导　　师： 陈志谦

学科专业： H0502

授予学位： 硕士

作　　者： ;

机构地区： 西南大学

摘　　要： 本文基于分析型嵌入原子势的分子动力学方法,模拟了体心立方和面心立方两种纳米金属材料在不同加载作用下的力学性能,还同时研究了温度、尺寸和应变率对其力学性能的影响。从应力-应变曲线、原子位置的变化图中解释了纳米材料产生的力学性能。 对BCC结构的纳米Mo丝研究表明：在拉伸负载下,位错和滑移是纳米丝变形机制的主要原因,从表面原子形成空位开始,随着缺陷的不断的扩展最后造成了纳米的断裂。在一定温度范围内,从100K到500K,随着温度的增加,铝单晶纳米丝拉伸过程中屈服应力逐渐降低,断裂应力逐渐增加；屈服应变逐渐减小,断裂应变在常温300K时达到最大。此外,100K和300K时,纳米丝的结构在拉伸过程中出现了明显的屈服变形,内部变形以滑移为主；在500K时,没有出现明显的屈服变形,原子排列不规则成无序状态,结构内部拌有大量的位错和滑移现象。纳米丝在不同尺寸的模拟结果表明,在弹性变形阶段,随着尺寸的增加,应力-应变曲线没有出现明显的区分变形；进入到纳米拉伸的塑性变形阶段,尺寸效应也变得越来越明显,随着尺寸的增加,屈服变形的区域变得越来越窄,所需要的屈服应变逐渐的减小,屈服强度反而逐渐的增加。而在不同应变率的模拟中,随着应变率的升高,屈服强度和断裂强度都增大。 对FCC结构的纳米Ag丝研究表明：对于单晶纳米丝,在拉伸过程中,在原子结构模型变形方面,内部结构以滑移为主,两侧伴有部分的位错,出现了堆垛现象,在距离纳米丝上下端大约1//3处出现了多重处颈缩现象。在多晶纳米丝拉伸的过程中,纳米丝处于弹性变形阶段时,纳米丝出现了多次屈服变形,应力随着应变的增加呈“之”字形上升和下降,纳米丝进入到塑性变形阶段,应力曲线随着应变的增加迅速的上升,然后出现了一个断裂屈服平台。随着应变的增加应力急速下降,此时纳米丝表现出明显的超塑性。 不同结构的纳米金属材料在拉伸过程中,BCC纳米丝比FCC纳米丝更早的进入到弹性变形阶段,BCC纳米丝比FCC纳米丝致密低,但是有更好的延展性。在弹性变形阶段和塑性变形阶段几乎拥有相等的应变间隔,屈服强度小于断裂强度,纳米丝表现出更强的延展性；相对于BCC纳米丝,而FCC的纳米丝主要以弹性变形为主,但是弹性变形也只占用了整个加载应变的1//4,进入塑性变形之后,原子机构模型就很快出现了颈缩,并开始断裂,屈服强度大于断裂强度。 在原子结构的变形方面,BCC结构的纳米丝从表面开始遭到破坏,原子模型的变形以表面位错和内部滑移为主,并出现少量的堆垛现在,最后,纳米丝从中间出现颈缩现象,开始断裂；而FCC结构的纳米从一开始就是内部结构出现的大量的滑移现象,表面出现不同程度的缺陷,原子结构的变形以滑移为主,最后在两头1//3出出现了双重颈缩现象。 The mechanical properties of two kinds of nanometal under different external load are studied by molecular dynamics/(MD/) simulation, it's based on the analytical embedded atom potential, the effect of temperature、size and strain rate of mechanical properties were studied in detail. It explains the unique mechanical properties of nanometal by studying the strain-stress curve, atomic location map. The simulation results of the body centered of the Mo nanocrystals indicate that:dislocations and slips are main mechanisms of the tension deformation of single Mo naro-wires. From the surface atomic beginning, as the expansion of atomic vacancies are created nano-wire fracture. When the temperature is increased from 100K to 500K, the yield strength and strain decrease, and the fracture strength and strain increase, respectively. The maximum fracture strength is at room temperature /(300K/). Furthermore, the structures of the stretched nanowires at 100K and 300K have apparent yield deformation and the interior deformation is caused by slips. At 500K, the structures of the stretched nanowires have no apparent yield deformation, and the atomic arrangement of the interior is irregular and disordered and mixed with a large number of dislocations and slips. The simulation results of different sizes show that the strain-stress curves have no obvious deformation in elastic deformation with the sizes increasing. In the yield deformation, size effects have an important role. The region of yield deformation is more and more narrow and the yield strain decrease, the yield strength increasing. In the simulation of strain rate show that the yield and fracture strength will increase with the strain rate increased. The simulation results of face centered nano-Ag wire show that:for single crystal, during the tensile load, slips are the main cause of the inside deformation and appear part of dislocation, forming lots of stows and emergent multiple necking at a distance of top and bottom nanowires about 1\3. during the tension of polycrystalline nanowires, in the elastic deformation stage, the nanowires appear several yield deformation and the stress is the“之”shape up and down with strain increasing. In the plastic deformation, the stress-strain curve increase rapidly as the strain increasing, then there is a fracture yield platform. The nanowire appeared super-plastically as the strain increased. Different structures of nanowires during tensile process indicate that:BCC nanowires go into the elastic deformation earlier than FCC nanowires and BCC nanowires dense low, but it has better scalability. During the elastic deformation and plastic deformation, it has almost equal strain intervals; the yield strength is less than breaking strength, showing greater scalability; relative to the BCC of nanowires, FCC nanowires are based on elastic deformation, however elastic deformation took 1//4 of the entire load strain; during plastic deformation, the atomic model appeare necking quickly and the yield strength greater than the breaking strength. In the deformation of the atomic structure, the nanowires of BCC structure destroy form the surface beginning, the deformation of the atomic model are based on the surface dislocation and internal slip and emergence of a small amount of stacking, finally, the nanowires are necking from the middle of structure and appeared breaking; but the nanowires of FCC structure appeared the vast internal slip of the surface from the beginning, the deformation of the atomic structure are based on slip and finally emergence of dual-necking.

关 键 词： 纳米金属 力学性能 分子动力学模拟

分 类 号： [U4]

领　　域： [交通运输工程]