Al—Li合金在慢拉伸条件下的形变与断裂行为

研究了Ａｌ－Ｌｉ合金在单向受力状态下的形变特征与断裂行为，测试了不同时效状态合金的慢拉伸裂纹扩展速率与应力强度因子的关系。结果表明，不同时效合金的范性形变机制、微观断裂模式和慢拉伸裂纹扩展阻力不同，它们视微观组织和晶界性态而异，应变局部化是低塑性的根源。     

超声辅助介观尺度纯钛塑性变形晶体塑性数值模拟

纯钛是以孪生为主要变形机制的密排六方金属,塑性变形时需要考虑到孪晶的影响。建立了考虑形变孪晶的晶体塑性有限元模型,模拟超声辅助下的介观尺度纯钛微镦粗时的变形规律。通过模拟结果与试验结果对比,证明了所建模型的合理有效。模拟结果发现随着超声振幅的增大,超声振动辅助介观尺度纯钛变形的应力下降值增大,而0～120 k Hz内超声频率对变形影响不大。超声会使介观尺度材料的形变孪晶体积分数降低,随着超声振幅的增加,形变孪晶体积分数下降值增大。晶体塑性有限元模型比各向同性有限元模型能更合理地反映材料真实变形。     

介观尺度上热变形奥氏体储存能演化的计算机模拟

为了定量描述热变形奥氏体在介观尺度上微观变形的非均匀性,采用晶体塑性有限元方法(crystal plasticity finite element method,CPFEM)模拟了C—Mn钢在不同变形条件下的变形行为,得到了在介观尺度上奥氏体的微观应力应变和变形储存能分布．模拟所得到的应力一应变曲线与文献测定的应力一应变曲线基本一致．通过对真应变为0．5,变形速率为50s^-1的热变形奥氏体的研究发现,即使在外部的均匀变形条件下,无论是在晶粒内部还是晶粒间,材料内部变形都非常不均匀．这种变形不均匀性主要是由晶粒的初始取向不同,近邻晶粒的取向差,以及变形时滑移系的运动特性不同所引起的．本文定量描述了介观尺度上奥氏体变形储存能不均匀分布,为结合介观尺度组织模拟,实现组织演变的多尺度耦合计算提供了参考．     

介观尺度模拟方法及其在金属材料微观组织演变中的应用

介观尺度模拟方法是表征金属材料微观结构变化或微观组织结构与力学性能之间关系的重要手段，可以在减少实验数量的基础上，将结果性实验向过程性实验转移，从而更直观地表征实验结果。针对金属材料常见的介观尺度模拟方法，对晶体塑性模型、蒙特卡罗法、相场法以及元胞自动机法进行了综述，回顾了不同方法的理论基础和发展历程，阐述了其特点及局限性；分别从金属材料织构演变、动态再结晶、固态相变和凝固相变等角度介绍了不同方法的典型应用场合，最后展望了该领域未来的发展趋势，以期为金属材料微观组织演变的介观尺度模拟提供更具参考性的方法。     

两院院士严东生如是说:寻找纳米材料应用新领域

在2003年国际新材料产业发展研讨会上,我国著名材料科学家、两院院士严东生就纳米材料与纳米科技作了具体介绍。纳米作为一种介观尺度,介于原子与分子之间,纳米的这种介观尺度存在诸多新的现象。因为纳米科技进入到非常小的尺度,纳米技术被认为在节约资源、减少生态环境压力、实现可持续发展方面有着巨大潜力。同时,纳米科技对实现我国传统产业的升级换代也起着很重要的作用。     

加载方向对γ/γ′-α-Mo定向共晶瞬时拉伸性能的影响

γ/γ′-α-Mo定向共晶具有良好的力学性能,特别是在垂直于生长方向上,尚有一定塑性。本文研究了加载方向对瞬时拉伸性能的影响。当加载方向偏离生长方向时,瞬时拉伸强度和塑性随之下降。即使在加载方向垂直于生长方向时,断裂强度仍在900MN/m~2以上,延伸率也能保持在3%左右。在动态拉伸过程中,观察到两种断裂形式:纤维本身的断裂和纤维与基体界面的开裂。在范性形变出现以后,基体的滑移可以通过α-Mo纤维。一般是较粗的纤维首先断裂,纤维与基体界面的开裂是导致试样最后断裂的主要原因。     

