TA15合金应变速率循环超塑性研究

采用应变速率循环法研究了TA15合金的超塑性.在变形温度分别为850、900、950℃,应变速率范同为1×10-5～1×10-3S-1的试验条件下.讨论了工艺参数对流变应力、m值及其超塑性的影响.结果表明,TA15合金具有较好的超塑性,最佳变形温度为900℃,伸长率为412%.     

变形温度对炉冷态TA15合金α＋β两相区流动应力和组织的影响

利用Thermecmastor-Z型热模拟试验机,在a 13两相区对炉冷态TA15合金进行等温恒应变速率压缩实验.获得高温塑性流动的真应力-真应变关系曲线特征,并采用金相显微镜对其变形后的显微组织进行观察.结果表明.变形温度对炉冷态TA15合金的流动应力的影响与应变速率大小密切相关;并且获得了试验参数范围内的热变形过程的较佳工艺参数范围,其热加工温度为850～900℃,应变速率为0.001～0.01 s-1.     

中碳V-N微合金钢热变形行为

用Gleeble-1500热模拟试验机对中碳V-N微合金钢在不同变形温度(900~1050℃)及不同变形速率(0.005~30 s-1)的奥氏体区热变形行为进行研究。通过建立真应力-真应变曲线、动态再结晶图、功率耗散效率因子(η)图和应变速率敏感因子(m)图综合分析其热变形行为。结果表明,试验钢在1050℃、1 s-1变形条件下发生了动态再结晶,其真应力-真应变曲线、动态再结晶图、m图等方法得出的结果相互吻合。其中η图与m图差异很小,但由于应变速率敏感因子具有合理的物理意义,因此建议利用m图分析材料的热变形行为和选取最佳热变形工艺参数。     

TA1/6061铝合金双金属热变形行为及热加工图

以TA1/6061铝合金双金属为研究对象，采用Gleebe-3800热模拟试验机，在变形温度为350～500℃、应变速率为0.01～1 s^-1、变形量为40%的条件下进行了单向热压缩复合试验，研究了TA1/6061铝合金双金属的热变形行为，建立了TA1/6061铝合金双金属本构方程及热加工图。结果表明，TA1/6061铝合金双金属热变形过程中的流变应力随着温度的上升和应变速率的降低而减小；基于试验数据建立的Arrhenius本构方程可以有效预测特定真应变下的真应力，其相关性系数为0.99642,热变形激活能为231434 J·mol^-1;基于热加工图、SEM图像和EDS线扫描图像，确定最优热加工工艺窗口为：变形温度为482～500℃,应变速率为0.011～0.192 s^-1。     

Ti53311S钛合金热变形的组织和机制分析

用Gleeble-1500型热模拟试验机对Ti53311S钛合金在温度为880～1080℃,应变速率为0.001～10 s-1的条件下进行高温压缩变形行为的研究.测试了其真应力.真应变曲线,采用双曲正弦本构方程计算出激活能,双相区为641 kJ/mol,β相区为244 kJ/mol.观察了变形后的显微组织,并分析了其变形机制.结果表明:该合金对温度和应变速率敏感,不同变形条件下应力值变化很大;应变速率敏感指数(m)随温度升高而降低,而变形激活能(Q)随温度升高而增大.合金的变形机制在双相区为晶界滑移和晶粒球化,在β单相区为动态回复.     

TA11钛合金热变形本构方程

在Gleeble-3500热模拟试验机上进行等温热压缩试验,得到TA11钛合金在温度为954~1074℃、应变速率为0.05~5 s~(-1)、变形量为60%条件下的真应力-真应变曲线。根据真应力-真应变曲线,分析流变应力随变形温度、应变速率和应变的变化规律。结果表明,流变应力与变形速率成正比,与变形温度成反比;利用Arrhenius双曲正弦方程和Z参数建立了TA11钛合金的热变形本构方程。经验证明,试验值与所建立的本构方程的预测值吻合较好,可用于预测TA11钛合金塑性变形过程中的变形抗力和作为有限元数值模拟的材料模型。     

TA15合金的热变形行为及加工图

研究了TA15合金的热模拟压缩实验。结果表明：变形温度的升高和应变速率的减小使峰值应力和稳态应力显著降低，变形温度会影响进入稳态流动所需变形量。以热模拟压缩实验为基础，建立的加工图表明：TA15合金高温变形时存在2个非稳定区域，一个是变形温度1300K以上和应变速率10．0s^-1以上的区域，另一个是变形温度1200K以下和应变速率0．006s^-1～1．995s^-1之间的区域。同时，建立的TA15合金高温变形时的流动应力模型表征了变形温度、应变速率和变形程度对流动应力的影响，模型的计算精度较高。     

