低层错能奥氏体钢的变形硬化特点

研究了低层错能奥氏体不锈钢和高锰钢低温下的变形行为,根据两者拉伸应力应变曲线硬化阶段的相似性,提出曲线由基体硬化、行为软化和结构硬化三部分合成,应力应变曲线的上凹特征是软化主导与硬化共同作用的结果。分析了两种材料变形过程的相似性与不同点及其原因。     

考虑晶粒尺寸影响的硬度预测模型

目的研究了晶粒尺寸对冷成形过程中材料硬度变化的影响。方法通过不同的退火处理,获得了具有不同晶粒尺寸的材料。采用压缩试验、压痕试验和有限元模拟相结合的方法,研究了不同初始晶粒尺寸条件下材料硬度与等效应变的关系。结果初始晶粒尺寸d对材料初始硬度HV0、硬化系数K、硬化指数n有较大影响,随着晶粒尺寸的增大,它们呈减小趋势。同时提出了一个考虑晶粒尺寸影响的硬度预测模型,并进行了实验验证,硬度的预测值与测量值吻合较好。结论该模型能准确预测具有不同初始晶粒尺寸的材料在冷成形过程中的硬度分布。     

成形加工和飞行振动对防爆铝箔网力学性能的影响

通过拉伸试验、硬度测试、扫描电镜观察(SEM)以及模拟飞行振动试验,研究了防爆铝箔网的力学性能以及飞行振动对其力学性能的影响.结果表明:防爆铝箔网的成形加工过程对其抗拉强度有显著影响.加工造成防爆铝箔厚度不均匀,使其抗拉强度测试数据表现出很大的分散性.与加工前相比,加工造成的冷作硬化效应使防爆铝箔网材料的平均抗拉强度显著提高.而飞行环境中的长期振动对防爆铝箔网的表面微观形貌、断裂行为、抗拉强度、显微硬度没有可察觉的影响.     

拉伸变形对电沉积镍镀层微观结构及力学性能的影响

采用电沉积法在低碳钢基体上制备了韧性较好的镍镀层,并对镀镍钢带进行了拉伸试验.通过扫描电镜(SEM)和X射线衍射(XRD)分别对镍镀层变形前后的表面形貌和织构进行了研究.用纳米压痕的方法对拉伸变形前后的镍镀层样品进行了载荷-深度曲线的测试,并测量了电沉积镍镀层变形前后的硬度与杨氏模量.结果表明,拉伸变形后的镍镀层表面粗糙度变大,出现起皱,但并没有产生新的织构,只是原有织构的强度发生了变化,其硬度和杨氏模量都明显减小.对于同一样品,加不同载荷会导致不同的压痕深度,随深度的增加,测得的压痕硬度和样式模量也有明显降低.     

H80薄板拉伸性能的尺寸效应

为了研究薄板拉伸性能的尺寸效应及其产生的微观机制,对晶粒尺寸相似但厚度不同的H80薄板试样进行了拉伸试验和显微硬度试验.通过建立晶界区域强化模型和晶粒间的弱弱组合变形机制,研究了薄板的厚度尺寸、晶粒尺寸及显微硬度分布对拉伸力学性能的影响.结果显示:强度指标值和塑性指标值均随厚度尺寸的减小而降低,呈现越小越弱的尺寸效应现象,但强度指标和塑性指标的变异因子值却呈现越小越强的相反规律.薄板的横向硬度均值随厚度尺寸的增大而增大,而横向硬度值的变异因子却随厚度尺寸的增大而减小.随厚度尺寸的减少,晶界区域所占百分比的减少和单个晶粒所占百分比的增加是产生拉伸性能尺寸效应现象的根本原因.此外,显微硬度分析表明:易变形区域形成的概率随厚度尺寸的减小而增大,易变形区域的集中变形是引起薄板试样过早断裂的原因.     

PLLA-PCL无规共聚物的冷变形形状记忆效应

以辛酸亚锡为引发剂,采用开环聚合法合成了一系列聚乳酸(PLLA)-聚ε-己内酯(PCL)无规共聚物,并通过红外光谱和差示扫描量热分析(DSC)对共聚物的结构与热性能进行了表征.共聚物在玻璃态拉伸时的力学行为表明,材料在屈服后出现的大应变,本质上是一种高弹形变.采用拉伸试验对共聚物的冷变形形状记忆效应进行了研究,研究结果表明,共聚物的组成、变形量以及定型温度对冷变形形状记忆效应都有较大的影响,随着变形量的增加,形状恢复率呈下降趋势,低的定型温度可获得较好的冷变形形状记忆效应.     