OFF-AXIS TENSILE PROPERTIES OF γ/γ'-α-Mo DS EUTECTIC

γ/γ′-α-Mo定向共晶具有良好的力学性能,特别是在垂直于生长方向上,尚有一定塑性。本文研究了加载方向对瞬时拉伸性能的影响。当加载方向偏离生长方向时,瞬时拉伸强度和塑性随之下降。即使在加载方向垂直于生长方向时,断裂强度仍在900MN/m~2以上,延伸率也能保持在3％左右。在动态拉伸过程中,观察到两种断裂形式:纤维本身的断裂和纤维与基体界面的开裂。在范性形变出现以后,基体的滑移可以通过α-Mo纤维。一般是较粗的纤维首先断裂,纤维与基体界面的开裂是导致试样最后断裂的主要原因。     

7075铝合金热变形多尺度数值模拟

为揭示7075铝合金热变形介观组织演变与宏观力学性能之间的关系，建立了可以并行计算的7075铝合金热变形多尺度模型。结合唯象型与物理型模型构建了新的并联型介观本构模型。采用ABAQUS有限元软件位移法的umat节点变量进行信息交互，实现不同尺度模型之间的数据耦合。通过实验与模拟结果的校准获得模型所需的材料参数，并在ABAQUS有限元软件上成功实现了7075铝合金试样单轴热拉伸过程的多尺度同平台并行数值模拟，通过对比数值模拟结果与实验结果发现，位错与应变分布趋势接近，均呈现出不均匀的带状分布，表层晶粒的位错明显多于内部晶粒，这主要是由于表层更容易发生滑移位错。     

CA法及其在材料介观模拟中的应用

元胞自动机(CA)是复杂体系的一种理想化模型,适合于处理难以用数学公式定量描述的复杂动态物理体系问题,如材料的组织演变等。本文概述了CA法的基本思想及原理,介绍了CA法的基本组成及特征,综述了其在材料介观模拟研究中的应用。研究表明,CA法在金属凝固结晶、再结晶及相变现象等材料介观尺度的组织模拟中表现出特有的优越性。     

微成形中金属材料韧性断裂行为研究综述

微成形技术在成形过程中,韧性断裂是经常出现的主要缺陷之一,极大地影响了微型零件的成形质量.在微介观尺度的变形过程中,由于尺寸效应的影响,传统的塑性变形以及韧性断裂理论不能完全应用于微介观尺度下金属材料的成形预测,尺寸效应对微介观尺度塑性变形和韧性断裂行为有着显著的影响.对当前微介观尺度下的塑性变形及断裂理论进行了综述,...     

Surface tension of solids. Structure-mechanical approach

Problems of applying the classic and generalized Lippmann equations to adsorption studies on solid electrodes are shortly reviewed. Long-time thermodynamic discussion results in no clear-cut decision. A new nonthermodynamic approach is proposed considering the principal possibility and role of elastic and plastic deformations of the electrode surface during adsorption. The extremely thin electrode surface layers affected electrically and mechanically by adsorbate are supposed to be free of dislocations because of volume restriction. The nearest structure-mechanical analogs of such layers are the whisker crystals whose side surface could have one-and two-dimensional defects, but no active dislocations. Like whiskers, surface metal layers should possess a high ultimate strength close to the theoretical one and a purely elastic deformation. Special attention has been paid to the concepts of “reversible wholly plastic deformation (as in cleavage)” used for derivation of the generalized Lippmann equations for solids. This concept was shown to be an idealized one having no relation to the real plastic deformation of metals. It is a case of mixed notions since “cleavage” is not a kind of deformation. It is a kind of irreversible fracture mostly without the steps of plasticity. Affected only by adsorbate, the thin surface electrode layer should be considered as absolutely elastic body, whose plastic deformation is impossible, i.e. the Lippmann equation and other equations containing terms of plastic deformation cannot be used in thermodynamics of adsorption on the solid metals. The text was submitted by authors in English.     

