A TEM study of deformation substructure in high strain rate and explosively shock-loaded polycrystalline iron

Residual microstructures developed in polycrystalline iron after deformation at high rates of strain were characterized using transmission electron microscopy (TEM). Long, screw- type dislocations were observed in Hopkinson bar and impacted specimens compared to deformation twinning and dislocation cell structures found in shocked specimens. Localized deformation bands were observed in the impacted specimen. No evidence for deformation twinning was found in the Hopkinson bar and impacted specimens. The effect of substructure produced in the specimens on the hardness of these specimens is also observed.     

TC21G钛合金平面应变变形行为及其机理

利用平面应变压缩实验,研究TC21G钛合金在变形温度为870～940℃、应变速率为0.1～1 s-1条件下的变形行为,并分析显微组织的演变过程.同时,研究加工参数对应变硬化指数n值的影响.结果表明:在应变速率一定的条件下,随着变形温度的升高,显微组织中 β 相的含量增加,合金的流变应力降低;而在变形温度一定的条件下,随...     

A study of surface roughening in fcc metals using direct numerical simulation

Grain-scale surface roughening due to plastic straining in polycrystalline aluminum is studied with the aid of a three-dimensional finite-element crystal-plasticity model. An improved understanding of the origin of surface roughening profiles in plastically strained aluminum is sought. Large-scale, Direct Numerical Simulation enables the computation of full-field solutions and the explicit consideration of the deformation of individual crystals as well as of their crystalline texture evolution and interaction with neighboring grains. Simulations are conducted on idealized flat sheet polycrystalline aluminum samples under uniaxial and biaxial loading conditions. The results obtained show that the ensuing surface profiles are controlled by several factors: applied boundary conditions, Taylor factor and shear tendency of the individual grains and the spatial distribution of grain neighborhood orientations. The conditions leading to different ridging profiles, e.g., corrugated and ribbed surface profiles, are discussed.     

多晶体钛拉伸变形表面褶皱的形成及晶粒变形

用原子力显微镜对多晶体钛拉伸塑性变形中表面褶皱的形成及单个晶粒的变形进行了连续观察和测量.结果表明,表面褶皱随着应变量的增加而增大,而凹凸的相对位置关系不随变形的进行而变化.从单个晶粒来看应变量在0.1附近时由于孪晶的形成产生了新的凹凸,但对整体的表面褶皱凹凸位置关系没有大的影响.表面平均粗糙度和表面最大高低差随应变量的增加近似呈直线性增大,应变量在0.1左右时由于孪晶的形成,使粗糙度有较明显的增大.     

2195铝锂合金热模拟平面应变压缩行为研究

2195铝锂合金被认为是航空航天领域的理想结构材料,吸引了广大研究学者的兴趣,但其热变形行为的研究却相对较少。本文通过热模拟平面应变压缩试验,研究了2195铝锂合金的热变形行为,变形温度为400~500 ℃,应变速率为0.01~10 s-1。研究表明,材料变形呈稳态流变特征,随变形温度增加,应变速率降低,流变应力逐渐减小,合金具有正应变速率敏感性。建立了2195铝锂合金材料本构方程,其激活能值为214.937KJ/mol。通过分析加工图,得到材料的适宜加工区为应变速率接近0.01 s-1,温度为475~500 ℃。最后通过分析动态软化过程中的应力规律,得到了材料软化机制判定方程。     

往复挤压变形ZK60镁合金应变分布的有限元数值模拟

利用有限元法研究ZK60镁合金在多道次往复挤压过程中的应变分布。为优化往复挤压工艺,研究摩擦条件和模具结构尺寸对ZK60合金总等效塑性应变分布的影响。结果表明：ZK60合金试样经往复挤压后内应变分布不均匀。试样两个端部的应变量低于试样中间部位的应变量。工艺参数对应变分布的影响很大。试样和模具间的摩擦不利于试样内应变量的均匀分布,因而应尽量降低摩擦。为了提高应变量的均匀分布,应该使用较大的过渡圆角半径和较低的挤压角度。     

Producing the nano/ultrafine grained low carbon steel by martensite process using plane strain compression

