非晶合金中锯齿流变动力学的研究

非晶合金在外力场的作用下会出现间接性的锯齿流,锯齿具有空间和时间的无序分布性,能够反映塑性变形过程中剪切带的演化过程。借助于混沌理论、自组织临界理论、统计分析、分形和平均场理论等数学方法进行了锯齿动力学研究。发现非晶合金的塑性流变行为与材料的本征结构、试样尺寸、加载试验机的刚度、温度和应变速率等密切相关,揭示了非晶合金的塑性变形过程中剪切带滑移不稳定性的演化特点。试样尺寸小、低温或高应变速率下加载的韧性非晶试样的塑性流变动力学呈现类自组织临界状态,锯齿的幅值分布具有无标度性特点,剪切带之间的交互作用强,剪切带过程相对稳定。低温下大的分形维数说明剪切带分叉速率快,触发了剪切带之间的交互作用。简单的平均场理论证实了非晶合金的塑性可受应变速率调控。这些结论为进一步探索非晶合金的塑性提供了新的思路。     

30MnCrNiMoB钢绝热剪切与动态再结晶

计算了30MnCrNiMoB用正交切削形成绝热剪切带（ASB）的应变和温升，结果表明随着切削速度的增加ASB内应变增大而温度升高．电镜观察发现切削速度小于1．123m·s-1时，ASB内组织严重碎化，而切削速度为1．57m·s－1时，ASB组织为再结晶组织表明在一定条件下绝热剪切过程伴随着动态再结晶，ASB中碳化物起促进再结晶作用．     

纳米压入下镁基非晶合金的间歇性塑性流动

镁基非晶合金通常表现出显著的宏观脆性,因此用常规拉伸、压缩等方法对该合金的变形行为进行研究具有很大困难。本研究利用具有高时间分辨率和高空间分辨率的纳米压痕技术观察了不同加载速率下镁基非晶合金的锯齿流变行为。结果表明,低加载速率促进锯齿的形成,而高加载速率则抑制锯齿的形成。其原因是在低加载速率下,单一剪切带足以耗散外加应变;而在高加载速率条件下,由于单一剪切带不能将应变耗散掉,因此需要更多的剪切带参与变形。为了进一步解释这一锯齿流变行为,本研究采用遍历处理对每个锯齿的应变突变进行了统计分析。结果表明,在不同的加载速率下,小的应变突变服从幂律分布,且幂指数为1.45;而大的应变突变则呈现指数衰减规律。最后,借助硬度对应变速率的敏感性,估算了镁基非晶合金在纳米压入条件下剪切转变区的体积,为4.5 nm3。     

强剪切对单层晶极薄带轧制变形行为的影响

为确定最优的极薄带轧制工艺,本研究深入分析了强剪切对轧制单层晶极薄带微观变形行为和晶体转动演化的影响。采用基于位错滑移机制的晶体塑性有限元模型进行模拟,最大异速比达到1.5。建立了晶粒取向随机分布的单层晶极薄带轧制模型,以探究少晶组织的晶界作用特性。结果表明：强剪切导致单层晶极薄带轧制微观变形和晶体转动表现出显著的局部化。强剪切促进了晶粒的剪切变形,使得晶界的协调变形能力增强。在轧制区施加强剪切变形,可使已启动滑移得到扩展,主滑移带缩窄分散形成新的次滑移带,滑移更加集中和各向异性。变形后晶粒取向主要绕箔材宽度方向发生转动和分散,强剪切使转动角度增大和分散点更加集中稳定。模拟表明强剪切严重影响单层晶极薄带轧制变形的各向异性,进而导致择优取向、滑移局部化以及非均匀应力-应变分布。     

金属材料在冲击荷载下局部变形的数值模拟及分析

高应变率载荷作用下金属材料的变形集中于很窄的区域内,即剪切变形局部化。局部化变形带内产生严重的塑性变形,削弱材料的承载能力,甚至导致材料断裂破坏。基于有限元分析软件FEAP(Finite Element Analysis Program),采用混合有限元方法,用Fortran语言编译适用于金属材料在高应变率下的剪切局部化问题的新单元;计算过程中采用与应变、应变率及温度相关的塑性本构关系来描述剪切带现象,同时在能量平衡方程中考虑剪切带形成过程中的热传导作用;同时考虑显式算法与隐式算法的时间离散方法,并将两种算法的结果进行对比。结果表明,虽然剪切带形成过程很短,一般为微秒量级,但剪切带形成过程中的热扩散项与塑性变形产生的热能量级相同,有效地缓解剪切带模拟的网格敏感性;对于金属材料热塑性剪切带问题,为了满足计算精度要求,显式算法需要的时间步太小,计算成本比隐式迭代高很多;而基于该单元采用隐式算法模拟热塑性剪切带问题迭代收敛稳定,计算精度高,且因为考虑了热传导作用,网格敏感性小。     

