非晶合金纳米线结构和力学性能的分子动力学模拟研究

发布时间：2018-06-26 14:14

本文选题：非晶合金纳米线 + 表面偏析　；参考：《清华大学》2015年博士论文

【摘要】：非晶合金纳米线因具有特殊的结构和纳米尺寸效应,可望应用于催化介质、数据存储介质、电子元器件和光学器件等领域。由于实验条件的限制,目前对于非晶合金纳米线微观结构和力学性能的认识还很有限。因此,开展相关研究具有重要意义。本论文以Cu64Zr36非晶合金纳米线为代表,利用分子动力学方法,模拟不同工艺制备非晶合金纳米线,研究其微观结构和力学性能之间的联系,并对非晶合金纳米线的尺寸效应进行了深入分析。首先,模拟聚焦离子束(Focused ion beam,FIB)切割、热压印和铸造方法分别制备了非晶结构的FIB纳米线、热压印纳米线和铸造纳米线。研究发现制备工艺对纳米线的内部结构有显著影响,其中FIB纳米线的内部结构非常接近块体非晶,热压印纳米线的内部结构与块体非晶稍有偏差,而铸造纳米线的内部结构与块体非晶以及其它两种纳米线差别很大。这三种纳米线的表面都存在不同程度的成分偏析。非晶合金纳米线表面的势能梯度是导致非晶合金纳米线表面出现成分偏析的原因。对纳米线进行拉伸和压缩模拟实验,结果表明,不同工艺方法制备的非晶合金纳米线的力学性能有显著差别。其中,二十面体含量最高的热压印纳米线强度最高,延展性最差,二十面体含量最低的铸造纳米线强度最低,延展性最好。在形变过程中,FIB纳米线和热压印纳米线中都出现了应变局域化,铸造纳米线的变形则比较均匀。非晶合金纳米线的力学性能与原子结构以及排列密度紧密相关。最后,以FIB纳米线和铸造纳米线为对象,研究非晶合金纳米线的尺寸效应。研究认为,非晶合金纳米线的表观力学性能由其内部和表面的力学性能共同决定。在拉伸情况下,表面起着缺陷的作用,而内部的残余压应力对非晶合金纳米线有强化作用,两者共同的作用使得纳米线整体的强度随纳米线尺寸的变化相对较小。在压缩情况下,表面对纳米线的内部变形没有影响,但是内部的残余压应力对纳米线有弱化作用。由于FIB纳米线和铸造纳米线的表面厚度差别很大,导致两者表现出不同的尺寸效应。FIB纳米线的拉伸强度随纳米线尺寸减小先缓慢下降然后上升,而铸造纳米线的拉伸强度随纳米线尺寸减小而缓慢上升。FIB纳米线的压缩强度随尺寸减小而减小的速率与其内部残余压应力随尺寸减小而增加的速率相同。而铸造纳米线的压缩强度随纳米线尺寸减小的变化却是有增有减。
[Abstract]:Amorphous alloy nanowires are expected to be used in catalytic media, data storage media, electronic components and optical devices due to their special structure and nanoscale effect. Due to the limitation of experimental conditions, the understanding of microstructure and mechanical properties of amorphous alloy nanowires is very limited. Therefore, it is of great significance to carry out relevant research. In this paper, Cu64Zr36 amorphous alloy nanowires were used to simulate different processes to prepare amorphous alloy nanowires, and the relationship between microstructure and mechanical properties was studied. The size effect of amorphous alloy nanowires was analyzed. Firstly, FIB nanowires with amorphous structure, thermal imprint nanowires and foundry nanowires were prepared by simulated focused ion beam (FIB) cutting, thermal imprint and casting methods, respectively. It is found that the preparation process has a significant effect on the internal structure of the nanowires, in which the internal structure of FIB nanowires is very close to that of bulk amorphous, and the internal structure of thermal imprinted nanowires is slightly different from that of bulk amorphous wires. However, the internal structure of cast nanowires is quite different from that of bulk amorphous and other two nanowires. The three nanowires have different degrees of segregation on the surface. The potential energy gradient of amorphous alloy nanowire surface is the cause of composition segregation on amorphous alloy nanowire surface. The tensile and compression experiments of nanowires show that the mechanical properties of amorphous alloy nanowires prepared by different processes are significantly different. Among them, the highest icosahedron content of thermal imprint nanowires have the highest strength and the worst ductility, and the lowest icosahedron content of the casting nanowires have the lowest strength and the best ductility. Strain localization occurs in FIB nanowires and thermal imprint nanowires during deformation, while the deformation of cast nanowires is uniform. The mechanical properties of amorphous alloy nanowires are closely related to atomic structure and arrangement density. Finally, the size effect of amorphous alloy nanowires was studied by using FIB nanowires and cast nanowires. The apparent mechanical properties of amorphous alloy nanowires are determined by their internal and surface mechanical properties. Under tensile condition, the surface acts as a defect, while the internal residual compressive stress strengthens the amorphous alloy nanowires. Together, the strength of nanowires varies relatively little with the size of nanowires. In the case of compression, the surface has no effect on the internal deformation of nanowires, but the internal residual compressive stress weakens the nanowires. Because the surface thickness of FIB nanowires and cast nanowires are very different, they show different size effects. The tensile strength of FIB nanowires decreases slowly and then increases with the decrease of FIB nanowires. However, the tensile strength of cast nanowires increases slowly with the decrease of the size of nanowires. The compression strength of FIB nanowires decreases at the same rate as the internal residual compressive stress increases with the decrease of the size of FIB nanowires. However, the compressive strength of cast nanowires increases and decreases with the decrease of nanowire size.
【学位授予单位】：清华大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG139.8

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