【作者】 郑斌；

【导师】 齐民； 王轶农；

【作者基本信息】 大连理工大学 ， 材料学， 2009， 博士

【摘要】 本论文使用分子动力学方法模拟研究了金属纳米颗粒和金属纳米线的变形行为,主要包括:Cu纳米粒子和Cu/Ag核壳纳米粒子的压缩变形,Cu纳米线、Ni纳米线和孪晶结构纳米线的拉伸变形,以及不同原子配比的Al-Cu合金纳米线的拉伸变形。从而获得了一些低维纳米材料的机械性能及变形机制。通过模拟尺寸为4 nm、6 nm和8 nm的Cu纳米粒子的压缩变形,发现Cu纳米粒子的压缩应力、温度及结构势能均随压缩应变增加而呈锯齿形上升特征。通过观察原子结构演化可得到:在压缩应力锯齿形上升时,纳米粒子内部的缺陷原子较多,是纳米粒子在自由表面限制下的蓄能过程;而压缩应力锯齿形下降时,缺陷原子迅速消失,意味着表面限制已被冲破,纳米粒子开始释放能量。把该锯齿形压缩应力与纳米尺度摩擦的锯齿形摩擦力进行对比,发现二者的产生原理相似。通过模拟Cu/Ag核壳纳米粒子的压缩变形,发现了核壳相界面对纳米粒子变形的影响规律。原子结构图显示,Cu/Ag核壳纳米粒子的压缩过程中,大量单一类型的Shockley不全位错在核壳界面处形核,并平行地滑过Cu核的{111}面,最终导致Cu/Ag核壳纳米粒子被压为长条状。在被压缩的纳米粒子中还观察到了孪晶结构,经分析,它通过一对反平行的Shockley不全位错运动而形成。原子应力分布图显示,核壳界面处部分原子在压缩方向的压强极高,可保证开动一对反平行的不全位错。通过模拟Cu纳米线和Ni纳米线的拉伸变形,发现在初始的结构弛豫中,表面应力能够驱动Cu、Ni纳米线的轴向从<100>晶向转变为<110>晶向。原子结构图显示,再取向过程是通过<112>/6型Shockley不全位错平行运动实现,其中,在Ni纳米线中观察到了滑移面偏转到轴向的运动。对再取向后的纳米线实施拉伸变形,结果显示,拉伸变形实质上是上述再取向过程的逆过程,若卸载,纳米线能够发生形状回复,在Cu纳米线中最高可获得50%的应变回复量。基于分子动力学方法,进一步模拟了Cu孪晶结构纳米线的拉伸变形,来分析孪晶界对纳米线机械变形的作用机制。基于无宏观变形的新孪生机制,构建了横截面尺寸从3到6 nm,孪晶带宽度从1.25到10 nm的一系列纳米线。结果显示,孪晶界能增强纳米线的拉伸强度,其最高增幅达60%。通过应力—应变曲线的斜率变化及原子结构演化,发现该强化由延长弹性变形和阻碍位错运动两个先后过程构成,二者为此消彼长的竞争关系,可建立孪晶界面面积（S_T）与自由表面面积（S_F）的比值（S_T/S_F）来表征该竞争关系,当（S_T/S_F）＞0.3时,孪晶界延长弹性变形处于优势地位,可被明显地观察到。使用分子动力学方法模拟Al-Cu合金纳米线的拉伸变形,通过改变原子比例来控制纳米线的机械性能和变形机制。首先检查对称度较高的AlCu纳米线,发现表面应力能驱使它处于高弹状态,在接下来的变形中发生结构无序转变,可显示较高的拉伸强度,但塑性差。而对于Cu原子比例较高的AlCu₃纳米线,其拉伸应力—应变曲线呈现多次屈服,原子结构分析表明,相邻两屈服点之间包括（0（?）1）孪晶扩展的塑性变形过程和纳米线中部的完整晶体的弹性变形过程,卸载时能获得4%～9%的弹性回复,纳米线的强度和塑性都较好。对于Al原子比例较高的Al₃Cu和Al₁₅Cu纳米线,它们的塑性较好,但拉伸强度明显下降。更多还原