应力状态和渗氢条件在非晶态NiCrFeSiB合金氢脆断裂中的作用

通过慢应变速率拉伸、断口观察和断裂力学分析等方法研究了非晶态NiCrFeSiB合金氢脆断裂过程和断口形态的关系.试样的应力状态,氢的渗入量及其在试样中的分布,以及非晶态合金塑性变形局部化的程度等因素对断裂过程都有影响.当渗氢量较低时,断裂方式主要由试样的应力状态所决定,发生近表面区的平面应力断裂和内部的平面应变断裂.当渗氢量较高,且氢由表面向内部分布的浓度梯度较陡时,发生表面区的"解理"型断裂和核心区的延性断裂.在渗氢量适中,氢浓度梯度平缓的条件下,出现由表面向内部塑性变形程度逐渐增大的"河流"形态.根据     

块状非晶态合金Zr65Al7.5Ni10Cu17.5的室温单轴压缩断裂行为

利用IUTM和SEM研究了Zr65Al7 5Ni10Cu17.5块状非晶合金的室温单轴压缩变形和断裂行为.该合金的室温压缩变形过程主要表现为弹性和塑性变形,并且塑性变形阶段没有加工硬化现象.宏观形貌观察发现样品形成与压缩方向呈约45°的变形带.塑性变形以剪切带粘性流层相对滑动的方式进行,在变形过程中形成脉络状断面形貌.裂纹于塑性变形积累到一定程度之后,在切变变形较大的区域表面形成.裂纹从萌生、扩展到断裂,时间极短.流变积累和应力集中综合作用的结果导致断裂.     

Concept of reversible cleavage in surface tension of solids

“Wholly plastic (as in cleavage)” surface area changing is the widely accepted concept formally allowing the application of Gibbs thermodynamics to surface tension of solids considered as a reversible process of partly plastic and partly elastic deformation as recommended by the IUPAC. However, this concept contains some internal contradictions that are serious enough to raise doubts about its correctness and, consequently in validity of the thermodynamic analysis of surface tension of solid electrodes based on this concept. This analysis leads to the generalized Lippmann equation and eventually approves using the classical Lippman equation for solids. The latter is shown to be erroneous since it is based on the misleading concept of “reversible cleavage” and contradicts all available in situ experimental data.     

Analytical model to determine the critical feed per edge for ductile-brittle transition in milling process of brittle materials

Brittle materials like glass are considered difficult-to-machine because of their high tendency towards brittle fracture during machining. The technological challenge in machining such brittle materials is to achieve material removal by plastic deformation rather than characteristic brittle fracture. In ductile mode machining, the material is removed predominantly by plastic deformation and any cracks produced due to possible fracture in the cutting zone are prevented from extending into the machined surface. This is achieved by selecting an appropriate cutting tool and suitable machining parameters. In ductile machining by milling process, fracture induced cracks are diverted away from final machined surface by selecting a suitable feed per edge less than a critical threshold value. Hence determination of critical feed per edge is of paramount importance to achieve ductile mode machining by milling process. This paper presents an analytical model based on fracture mechanics principles to predict the critical feed per edge in milling process of glass. The size and orientation of cracks originating from brittle fracture during machining have been quantified by using indentation test results and the critical value of feed per edge has been determined analytically as a function of intrinsic materials properties governing brittle fracture and plastic deformation. Furthermore, an equivalent tool included angle has been suggested for machining operation as against the indenter included angle to correlate the indentation and machining test results with improved degree of accuracy. Experimental results validated the proposed model fairly accurately. It has been established that if the longest cracks oriented in radial direction to the cutting edge trajectory are prevented from reaching the final machined surface by selecting a feed per edge less than or equal to a critical value, a crack-free machined surface can be achieved.     

