Stress-induced structural transformation and shear banding during simulated nanoindentation of a metallic glass

Simulated nanoindentation tests on a three-dimensional model of a binary metallic glass-forming alloy reveal how the stress field and material structure interact to control deformation beneath the indenter. Initially, the stress field follows the Hertzian solution with fluctuations due to heterogeneities. Homogeneous or localized plastic deformation arises, depending on the processing history of the material. In the case of localized deformation, the first shear band initiates sub-surface, relaxing the local shear stress. The subsequent shear band morphology is observed to be rate-dependent. Deformation induced changes in material structure are characterized in terms of short-range ordering and free volume, with the former providing significantly greater signal-to-noise.     

热处理对粉末冶金Cu-15Ni-8Sn合金组织与性能的影响

以Cu-15Ni-8Sn合金粉为原料制备了粉末冶金试样,研究其在不同的固溶温度、冷压变形、时效温度和时效时间条件下的硬度,着重研究了840℃×15 min固溶+40%冷压变形条件下时效温度和时间对硬度及剪切强度的影响规律,采用金相及扫描电镜分析了相应的微观组织。结果表明,影响Cu-15Ni-8Sn合金硬度的主次因素为:冷压变形量>时效时间>时效温度>固溶温度,较优的工艺参数为840℃×15 min固溶+40%冷压变形+400℃×4 h时效,可获得37.6～38.3 HRC的高硬度和570～628 MPa的抗剪切强度。     

Accounting for shear deformation in fast models for plate rolling

For on-line prediction of roll force and torque, fast models have been available for a long time, which are mainly based on the slab method or other solution methods that allow for computing times in the range of seconds. Such fast models typically treat the rolling process as a plane strain problem and neglect shear deformation, which is always present in real processes. The shear deformation results in inhomogeneous strain profiles and thus might lead to inhomogeneity in microstructure over plate thickness. In this paper, a novel method is presented that allows for superposition of shear strain onto the strain state obtained from the slab method. The shear strains are interpolated from an extensive finite element (FE) parameter study of rolling processes that covers the entire parameter range of today’s plate rolling. For the regimes of very thick and thin plates different interpolation functions are introduced. It is shown that when the proposed shear strain model is combined with the slab method, similar results are obtained as with a full-scale FE calculation of the rolling problem but with a calculation time in the range of seconds.     

Effect of Phase Transformation Conditions on the Microstructure and Shear Performance of TB8 Titanium Alloy

研究了TB8钛合金在不同热处理条件下的组织转变规律,并通过剪切实验分析了显微组织差异对于该合金剪切行为的影响。结果表明,热处理条件会影响TB8钛合金的显微组织,也会影响剪切变形行为。随着固溶处理温度升高,合金的晶粒明显变大,析出物的形貌也发生变化,剪切变形变得困难。固溶处理之后的时效温度对合金的剪切变形行为也有显著影响,在840 ℃固溶处理1 h之后,如果接着进行450 ℃/8 h时效处理,合金剪切变形所需的驱动力会降低;如果把时效热处理的温度提高到550 ℃,甚至更高（保温时间不变）,那么剪切变形就会发生穿晶断裂。根据剪切检测结果可知,合金经过840 ℃/1 h固溶处理接着再进行500 ℃/8 h的时效处理可以获得优异的剪切性能,而且具有合理的显微组织。     

动态冲击条件下Ti-15Mo时效钛合金的变形机制

通过固溶时效处理Ti-15Mo合金获得片层组织,采用分离式霍普金森压杆（SHPB）研究应变速率对变形机制产生的影响,结合绝热温升、显微组织和硬度分析表明：由于位错与第二相的相互作用,导致流变应力曲线发生波动。提高应变速率,一方面造成应变速率强化;另一方面促进绝热升温软化。合金温度达到379K时,热软化效应超过应变硬化效应,变形方式由均匀塑性变形变为绝热剪切变形。绝热剪切带的宽度随切应变的增加而增大,通过亚晶旋转再结晶机制产生等轴晶粒。再结晶的界面强化导致组织硬度由高到低为：混合组织>条状组织>基体组织。时效处理抑制应力诱发孪生（TWIP）效应,造成合金较低的应变硬化能力,劣化材料的动态力学性能。     

Shear deformation and grain refinement in pure Al by asymmetric rolling

Asymmetric rolling（ASR）, as one of severe plastic deformation（SPD） methods, was widely used to make ultra-fined materials with enhanced performance. Internal marks were used to show the shear deformation during asymmetric rolling with pure aluminium as a model material. Effects of reduction ratio and mismatch ratio on the shear deformation were studied. With the observed shear deformation results, equivalent strain was calculated. For lager shear deformation, rolling equipment was modified to increase friction between specimen and the rollers. Consequently, extremely fine grains with size of 500 nm are obtained in pure aluminium. With improved asymmetric rolling, the ability of grain refinement of ASR is greatly improved.     