TRIP钢板在单向拉伸变形下的相变行为研究

TRIP钢的塑性取决于残余奥氏体的含量及其相变速度.研究TRIP钢中残余奥氏体在单向拉伸变形下的相变行为,首先回顾了TRIP钢板的研究进展,然后对一种低碳硅锰系TRIP700钢板在单向拉伸变形下的力学性能和相变性能进行了试验研究,并用扫描电镜方法分析了该TRIP钢在变形过程中微观组织的变化.结果表明,该低碳硅锰系TRIP钢板兼有高强度和高韧性的特点,在单向拉伸变形中残余奥氏体的含量与等效应变的关系可用来logfγ0-logγ=kε.表示.     

SiC晶须对SiCw/6061Al复合材料拉伸和压缩变形行为的影响

通过试验研究了SiCw/6061Al复合材料和6061Al基体的拉伸和压缩变形行为.结果表明,SiCw/6061Al复合材料和6061Al的拉伸变形行为相同,而压缩变形曲线上出现应力峰,这与不同应力状态下SiC晶须的转动有关.拉伸时SiC晶须逐渐转向与外力平行方向使SiCw/6061Al复合材料的应力增大,而压缩时SiC晶须转向与外力垂直方向使其应力减小,而不是由动态再结晶引起.     

高密度聚乙烯材料在大变形条件下的数值模拟研究

针对高密度聚乙烯(HDPE)在大变形条件下有限元模型不易收敛、本构关系较为复杂的问题,对HDPE片材进行了单轴拉伸试验和数值模拟研究。通过对比试验结果和模型计算结果发现:非线性粘弹性本构模型与小变形条件下HDPE的单轴拉伸试验结果较为吻合,但与大变形条件下的试验结果相差较大;而Kwon模型的计算结果与大变形和小变形条件下的试验结果均较为吻合。同时,对Kwon模型的参数选择进行了优化,得到了100mm/min和150 mm/min拉伸速率下的优化参数,对大变形下片材的数值模拟具有较好的参考价值。此外,通过对条带单元的应力应变分析,可知HDPE条带在单轴拉伸下的应力应变呈不均匀分布,中心点区域是片材最大应力应变的集中点,这也解释了HDPE条带断裂多出现在中心区域的原因。     

SiCw／6061A1复合材料温度循环拉伸变形行为

本文采用热循环拉伸试验方法研究了ＳｉＣｗ／６０６１Ａ１复合材料的变形行为．结果表明，ＳｉＣｗ／６０６１Ａ１复合材料温度循环拉伸变形行为与蠕变类似分为初始变形阶段、稳态变形阶段和快速断裂三个阶段；温度循环拉伸变形稳态流变速率明显提高；温度循环拉伸变形的应力指数低于恒温蠕变的应力指数．     

Estimation of the elasto-plastic properties of metallic materials from micro-hardness measurements

Although hardness testing is a cost-effective and reliable technique to quickly estimate the overall mechanical properties of materials, hardness is not a fundamental mechanical parameter and it is rarely used in constitutive modeling or in engineering design. This work proposes a procedure to derive the elasto-plastic properties of metallic materials from the Vickers and the Knoop hardness measurements. Through dimensional analysis and finite-element simulations, relationships between hardness testing variables and mechanical parameters (Young’s modulus, yield strength, and strain hardening exponent) are built up. An inverse procedure is developed to derive the mechanical parameters within the framework of genetic algorithm. The method is verified against two bulk steel materials and it is shown the derived stress–strain curves are in good accord with those measured by tensile tests. Then the method is successfully applied to measure the elasto-plastic properties of the pore walls of a metallic foam material. The present method can achieve a performance as good as those based on the instrumented indentation test and it is more readily accessible to the industry considering all the measurements can be performed on a cost-effective micro-hardness tester.     