挤压态42CrMo钢热塑性加工图及稳态变形参数识别

通过挤压态42CrMo钢多组试样的热压缩实验获得应变速率为0.01～10 s-1、变形温度为1123 ～1148 K条件下的真应力-应变数据,以此作为计算应变速率敏感指数(m值)、能量耗散因子(η值)和失稳判据(ξ值)三重判据的底层材料数据.利用三重判据构建的加工图对挤压态42CrMo高强度钢的热成形过程进行分析,得到了挤压态42CrMo强度钢在成型过程中的稳定区域和流变失稳区.结果表明:42CrMo钢的最优变形热力参数处于具有较高η值和较高ξ值的区域内.     

铸造Mg-4Al-2Sn-Y-Nd镁合金的热压缩行为与加工图

利用热压缩实验研究一种新型的具有优异室温塑性的Mg-4Al-2Sn-Y-Nd镁合金的高温流变行为,变形温度为200～400℃,应变速率为1.5×10-3～7.5 s^-1。结果表明：合金的应变速率敏感因子（m）在不同变形温度下均明显小于AZ31镁合金的m值,因此该合金适合在高应变速率下进行热加工。在真应力-应变曲线基础上,建立Mg-4Al-2Sn-Y-Nd 镁合金高温变形的本构方程,并计算得到合金的应力指数为10.33,表明合金在高温下主要的变形机制为位错攀移机制。同时,利用加工图技术确定合金的最佳高温变形加工窗口,即变形温度在350～400℃之间,应变速率在0.01～0.03 s^-1。     

5083铝合金热压缩应力-应变曲线修正与热加工图

在Gleeble-3500热模拟试验机上对圆柱体5083铝合金试样进行温度为300～500℃、应变速率为0.001～1 s~(-1)条件下的热压缩试验。对实验获得的真应力应变曲线进行摩擦修正,依据摩擦修正后的应力应变曲线计算本构方程,采用包含Zener-Hollomon参数的本构方程描述摩擦修正后的5083铝合金流变应力行为,其热变形激活能为164.17 kJ/mol。根据摩擦修正后的真应力-应变曲线绘制热加工图,随着真应变的增加,失稳区域向着高应变速率、高变形温度区域扩展,5083铝合金适宜热变形工艺参数:变形温度为400～500℃、变形速率为0.01～0.1s~(-1)与340～450℃、变形速率为0.001～0.01 s~(-1)。随着变形温度升高与应变速率降低,晶粒内位错密度减少,主要软化机制逐渐由动态回复转变为动态再结晶。     

Strain Rate Effects in Tungsten

The yield strength of annealed tungsten was found to have a strain rate exponent 12 times as great as that of low carbon steel. The effects of temperature and strain rate could be correlated through the Zener-Hollomon parameter with a heat of activation as-sociated with yielding of 32,000 cai per g-atom. This heat of activation is independent of strain although both the temperature and strain rate dependence of the yield strength vary with strain.     

Strain Rate Dependency of Simulated Welding Residual Stresses

In the present work, the influence of taking into consideration the strain rate-dependent nature of steel S355 during a calculation of the welding residual stresses through a finite element simulation is investigated. The Perzyna material model is calibrated using experimental values found in the literature and is introduced to a validated weld simulation model, where the strain rate dependency had not been taken into consideration before this study. The calculated profiles of the welding residual stresses, for strain rate-dependent and independent behavior, are then compared with experimentally measured profiles. The results of this first-step analysis show that taking into consideration strain rate dependency during a welding simulation of the investigated S355 has non-negligible influence on the calculated welding residual stresses.     

应变率和应变历史对Cu-Al-Zn形状记忆合金力学行为的影响

本文研究了不同应变率和应变历史对单相Cu-Al-Zn形状记忆合金的力学行为的影响.结果表明,在试验应变率0.001/s～0.1/s范围,单相记忆合金是一种应变率相关材料,抗拉强度Rm和断裂应变∈f随应变率的增加而降低,呈现应变率弱化效应;但是,应变率改变对母相的弹性变形、热弹马氏体相变的起始应力没有显著影响.实验结果还表明,合金存在明显的相变诱发应变滞后和应力松弛现象.     