In this study, a simple experimental method was developed to produce the nano/ultrafine grained structure in a low carbon steel. This method involved cold plane strain compression of martensite starting structure and subsequent annealing. The martensitic specimens were first plane strained to 75% reduction in thickness (equivalent strain of 1.5), followed by subsequent annealing at the temperature range of 400-550 °C for 0-180 min. The microstructures and mechanical properties of specimens were characterized. The microstructure of the specimen annealed at 500 °C for 65 min showed cementite precipitates in the ferritic microstructure with the grain size of about 145 nm. The results showed superior mechanical properties with the estimated values of ultimate strength of 1135 MPa and uniform elongation of 11.6% for the fully recrystallized specimen.     

应变速率对QT450-10球墨铸铁热塑性及石墨颗粒的影响

利用Gleeble-1500热模拟机研究了QT450-10球墨铸铁在不同压缩应变速率下的热塑性行为。结果表明:随着应变速率的增加,球墨铸铁在950℃下的极限压下量呈递减趋势。流变曲线随着真应变的增加,迅速达到一峰值后发生软化,即存在动态再结晶行为。利用光学显微镜观察了微观石墨球在明/暗场中的形貌,发现石墨颗粒在三向不等压应力作用下具有一定的塑性,但是应变速率对石墨颗粒的形貌及分布的直接影响并不明显。     

Enhancement of tool geometry for a plane strain punch stretching test

In this paper, the tool geometry of the plane strain punch stretching (hereafter PSS) test was optimized in order to assure the reliability of the test. Considering several factors for optimization of the tool geometry, a computer-simulation technique using the three-dimensional finite element method (FEM) was used. Three design variables — the punch length, punch crown and punch corner radius — are chosen to be optimized according to the Taguchi’s experimental technique with the L₉, orthogonal array. The optimum condition to ensure the plane strain mode over the overall area of the specimen was clarified. The simulation results have been confirmed by experiment.     

电致塑性变形中试样尺寸与表面电场相关性研究

为了探究电场作用下金属圆柱体塑性压缩试验试样尺寸变化对试样表面电场稳定性的影响,视电致塑性压缩试验过程为动态的同轴圆柱形电容器充放电过程,通过理论推导和Mathematica数学软件分析,研究了电场作用下金属塑性压缩时试样尺寸与其表面电场的相关性.研究结果表明:当试样半径和电极半径之比R2/R0< 2时,试样变形过程中表面电场变化显著;当2< R2/R0< 3.5时,对于试样真应变较小(ε≤0.1)的情况,试样表面处电场可近似认为恒定;当3.5< R2/R0< 5时,试样变形中表面电场稳定性较好.为此类试验研究及以此为基础的各种材料成形技术的试验设计提供了理论和试验依据.     

温压缩变形过程中渗碳体的球化行为

在Gleeble-3500热模拟试验机上对原始组织为层片状珠光体的高碳钢进行温度为640～700℃、应变速率0-01～10s-1,最大真应变为1的温压缩变形.采用扫描电镜研究了该过程中渗碳体片层的组织演变情况,并用DEFORM-3D软件模拟了温变形过程中试样内部等效应变的分布情况.结果表明,温压缩变形过程中存在着形变不均匀性,使得试样心部和边缘组织形态有明显差异,随着应变速率的降低,边缘层片状珠光体组织球化为粒状珠光体;在变形过程中珠光体组织主要发生渗碳体片层的球化,随着应变量、应变速率以及变形温度的增加,渗碳体球化率明显增大.     

Calibration of anisotropic yield function by introducing plane strain test instead of equi-biaxial tensile test

The equi-biaxial tensile test is often required for parameter identification of anisotropic yield function and it demands the special testing technique or device. Instead of the equi-biaxial tensile test, the plane strain test carried out with the traditional uniaxial testing machine is suggested to provide the experimental data for calibration of anisotropic yield function. This simplified method by using plane strain test was adopted to identify the parameters of Yld2000-2d yield function for 5xxx aluminum alloy and AlMgSi alloy sheets. The predicted results of yield stresses, anisotropic coefficients and yield loci by the proposed method were very similar with the experimental data and those by the equi-biaxial tensile test. It is validated that the plane strain test is effective to provide experimental data instead of equi-biaxial tensile test for calibration of Yld2000-2d yield function.     

