金属拉伸试样原始力学性能测量的讨论

金属拉伸力学性能因拉伸温度和拉伸速率的改变引起测试不确定性，导致在一个温度一个应变速率下的拉伸试验结果，一般不是被测金属的原始力学性能。拉伸试验新技术体系表明，越快的拉伸应变速率，“力学性能 拉伸应变速率”曲线上的力学性能越接近原始力学性能。存在充分快的拉伸应变速率，在“力学性能 拉伸应变速率”曲线上能获得原始力学性能。同时给出了确定这个能获得原始力学性能的充分快应变速率的方法。     

金属“力学性能 -拉伸应变速率”曲线

金属弹性变形的微观理论揭示出测试不确定性是拉伸试验的弹性变形阶段张应力改变了被测金属原子水平上的微观结构引起的。在此基础上，提出了一个拉伸试验新技术体系框架：测试一定温度下“力学性能 -拉伸应变速率”曲线，表征被测金属的原始力学性能，服役力学性能和金属加工变形过程中的力学性能。     

含缺陷金属Ti力学性能的模拟研究

利用分子动力学模拟方法分别研究了空位、自间隙杂质原子、杂质He原子等缺陷对金属Ti样品的力学性能的影响.对完整晶格的金属Ti在不同拉伸应变速率下的应力-应变曲线进行计算,发现拉伸过程可分为弹性形变、塑性形变及断裂3个阶段.分别研究了含有不同浓度的空位、自间隙杂质原子、杂质He原子缺陷的金属Ti样品在2×109s-1拉伸应变速率下的应力-应变曲线,并对不同情况下的Young's模量进行了统计.还分别对含有自间隙杂质原子和杂质He原子的金属Ti的拉伸断裂过程进行了观察与分析.     

AZ91D变形镁合金的动态应变时效现象

在应变速率为1.11×10-4～1.67×10-3s-1、温度为248～523K的条件下,对固溶态AZ91D变形镁合金进行拉伸试验。结果表明:在一定的拉伸应变速率和温度区间拉伸时,AZ91D镁合金在形变过程中发生动态应变时效(DSA)现象,其典型特征表现为其拉伸曲线出现锯齿波,所对应的锯齿波类型分别呈A型及A+B混合型;应变速率敏感性系数为负值;且出现加工硬化速率峰值;出现锯齿屈服的临界应变量随变形温度升高而减小,而随应变速率增加而增大;当形变温度大于323K时,加工硬化速率随着温度升高反而急剧增大,在368K时达到峰值。     

00Cr21Ni14Mn5Mo2N不锈钢耐海水应力腐蚀性能

采用慢应变速率拉伸试验方法研究了00Cr21Ni14Mn5Mo2N不锈钢的耐海水应力腐蚀性能,分析了试验温度、拉伸应变速率对应力腐蚀行为的影响。结果表明,在35～65℃温度区间,00Cr21Ni14Mn5Mo2N不锈钢在3.5%NaCl溶液中对应力腐蚀不敏感,试验温度对00Cr21Ni14Mn5Mo2N不锈钢的应力腐蚀行为无明显影响。拉伸应变速率对其应力腐蚀行为有影响,应变速率为1.0×10-6/s时应力腐蚀敏感因子相对较高。     

Cu对5052铝合金热拉伸变形行为的影响

利用Gleeble-3500热模拟试验机与透射电镜(TEM)研究了Cu对5052铝合金热拉伸变形行为的影响。研究表明:Cu的添加导致5052铝合金在等温拉伸温度为100℃、拉伸速率为0.01 mm·s-1与拉伸温度为250℃、拉伸速率为0.01 mm·s-1与0.1 mm·s-1的真应力-真应变曲线上出现由动态应变时效引起的锯齿波动,不同升温速率与拉伸速率组合下的非等温拉伸真应力-真应变曲线上出现由动态析出引起的"屈服"平台。     

铸态AZ31镁合金高温拉-压不对称性研究

利用Gleeble-1500在250～500℃和应变速率0.001～1 s-1对铸态AZ31镁合金进行热压缩及热拉伸试验,对热变形过程中拉-压不对称性进行研究;基于Hollomon公式,分析了铸态AZ31镁合金热拉伸-压缩过程中应变硬化指数的变化规律.结果表明:铸态镁合金在热拉伸-压缩过程中,存在明显拉-压不对称性,热拉伸应变硬化指数均比热压缩应变硬化指数大;升高形变温度及降低应变速率有利于减小铸态镁合金的拉-压不对称性.     

