纳米尺度单晶铜材料表面切削特性分子动力学模拟

采用分子动力学模拟方法研究单晶铜材料表面纳米切削特性。通过对单晶铜纳米切削过程进行分子动力学建模、计算与分析,研究了不同切削速度及切削厚度对单晶铜材料表面纳米切削过程中微观接触区域原子状态和切削力变化的影响规律。研究结果发现：在单晶铜表面纳米切削过程中,切削速度越高,切屑堆积体积越大,切屑里原子的排列越紧密,位错缺陷分布区域越大;在同种切削速度下,切削厚度越大,在刀具前方堆积的切屑体积越大,位错缺陷越多。不同切削速度及切削厚度下,切削力曲线均在切削初期呈上升趋势,达到稳定切削状态后围绕稳定值进行波动,但在切削初期,切削速度及切削厚度越大,切削力上升幅度越大;达到稳定切削状态后,切削速度、切削厚度越大,切削力越大。     

双峰结构纳米晶铜的力学行为

通过分子动力学模拟、黏塑性本构模型和纳米压痕试验验证相结合的研究方法,系统研究了双峰结构(晶粒尺寸服从统计学中双峰分布)纳米晶铜的变形机理与力学性能。结果表明：在塑性变形过程中位错首先在纳米晶铜的细晶区形核和扩展,且方向互相平行;而粗晶区的位错滑移方向相互交叉,且粗晶尺寸越大,越容易发生位错缠绕和交滑移。双峰结构纳米晶铜的流变应力随着粗晶尺寸的增大而增大,硬度随着粗晶体积分数的增大而减小。由黏塑性本构方程计算得到的应力变化规律与由经验公式和分子动力学模拟得到的结果一致,且本构方程计算得到的流变应力和经验公式所得结果的相对误差小于5%。     

纳米多晶铜单点金刚石切削亚表层损伤机理

基于Poisson-Voronoi和Monte Carlo方法构建了多晶铜分子动力学模型,研究了纳米切削中多晶铜材料去除、切削力变化及晶界与位错间的相互转化机制。研究结果表明：晶界的阻碍作用使得切屑流向发生了改变,并在已加工表面形成凹槽和毛刺;切削过程中晶界前方材料变形能的逐渐积聚及晶界的最终断裂,造成了切削力发生由最大峰值到最小谷值的大幅波动;晶界附近的材料去除经历了材料变形积聚、位错穿越晶界、晶界转变为位错及晶界最终断裂等过程。通过详细分析多晶铜纳米切削中位错与晶界间的演化过程,揭示了晶界与位错间的相互转化机制,丰富了多晶铜亚表层损伤机理的内涵。     

带孔纳米单晶铜悬臂梁弯曲的分子动力学模拟

应用分子动力学方法模拟了带孔纳米单晶铜悬臂梁的弯曲过程。通过一端固定另一端施加横向作用力驱使原子运动,得到纳米单晶铜悬臂梁弯曲的变形图。对其不同于宏观连续介质理论的位移-载荷曲线进行分析,给出了合理的解释。结果表明:纳米尺度下的微缺陷对纳米单晶铜悬臂梁的性能具有明显的影响;尺寸效应和表面效应的影响,以及位错滑移和弛豫的综合作用,使得纳米单晶铜悬臂梁在纳米尺度下表现出与宏观尺度下不同的力学特性。     

有孔纳米单晶铜薄膜拉伸断裂特性的分子动力学模拟

运用分子动力学方法模拟有孔纳米单晶铜薄膜的拉伸断裂过程.通过施加拉伸应变驱动原子运动和小孔变形,展示有孔纳米单晶铜薄膜随应变增加的原子位形直观分布;超过弹性极限后,位错发生于小孔的应力集中处.计算所得原子平均能量随变形的增长趋势,结果表明,有孔纳米单晶铜薄膜的变形机制可以显著地分为三个阶段,弹性延伸、塑性滑移、沿位错线开裂.     

