Fe、Ni、Zr纳米晶体微观结构与力学性能的分子动力学模拟
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摘要
纳米晶体材料是指由晶粒尺寸在1—100nm的微晶粒组成的多晶体系。由于它具有独特的性能而备受关注。本文采用Voronoi元胞法构建纳米晶体初始位形，应用分子动力学弛豫技术和分析型嵌入原子方法(AEAM)多体势模拟纳米晶体Fe、Ni和Zr的微观结构和力学性能。从原子能量、径向分布函数和局域晶序等角度对纳米晶体的结构进行分析。
对Fe、Ni和Zr纳米晶体微观结构模拟表明：晶界结构与平均晶粒尺寸基本无关。随着平均晶粒尺寸降低，晶界所占比例增加，晶粒内部品格扭曲加剧，系统有序度愈加接近晶界的结构有序度。由于晶界原子间的结合较晶粒内部弱，纳米晶体的结合能较常规晶体低。受晶格扭曲和晶界比例增加的影响，纳米晶体的密度小于理想晶体密度。
对Fe、Ni纳米颗粒模拟表明：受自由表面的影响，纳米颗粒的结合能比同尺度的纳米晶体的结合能更低。受表面张力的作用，纳米颗粒出现收缩，且随着颗粒尺寸的降低，收缩量增加，晶格常数降低。
对Fe、Ni、Zr纳米晶体单向拉伸形变模拟表明：小品粒尺寸纳米晶体的弹性模量小于普通多晶体，随着晶粒尺寸的降低，纳米晶体的弹性模量下降。纳米晶体的力学强度随着晶粒尺寸的减小而减小，屈服强度与晶粒尺寸间存在反Hall-Petch关系。小晶粒尺寸纳米晶体内部很少存在位错，纳米晶体的塑性变形主要依靠晶界滑移和晶粒的转动来实现。
纳米晶体Fe的单一轴向压缩模拟表明，纳米晶体的压缩形变过程表现为三个典型的特征区域：弹性形变阶段、塑性流变阶段和应变强化阶段。纳米晶体材料在塑性流变阶段表现出极好的压缩延性。纳米晶体压缩形变过程也主要是通过晶界的滑移和晶粒的转动来实现的。
Nanocrystalline materials are polycrystals with mean grain size ranging from 1nm to 100nm. Due to its unique properties, great attention to nanocrystalline materials has been increased in past years. In the present paper, the initial structural models of nanocrystallites are constructed with Voronoi cell method. The microstructure and mechanical properties of nanocrystalline Fe, Ni and Zr are simulated with the molecular dynamic (MD) simulation and the analytic embedded-atom method (AEAM). The microstructures of nanocrystallites are analyzed with atomic energy method, radius distribution function and common-neighbor analysis technique.
The simulation result of microstructures of nanocrystallite Fe, Ni and Zr reveal that the structure of grain boundary in nanocrystalline is almost independent on average grain size. With reducing grain size, the fraction of grain boundary increases and the lattice distortion in grain interior enhances, and the structural difference between grain boundary and grain interior diminishes gradually. As the bonds between grain-boundary atoms are weaker than atoms in grain interior, the cohesive energy of nanocrystalline materials is lower than general microcrystal and decreases with reducing average grain size. Affected by the lattice distortion in grain interior and the high proportion grain boundary, the density of nanocrystallite can't approach that of the perfect crystal.
The simulation result of nanoparticle Fe and Ni indicate that the cohesive energy of nanoparticle is lower than that of nanocrystallite with the same grain size for the free surface of nanoparticle. Under the function of surface tension, the nanoparticles show lattice contraction, and with reducing the particle size, the contraction increases and the lattice constant of nanoparticle decreases.
The uniaxial tension simulations of nanocrystalline Fe, Ni and Zr show that the elastic modulus of nanocrystallite with small average grain size is lower than the conventional microcrystal and decreases with reducing grain size. The mechanical strength of nanocrystalline decrease with grain size reducing and there exists a reverse Hall-Petch relation between the yield stress and the mean grain size. As known from the dislocation theory, dislocations seldom appear in the grain interior, and the plastic
deformation of nanocrystallite mainly carries out by the grain boundary sliding and grain rotation.
The uniaxial compression simulation of nanocrystalline Fe indicates that the compressive deformation process of nanocrystallite can be described as three characteristic regimes: quasi-elastic deformation, plastic flow deformation and strain strengthening regimes. During the plastic flow deformation process, the nanocrystallite show very good compressive ductibility. As discussed in the tensile deformation, the deformation mechanisms in the compressive deformation of nanocrystallite are mainly from atomic sliding and rotation of grain boundary.