AZ31镁合金棒材拉矫残余挠度及残余应力场数值模拟

以AZ31镁合金棒材为研究对象,分别测量了AZ31镁合金棒材在不同温度下的应力应变曲线及不同温度、不同残余拉伸应变下的残余挠度.建立有限元模型,模拟了上述试样拉伸矫直后的残余挠度和残余应力场,并将模拟的不同温度、不同残余拉伸应变下的残余挠度和测量结果进行了比较,二者吻合较好,证明所建立的模型是有效的.在此基础上,分析了拉伸矫直残余应力场,随拉伸应变、拉伸温度的变化规律,研究表明在矫直温度220℃、残余拉伸应变为2%时进行拉伸矫直,夹持部位不易打滑、且在夹持端的残余应力最小及残余应力影响区最小,并可采用小拉伸量、多次拉伸的方式进一步减小其残余挠度.     

细晶TA15钛合金板材制备工艺及其超塑性研究

研究了β淬火和换向轧制对TA15钛合金板材显微组织和力学性能的影响,并对显微组织最为均匀细小的板材进行了超塑性能测试。结果表明,增加β淬火工艺,可以提高板材显微组织的均匀性,细化晶粒尺寸,提高板材的室温拉伸强度;采用换向轧制工艺,能够显著减小横纵向组织差异,提高组织均匀性,使板材横纵向性能差异减小;对同时采用β淬火和换向轧制工艺制备的板材进行超塑性拉伸试验,在850-920℃、0.001-0.01 s-1试验条件下,板材具有良好的超塑性能,且超塑性能对拉伸温度和拉伸应变速率均较为敏感。不同的应变速率下,温度对超塑性能的影响规律不同。     

TC4钛合金电热拉伸变形行为有限元模拟

为研究拉伸速率和温度对TC4钛合金性能的影响,采用电-热-力完全耦合方法,运用林建国统一粘塑性本构模型,使用有限元分析软件ABAQUS,对不同温度和拉伸速率下的TC4钛合金电热拉伸过程进行模拟研究,并选取目标温度为750℃、拉伸速率为1 mm·min;的这一组模拟结果与试验结果进行对比。对比结果显示:有限元模拟中通电加热后得到的温度场与试验中同一阶段采用热像仪测得的温度场分布相似,标距段中间部位水平线上的平均温度相差仅为2.1℃;此外,有限元模拟得到的拉伸力-位移曲线与试验曲线的变化趋势相近,最大拉伸力相差0.014 kN,误差约为2.41%,最大拉伸力出现时的伸长量相差0.148 mm。可见有限元模拟结果与试验结果较为吻合,证明了有限元模拟方法的可靠性。有限元模拟得到的拉伸力-位移曲线和试验曲线均表明:相同应变速率下,温度越高,钛合金塑性越好,在应变速率较高时,温度的影响作用减弱;在相同温度下,应变速率越低,钛合金的塑性越好,在温度较低时,应变速率的影响作用减弱。     

不同Zn/Cu质量比对Mg-Zn-Cu显微组织和性能的影响

论文采用光学显微镜、X射线衍射仪、扫描电子显微镜及显微硬度测试、室温和高温拉伸性能测试、蠕变性能测试研究了Ce和不同的Zn /Cu质量比对Mg-Zn-Cu显微组织和室温及高温力学性能的变化规律、高温变形性能、强化机制和抗蠕变性能的影响。研究结果表明,室温下挤压态Mg-8Zn-8Cu-Ce的拉伸强度和屈服强度分别为320 MPa和291 MPa,在423K温度下,拉伸强度仍高于220MPa。合金具有优良的蠕变性能,稳态蠕变速率为1.21×10_-8 s_-1,蠕变量仅为0.562%。在相同的变形温度下,铸造Mg-7Zn-3Cu-Ce的真实应力随着应变速率的增大而增大,表明合金是应变速率敏感材料。相同的应变速率下,合金的真实应力随着温度的升高而减小,但没有明显的动态再结晶和软化现象。     