单晶硅纳米级磨削过程的理论研究

对内部无缺陷的单晶硅纳米级磨削过程进行了分子动力学仿真,从磨削过程中瞬间原子位置、磨削力、原子间势能、损伤层深度等角度研究了纳米级磨削加工过程,解释了微观材料去除、表面形成和亚表面损伤机理。研究表明：磨削过程中,单晶硅亚表面损伤的主要形式是非晶结构形式,无明显的位错产生,硅原子间势能的变化是导致单晶硅亚表面损伤的重要原因;另外,发现磨粒原子与硅原子之间有黏附现象发生,这是由于纳米尺度磨粒的表面效应而产生的。提出了原子量级条件下单晶硅亚表面损伤层的概念,并定义其深度为沿磨削深度方向原子发生不规则排列的原子层的最大厚度。     

基于原子力显微镜和分子动力学的纳米压痕技术研究

利用原子力显微镜对真空蒸发镀膜技术制得的单晶铜薄膜试件进行了纳米压痕试验。通过进行各种压痕深度下的试验,获得了压痕深度对试件力学性能的影响关系。试验得到的试件弹性模量为67.0 GPa±6.9 GPa。试件的硬度值随着压痕深度的减小而不断增大,表现出强烈的尺寸效应。在原子力显微镜试验的同时,使用分子动力学仿真方法对单晶铜薄膜的纳米压痕过程进行了研究。仿真结果表明单晶铜薄膜的纳米压痕的力学机理不是位错在晶体中运动产生的塑性变形,而是非晶态产生的变形,从而解释了尺寸效应产生的原因。     

基于准连续介质理论的单晶铜纳米切削过程仿真

为了提高计算精度和扩大计算尺寸,克服分子动力学模拟方法计算效率低、模拟尺寸小、边界条件影响大等特点,本文采用多尺度准连续介质力学数值方法对单晶铜纳米切削过程进行仿真,探究单晶铜的纳米切削机理。验证了不同的刀具前角、切削厚度对切削过程中的位错、切削力和残余应力的影响。实验结果表明,当采用同一把刀具时,随着切削厚度的增加切削过程中的切削比能逐渐减小而位错深度、残余应力均相应增加。当采用同一个切削厚度,不同的刀具前角时发现,采用负前角切削过程中的切削力波动范围最大。     

基于单晶铜的纳米压痕的机理分析

为了进一步深入研究纳米压痕的机理，提高纳米压痕的测量精度和应用范围，对单晶铜纳米压痕的机理展开详细分析与研究。从纳米压痕的理论方法入手，建立单晶铜的分子动力学模型，利用该模型对压力和势能进行仿真分析，并得到相应的曲线。在单晶铜的纳米压痕过程中，压力先增加后下降，其中，曲线的波动主要受表面粗糙度、表面化学反应膜、表面力、表面加工硬化等因素的影响；原子势能一直上升，最后趋于稳定，推断出纳米压痕没有出现位错的现象。最后，对纳米压痕的局限性进行总结，并针对不同的问题，提出行之有效的措施，为纳米压痕的精度提高奠定良好的基础。     

小切深磨削条件下工件表面硬化机理

以位错运动造成塑性变形的理论为基础,深入分析了小切深条件下磨削力机械作用硬化机理和材料热相变硬化机理。通过不同磨削参数的小切深磨削硬化试验,分析磨削硬化过程中不同磨削参数条件对工件表面强化层形成的影响及其金相组织转变的情况,深入研究磨削强化层组织的形成机理。试验结果表明,小切深条件下磨削加工试件表面的硬化主要以位错运动而产生的强化层为主,提高磨削深度和降低工件进给速度会增大工件表面显微残余应力,增强试件表层硬化层的形成效果。     

分子动力学仿真过程中硅晶体位错模型的构建

基于位错形成机理,在单晶硅晶体结构基础上描述了硅晶体位错形成的过程。应用偶极子模型,构建了60°滑移位错芯和螺旋位错芯,进而得到硅晶体含有60°滑移位错的模型和含有螺旋位错的模型。对含有螺旋位错的硅晶体模型进行了分子动力学仿真计算,分析了含有螺旋位错的硅晶体超精密磨削的加工过程,研究了含有螺旋位错缺陷的硅晶体纳米级磨削机理。     

Influence of anisotropy of nickel-based single crystal superalloy in atomic and close-to-atomic scale cutting