Kinetics of graphite expansion during eutectic solidification of cast iron

The paper introduces a new linear displacement analysis (LDA)/thermal analysis (TA) experimental device for measuring linear displacement during the solidification of cast iron. The experimental device comprises a sand mould encased in a steel shell that prevents mould wall movements. Thus, only the linear displacement caused by the shrinkage or expansion of the metal is recorded by the transducers. Two quartz rods introduced directly at different heights into the liquid metal and connected to two transducers record the linear displacement during the liquid–solid transformation and subsequent cooling. Two thermocouples positioned at the same height with the quartz rods allow for the concomitant TA and LDA and thus for the direct correlation between expansion/contraction and the temperature change during solidification events such as graphite formation. The LDA device was used to study the differences in the solidification mechanisms of irons with different graphite morphologies (lamellar, compacted/vermicular and spheroidal) at carbon equivalent in the range of 3·7–4·4%. The analysis included the LDA and TA curves and full metallographic characterisation of the cast irons. In general, graphite expansion increased as the graphite shape changed from lamellar, to compacted and then to spheroidal. The most important process variables are the magnesium and carbon contents. Higher Mg residual and C in the iron produced more graphite expansion. Compacted graphite (CG) iron was particularly sensitive to the Mg residual. Indeed, the high Mg CG irons exhibited similar graphite expansion to that of spheroidal graphite (SG) iron, while the low Mg CG iron expansion was closer to that of the lamellar graphite (LG) iron. Graphite expansion increased for all data with the time interval over which graphite expansion occurred. It also increased with both carbon and carbon equivalent. The time for graphite expansion increased noticeably with the carbon content of the iron. It did not depend on the graphite shape. By combining TA and LDA, it was possible to plot the evolution of graphite expansion as a function of the fraction solid and thus to understand the kinetics of graphite expansion. The amount of expansion available at the end of solidification was quantified. Such data, when correlated with process variables, will be useful in decreasing microshrinkage and in producing riserless compacted and SG irons.     

Characteristics of cementite formation in the surface layer of flake graphite cast iron formed by plasma transferred arc remelting

Using the plasma transferred arc (PTA) process to conduct surface remelting, the surface region of flake graphite cast iron can be hardened. To study the effect of silicon content on the microstructure of the hardened surface layer, flake graphite cast iron specimen of about 3.1 ～ 3.2 wt pet carbon and various silicon contents from 1.47 to 3.41 wt pet were selected for the PTA surface remelting experiment. The remelting current and voltage were set at 100 A and 32 V. The results show a rapidly solidified microstructure with large amounts of eutectic cementite. Reducing the silicon content increases the volume fraction of eutectic cementite and thus increases the hardness. The increase of cementite volume fraction with decreasing silicon content is linear. Reducing the silicon content is also effective in suppressing the formation of porosities.     

交变载荷作用下平面应力状态焊接接头焊趾处应力应变理论解的推导

基于弹塑性力学理论计算对接接头交变载荷作用下焊趾处应力和应变,以准确预测交变载荷作用下平面应力状态对接接头焊趾处的应力和应变值为目标,考察了不同加载情况下平面应力状态对接接头焊趾处应力和应变的理论计算方法,并给出了交变载荷作用下平面应力状态对接接头焊趾处应力和应变的理论解.结果表明,在材料参数、载荷大小以及焊趾应力集中系数已知的前提下,根据提出的理论解的计算式可获得相应载荷时接头焊趾处的应力应变,为焊接接头在交变载荷下的疲劳失效预测提供理论指导.     

Variation effect of strain rate on microstructure in isothermal compression of Ti-6Al-4V alloy

Isothermal compression of the Ti-6Al-4V alloy at the deformation temperatures of 950 and 980℃,height reductions of 30％ and 60％,and strain rates of 0.001,0.010,0.100 and 1.000 s-1 was conducted,wherein the variations of microstructure with strain rate were investigated.The experimental results showed that the variation of the microstructure with the strain rate under one condition was significantly different from that under another condition,which meaned that the interaction between the processing parameters was great.The optimization of the strain rate under one condition was not suitable for another condition.Therefore,selecting the forging equipment and optimizing the strain rate should be based on simultaneously considering the deformation temperature and height reduction.     