工业纯钛室温下的应变速率敏感性及Hollomon经验公式的改进

通过实验研究了工业纯钛TA2在室温下应变速率范围为1×10-4～1×10-2s-1的拉伸力学性能。发现TA2的拉伸力学性能存在显著的应变速率敏感性,随着应变速率的增加,材料的强度提高、塑性下降,应变速率敏感性指数较高。通过对Hollomon经验公式σ=Kεnεm的推导和TA2实验数据的分析发现应变速率敏感性指数m和应变硬化指数n分别会受到应变和应变速率的影响,并且它们之间均呈指数关系。因此对Hollomon经验公式提出了改进,得到了TA2在室温下改进的Hollomon模型。与传统的Hollomon经验公式及Johnson-Cook模型相比,改进的Hollomon模型的预测结果与实验结果更加吻合,能更准确地表现材料的拉伸力学性能。     

铌微合金化高强塑积冷轧DP780钢的I&Q&P工艺

基于亚稳奥氏体形变诱导相变理论,在实验室采用盐浴炉对800 MPa级冷轧双相钢DP780的I&Q&P(临界退火与淬火配分)工艺进行了探讨,并采用光学显微镜、扫描电镜、拉伸试验机与XRD对不同工艺下试验钢的组织性能进行了研究。结果表明,在I&Q&P工艺试验条件下,试验钢的显微组织由铁素体、马氏体与残留奥氏体组成;830 ℃退火时铁素体晶粒尺寸以＞5 μm为主,860 ℃退火下其晶粒尺寸以＜5 μm为主。830 ℃退火时试验钢的力学性能随淬火温度的变化波动较大,860 ℃退火时试验钢的力学性能随淬火温度的变化波动较小。860 ℃退火+260 ℃淬火时,试验钢的综合力学性能最佳,其抗拉强度、伸长率与强塑积分别为802 MPa、26.8%与21.5 GPa·%,钢中残留奥氏体含量高达13.89%。     

基于BP神经网络的置氢TC21合金力学性能预测

基于神经网络的非线性映射和泛化能力,采用人工神经网络方法,建立了置氢TC21合金力学性能预测的BP神经网络模型。模型的输入参数包括高温拉伸试验温度和置氢含量,输出参数为合金的常用力学性能指标,即抗拉强度和屈服强度。通过检验样本验证了ANN模型的准确性。结果表明:该模型具有容错性好、通用性强等优点,可以预测置氢TC21合金在不同拉伸温度和不同置氢含量下的机械性能。同时,将神经网络技术应用于材料制备工艺设计领域,可以明显地提高工艺设计效率,缩短实验周期。     

The effect of nitrogen on the cold forging properties of 1020 steel

We have studied the effect of nitrogen on the cold forging properties of a low carbon steel as a function of temperature. Five AISI 1020 steels with nitrogen contents from 12 to 180 ppm were examined by tensile testing from 25 to 371 °C. Yield strength, tensile elongation (ductility), ultimate tensile strength (UTS), strain hardening exponents and strength coefficients were determined. The influence of nitrogen on the mechanical property behavior of this low carbon steel exhibits trends as expected—when nitrogen content increases, the strength of the steel increases and the ductility decreases. Likewise, as the temperature increases, the strength of the steel generally decreases; however, the ductility initially decreases, then exhibits an increasing trend. Additionally, there is an intermediate temperature range for these alloys where anomalous behavior is observed. Serrated stress–strain curves seen in this temperature range are indicative of dynamic strain aging. It is probable that this anomalous mechanical property trend is due to dynamic strain aging.     

长期时效对GH4169合金动态拉伸变形行为的影响

研究了长期时效对GH4169合金的显微组织和动态拉伸性能及变形行为的影响规律及机制.结果表明,应变速率为10¹—10² s^-1时,合金强度受时效时间影响显著,断裂延伸率随时效时间的延长呈降低趋势,在时效500 h后基本保持不变;高应变速率(10³ s^-1)条件下,长期时效对合金强度无明显影响,而断裂延伸率受时效时间的影响显著,长期时效造成的合金塑性劣化现象提前发生.高应变速率变形过程中,位错运动受阻来不及释放,在时效0—1000 h范围内,合金未出现强化相峰值尺寸效应,强度受时效时间的影响并不明显.长期时效后GH4169合金晶界δ相附近无析出带的产生,导致动态载荷下晶界塑性变形的协调能力降低,应变速率为10³ s^-1时,合金塑性在短时间时效后迅速下降.     

