研究。从分子动力学方法在镁及其合金的势函数、位错滑移、孪晶、晶界、固溶及第二相等方面的应用进行概述,并对分子动力学模拟方法运用于镁及其合金进行了展望。

As the lightest metal structural materials,magnesium alloy has great application potential due to its high specific strength and specific stiffness.However,magnesium alloy is a hexagonal close packed structure with a limited number of active slip systems so that low ability of plastic deformation at room temperature becomes one of the major reasons limits the wider applications of magnesium alloy.Investigating and understan- ding the microscopic plastic deformation mechanism of magnesium and its alloys can provide further innovation to design alloys and optimize process. The complex behavior and the mechanism of plastic deformation in magnesium and its alloys result in clarifying the relationships between microstructure,deformation condition and materials properties difficultly only using experimental methods.Since molecular dynamics (MD) is an important method to understand various properties and phenomena of atoms or molecules under microscale,it attracts increasing attention to be applied to study magnesium and its alloys. It can calculate various thermodynamic and kinetic properties,and simulate atomic motion under specific loading conditions by using MD.Accordingly,recent reports pay more attention to the problems of dislocation and slips,twinning,and grain boundary about the microscopic deformation mechanism of magnesium and its alloys which basic slip system is basal slip,and the potential slip systems are prismatic and pyramidal slips.Generally,MD can be performed to explore the activity of slip systems under tension or compression,especially the slide and dissociation of 〈 c+a 〉 dislocation.In comparison with face-centered cubic and body-centered cubic metal,the contribution of twinning to plastic deformation is more obvious in magnesium and its alloys because of the less slip systems.There have been studies that discuss the nucleation conditions and the types of twins through setting initial defect,microstructure,solute atoms or other cases to simulate the nucleation and growth of twins.The study of grain boundaries can be related to the mechanism of polycrystalline plastic deformation such as fine grain strengthening and texture.Ho- wever,the size scale that can be achieved by MD methods is still difficult to simulate micron-sized polycrystals so that researchers tended to rea- lize the migration behavior and the interaction with other microstructures of grain boundaries in bi-crystals and nanopolycrystals.Important here is that the reliability of MD depends mainly on the accuracy of the interatomic potentials.In the early days,MD is only used in pure magnesium with the lack of interatomic potentials.And in recent years,with the improvement of interatomic potentials,especially the second nearest neighbor modified embedded-atom method (2NN MEAM),the investigations of magnesium and its alloys are increasing together with the development of interatomic potentials which can be used to describe binary magnesium alloys. In this paper,the MD theory and method are introduced briefly,and go further to review the MD to study the microscopic plastic deformation mechanism of magnesium and its alloys.It summarized the application of MD in interatomic potentials of magnesium and its alloys,dislocation and slips,twinning,grain boundary,solute atoms and second phases.At the end,it presented several prospects about MD applied to magnesium and its alloys.