TC11钛合金应变率相关的拉伸行为

为了获取TC11钛合金拉伸性能随应变率的变化规律,对该材料开展了宽应变率范围下的单轴拉伸试验。结果表明,随着应变率从准静态增加到动态,TC11钛合金的屈服强度略有上升,而应变硬化模量下降。此外,在准静态和动态拉伸下,TC11钛合金均发生了剪切断裂,但动态断裂面上韧窝尺寸小于准静态断面上韧窝尺寸。进一步对材料在变形过程中的温升进行了分析,结果发现,高应变率下材料断裂面上更小尺寸的韧窝和材料更容易发生应变软化归因于动态加载情况下材料中产生了更高的温升。     

Analytical Modeling of Strain Rate Distribution During Friction Stir Processing

Friction Stir Processing (FSP) is becoming an acceptable technique for modifying the grain structure of sheet metals. One of the most important issues that hinder the widespread use of FSP is the lack of accurate models that can predict the resulting microstructure in terms of process parameters. Most of the work that has been done in the FSP field is experimental, and limited modeling activities have been conducted. In this work, an analytical model is presented that can predict the strain rate distribution and the deformation zone in the friction stir processed zone as a function of process parameters. In the model, the velocity fields within the processed zone are determined by incorporating the effects of both the shoulder and the pin of the tool on the material flow. This is achieved by introducing state variables and weight functions. The model also accounts for different interfacial conditions between the tool and the material. The effects of different process parameters and conditions on the velocity fields and strain rate distributions are discussed. The results clearly show that the model can successfully predict the shape of the deformation zone and that the predicted strain rate values are in good agreement with results reported in the literature. This article was presented at the AeroMat Conference, International Symposium on Superplasticity and Superplastic Forming (SPF) held in Baltimore, MD, June 25-28, 2007.     

应变速率对TA15钛合金超塑性的影响

采用恒应变速率拉伸方法研究了应变速率对TA15合金超塑性的影响。结果表明,在变形温度为900℃,应变速率为3.3×10-4～1.1×10-2s-1时,随应变速率的降低,伸长率逐渐增大,最大伸长率为1074%。同时,在高应变速率条件下也获得了良好的超塑性能。此外,应力-应变曲线中出现了较长的应变硬化阶段,应变速率越低,应变硬化阶段越长,并且有利于超塑性变形。微观组织观察表明应变速率对TA15合金显微组织演变有着显著的影响,应变速率越低,显微组织粗化越严重。高应变速率条件下,由于动态再结晶的作用,试样变形区出现了很多新的细小等轴α相。     

高应变速率对纯钛塑性变形的影响

利用动态塑性变形(DPD)和准静态压缩变形(QSC)技术对纯钛圆柱样品进行对比压缩试验,研究了不同应变速率下纯钛形变孪晶和微结构演变。结果发现:2种变形方式的变形机制相似,低应变时以形变孪生为主,孪生饱和后转变为位错滑移主导;高应变速率促进了形变孪晶的产生,激发{4211}压缩孪晶的形成,同时使变形机制转变临界应变提前至0.2;纯钛在高应变速率和高应变(ε≥0.6)下出现绝热剪切带(ASB)。     

高应变速率下超高强冷轧双相钢的拉伸性能研究

测试超高强冷轧双相钢在不同应变速率下的准静态和动态拉伸力学性能,并通过数学模型研究了高应变速率下材料的变形行为。结果表明,Johnson-Cook模型拟合度较差,但是根据可决系数修正以后的模型预测结果与实验结果具有较好的吻合度。     

Carbon-Nanofiber-Reinforced Syntactic Foams: Compressive Properties and Strain Rate Sensitivity

The current study is focused on exploring the possibility of reinforcing syntactic foams with carbon nanofibers (CNFs). Syntactic foams are hollow, particle-filled composites that are widely used in marine structures and are now finding applications in other modes of transportation due to their high stiffness-to-weight ratio. The compressive properties of syntactic foams reinforced with CNFs are characterized over the strain rate range of 10^?4 to 3,000 s^?1, which covers seven orders of magnitude. The results show that despite lower density with respect to neat epoxy, CNF/syntactic foams can have up to 7.3% and 15.5% higher quasi-static compressive strength and modulus, respectively, for the compositions that were characterized in the current study. In addition, these properties can be tailored over a wide range by means of hollow particle wall thickness and volume fraction, and CNF volume fraction. The compressive strength of CNF/syntactic foams is also shown to generally increase by up to a factor of 3.41 with increasing strain rate when quasi-static and high-strain-rate testing data are compared. Extensive microscopy of the CNF/syntactic foams is conducted to understand the failure and energy absorption mechanisms. Crack bridging by CNFs is observed in the specimens, which can delay final failure and increase the energy absorption capacity of the specimens. Deformation of CNFs is also noticed in the material microstructure. The deformation and failure mechanisms of nanofibers are related to the test strain rate and the structure of CNFs.