方盒形件拉深破裂预测的研究

根据盒形件拉深变形的特点 ,借助轴对称拉深件的变形规律与盒形件圆角区剪应力零线相同的假设 ,得到了方盒形件主要变形区法兰和侧壁的应力解析表达式 ,从而对方盒形件的拉深破裂进行了预测。理论解析的可靠性用有限元数值模拟方法得到验证     

Simulation and experimental study on thermal deep drawing of carbon fiber woven composites

Carbon fiber woven composites are composed of carbon fiber woven and resin matrix. To reduce the manufacture cost, thermal stamping, a new forming technology, was proposed and investigated to fabricate composite part. The mechanical properties of carbon fiber have great influence on the deformation of carbon fiber composites. In this study, shear angle–displacement curves and shear load–shear angle curves were obtained from picture frame test. Thermal deep drawing experiments and simulation were conducted, and the shear load–displacement curves under different forming temperatures and shear angle–displacement curves were obtained. The results show the compression and shear between fiber bundles are the main deformation mechanism of carbon fiber woven composite. The maximum shear angle for the composites in this study is 33°. In the drawing process, the forming temperature affects the drawing force, which drops rapidly with the increasing temperature. The suitable forming temperature in deep drawing of the carbon fiber woven composite is approximately 170 °C.     

Shock-induced deformation twinning and omega transformation in tantalum and tantalum–tungsten alloys

Effects of high strain-rate and high plastic-strain deformation on the development of deformation substructures in tantalum and tantalum–tungsten alloys (Ta–2.5 wt.% W and Ta–10 wt.% W) shocked at 15 and 45 GPa have been investigated. In addition to dislocation cells/walls, and {112}<111>-type deformation twinning, a shock-induced omega phase (hexagonal) is also found within polycrystalline tantalum shocked at 45 GPa. The orientation relationships between the omega phase and parent (bcc) matrix are

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. The lattice parameters of omega phase are and

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Since both deformation twinning and omega transformation occur preferably in the {211}_b planes with high resolved shear stresses, it is suggested that both can be considered as alternative paths for shear transformations in shock-deformed tantalum. A greater volume fraction of twin and omega phase formed in Ta–W than in pure Ta reveals that shock-induced shear transformations can be promoted by solid solution alloying. While deformation twinning is resulted from homogeneous shear in consecutive {211} planes, omega transformation can be attributed to the

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inhomogeneous shear in consecutive {211} planes. Dislocation mechanisms for shock-induced twinning and omega transformation are proposed and critically discussed.     

板材取样剪剪切过程数值模拟及实验研究

金属板材剪切过程是一种复杂的弹塑性大变形过程,本文根据板材剪切过程变形特点,利用ANSYS/LS - DYNA建立了金属板材取样剪剪切板材的有限元模型,进行了弹塑性有限元分析.通过仿真计算,获得了板材剪切过程的应力-应变状态,变形和力能参数,并在某钢铁集团公司取样剪上对剪切力进行了测试,结果表明,有限元模拟结果可靠,可作为取样剪的设计制造的依据.     

切边圆盘剪剪切过程的数值模拟和实验研究

根据钢板剪切过程的变形特点,利用ANSYS/LS-DYNA建立切边圆盘剪剪切钢板的有限元模型,并进行弹塑性有限元分析。通过仿真计算,获得了钢板剪切过程中的应力-应变状态、变形和力能参数,并在某钢铁集团公司切边圆盘剪上对剪切力进行了测试,结果表明,有限元模拟结果可靠,可作为圆盘剪设计制造的依据。     

钢格板剪床剪切过程有限元仿真研究

钢格板剪切过程是一个复杂的弹塑性大变形过程,其主要特点是变形区域非常小、变形力集中,同时还涉及断裂问题。为了更好地研究钢格板剪切机制,运用大型有限元软件ANSYS/LS-DYNA对剪切过程进行仿真,得到剪切过程中变形区的应力应变分布规律和力能参数,并与实验数据和经验公式得到的结果进行比较,证明了有限元模拟的可靠性,为优化剪切参数、提高剪切质量和刀具寿命提供合理依据。     

T2/QBe2爆炸复合界面结合层内的形变特征

采用平板爆炸复合方法制备T2/QBe2复合板材,并利用金相和扫描电镜电子背散射衍射技术对复合界面附近处QBe2和T2侧产生的绝热剪切变形及形变孪生的微观特征进行了观测.结果表明:由于T2和QBe2在物理性能、力学性能和热学性能上的差异,在爆炸载荷条件下只在QBe2中产生绝热剪切带,并且绝热剪切带内分布有十分细小的等轴晶;带内晶粒取向基本相同,带与带之间基体中的晶粒取向也基本相同,且两者还存在较大的取向差;爆炸冲击加载后,QBe2和T2两侧均产生了形变孪晶.     