Experimental observation and numerical modeling of formation of local plastic zones in hardened surface layers due to contact overloading

The problem of formation of plastic zones in case-hardened metallic bodies due to contact overloading is studied both experimentally and numerically. Metallic materials exposed to surface hardening demonstrate spatial variation of the material hardness and yield strength with a decreasing profile with depth and belong to the class of so-called plastically graded materials. The presented experimental program employs micro-Vickers hardness tests to map the variation in material hardness and corresponding yield strength for both virgin and loaded case-hardened specimens made of a chromium tool steel. It is shown that, depending on the profile of the yield strength in the near-surface zones and contact parameters, a plastic deformation can originate underneath the hardened layer. The distribution of the effective plastic strain extracted from the micro-hardness increment measurements are found in good agreement with the results of finite element simulations of a plastically graded material subjected to similar loading conditions. Numerical analysis reveals significant perturbations in the stress field distribution within the hardened layer due to formation of a closed-shaped plastic zone in the gradient layers, including development of a tensile stress on the boundary between the elastic and plastic zones as well as an overall increase in the effective stress intensity. It is shown that the hardened layer behaves similar to an elastic beam on a compliant foundation. These stress field perturbations in the hardened layers with low deformation capacity can greatly affect the durability and serviceability of surface treated mechanical parts.     

Numerical study on patterning of shear bands in a Cosserat continuum

Summary Numerical simulation of patterns of shear bands in biaxial compression tests using an elasto-plastic Cosserat constitutive equation is presented. Random distribution of the material properties acts as a trigger for the localized deformation. Two types of stress-strain curves, namely strain softening and strain softening followed by strain hardening, are investigated. It is shown that the characteristic of the stress-strain curve is crucial for the patterning of shear bands. While calculations with the stress-strain curve with solely softening yield only one single shear band, a flock of shear bands can be obtained with the stress-strain curve with softening followed by hardening. Benefited from the characteristic length provided by the Cosserat elasto-plastic constitutive equation, the dependence of the calculation on the mesh-size is avoided.     

Recrystallisation of austenite grain when non-isotherm steel working: Effects of thermal kinetics and deformation-based mechanisms

The effects of high temperature deformation on the recrystallisation of austenite grains and hardening occurring during hot forging of steels were studied. Three commercial steels containing various carbon weight percentages were heated beyond the austenitising temperature and free forged up to desired deformation ratios. The specimens were then air-, or oil-cooled. Two zones were distinguished according to the grain-size: a zone with fine grains, associated with highest plastic deformation and, a zone with coarse grains located within the subsurface layers. Unexpectedly, the highest values of microindentation hardness were obtained in the coarse-grain zone. Consequently, the interaction between the grain-size gradient induced by thermal kinetics of cooling and the local hardening governed by dislocation kinetics was studied by means of microindentation hardness inspections. Analysis of stress–strain curves confirmed that while forging enhances mechanical strength, it has a detrimental effect on ductility of steel.     

BAl7-7-2-2铜合金冷变形加工硬化行为

采用室温压缩试验研究了BAl7-7-2-2铜合金冷的变形加工硬化特性。结果表明,修正后的Ludwik模型可以准确地对BAl7-7-2-2铜合金的真应力-真应变数据进行拟合,相关材料常数K=1 001.736、m=0.295、n=0.044,拟合方程的相关系数可以达到0.96。当真应变ε＜0.02时,加工硬化率很高且迅速增大,此阶段为线性硬化阶段;当0.02≤ε≤0.2时,真应力随着真应变的增加而增大,加工硬化率则逐渐减小,这一阶段为抛物线硬化阶段;ε＞0.2时,随着应变的增加,加工硬化率趋于常数,为弱硬化阶段。结合微观组织和硬度测试表明,材料在不同变形阶段的变形机制有所不同。      

超声冲击对铝合金搅拌摩擦焊接头组织及性能的影响

目的分析超声冲击对铝合金搅拌摩擦焊成形后接头的组织及耐蚀性的作用效果。方法采用超声冲击设备对2A12铝合金搅拌摩擦焊接头表面进行超声冲击处理,并对超声冲击前后接头的显微组织、显微硬度以及耐腐蚀性能进行了分析。结果经过超声冲击处理后的铝合金接头上表面会产生一层塑性变形层,并且塑性层内位错密度增大,使表层金属得到一定程度的加工硬化,促使冲击后接头各区域的表面硬度明显提高,冲击后接头热机械影响区和热影响区硬度提高达60%以上;腐蚀浸泡试验发现,超声冲击后接头的点腐蚀程度较超声冲击前明显减缓,腐蚀速率约是冲击前的1/2。结论超声冲击有效改善了铝合金搅拌摩擦焊接头区域材料过时效的软化现象,并且有效改善了接头的抗腐蚀性能。     

