加工参数对单晶γ-TiAl表面质量和亚表层损伤的影响机理

为研究加工工艺参数对纳米切削单晶γ-TiAl合金表面质量和亚表层损伤的影响机理,以分子动力学（molecular dynamics, MD）为基础理论,采用非刚性金刚石刀具建立三维纳米切削模型,通过研究切屑体积、表面粗糙度、静水压分布、位错密度、位错演化、相变原子数,详细分析不同切削速度和切削深度对表面和亚表面结构的影响。结果发现:随着切削速度的增加,切屑体积增大,加工效率提升,且存在切削速度为100 m/s的临界值。表面粗糙度先减小后增大,同样存在切削速度为100 m/s的临界值。位错的复杂程度降低,位错密度减小,塑性变形程度增加;随着切削深度的增加,切屑体积增大,加工效率提升,表面粗糙度、位错密度以及塑性变形程度显著增加。在切削过程中,发现位错主要分布在刀具前方和下方,在刀具前方45°方向存在V形位错和梯杆位错以及位错间的相互反应,且切削完成后残留下空位和原子团簇等稳定缺陷。      

On tool wear and its effect on machined surface integrity

This paper presents the results of an investigation of induced residual stress, induced strain, and induced subsurface energy in machined surfaces due to the machining process. The influence of tool wear on residual stress, strain, and energy is also reported. The exact elasticity solution for a split ring was extended and used to calculate the residual stress in the machined surface by using ring dimension changes caused by the electrochemical removal of a thin layer of residually stressed surface. The strain distribution beneath the machined surface was determined by using the grid technique. The subsurface energy stored in the machined surface was then obtained from the data of residual stress and strain. For the materials studied, this investigation showed that such energy could not be neglected when establishing the total energy needed for machining a unit volume of material. Tool coatings having different surface roughness and tools having various magnitudes of flank wear were investigated. The experimental results show that tool wear is a dominant factor affecting the values of induced residual stress, strain, subsurface energy, and the quality of the machined surface. The increase of tool wear caused an increase of residual stress and strain beneath the machined surface. It was also found that the overall energy stored in the machined subsurface increases as the tool wear increases and as the tool surface gets rougher. When the cutting tool is severely worn, the machined surface not only becomes very rough, but also contains many partially fractured laps or cracks. This makes tool wear a key factor in controlling the quality of the machined surface.     

低氢含量工业纯钛循环变形的微观结构Ⅱ.氢化物的溶解和应变诱发氢化物的形成

对均匀弥散分布着γ氢化物的低氢含量的工业纯钛进行循环疲劳试验．发现滑移带能够穿过氢化物的共格界面，使氢化物发生塑性剪切变形．由于位错周围的氢气团能够在位错的拖拽下，随同位错一起运动．所以位错的运动能够促使氢原子沿滑移带扩散．滑移带穿过氢化物的过程伴随有氢原子沿滑移带的扩散，材料中原有的氢化物在滑移带的冲击下，由于局部的氢原子浓度太低，而重新溶解．同时位错也会带着氢气团在氢化物处塞集，引起氢原子的局部富集；并导致应变诱发氢化物的产生．研究表明这个可逆相变过程由位错运动诱发的氢原子扩散所控制．     

Dynamic characteristics of nanoindentation using atomistic simulation

Atomistic simulations are used to investigate how the nanoindentation mechanism influences dislocation nucleation under molecular dynamic behavior on the aluminum (0 0 1) surface. The characteristics of molecular dynamics in terms of various nucleation criteria are explored, including various molecular models, a multi-step load/unload cycle, deformation mechanism of atoms, tilt angle of the indenter, and slip vectors. Simulation results show that both the plastic energy and the adhesive force increase with increasing nanoindentation depths. The maximum forces for all indentation depths decrease with increasing multi-step load/unload cycle time. Dislocation nucleation, gliding, and interaction occur along Shockley partials on (1 1 1) slip planes. The indentation force applied along the normal direction, a tilt angle of 0°, is smaller than the force component that acts on the surface atoms. The corresponding slip vector of the atoms in the (1 1 1) plane has low-energy sessile stair-rod dislocations in the pyramid of intrinsic stacking faults.     

