【作者】 王欣；

【导师】 单德彬；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2008， 硕士

【摘要】 加工硬化现象普遍存在于金属塑性加工过程中。长久以来,这种现象一直吸引着人们的注意力。当在低温(T<0.3Tm)条件下施加应力时,加工硬化可以在2个甚至3个数量级内改变韧性材料的强度,因此许多学者对它进行了大量研究,但其中大多数研究仅局限于宏观范畴。晶体是由大量的原子有序排列而成,材料的强度来源于原子间的相互作用,塑性来源于原子间的相互运动,因此,直接从原子尺度上对材料的微观力学行为进行研究非常有必要。分子动力学方法可以直接对原子的运动进行观察,从而分析材料的变形机理。为了研究纳米单晶材料的加工硬化现象,本文以分子动力学为主要研究手段,模拟了单晶Cu的三维拉伸、压缩过程,分析微观塑性变形机制,并研究了缺陷对材料力学性能的影响。本文模拟采用EAM势函数描述原子间的相互作用情况,共轭梯度算法用于求解运动方程,并采用中心对称参数表征晶体缺陷类型。在此基础上研究了单晶Cu纳米线0K下的单向拉伸、压缩过程。分别从位错机制、硬化现象及拉、压不对称性三个方面对模型的微观变形过程进行分析。结果表明:单晶铜纳米线的屈服通过{111}<112>部分位错的形核和扩展实现,塑性变形由堆垛层错的形成所主导。位错交割及位错与层错的相互作用可以引起屈服应力升高。研究最终发现,单晶纳米线没有加工硬化现象发生。带孔洞的单晶Cu拉伸模拟主要研究了缺陷对材料弹性模量和屈服应力的影响规律。结果表明:随孔洞体积分数的增大,材料的弹性模量和屈服应力近线性降低,塑性变形提前。更多还原

【Abstract】 The phenomenon of work-hardening prevalently consists in metal plastic forming. This phenomenon has attracted people’s attention for a long time. Work-hardening can change the strength of ductile material by two or three orders of magnitude when given stress in low temperature conditions(T<0.3Tm). Therefore, extensive studies have been carried out on it. While most of the studies only confine to the microscopic category. As the strength of materials results from the interactions of dislocations, the plastic deformation results from the mutual movement of dislocations, it is important to investigate the microscopic mechanical behavior on atomic scale. While the molecular dynamics simulation method is able to observe the motion of atoms, it can be used to investigate the deformation mechanism.In order to study the work-hardening phenomenon of single crystal nano-materials, molecular dynamics simulations are carried out on the tension and compression process of single crystal copper. The effects of crystal defects on the material mechanical properties were studied. Some analysises on the plastic forming mechanism were given.Embedded atom method and conjugate gradient method are used in this simulation. The centrosymmetry parameter is used to characterize crystal defects. With the molecular dynamics simulation mentioned, we then study the uniaxial tension and compression processes of single crystal Cu nano-wire in three dimensions under absolute zero temperature condition. In this dissertation, the research focuses on three aspects around the microscopic deformation mechanism: stress-strain analysis, dislocation mechanism and the unsymmetry of tension and compression. The results indicate that, nanowires yield via the nucleation and propagation of the {111}<112> partial dislocations. Formation of stacking fault is the dominant mechanism in the microscopic deformation of single crystal. Interaction of stacking fault and dislocation is the principal factor leading to the rising of yield stress. But we find that, there is no work-hardening in single crystal nano-wire.The tension simulation of single crystal copper with void is mainly to study the influence of void volume fraction on elastic modulus and yield stress. The results show that, elastic modulus and yield stress decrease with increasing void volume fraction, the plastic deformation occurs in advance.更多还原