电润湿作用下水在石墨烯表面流动的边界滑移及界面摩擦

采用分子动力学方法结合流体边界滑移理论对电润湿作用下水在石墨烯表面流动的边界滑移及摩擦特性进行数值模拟和理论分析。采用石墨烯-水Couette剪切模型,获取电润湿作用下和不带电情况下的剪切速率轮廓、石墨烯表面的剪切应力、边界滑移速率、滑移长度及界面摩擦系数,着重研究界面滑移长度和界面摩擦系数与剪切应变率之间的变化。结果表明:当剪切应变率超过临界剪切应变率时,滑移长度迅速增加,且电润湿作用下的临界剪切应变率明显高于不带电情况下的临界剪切应变率,而界面摩擦系数随着剪切应变率的增大而减小;石墨烯电润湿作用明显增强了石墨烯-水界面的摩擦。无论在电润湿作用下还是不带电情况下,石墨烯-水Couette模型中水的黏性系数与剪切应变率无关。     

帽型试样的绝热剪切数值模拟与温度场研究

通过选择恰当的模型和科学的空间离散化,对SHPB加载过程进行了2D数值模拟,数值重现了帽型试样高应变条件下的绝热剪切变形历程.基于应力塌陷绝热剪切形成判据分析了材料绝热剪切变形规律;利用模拟结果的应力一时间、应变一时间曲线进行了温度场的计算,确定了剪切带的类型,计算结果与实验结果吻合.     

动态载荷下7005铝合金力学行为及数值模拟

本工作研究了7005铝合金在应变速率为(1～5)×103s-1条件下的力学行为。结果表明,7005铝合金有明显的应变速率敏感性。利用最小二乘法求得了材料的Johnson-Cook本构参数;应用ABAQUS软件,研究了高应变速率下的帽型试样的绝热剪切变形历程。数值模拟得到的应力-应变曲线与实验结果吻合。温度场的计算为绝热剪切带内微观组织是否发生动态再结晶提供了依据,当应变速率为15 000 s-1时,在120～240μs内,相比初始温度,试样的平均温升达到405℃。通过对冲击后试样的微观组织观测发现,绝热剪切带中有大量的等轴晶,具有典型的再结晶组织特征。7005铝合金在高应变速率下的变形温升和动态再结晶软化行为,将为其在汽车碰撞构件中的应用提供指导作用。     

高速切削淬硬钢切屑变质层微结构及形成机理

为了揭示高速切削过程中切屑形成以及刀具-切屑界面摩擦机理,对剪切区内剪切带及白层进行了微观观察和分析.利用光学显微镜,SEM,电子探针,TEM和X射线衍射等方法对高速切削淬硬钢锯齿状切屑剪切变形区内变质层的微细组织进行了观察,对变质层的微结构本质及其形成机理进行了分析.研究表明:在锯齿状切屑锯齿之间的第一变形区存在绝热剪切带,由切屑基体到剪切带中心依次出现马氏体板条、沿剪切方向拉长并被位错分割的板条和等轴晶粒等不同的微结构特征;在切屑底部的第二变形区存在白层,其微结构显示了非晶组织特征.剪切带形成过程中没有相变发生,其形成是基于一种旋转式动态再结晶机制,白层的形成过程中发生了非晶转变和马氏体相变,形成机理属于相变机制.     

考虑一阶和二阶位移梯度的数字图像相关方法在剪切带测量中的比较

当剪切带中存在二阶位移梯度或应变梯度时,研究了两种数字图像相关(DIC)方法在测量位移、应变及应变梯度中的表现。以基于梯度塑性理论的剪切带的位移解作为基础,通过Matlab仿射变换制作了具有不同平均剪切应变和应变梯度的虚拟剪切带。通过对其计算和分析得到了下列结果:当剪切带的平均剪切应变和应变梯度较高时,与只考虑一阶位移梯度的DIC方法相比,考虑二阶位移梯度的DIC方法优势明显,获得的位移、应变及应变梯度结果与理论解比较吻合,由于DIC方法测量的是平均应变,因此,剪切带中心的峰值应变将被低估;当剪切带的平均剪切应变和应变梯度较低时,剪切带中心的峰值应变可能被高估,受标准偏差的影响,考虑二阶位移梯度的DIC方法没有优势。基于上述研究结果,在单向压缩应力控制加载条件下,对砂样从开始加载至宏观裂纹出现之前变形过程中的3种应变场的4种结果进行了分析。由于应变不超过0.25,因而考虑二阶位移梯度的DIC方法的结果并无优势。     