AZ31B镁合金压-压循环载荷下变形行为及变形机制演化EI北大核心CSCD

为探讨AZ31B挤压态镁合金棒材沿径向取样的循环变形特征,开展了0.75%,1.0%,2.0%和4.0%应变幅下应变控制的非对称压-压循环变形实验。结果表明:在小应变幅(0.75%,1.0%)下,循环变形的滞回曲线表现出较好的对称性;在大应变幅(2.0%,4.0%)下,滞回曲线对称性差,且在滞回曲线上出现拐点;随着循环周次增加,塑性应变幅呈现下降趋势,材料均表现出循环硬化行为,在小应变幅下循环拉伸阶段对材料硬化率远大于压缩阶段的硬化率,而在大应变幅下这种区别并不明显。分析表明,沿径向取向的〈1120〉丝织构镁合金,小应变幅下位错滑移在整个寿命周期内作用更大;大应变幅下,随着塑性变形的增加,循环过程中变形机制发生演化,较低临界剪切应力(critical resolved shear stress,CRSS)的基面位错和拉伸孪生不能完全满足变形要求,较高CRSS滑移系启动及残余孪晶使得滞回曲线出现拐点;循环变形过程中不完全的孪生-去孪生过程使基体中存在大量残余孪晶,影响了循环变形过程的硬化率,同时降低了疲劳寿命。     

30%Cr-Cu复合材料热变形及动态再结晶临界条件

采用放电等离子烧结法(SPS)制备出30%Cr-Cu复合材料,对其致密度、硬度和导电率等相关性能进行测试,并观察分析该复合材料的显微组织。利用Gleeble-1500D型热模拟试验机在变形温度650~950℃、应变速率0.001~10s-1、变形量60%的条件下对30%Cr-Cu复合材料进行热模拟压缩试验。对热压缩试验得到的真应力-应变数据进行拟合、计算和分析,构建该复合材料的本构方程,同时得到材料的加工硬化率θ,利用材料的lnθ-ε曲线出现有拐点和-(lnθ)/ε-ε曲线对应有最小值这一判据,分析该复合材料的动态再结晶临界条件。结果表明:30%Cr-Cu复合材料的真应力-应变曲线主要以动态再结晶软化机制为特征,峰值应力随应变速率的增加和温度的降低而升高;该复合材料的lnθ-ε曲线出现拐点,-(lnθ)/ε-ε曲线对应有最小值,该最小值所对应的应变为临界应变εc,且εc随变形温度的升高和应变速率降低而减小,εc与Zener-Hollomon参数Z的函数关系为εc=2.38×10-3 Z0.1396。     

Some useful approximations for wrought aluminum alloys based on monotonic tensile properties and hardness

The objective of the present paper is to derive some useful approximations for estimating the strain‐controlled fatigue properties and cyclic deformation of wrought aluminum alloys from hardness and monotonic tensile properties. A variety of relationships and correlations among monotonic tensile properties, Brinell hardness, cyclic deformation and strain‐controlled fatigue properties are developed for wrought aluminum alloys. A simple method is proposed for prediction of the strain‐life curve requiring only ultimate tensile strength and modulus of elasticity. Prediction capability of the proposed method is evaluated for 25 kinds of wrought aluminum alloys with ultimate tensile strength between 120 MPa and 650 MPa. The proposed method provides good approximations of the strain‐life curve.     

Tensile deformation behavior of high manganese austenitic steel: The role of grain size

The tensile deformation behavior and microstructural evolutions of twinning induced plasticity (TWIP) steel with the chemical composition of Fe–31Mn–3Al–3Si and average grain sizes in the range of 2.1–72.6 μm have been analyzed. For each grain size, the Hollomon analysis and also the Crussard–Jaoul (C–J) analysis as an alternative method to describe the work hardening behavior were investigated. The results indicated that the optimum mechanical properties as a function of work hardening capacity can be obtained by changing the grain size. The microstructural observations showed that the pile-ups of planar dislocations are necessary for triggering the mechanical twinning and grain refinement suppresses the mechanical twinning in TWIP steel. Furthermore, the mechanical twinning increases with increasing applied strain. As a result, a high instantaneous work hardening due to the mechanical twin boundaries enhances the uniform elongation. The contribution from the strain of twinning and hardening due to an increase in the hardness of the twinned regions (i.e., the Basinski mechanism) may be also useful in achieving the high strength–ductility in TWIP steels.