Nickel-based single crystal superalloy is widely used in the field of aerospace and nuclear reaction equipment due to its good properties. Ultra-precision machining technology is an important means to ensure the surface quality of parts. However, the anisotropy of materials has great influence on the evolution of surface and subsurface defects and the removal of materials in the process of machining. In this paper, The MD (molecular dynamics) modeling and simulation verification of cutting anisotropic nickel-based single crystal superalloy workpiece with silicon nitride tool is carried out by using the mixed potential function simulation. Through cutting simulation and visualization, the types, number, deformation area and dislocation evolution of the machined surface defects and inside of the workpiece defect of nickel-based single crystal superalloy with different crystal orientations are analyzed. The evolutionary mechanism of the machined surface defects and the law of material removal are discussed. The research content provides a theoretical basis for parameter optimization and improvement of machining quality in the atomic and close-to-atomic scale (ACS) cutting process, and technical support for efficient and precise machining process of the nickel-based superalloy.     

Subsurface Damage in Scratch Testing of Potassium Dihydrogen Phosphate Crystal

Potassium dihydrogen phosphate (KDP) is an important electro-optic crystal, often used for frequency conversion and Pockels cells in large aperture laser systems. To investigate the influence of anisotropy to the depth of subsurface damage and the profiles of cracks in subsurface of KDP crystal, an experimental study was made to obtain the form of subsurface damage produced by scratches on KDP crystal in [100], [120] and [110] crystal directions on (001) crystal plane. The results indicated that there were great differences between depth and crack shape in different directions. For many slip planes in KDP, the plastic deformation and cracks generated under pressure in the subsurfacewerecomplex. Fluctuations of subsurface damage depth at transition point were attributed to the deformation of the surface which consumed more energy when the surface deformation changed from the mixed region of brittle and plastic to the complete brittle region along the scratch. Also, the process of subsurface damage from shallow to deep, from dislocation to big crack in KDP crystal with the increase of radial force and etch pit on different crystal plane were obtained. Because crystallographic orientation and processing orientation was different, etching pits on (100) crystal plane were quadrilateral while on (110) plane and (120) plane were trapezoidal and triangular, respectively.     

基于三类单胞模型的开、闭孔泡沫材料弹性性能的有限元分析

在对发泡过程进行假设的基础上,根据泡沫材料微结构的特点,建立开孔和闭孔泡沫材料的简单立方、面心立方和体心立方单胞模型,导出相对密度和胞体结构参数的定量关系.由这三类单胞模型描述的泡沫材料均具有周期性的特点,且单胞还具有结构对称性.文中利用有限元方法,在适当的边界条件下计算整个密度范围内泡沫材料的弹性模量和泊松比,数值拟合模量计算公式.通过与已有的实验资料和理论预测结果的比较,说明有限元数值计算的有效性.     

Influence of Crystal Structure on Friction Characteristics of Rare-Earth and Related Metals in Vacuum to 10−10 mm of Mercury

Friction, wear, and metal-transfer characteristics were determined for rare-earth and related metals in vacuum to 10^?10 mm of mercury. The metals studied were lanthanum, neodymium, praseodymium, cerium, holmium, erbium, gadolinium, dysprosium, samarium, yttrium, and thallium. Friction and wear experiments were conducted with the rare-earth or related metals generally sliding against 440-C stainless steel.

The results of the investigation indicate that crystal structure considerations and polymorphism can explain the friction, wear, and metal-transfer characteristics of the rare-earth and related metals in vacuum. Close-packed hexagonal crystal forms of the rare earths and thallium had much lower friction, wear, and metal-transfer characteristics than face-centered or body-centered cubic structures.     

Subsurface deformation of germanium in ultra-precision cutting: characterization of micro<Emphasis Type="Bold">-</Emphasis>Raman spectroscopy

Germanium is widely used for infrared components and high-speed transistors. Ultra-precision single-point diamond turning (SPDT) can be employed to achieve its nanometric surface, while subsurface damage has a significant influence on surface integrity due to its brittle feature. This study investigated its subsurface deformation after SPDT. A characterization model of micro-Raman spectroscopy was established with the aim to characterize the subsurface deformation, including phase transformation and residual stress, where the transformation from single crystal structure to amorphous structure is dominant. A spectral fitting method was utilized to analyze the variation of subsurface deformation in various machining parameters. The degenerated single crystal Raman peak was widened due to anisotropic stress. It was discovered that the turning operation at a moderately low spindle speed and tool feed rate reduces phase transformation and residual stress based on the value of Raman ratio and Raman shift. These findings provided significant basis for the manufacturing process of optical components with good surface integrity.     