【Abstract】 In this thesis,deformation behaviors of nanomaterials,including Cu nanoparticles, eore/shell-type Cu/Ag nanoparticles,defect-free nanowires,twinned nanowires and Al-Cu alloy nanowires were studied by molecular dynamics simulation method.Then,some mechanical properties and deformation mechanisms of lower-dimension nanomaterials were obtained.The compressive deformation of Cu nanoparticles with diameter of 4 nm,6 nm and 8 nm was examined by using molecular dynamics simulation.The compressive stress increased in a zigzag fashion with strain,accompanying with the temperature zigzag rise and the potential energy zigzag rise.The analysis of atomic structure demonstrated that the compressive stress rise and drop corresponded to that the defective atoms aggregated to form clusters and that the clusters decomposed into single atoms,respectively.The essence of compressive deformation of nanoparticles is a repeated process of storing and releasing energy,as is similar to the stick-slip motion in nanoscale friction process.The tmiaxial compressive deformation of core/shell-type Cu/Ag nanoparticles was simulated by molecular dynamics,revealing the role of nano-phase boundaries in the mechanical deformation.The simulations show that single type of Shockley partial dislocations glide across {111} slip planes and then resulted in elongated Cu cores.Twinnings, which originated from the movement of a pair of anti-parallel dislocations,were observed in the compressed core/shell-type Cu/Ag nanoparticles.The ultra-high atomic level stress in the phase boundary can ensure the movement of a pair of anti-parallel dislocations.The uniaxial tensile loading of defect-free Cu and Ni nanowies was carried out by utilizing molecular dynamics simulation.The results of structure relaxation showed that the surface stress can drive crystallographic lattice reorientation,in which the axial crystal orientation of nanowires transformed from <100> to <110> with the help of Shockley partial dislocations of Burgers vector <112>/6.Especially,we observed the rotation movement of slip plane in Ni nanowires.The axial tensile loading can drive reverse process of above reorientation.Under tensile loading and unloading,the Cu nanowires exhibit recoverable strains up to 50%.The uniaxial tensile deformation of twinned Cu nanowires was examined to reveal the strengthening mechanism of twinning boundary by utilizing molecular dynamics simulation. The initial configurations of nanowires,including the side length of cross section varied from 3 to 6 nm and twin thickness varied from 1.25 to 10 nm,were created basing on new twinning mechanism with zero macroscopic strain.The results showed that twinned nanowires were strengthened by twinning boundaries and the highest increment reached 60%.The slop of stress-strain curves and atomic structural evolution indicated that the strengthening process consists of two competitive processes,including extending elastic deformation and twinning boundaries blocking the movement of dislocations.Therefore,a theory model,that the ratio of twinning boundary area to surface area in twinned nanowires decided the energetic nucleation of dislocation,was built.When the ratio is more than 0.3,the process of extending elastic deformation can be observed in our simulation.A molecular dynamics simulation was utilized to study Al-Cu alloy nanowires for obtaining mechanical properties and deformation mechanisms by changing the ratio of Al atom to Cu atom.The simulation results showed that surface stress can force AlCu nanowires into high-elastic states and induce slip and clusters in AlCu₃ nanowires.Two distinct types of deformation mechanisms were observed.One is that the AlCu nanowires underwent the changes of elastic deformation,amorphous transition,necking,and breaking.The other is because the（0（?）1） twinning and crystal region coexist,the calculated tensile curves of AlCu₃ nanowires show many yield points,and unloading can lead to elastic recovery of 4%～9%.We increased the percentage of Al atom to build Al₃Cu and Al₁₅CU nanowires,which showed a good plastic but lower strength than other nanowires.更多还原