Deformation of specimen during laser surface remelting

During laser surface remelting, thermal expansion in heating, and contraction of material in cooling, and due to microstructure changes in the heated surface layer, the specimen deforms. This induces volume changes during the process of remelting the thin surface layer and causes internal stresses. By measuring the deformation of the specimen during the remelting process as well as after the laser beam interaction, i.e., to the moment when the specimen cools down to the ambient temperature, it is possible to follow the progress of deformation in the specimen material. Detailed information on the deformation events in the specimen material during the laser remelting process of a thin surface layer enables the engineer to optimize the process variables in terms of amount of deformation or magnitude of residual stresses on completion of cooling.     

加热温度和保温时间对镜面纯铁表面粗糙度的影响

精密零件的扩散连接中针对主要扩散工艺参数对镜面纯铁的表面粗糙度影响,选取99.99%的纯铁为研究对象,分别研究了加热温度和保温时间对纯铁表面粗糙度的影响.结果表明,在550℃以下,保温1 h,温度对纯铁表面粗糙度影响不大,在10 nm之内,但有增大趋势,在550℃以上时,纯铁表面粗糙度缓慢增大,直到912℃,纯铁发生多晶型转变,表面粗糙度有突变,由原来的几个纳米突变到700 nm,在400℃时保温时间从60~240 min,表面粗糙度变化非常小,说明在此温度下保温时间对表面粗糙度影响有限.     

马氏体相变的晶体学特征—不变平面应变

自20世纪20年代以来,基于马氏体相变产生的浮凸和在母相中预先刻制的直线变成在相界面上连续的折线,提出了以切变为基础的马氏体相变的晶体学特征—"不变平面应变"的概念。随后,以该概念为基础建立了马氏体相变晶体学表象理论(PTMC)。然而,刘宗昌等基于相同的实验,即马氏体相变后的直线刻痕仍为直线和浮凸形态为帐篷形,分别在2010年和2013年《热处理》杂志上发表文章,否定马氏体相变的"切变"机制进而否定马氏体相变的"不变平面应变"。如所周知,否定马氏体相变的"切变机制"就是否定"不变平面应变",因此刘宗昌等于2013年发表的文章彰显出他们的轻率。本文作者已在3篇文章中列举用原子力显微镜和透射电镜观察的结果驳斥了他们的错误观点,至少可以说,他们没有理解我们文章中的实验和理论。为此,本文再次引用Yang和Wayman的透射电镜实验结果,即单变体马氏体使预存在的层错迹线(直线)变成折线,而自协调的多个马氏体可使迹线仍为直线;单变体马氏体的浮凸为N形,但多变体马氏体的浮凸可以是帐篷形或更为复杂的形态,由此可以说明刘宗昌等错误观点的原因。最近,本文作者及其合作者基于PTMC计算了Mn80Fe15Cu5热弹性合金马氏体相变的惯习面,并与实验结果相符,由此确认了"不变平面应变"是马氏体相变晶体学特征的正确性。     

基于应变影响的7A09铝合金等温压缩流动应力模型

在Gleeble-1500型热模拟压缩机上研究7A09铝合金在温度为633~733 K、应变速率为0.01～10.0s-1、最大变形程度为60%条件下的高温流动行为;基于7A09铝合金高温压缩时的流动应力特征,建立反映应变影响的7A09铝合金流动应力模型.结果表明:随着变形温度的升高和应变速率的降低,合金的流动应力显著降低;当应变超过一定值后,随着应变的增加,高、低应变速率下合金的流动应力变化趋势不同;建立的流动应力模型的计算值与实验值之间的最大误差为7.77%,平均误差为2.69%;与不考虑应变影响的流动应力模型相比,该模型的拟合精度高,能较好地描述7A09铝合金高温变形过程中的流动行为,为铝合金高温变形过程的数值模拟奠定了较好的基础.