对Fe、Ni和Zr纳米晶体微观结构模拟表明：晶界结构与平均晶粒尺寸基本无关。随着平均晶粒尺寸降低，晶界所占比例增加，晶粒内部品格扭曲加剧，系统有序度愈加接近晶界的结构有序度。由于晶界原子间的结合较晶粒内部弱，纳米晶体的结合能较常规晶体低。受晶格扭曲和晶界比例增加的影响，纳米晶体的密度小于理想晶体密度。
对Fe、Ni纳米颗粒模拟表明：受自由表面的影响，纳米颗粒的结合能比同尺度的纳米晶体的结合能更低。受表面张力的作用，纳米颗粒出现收缩，且随着颗粒尺寸的降低，收缩量增加，晶格常数降低。
对Fe、Ni、Zr纳米晶体单向拉伸形变模拟表明：小品粒尺寸纳米晶体的弹性模量小于普通多晶体，随着晶粒尺寸的降低，纳米晶体的弹性模量下降。纳米晶体的力学强度随着晶粒尺寸的减小而减小，屈服强度与晶粒尺寸间存在反Hall-Petch关系。小晶粒尺寸纳米晶体内部很少存在位错，纳米晶体的塑性变形主要依靠晶界滑移和晶粒的转动来实现。
纳米晶体Fe的单一轴向压缩模拟表明，纳米晶体的压缩形变过程表现为三个典型的特征区域：弹性形变阶段、塑性流变阶段和应变强化阶段。纳米晶体材料在塑性流变阶段表现出极好的压缩延性。纳米晶体压缩形变过程也主要是通过晶界的滑移和晶粒的转动来实现的。
Nanocrystalline materials are polycrystals with mean grain size ranging from 1nm to 100nm. Due to its unique properties, great attention to nanocrystalline materials has been increased in past years. In the present paper, the initial structural models of nanocrystallites are constructed with Voronoi cell method. The microstructure and mechanical properties of nanocrystalline Fe, Ni and Zr are simulated with the molecular dynamic (MD) simulation and the analytic embedded-atom method (AEAM). The microstructures of nanocrystallites are analyzed with atomic energy method, radius distribution function and common-neighbor analysis technique.
The simulation result of microstructures of nanocrystallite Fe, Ni and Zr reveal that the structure of grain boundary in nanocrystalline is almost independent on average grain size. With reducing grain size, the fraction of grain boundary increases and the lattice distortion in grain interior enhances, and the structural difference between grain boundary and grain interior diminishes gradually. As the bonds between grain-boundary atoms are weaker than atoms in grain interior, the cohesive energy of nanocrystalline materials is lower than general microcrystal and decreases with reducing average grain size. Affected by the lattice distortion in grain interior and the high proportion grain boundary, the density of nanocrystallite can't approach that of the perfect crystal.
The simulation result of nanoparticle Fe and Ni indicate that the cohesive energy of nanoparticle is lower than that of nanocrystallite with the same grain size for the free surface of nanoparticle. Under the function of surface tension, the nanoparticles show lattice contraction, and with reducing the particle size, the contraction increases and the lattice constant of nanoparticle decreases.
The uniaxial tension simulations of nanocrystalline Fe, Ni and Zr show that the elastic modulus of nanocrystallite with small average grain size is lower than the conventional microcrystal and decreases with reducing grain size. The mechanical strength of nanocrystalline decrease with grain size reducing and there exists a reverse Hall-Petch relation between the yield stress and the mean grain size. As known from the dislocation theory, dislocations seldom appear in the grain interior, and the plastic
deformation of nanocrystallite mainly carries out by the grain boundary sliding and grain rotation.
The uniaxial compression simulation of nanocrystalline Fe indicates that the compressive deformation process of nanocrystallite can be described as three characteristic regimes: quasi-elastic deformation, plastic flow deformation and strain strengthening regimes. During the plastic flow deformation process, the nanocrystallite show very good compressive ductibility. As discussed in the tensile deformation, the deformation mechanisms in the compressive deformation of nanocrystallite are mainly from atomic sliding and rotation of grain boundary.