What can be learnt on the yield stress anomaly of Ni3Al using AFM observations

We examined by in situ scanning probe microscopy the slip traces at the free surface of Ni₃(Al,Ta) single crystals deformed in ultra high vacuum environment at various temperatures. It is evidenced that the mean free path of superdislocations in the primary octahedral planes strongly decreases with increasing temperatures. This is quantified by the decrease in the number of dislocations along slip traces corresponding to the primary (111) slip planes. Correlatively the number of cross-slip events increases with increasing temperature, together with a drastic increase of the distance travelled by the dislocations on the cube cross-slip plane. The experimental results are discussed in the frame of previous theoretical models that have been proposed to predict the yield stress anomaly of L1₂ intermetallic compounds.     

Effect of crystallography on residual stresses during ultra-precision machining of sapphire

Residual stresses play an important role in determining the quality of a machined surface during ultra-precision machining (UPM) of ceramics by initiating cracks at premature loads. In this study, the anisotropy in residual stresses resulting from UPM of two crystallographic planes of sapphire is investigated using micro-Raman spectroscopy. An analytical model is then used to improve our understanding of plastic deformation during UPM and its contribution to residual stress. Results show a strong dependence of residual stress magnitude on the slip and fracture systems activated during machining which are anisotropic based on the cutting direction.     

位错和位错偶沿单─滑移系从裂纹尖端的发射

采用计算机模拟了位错和位错偶沿单一滑移系从裂纹尖端的发射,考察了滑移面取向、外加载荷、晶格摩擦力以及位错发射的临界应力强度因子对所发射的位错数量、塑性区与无位错区大小以及裂关残余应力强度因子的影响研究表明,位错从裂纹尖端发射的临界应力强度因子对无位错区的存在和其大小起决定作用,而外加载荷与晶格摩擦力主要影响位错发射的数量以及塑性区大小．在Ｉ型载荷作用下,滑移面与裂纹面的夹角越大,从裂尖发射出的位错数量越多,位错对裂纹的屏蔽效应也越大当裂纹发射位错后的残余应力强度因子仍然较大时,位错偶就有可能在裂纹尖端附近产生井沿着几个滑移面发射,但发射出的位错偶对裂纹没有明显的屏蔽作用在滑移面不垂直于裂纹面时,发射出的位错或位错偶关于裂纹面呈不对称分布     

WC涂层纳米划痕过程中摩擦学性能及演化特征的分子动力学研究

为研究纳米划擦过程中WC涂层纳米级摩擦演化特征,利用大型原子/分子大规模并行模拟器建立了不同条件（载荷、划痕深度、划痕速度）下的分子动力学模拟模型。结果表明：摩擦力和摩擦系数随着划痕深度的增加而增大;当压头划擦试样时,沿划痕方向在压头前方及凹槽两侧的原子被挤压、剪切、堆积。瞬时摩擦曲线在初始阶段和稳定阶段表现出明显的摩擦学特征,摩擦过程中压头下方区域晶体出现错位、滑移、间隙或空位。随着划痕速度的增加,体系应变能超过原子间相互约束,原子突破约束,在划痕沟槽两侧堆积,堆积的表面形貌和外缘变得粗糙,亚表面晶体结构产生缺陷。本研究有助于在纳米尺度了解WC涂层摩擦过程的微观磨损机理。     

FLOW STRESS MODEL FOR HARD MACHINING OF AISI H13 WORK TOOL STEEL

1. Introduction The manufacturing process of dies/molds is one of the most demanding tasks in manufacturing en- gineering. Complex workpiece geometries, high material hardness as well as short lead time are among the main obstacles. At the same time, quality requirements become more and more important due to in- tensified competition and quality awareness. Traditionally, the production of molds/dies generally in- volves conventional machining in the annealed (soft) state, followed by heat trea…     

Dislocation processes during the deformation of NiAl–0.2at.%Ta

Macroscopic compression tests and in situ straining experiments in a high-voltage electron microscope were performed on NiAl–0.2at.%Ta at room temperature and at elevated temperatures. At room temperature in soft orientations, dislocations of a〈100〉 Burgers vectors bow out between jogs. In contrast to pure NiAl, the dislocations move in a viscous way between the pinned configurations. At 475°C in a hard orientation, dislocations with a〈110〉 Burgers vectors move in a viscous way in configurations strongly depending on the respective slip plane. Preferred orientations of dislocations are of mixed character, most pronounced as very straight dislocations oriented along 〈111〉 directions on {110} planes. These configurations cannot be explained on the basis of the existing atomistic theories. The flow stress is interpreted in terms of the back stress of the dislocations bowing out between jogs at room temperature, the statistical theory of solid solution hardening, and the formation of atmospheres containing Ta atoms at elevated temperatures.     