Effect of hot plastic deformation on microstructure and mechanical property of Mg-Mn-Ce magnesium alloy

Hot plastic deformation was conducted using a new solid die on a Mg-Mn-Ce magnesium alloy. The results of microstructural examination through OM and TEM show that the grain size is greatly refined from 45 μ to 1.1 μm with uniform distribution due to the occurrence of dynamic recrystallization. The grain refinement and high angle grain boundary formation improve the mechanical properties through tensile testing with the strain rate of 1.0× 10^-4 S^-1 at room temperature and Vickers microhardness testing. The maximum values of tensile strength, elongation and Vickers microhardness are increased to 256.37 MPa, 17.69% and HV57.60, which are 21.36%, 133.80% and 20.50% more than those of the as-received Mg-Mn-Ce magnesium alloy, respectively. The SEM morphologies of tensile fractured surface indicate that the density and size of ductile dimples rise with accumulative strain increasing. The mechanism of microstructural evolution and the relationship between microstructure and mechanical property of Mg-Mn-Ce magnesium alloy processed by this solid die were also analyzed.     

Dynamic tensile behaviors of AZ31B magnesium alloy processed by twin-roll casting and sequential hot rolling

The dynamic tensile behavior of twin-roll cast-rolled and hot-rolled AZ31B magnesium alloy was characterized over strain rates ranging from 0.001 to 375 s⁻¹ at room temperature using an elaborate dynamic tensile testing method, and the relationship between its mechanical properties and microstructures. It is observed that the sheet has a strong initial basal fiber texture and mechanical twinning becomes prevalent to accommodate the high-rate deformation. The yield strength and ultimate tensile strength monotonically increase with increasing the strain rate, while the strain hardening exponent proportionally decreases with increasing the strain rate due to twinning-induced softening. The total elongation at fracture distinctly decreases as the strain rate increases under quasi-static tension, while the effect of strain rate on the total elongation is not distinct under dynamic tension. Fractographic analysis using a scanning electron microscope reveals that the fracture is a mixed mode of ductile and brittle fracture.     

Evaluation of tensile stress-strain curve of electroplated copper film by characterizing indentation size effect with a single nanoindentation

Nanoindentation has been widely used to measure mechanical properties for instance elastic modulus and hardness due to relatively simple sample preparation and experimental procedure. Primary limitation of nanoindentation is that it does not measure quantitative mechanical properties such as yield strength, ultimate tensile strength and fracture strain unlike uni-axial tensile testing. We investigate the tensile stress-strain curve of electroplated copper using a single nanoindentation with a Berkovich indenter. Micro-tensile testing and nanoindentation were performed for three electroplated copper samples with different microstructures by post heat treatments. We find a linear relationship between the strain-hardening exponent as measured by micro-tensile testing and the log value of the characteristic length for the indentation size effect as measured by nanoindentation. By defining a representative flow stress-strain point corresponding to the Berkovich indenter along with the elastic modulus measured by nanoindentation, we obtain complete tensile stress-strain curves for electroplated copper that are in good agreement with those measured by micro-tensile testing.     

Testing the tensile properties of ceramic-matrix composites

This article reports the development of a mechanical testing methodology (including fixtures, extensometry, temperature control, and calibration) and procedure (including control mode options, and analysis) for ceramic-matrix composites (CMCs). Six different CMCs were tensile tested at room temperature (RT) and 1,000°C. An appreciable reduction in fracture resistance was measured for most of the systems at the high temperature, but there was little effect of an order-of-magnitude change in strain rate at either temperature. The reduced properties at 1,000°C could be essentially duplicated at RT for several of the composites by prior exposure in air at 1,000°C. The same exposure in vacuum was much less damaging; thus, oxygen would appear to be the culprit. Fractographic evidence indicated that fiber pullout in any system was a function of test temperature rather than fracture strength. Moreover, the incidence and magnitude of energy-release events during testing varied among the systems and revealed no unique connection with the state of damage induced by the prior exposures. These observations do not fit readily into any simple theoretical model. However, they do serve to identify potential limitations for this class of composite in terms of sensitivity to thermal transients in a non-inert environment.