Adiabatic shear band in a Ti-3Al-5Mo-4.5V Titanium alloy

An investigation has been carried out on the adiabatic shear band (ASB) in a Ti-3Al-5Mo-4.5V (TC16) alloy deformed at high strain rate by a split Hopkinson pressure bar (SHPB). ASB in TC16 alloy is a “white” band with a width of about 13 μm. Microhardness of the ASB is larger than that of the matrix. The elongated cell structures of width about 0.2–0.5 μm with thick dislocation exist in the boundary of the shear band. Results suggest that the fine equiaxed grains with α-phase and α″-phase coexist in the shear band. The “white” band is a transformation band. Calculation of the adiabatic temperature rise indicates that the maximum temperature within ASB is about 1,069 K that is above the phase transformation temperature. Finally, formation of an ASB in the TC16 alloy and its microstructure evolution are described.     

Development of adiabatic shear band in cold-rolled titanium

The evolution of adiabatic shear localization in commercial titanium subjected to heavy cold rolling was investigated. The evolution of the morphology, microhardness, local shear strain, and local temperature increments were systematically studied and estimated. A shear band with about 25 μm in width was formed and fine nanograins with a range of dimensions varying from 20 to 160 nm and had a mean size of about 70 nm were observed inside the centre of shear band after 83% cold-rolling. Microhardness test shows that hardness within the shear band is markedly higher than that of the surrounding matrix. The calculated shear strain and maximum temperature increase within the shear band are much higher than that of the overall deformed sample. The initiation of shear localization may depend on geometric perturbation instead of thermal ones.     

An adaptive mesh refinement technique for two-dimensional shear band problems

We have developed an adaptive mesh refinement technique that rezones the given domain for a fixed number of quadrilateral elements such that fine elements are generated within the severely deformed region and coarse elements elsewhere. Loosely speaking, the area of an element is inversely proportional to the value of the deformation measure at its centroid. Here we use the temperature rise at a material point to gauge its deformations which is reasonable for the shear band problem since the material within the shear band is deformed intensely and is heated up significantly. It is shown that the proposed mesh refinement technique is independent of the initial starting mesh, and that the use of an adaptively refined mesh gives thinner shear bands, and shaper temperature rise and the growth of the second invariant of the plastic strain-rate within the band as compared to that for a fixed mesh having the same number of nodes. The method works well even when the deformation localizes into more than one narrow region.     

Analyses of Adiabatic Shear in Some Metals from Calculations of the Temperature Distribution and the Cooling Rate

The temperature distribution under shearwith a high strain rate and the cooling rateof the shear bands of Al, Cu and 0.06C steelhave been calculated using a computer. The resultsshow that the temperature of shear band increaseswith the increase of the average strain rate(_o). When _o is in the range 8×10~5 to 10~6 1/s,the structure of the adiabatic shear band ina 0.06C steel is martensite but it becomesmetallic glass if _o=10~6 1/s. As to A1 andCu, the structure of the adiabatic shear bandscan also be of metallic glass if _o is greaterthan 1.8×10~6 and 5.5×10~7 1/s respectively.It explains that Cu tends most difficultly toform adiabatic shear band, while 0.06C steelmost readily among the three metals.     

温轧温度对Cr-Ti-B系低碳钢组织与织构的影响

使用扫描电镜、电子背散射衍射、透射电子显微镜和固体内耗仪研究了温轧温度对Cr-Ti-B系低碳钢组织和织构的影响.结果 表明,温轧后钢的组织由变形铁素体和少量珠光体所组成,随着温轧温度的提高铁素体晶内剪切带的含量呈现先提高后降低的趋势,在450℃温轧剪切带的含量最高.剪切带的形成,与Ti和B元素的偏聚密切相关.在350℃...     

