纳米晶金属加工硬化的分子动力学模拟-材料加工工程专业论文.docxVIP

模拟了层状多晶 Ag 纳米晶单轴拉伸过程中的加工硬化。在层状多晶 Ag 纳米晶块体的单轴拉伸过程中观察到了压杆位错和 Lomer-Cottrell 位错锁等阻碍位 错运动的障碍，因而出现了加工硬化现象，同时具有较稳定的塑性流变。而在层 状多晶 Ag 纳米晶薄膜的拉伸过程中，由于受到 Z 方向自由表面的影响，因而观 察到更少的压杆位错和 Lomer-Cottrell 位错锁等阻碍位错运动的障碍，因而出现 的加工硬化现象很微弱，但也出现了稳定的塑性流变。然而在层状多晶 Ag 纳米 线的单轴拉伸过程中则没有观察到加工硬化现象，其原因是自由表面对位错活动 的影响，位错没能够发生强烈的相互作用，没能够形成阻碍位错运动的障碍。模拟了平均晶粒尺寸为 1.769 nm，5.562 nm 和 8.566 nm 的多晶 Mo 纳米线 单轴拉伸过程中的加工硬化。发现这些多晶 Mo 纳米线在单轴拉伸过程中首先以 BCC 结构到 FCC 结构的相变开始塑性变形，然后随着相变的进一步深化出现了 位错的活动。但是很快晶界中纳米孔洞的萌生和长大以及相互间的合并导致了纳 米线的失效，使得多晶 Mo 纳米线丧失了加工硬化的能力。关键词：双晶 Ag 纳米线，多晶 Ag 纳米线，层状多晶 Ag 纳米晶，多晶 Mo 纳米 线，分子动力学模拟IIAbstractNanocrystalline metals are widely applied in microelectronic systems, integrated microelectromechanical systems, nanocomponents, mask aligners, surface engineering and biomedicane etc., as a result of their extremely high strength and predominant physics-chemical properties. Precious metal silver is also extensively applied in connection wires of super integrated circuits, due to its high thermal and electrical conductivity. However, an obvious disadvantage for nanocrystalline metals emerges as the grain size reduced to nanometer size -- low ductiliity, which attributes to their low work hardening rate. Fabrication of nanocrystalline materials with high strength as well as high ductility through material design will make a big difference in the engineering application and broadening the application of nanocrystalline materials. Therefore, this thesis is focused on the work hardening of nanocrystalline metals.Molecular dynamics simulation is the main method adopted in the study for work hardening of nanocrystalline metals in this thesis.Bicrystalline silver nanowires with asymmetric tilt grain boundary of various inclination angle, symmetric tilt grain boundary and random high angle grainboundary as well as ∑3 coherent twin grain boundary were simulated under uniaxialtensile deformation to investigate their work hardening. The results indicated that only bicrystalline silver nanowires with symmetric