Residual stresses in face finish turning of high strength nickel-based superalloy

This paper investigates the residual stresses distributions introduced in a new generation nickel-based superalloy RR1000 by surface finish turning. The residual stresses introduced as a function of depth have been analysed for a series of machining trials with round and rhombic inserts, coated and uncoated inserts, new and worn tools, and chipped tool. The residual stress depth profiles obtained by X-ray diffraction, and layer removal show that the tool type, tool coating, tool wear and tool breakage influence the residual stress. The extent of plastic deformation for different cutting conditions has been inferred from the different peak width. Overall, residual stresses tend to have a tensile character at all depths in the hoop direction, but exhibit a significant compressively stressed zone in the radial direction.     

Exploring Plastic Deformation Mechanism of Multilayered Cu/Ti Composites by Using Molecular Dynamics ModelingEI北大核心CSCD

Multilayered metallic composites have attracted great interest because of their excellent characteristics. In recent years, the mechanical behavior of Cu/Ti composites is described in terms of macroscopic or mesoscopic scales, but the micromechanism regarding dislocation slip, twinning and shear banding at heterogeneous interfaces remains unclear. In this work, the molecular dynamics method is used to study the uniaxial tensile and plane strain compression deformation of the Cu/Ti multilayered composites with characteristic initial crystal orientations. The simulation results show that under the tensile load, dislocations are preferentially nucleated at the heterogeneous interface between Cu and Ti, and then slip along {111} plane within the Cu layers. The corresponding mechanism is confined layer slip. With the multiplication of dislocations, dislocations interact with each other, and intrinsic stacking faults and deformation twins are formed in Cu layers. However, no dislocation slip or twinning is activated within the Ti layers at this stage of deformation. As the load increases, the stress concentration at the Cu/Ti interface leads to the fracture of the composites. For the composites under plane strain compression, the stress concentration at the Cu/Ti interface triggers the formation of shear bands in the Ti layer, and there are only very limited dislocations within the shear bands and their adjacent area. With the increase of applied strain, the common action of various deformation mechanisms causes the grains to rotate, and the disorder degree of complex atoms increases. In addition, the micro-plastic deformation mechanism and mechanical properties of Cu/Ti complex with different initial orientations and strain rates are significantly different. The results reveal the microscopic deformation mechanism of the laminated composites containing hcp metals.     

TiAl裂纹形核和塑性变形关系的研究

对层状结构ＴｉＡｌ在扫描电镜中原位拉伸,发现裂纹前端首先出现滑移带,只有当局部塑性变形发展和临界状态,位错塞积应力等于原子键合务时才会使微米尺寸卑贱 裂纹不连续形核,裂纹可沿滑移带形核,可沿其它是面形核,方扩展,新的不连续裂纺的阻力不为民增大     

Study of polycrystalline Al2O3 machining cracks using discrete element method

Discrete element method (DEM) was introduced to simulate crack initiation and propagation of polycrystalline alumina during the brittle model machining process. A bonded particle model (BPM) was employed in the DEM simulations procedure to generate a particle assembly system similar to the micro-structure of the polycrystalline alumina. Particle and parallel bond properties, which were calibrated through a series of numerical tests, were subsequently used in the simulations of polycrystalline alumina cutting process and scratching tests. It is found that the cracks initiated right under or in front of the machining tool. There were many micro-cracks remained on the machined surface, some of them propagated downwards to form macro-cracks or forwards to lead material removal. Both DEM simulations and acoustic emission measure experiments have found that the fracture became acute when the normal and the tangential force changed suddenly, causing the crack number to increase. In 3D DEM scratching simulation, the surface cracks length and subsurface cracks depth linearly increased with the scratching depth, the value agreed well with the experimental results, and the surface-damage width decreased gradually with the depth to the surface, looking like half of a coin.     

Machining induced residual stress in structural aluminum parts

Machining operations of aluminum structural parts are typically carried out under high feeds and high cutting speeds. Under these conditions, high thermomechanical loads are exerted on the workpiece, which may result in changes in the subsurface material. Residual stresses can be one of the machining induced changes and can lead to considerable rejection rates caused by part distortion. Due to their significant economic importance, it is essential to understand the influence of the machining process on the residual stresses in aluminum. This paper presents the influence of the machining parameters as well as the cutting edge geometry on residual stress of workpieces made out of a forged aluminum alloy.     