SEM imaging of the cell structure of dislocation networks in cold worked cu single crystals

In certain metallurgical applications of SEM, such as the observation of deformation effects, and the recovery and recrystallization processes in bulk specimens, it is of special interest to obtain information about local inhomogeneities of the dislocation densities in a sample. This can be obtained using TEM on thin foils. Previously information on distortion in cold-worked specimens was obtained by measuring the dependence of the deterioration of the resolution of the selected area channelling patterns (SACP) on the degree of deformation. This method, however, is not applicable to highly deformed materials. Direct SEM imaging of the cell structure of the dislocation network has been achieved without resolving single dislocations by using a suitable arrangement of specimen and detector, such that only backscattered electrons (BSE) leaving the surface at a large angle to the normal contribute to the image signals. The signal-to-noise ratio was improved to the degree that it was possible 1) to image the cell structure of the dislocation network and 2) on heating the specimens, to record the growth of recrystallized regions with TV-frame frequency. The imaging of the cell structure gives qualitative information about the degree of crystal deformation and the crystallographic direction of grain boundary motion during recrystallization.     

INVESTIGATION INTO THE CHARACTERISTICS OF WHITE LAYERS PRODUCED IN A NICKEL-BASED SUPERALLOY FROM DRILLING OPERATIONS

The nature of white layers generated during metal cutting operations have for some time been the object of scientific controversy in reference to their metallurgical nature/structure. This research aims to study the structure of the white layer from an abusive cutting operation (i.e., drilling) in a nickel-based superalloy at both macro and micro scale levels. This has been achieved by using (1) a Focus Ion Beam to mill a sample for Transmission Electron Microscopy to analyse the grain size within the white layer, (2) Scanning Electron Microscopy to see shape characteristics of the white layer generated, (3) X-ray diffraction to see any alterations to the crystallinity of the structure, and (4) nano-indentation within this layer to compare its hardness with that of the bulk material. The in-depth analysis of white layers generated from non-standard drilling parameters of alloy RR1000 has shown that the layer is of the same structure as the bulk material: face centre cubic (FCC). The analysis has also shown the layer possesses a greatly reduced grain size of approximately 50 nm as opposed to the bulk material which has an average grain size of 22–63 μm (ASTM 8-5). Although nano-indentation has also shown that the analysed layer has a 45% higher hardness than that of the bulk material. This analysis indicates that the white layer created from machining is a combination of both mechanical and thermal effects and not only thermal as previously shown.     

Crystal plasticity and the delamination theory of wear

Some implications of crystal plasticity effects with respect to the delamination theory of wear are discussed. Critical experiments to clarify the soft versus hard surface question, of interest with respect both to wear and to deformation in general, are suggested. A specific dislocation model for hexagonal close-packed (h.c.p.) metals under wear conditions is proposed and a correlation with stacking fault energy for face-centred cubic (f.c.c.) and h.c.p. metals is discussed.     

High-temperature dislocation plasticity in the single-crystal superalloy LEK94

The evolution of the dislocation structure that forms during uniaxial creep deformation in the single‐crystal superalloy LEK94 of low density and with Re additions was analysed using transmission electron microscopy. The material has a γ/γ′‐microstructure consisting of γ′‐cubes (L12 phase, 80 vol.%) separated by thin γ‐channels (face‐centred cubic). <100> tensile creep tests were performed at 980 and 1020 °C at stresses of 200 and 240 MPa. The microstructure was investigated at three characteristic stages of creep (directly after loading, at 5% strain and after rupture) to show the evolution of the dislocation structure during high‐temperature creep. It was found that in the early stages of creep, a₀/2<011> dislocations form within the γ‐channels. Later on, dislocation networks form and γ′ cutting processes with a₀/<001> superdislocations are observed. The results are in line with observations made for other superalloy single crystals in the high‐temperature low‐stress creep regime.