引文
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[3] H Gleiter. Nanostructured materials: Basic concepts and microstructure. Acta Mater, 2000, 48(1): 1-29
[4] 张立德，牟季美．纳米材料和纳米结构．第一版．北京：科学出版社，2001，51-67
[5] 巩雄，张桂兰，汤国庆．纳米晶体材料研究进展．化学进展，1997，9(4)：349-360
[6] 杨卫，马新玲，王宏涛，等．纳米力学进展．力学进展，2002，32(2)：161-174
[7] Thomas G J, Siegel R W, Eastman J A. Grain boundary in nanophase palladium: high resolution electron microscope and image simulation. Scripta Metallurgica et Materiallia, 1990, 24(1): 201-206
[8] Li D X, Ping D H, Ye H Q, et al. HREM study of the microstructure in nanocrystalline materials. Material Letters, 1993, 18(1-2): 29-34
[9] Zhao Y H, Lu K, Liu T. EXAFS study of structural characteristics of nanocrystalline selenium with different grain sizes. Phys Rev B, 1999, 59(17): 11117-11120
[10] Wang J, Wolf D, Phillpot SR. Computer simulation of the structure and thermo-elastic properties of amodel nanocrystalline materials. Phil Mag A, 1996, 73(3): 517-555
[11] 常明，杨保和，常皓．纳米晶体微观畸变与弹性模量的模拟研究．物理学报，1999，48(7)：1215-1222
[12] 常明，孙伟，邢金华等．模拟纳米晶体原子分布及X射线散射理论图案．物理学报，1997，46(7)：1319-1325
[13] 常明，孙伟，郭长海等．纳米晶体结构与性能的模拟研究．物理学报，1997，46(7)：1326-1331
[14] Lu K, Sun N X. Grain-boundary enthalpy of nanocrystalline selenium. Phil Mag Lett, 1997, 75(6): 389-395
[15] Lu K, Luck R, Predel B. The interracial execess energy in nanocrystalline nickel-phosphorus materials with different grain size. Scripta Metallurgica et Materiallia, 1993, 28(11): 1387-1392
[16] A I Gusev. Effects of the nanocrystallines state in solids. Physics-Uspekhi, 1998, 41(1): 49-76
[17] 孙伟，常明，杨保和．分子动力学模拟纳米晶体铜的结构和性能．物理学报，1998，47(4)：591-597
[18] Qin X Y, Wu X J, Cheng L F. The microhardness of nanocrystalline silver. Nanostructured Materials, 1995, 5(1): 101-110
[19] Zhang H Y, Hu Z Q, Lu K. Hall-Perch relationship in the nanocrystalline Selenium prepared by crystallization from the amorphous state. Journal of Applied Physics, 1995, 77(6): 2811-2813
[20] Qin X Y, Zhang X R, Cheng G S, et al. The elastic properties of nanostructured Ag measured by laser ultrasonic tecnique. Nanostructured Materials, 1998, 10(3): 661-672
[21] J Schitz. Strain-induced coarsening in nanocrystalline metals under cyclic deformation. http://www, fysik, dtu. dk/~schiotz/papers/coarsening-preprint, pdf, 2003-11-6
[22] Zhang .J F, Wu X J. Thermal stability and grain growth of nanocrystalline material silver. Acta Physica Sinica, 1993, 2:583-590
[23] Krasilnikov N A, Raab G I. Grain boundary effects in nanocrystalline copper. Materials Science Forum, 1999, 294-296:701-706
[24] M Tortes, F Pastor, I Jimenez, et al. Configurational entropy of rational approximates of a decagonal quasilattice in a pure Phason approach. Scripta Metallurgica et Materiallia, 1992, 27(1): 83-88
[25] Nieman G W,Weertman J R, Siegel R W. Tensile strength and creep properties of naocrystalline palladium. Scripta Metall Mater,1990, 24 (1) : 145-150
[26] Gunther B, Baalman A, Weiss H. Preparation and mechanical properties of ultrafine grained metals. Mater Res Soc Syp Proc, 1990, 195:611-615
[27] Koblev N P, Siofer Ya M, Andrievski R A, et al. Micro-hardness and elastic properties of nanocrystalline silver. Nanstructured Materials, 1993, 2(5): 537-544
[28] Kizuka T, Ichinose H, Ishida H. Structure and hardness of nanocrystalline silver, J Mater Sci, 1997, 32(6): 1501-1507
[29] Hall O. The deformation and ageing of mild steel: Ⅲ discussion of results. Proc Phys Soc London B, 1951, 64:?47-753
[30] Petch N J. The cleavage strength of polycrystals. J Iron Steel Inst, 1953, 174:25-28
[31] 卢柯，卢磊．金属纳米材料力学性能的研究进展．金属学报，2000，36(8)：785-789
[32] Schiotz. Simulations of nanocrystalline metals at the atomic scale. What can we do? What can we trust? arXiv:cond-mat/O109320 v1 18 Sep 2001
[33] Nieman G W, Weertman J R, Siegel R W. Mechanical behavior of nanocrystalline Cu and Pd. J Mater Res, 1991, 6(5): 1012-1027
[34] McFadden S X, Mishra R S, Valiev R Z, et al. Low-temperature superplasticity in nanostructured nickel and metal alloys. Nature, 1999, 398:684-686
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