冷镦轴承滚子模具失效分析及改善方法

Crl2MoV钢冷镦轴承圆锥滚子模具使用寿命较低。模具失效主要原因是型腔严重磨损、纵向及横向胀裂、圆角处塌陷或产生疲劳蚀坑。作者从塑性变形、微动磨损、疲劳断裂的基本原理出发，分析了模具工作过程中的微塑性变形、挤压磨损和挤压胀裂的失效机制。提出了根据模具设计和应用条件，分别采用硬质合金和新型高强韧性冷作模具钢模具提高使用寿命的途径。     

Fracture in strain gradient elasticity

Recent experiments have shown that the microscale material behavior is very different from that of bulk materials, and displays strong size effects when the characteristic length associated with the deformation is on the order of microns. Conventional continuum theories, however, can not predict this size dependence because they do not have an intrinsic length in their constitutive models. A new continuum theory, namely the strain gradient theory, has been proposed to investigate the deformation of solids at the microscale. For materials undergoing plastic deformation, the basis of strain gradient theory is the dislocation theory in materials science, and strain gradient plasticity has agreed remarkably well with experiments. For elastic materials with microstructures, it has also been established that the material behavior can be represented by an elastic strain gradient theory. A general approach to investigate fracture of materials with strain gradient effects is established. Both the near-tip asymptotic fields and the elastic full-field solutions are obtained in closed form. Due to stain gradient effects, stresses ahead of a crack tip are significantly higher than those in the classical K field. The plastic zone size surronunding a crack tip is estimated by elastic near-tip fields, as well as by the Dugdale model. It is established that the plastic zone is, in general, much more round and larger than that estimated from the classical K field.     

Tensile fracture of as-cast and hot rolled Mg-Zn-Y alloy with long-period stacking phase

An experimental Mg97Zn1Y2(molar fraction,%)alloy was produced by rolling the as-cast alloy.The microstructure of the alloy is composed of theα-Mg(also marked as 2H-Mg with reference to long-period stacking structure according to hexagonal close packed structure)and long-period stacking(LPS)phase.Tensile tests of Mg97Zn1Y2 alloy in comparison with pure Mg were conducted.The fracture morphologies of the tensile specimens were characterized and the microstructures near fracture surface were observed.The results show that the rolled Mg97Zn1Y2 alloy shows a mixed fracture mode including dimples indicating a ductile fracture pattern and a small fraction of cleavage planes indicating a brittle fracture pattern,which is different from the single brittle fracture of the as-cast alloy.In addition,the plastic deformation is mainly from dislocations induced strain with small strengthening effect during plastic deformation in the as-cast Mg97Zn1Y2 alloy,and the strain hardening rate is similar to that of the as-cast pure magnesium.The deformation mechanism of Mg97Zn1Y2 alloy is different from that of the pure magnesium according to a metallographical observation that whether twins are found or not.The strengthening effect hardly exists in the rolled Mg97Zn1Y2 alloy under the same dislocations induced strain,which is different from that of the as-cast alloy with moderate strengthening effect.     

Effect of rotating speed on rotating tool spot joining of 5052 alloy

ABSTRACT

Rotating tool spot joining in low rotating speed range is investigated in this study. Torque of the motor increased with the decrease in rotating speed of the motor. Joint strength increased significantly with the decrease in rotating speed. Shoulder plug fracture was observed as a characteristic fracture mode under the long joining time and low rotating speed joining conditions and it obtained the large plastic deformation. The rotational motion of the joining interface was similar with the Rankin vortex. The relationship between the joining interface deformation by the external force and the plastic deformability by the friction heat on this joining process was considered from the experimental results.     

Al_2O_3sf／LY12复合材料在高温下的流动应力与塑性变形行为

采用固态高温压缩试验、高温金相观察等试验手段，实验研究了Ａｌ２Ｏ３ｓｆ／ＬＹ１２复合材料在高温下的流动应力与塑性变形行为。结果表明，随着变形温度的提高，由于基体的变形抗力以及基体对阻碍其变形的纤维的剪切作用力大大降低，纤维不易产生断裂并易于以偏转的形式调整自己的方位来适应基体的塑性流动，从而显著降低复合材料的流动应力和大大提高其塑性和变形能力，有利于复合材料的成形。