Multiscale modeling of plastic deformation of molybdenum and tungsten: I. Atomistic studies of the core structure and glide of 1/2〈1 1 1〉 screw dislocations at 0 K

Owing to their non-planar cores, 1/2〈1 1 1〉 screw dislocations govern the plastic deformation of body-centered cubic (bcc) metals. Atomistic studies of the glide of these dislocations at 0 K have been performed using Bond Order Potentials for molybdenum and tungsten that account for the mixed metallic and covalent bonding in transition metals. When applying pure shear stress in the slip direction significant twinning–antitwinning asymmetry is displayed for molybdenum but not for tungsten. However, for tensile/compressive loading the Schmid law breaks down in both metals, principally due to the effect of shear stresses perpendicular to the slip direction that alter the dislocation core. Recognition of this phenomenon forms a basis for the development of physically based yield criteria that capture the breakdown of the Schmid law in bcc metals. Moreover, dislocation glide may be preferred on {1 1 0} planes other than the most highly stressed one, which is reminiscent of the anomalous slip observed in many bcc metals.     

Residual Stress Removal Under Pulsed Electric Current

The effect of a pulsed electric current on the residual stress evolution of metal materials has been investigated.It was found that the surface and internal residual stresses in the as-quenched samples were reduced dramatically by electropulsing.A large number of experimental data show that the residual stress reduction is proportional to the initial residual stress and related to the material properties and electropulsing parameters.Under the combined actions of drift electrons,Joule heating,and residual stress,the dislocation mobility was enhanced,resulting in plastic strain and the decrease in residual stress.Drift electrons played a unique role in the electropulsing treatment,acting as an additional force pushing dislocations forward.The dislocations ultimately accumulated at a grain boundary,forming a parallel arrangement.Finally,the phenomenological equation of the residual stress evolution under electropulsing was derived from the experimental data.     

Atomistic aspects of the deformation of NiAl

Properties of dislocations in B2-NiAl have been studied atomistically using an embedded atom potential. The response of dislocation cores to applied homogeneous shear stresses is investigated and the Peierls stresses of straight dislocations are determined. The results are in many details in excellent agreement with experimental observations. Specifically, the behaviour of the 〈1 1 1〉 dislocations, their slip planes, cross-slip behaviour and the limiting role of the screw dislocation can be explained. Similarly, the appearance of the {2 1 0} plane as a secondary slip plane for the 〈1 0 0〉 dislocations can be rationalised. Furthermore, the interaction of the dislocations with structural point defects is studied. Comparison with the flow stress of off-stoichiometric NiAl from the literature shows that the individual point defects cannot be made responsible for the strong increase of the flow stress, suggesting that more complex defect structures may play an important role.     

灰铸铁机床床身应力与变形的测试研究

利用数值模拟软件对铸件的充型、凝固过程及铸件内部热应力分布情况进行模拟分析。通过测试灰铸铁机床床身多个测点在热时效前后、加工前后及振动前后的残余应力值,发现残余应力随CE和Si/C比提高而降低;热时效能大幅度降低铸件残余应力,热时效温度越高,铸件残余应力降幅越大,且降温时段降温速度对残余应力有较大影响;加工会产生较大残余应力,热时效后进行其它时效对残余应力影响不大。     

Molecular dynamics simulation on mechanisms of plastic anisotropy in nanotwinned polycrystalline copper with {111} texture during tensile deformation

Based on molecular dynamics (MD) simulation, the mechanisms of plastic anisotropy in nanotwinned polycrystalline copper with {111} texture during tensile deformation were systematically studied from the aspects of Schmid factor of the dominant slip system and the dislocation mechanism. The results show that the Schmid factor of dominated slip system is altered by changing the inclining angle of the twin boundaries (TBs), while the yield stress or flow stress does not strictly follow the Schmid law. There exist hard and soft orientations involving different dislocation mechanisms during the tensile deformation. The strengthening mechanism of hard orientation lies in the fact that there exist interactions between the dislocations and the TBs during plastic deformation, which leads to the dislocation blocking and reactions. The softening mechanism of soft orientation lies in the fact that there is no interaction between the dislocations and the TBs because only the slip systems parallel to the TBs are activated and the dislocations slip on the planes parallel to the TBs. It is concluded that the plastic anisotropy in the nanotwinned polycrystalline copper with {111} texture is aroused by the combination effect of the Schmid factor of dominated slip system and the dislocation mechanism.