Deformation Analysis of Elliptical Cross-Section Spiral Equal-Channel Extrusion Technique

采用简化的切片平面应变假设,利用增量叠加法对椭圆截面螺旋等通道挤压(ECSEE)过程的应变累积进行了分析计算。通过对横截面上某一质点进行追踪,将ECSEE变形过程分解为两种基本变形方式:圆-椭圆/椭圆-圆截面过渡变形和椭圆截面扭转过渡变形,然后利用MATLAB软件编程求解,得到了组合后材料内部追踪点的累积等效应变沿模具通道长度的变化规律和横截面上等效应变累积的分布规律,并与Deform-3D有限元软件模拟结果进行了对比。结果表明:ECSEE扭转变形所得累积应变要大于其他变形形式累积的应变,ECSEE变形方式主要是剪切变形;坯料横截面的等效应变从坯料外围到中心呈梯度减小趋势;有限元模拟结果也验证了解析解的工程准确性。     

Size-dependent shear fracture and global tensile plasticity of metallic glasses

The tensile ductility or brittleness of metallic glasses is found to depend strongly on the critical shear offset. Based on experimental observations, the tensile shear fracture processes of metallic glasses can be divided into three stages: multiplication and coalescence of the free volume, formation of void and the final fast propagation of a shear crack. Accordingly, the size effect on the tensile shear deformation processes of metallic glass can be well understood: with decreasing specimen size smaller than the equivalent critical shear offset, the shear deformation of metallic glass is changed from unstable to stable, which leads to a transition from global brittleness on the macroscale to large global plasticity or even necking on the microscale. These results are fundamentally useful in understanding the physical nature of tensile shear deformation of various metallic glasses and even in the design of new metallic glass materials with good plasticity.     

Effect of shear deformation on the microstructure and mechanical properties of a steel St3

The effect of the methods of severe plastic shear deformation on the microstructure formation in a steel St3 has been studied. The shear deformation in hot and cold states allows one to obtain different levels of mechanical properties of the metal with different characteristic features of fine structure. It is shown that cold plastic shear deformation is accompanied by a rise in the level of internal stresses and a considerable increase in the fraction of low-angle boundaries, which is reflected in the enhancement of strength properties. Upon hot plastic shear deformation, numerous finely dispersed grains with high-angle boundaries and small density of dislocations are formed due to the concurrent development of fragmentation and recrystallization in situ. This results in a low level of stresses in the crystal lattice and favors enhancement in plasticity.     

一点的三维剪应力图及其与金属变形类型的对应关系

通过剪应力数值的数学描述,给出了一点剪应力分布图在三维空间中的变化规律,指出剪应力图在空间的分布与金属变形类型类型之间存在着对应关系,从深层次上剪应力对塑性变形的影响实质。     

上限原理在重轨万能轧制过程平均温度计算中的应用

在重轨万能轧制过程中,首先建立了简化的三维几何模型,然后分别给出了轨腰、轨头及轨底的运动学许可速度场以及相应的应变速度场和剪应变速度强度。根据上限原理分别求出了轨腰、轨头及轨底在运动学许可速度场下的塑性变形温升和摩擦温升,并且求得了轧件在相邻机架之间的穿梭温降。根据水平辊和立辊的能量平衡条件求出了接触温降,得到了轧件在单个机架上变形前后的温度变化。为了验证理论模型,对轧制过程的温度变化进行了刚塑性有限元仿真,得到了钢轨各部分的平均温度的有限元解。通过比较理论解、有限元解和实测表面温度可知,上限原理求得的平均温度值大于表面温度实测值,而且接近于有限元解,能更准确地表示轧件温度。因此,根据上限原理控制轧制温度从而控制钢轨的微观组织变化以得到高质量钢轨是可行的。     

Development of a thermomechanical cutting process model for machining process simulations

A thermomechanical model for cutting processes is presented. The deformation in the shear zone is represented using Johnson-Cook material model. The rake contact is modeled using sticking and sliding zones, and their lengths are also predicted. The parameters of the material model and the friction coefficient on the rake are directly identified from a few number of orthogonal cutting tests. The model can predict cutting forces, shear angle and stress, pressure distribution and contact lengths on the rake face and temperature distribution. The application of the model to common operations such as turning and multi-axis milling is also presented with experimental verification, and satisfactory results are obtained.     

枝叉管形锻件的剪—挤变形规律模拟实验研究

给出枝叉管形锻件剪－挤变形工艺的理论基础及其工艺性要求。通过物理模拟实验得出一种枝叉管形件－－截止阀体锻件的剪－挤变形关键工艺参数（即成形条件）;通过网格试验研究分析了枝叉管形件剪－挤工艺过程的金属流动情况及其变形规律;由剪－挤和镦－挤成形枝叉管形件的挤压力对比模拟实验,得出剪－挤成形过程挤压力降低规律,并验证了该工艺节能省力的特点。结果表明,利用剪－挤成形新技术可以在千吨级中型锻造设备上开发大型枝叉管形锻件的成形工艺。