Adiabatic shear banding in a bimetallic body

Summary Thermomechanical deformations of a body made of two different materials and under-going simple shearing deformations are studied with the objectives of finding out when and where adiabatic shear bands will initiate and how they will subsequently grow. Each material is modeled as strain and strain-rate hardening but thermally softening. A shear band is presumed to have formed if the introduction of a temperature perturbation centered around the common interface between the two materials results in an eventual localization of the deformation into a region of width considerably smaller than the width of the initial temperature bump. For a fixed set of material properties the effect of the applied overall strain-rate, and for a fixed applied strain-rate the effect of varying the shear modulus, thermal conductivity, and the coefficient of thermal softening of one material relative to the other have been examined. It is found that a shear band forms in the material that softens more rapidly.With 8 Figures     

Plastic banding in glassy polycarbonate under plane simple shear

Samples of amorphous polycarbonate were tested in plane simple shear at various temperatures (–100 to +150 C) and shear rates (3×10^–5 to 3×10^–2sec^–1). In the glassy state, it was observed that the deformation concentrated at yield inside a single shear band whose elongation and widening phases correspond to well marked stages of the recorded stress-strain curve. Birefringence and X-ray diffraction in the growing band show that the molecular orientation follows a pseudoaffine evolution with the local plastic strain. Although the shear is inhomogeneous during the growth of the shear band, it is fairly uniform inside the band itself, down to the scale of 100 nm. After the band has completed its widening (for an overall shear of about 0.9) the overall shear in the whole specimen is homogeneous and then one can deduce the constitutive equation for steady state plasticity of the glassy material, up to shear strains as large as 2.0. It is characterized by a linear strain hardening whose value, such as the extrapolated yield stress, decreases gradually with temperature.     

A Finite-Deformation Constitutive Model of Bulk Metallic Glass Plasticity

A constitutive model of bulk metallic glass (BMG) plasticity is developed which accounts for finitedeformation kinematics, the kinetics of free volume, strain hardening, thermal softening, rate-dependency and non-Newtonian viscosity. The model has been validated against uniaxial compression test data; and against plate bending experiments. The model captures accurately salient aspects of the material behavior including: the viscosity of Vitreloy 1 as a function of temperature and strain rate; the temperature and strain-rate dependence of the equilibrium free-volume concentration; the uniaxial compression stress-strain curves as a function of strain rate and temperature; and the dependence of shear-band spacing on plate thickness. Calculations suggest that, under adiabatic conditions, strain softening and localization in BMGs is due both to an increase in free volume and to the rise in temperature within the band. The calculations also suggest that the shear band spacing in plate-bending specimens is controlled by the stress relaxation in the vicinity of the shear bands.     

A Workability Criterion for the Transformed Adiabatic Shear Band Phenomena during Cold Heading of 1038 Steel

A criterion for predicting the workability limits for internal britde failure was developed for cold heading of 1038 steel. The criterion considers internal defects caused by microstructural changes generated by adiabatic shear. This transformation is termed the transformed adiabatic shear band (TASB) phenomena. The defect that develops is the formation ofbritde martensite as a result ofthe temperature rise and fall inside the adiabatic shear band (ASB). In this work, the material is considered to have a TASB defect when the temperature inside the ASB exceeds the phase transformation temperature (A_C3). The empirical formulas provided by Andrews^[1] were used to determine transformation temperatures. Microhardness testing and etching with 2% Nital and Le Pera etchants were performed on the sectioned specimens to locate and study the TASB. In order to simulate the cold heading process, a drop weight compression test was used and modeled with finite-element analysis (implemented within ABAQUS/Explicit).     

Numerical investigation of dynamic shear bands in inelastic solids as a problem of mesomechanics

The main objective of the present paper is to discuss very efficient procedure of the numerical investigation of the propagation of shear band in inelastic solids generated by impact-loaded adiabatic processes. This procedure of investigation is based on utilization the finite element method and ABAQUS system for regularized thermo-elasto-viscoplastic constitutive model of damaged material. A general constitutive model of thermo-elasto-viscoplastic polycrystalline solids with a finite set of internal state variables is used. The set of internal state variables is restricted to only one scalar, namely equivalent inelastic deformation. The equivalent inelastic deformation can describe the dissipation effects generated by viscoplastic flow phenomena. As a numerical example we consider dynamic shear band propagation in an asymmetrically impact-loaded prenotched thin plate. The impact loading is simulated by a velocity boundary condition, which are the results of dynamic contact problem. The separation of the projectile from the specimen, resulting from wave reflections within the projectile and the specimen, occurs in the phenomenon. A thin shear band region of finite width which undergoes significant deformation and temperature rise has been determined. Shear band advance, shear band velocity and the development of the temperature field as a function of time have been determined. Qualitative comparison of numerical results with experimental observation data has been presented. The numerical results obtained have proven the usefulness of the thermo-elasto-viscoplastic theory in the investigation of dynamic shear band propagations.