EFFECT OF STRAIN RATE(?) ON STRAIN HARDENING EXPONENT n OF SOME METALLIC MATERIALS

Variable strain rate tension tests for 4 metallic materials show that as the strain rate in creases the strain hardening exponent n decreases. The trend follows a two stage linear relation between n and Ig (?). When (?) < (?)cp, i.e. under quasi-static loading, n can be considered as a constant, but when (?)>(?)cp, n decreases rapidly till an ideal plastic state. n = 0. The characterizations and mechanisms of softening induced by high (?) are discussed.

On the homogenization of metal matrix composites using strain gradient plasticity

The homogenized response of metal matrix composites（MMC） is studied using strain gradient plasticity.The material model employed is a rate independent formulation of energetic strain gradient plasticity at the micro scale and conventional rate independent plasticity at the macro scale. Free energy inside the micro structure is included due to the elastic strains and plastic strain gradients. A unit cell containing a circular elastic fiber is analyzed under macroscopic simple shear in addition to transverse and longitudinal loading. The analyses are carried out under generalized plane strain condition. Micro-macro homogenization is performed observing the Hill-Mandel energy condition,and overall loading is considered such that the homogenized higher order terms vanish. The results highlight the intrinsic size-effects as well as the effect of fiber volume fraction on the overall response curves, plastic strain distributions and homogenized yield surfaces under different loading conditions. It is concluded that composites with smaller reinforcement size have larger initial yield surfaces and furthermore,they exhibit more kinematic hardening.

Isolation and Physiological Characterization of Synechococcus cedrorum 1191 Strain Tolerant to Heavy Metals and Pesticides

The toxicity of heavy metals (Hg2 + , Zn2 + ) and pesticides has been investigated by comparing the physiological properties in wild and tolerant strains of Synechococcus cedrorum 1191. The differential pattern of growth, absorption spectra of pigments and nutrient uptake was observed in tolerant strain.

Mobility enhancement of strained GaSb p-channel metal-oxide-semiconductor field-effect transistors with biaxial compressive strain

Various biaxial compressive strained GaSb p-channel metal-oxide-semiconductor field-effect transistors （MOSFETs） are experimentally and theoretically investigated, The biaxial compressive strained GaSb MOSFETs show a high peak mobility of 638 cm2/V.s, which is 3.86 times of the extracted mobility of the fabricated GaSb MOSFETs without strain. Meanwhile, first principles calculations show that the hole effective mass of GaSb depends on the biaxial compressive strain. The biaxiai compressive strain brings a remarkable enhancement of the hole mobility caused by a significant reduction in the hole effective mass due to the modulation of the valence bands.

Strain-induced insulator–metal transition in ferroelectric BaTiO_3(001) surface:First-principles study

The electronic properties of TiO2-terminated BaTiO3（001） surface subjected to biaxial strain have been studied using first-principles calculations based on density functional theory. The Ti ions are always inward shifted either at compressive or tension strains, while the inward shift of the Ba ions occurs only for high compressive strain, implying an enhanced electric dipole moment in the case of high compressive strain. In particular, an insulator–metal transition is predicted at a compressive biaxial strain of 0.0475. These changes present a very interesting possibility for engineering the electronic properties of ferroelectric BaTiO3（001） surface.

High strain rate induced embrittlement of Zr-based bulk metallic glass

Quasi-static and high strain rate mechanical behavior of the Zr_ 1.25Ti_ 13.75Ni_ 10Cu_ 12.5Be_ 22.5 bulk metallic glass was determined covering strain rates from 1.02×10 -4s -1 to 3.258×103s -1. By use of split hopkinson pressure bar(SHPB) equipment, it is found that the alloy fractures in the high strain rate period with a strength well below that of its quasi-static counterpart and thus a strain rate induced embrittlement happens. Considering the glassy nature of the alloy and with careful analysis of the fracture morphology, dynamic damage accumulated in the high speed deformation period is suggested to be the reason for this embrittlement.

Scaling law of hydrogen evolution reaction for InSe monolayer with 3d transition metals doping and strain engineering

Recently, two dimensional In Se attracts great attentions as potential hydrogen production photocatalysts.Here, comprehensive investigations on the hydrogen evolution reaction activity of In Se monolayer with3 d transition metal doping and biaxial strain were performed based on the density functional theory.Transition metal dopants significantly increase the bonding strength between H and Se, and then adjust the hydrogen adsorption free energy to 0.02 e V by Zn doping. The enhanced hydrogen evolution reaction activity results from less electron occupying H 1 s-Se 4 pzanti-bonding states, which is well correlated with the pzband center level. Importantly, the universal scalling law was proposed to descript the evolution of hydrogen adsorption free energy including both doping and strain effects. Moreover, with appropriate band alignment, optical absorption, and carriers separation ability, Zn doped In Se monolayer is considered as a promising candidate of visible-light photocatalyst for hydrogen production.

High strain rate superplasticity of TiN_p/1N90Al composite prepared by powder metallurgy method

TiN_p/1N90Al composite was fabricated by powder metallurgy method with a reinforcement volume fraction of 15%. The tensile experiment, DSC and SEM were used to study the high strain rate superplasticity of the TiN_p/1N90Al composite. The DSC result shows that the incipient melting temperature of the TiN_p/1N90Al composite is 906K. The tensile tests were carried out over a range of deformation temperature from 918 to 928K and strain rate from 1.7 to 1.7×10 -3 s -1. The maximum elongation of 201% is realized at 923K with a strain rate of 1.7×10 -1 s -1. Otherwise all the elongations are higher than 100% in the strain rate range of 3.3×10 -2-6.7×10 -1 s -1 at the three deformation temperatures. The curves of m value of the TiN_p/1N90Al composite can be divided into two stages with the variation of strain rate at different deformation temperatures and the critical strain rate of 10 -1 s -1. When the strain rate is higher than 10 -1 s -1, the m values of the three curves are smaller than 0.3, but the m values of the three curves are about 0.37 when the strain rate is higher than 10 -1 s -1.

Comprehensive Analysis of Stress-Strain in Typical Metal Forming Processes

The corresponding positions of different metal plastic working processes in the plane stress state on a Mises yield ellipse are described.The paper points out that the forming of a workpiece inside its die is corresponding to a mass point moving along the ellipse and that its dimensional variation tendency in all directions can be judged according to a correspondence rule of the stress-strain order offered by the author.In addition,the paper also presents the corresponding three-dimensional positions of different metal plastic working processes on the Mises cylinder,from which the deformation force and technological plasticity required in different processes can be compared,and the dimensional variation tendency can be predicted.The analysis method used in this paper is not to calculate the variation of each stress component(σx,σy,σz) by convention,but to emphasize the variation of the stress state in the typical position,which is expressed correspondingly as the varying locus of points in principal stress space,and then based on the analysis the strain result is judged.

Different influences of Schottky metal on the strain and relative permittivity of barrier layer between AlN/GaN and AlGaN/GaN heterostructure Schottky diodes

Ni/Au Schottky contacts on AlN/GaN and AlGaN/GaN heterostructures are fabricated. Based on the measured current-voltage and capacitance-voltage curves, the polarization sheet charge density and relative permittivity are analyzed and calculated by self-consistently solving Schrodinger＇s and Poisson＇s equations. It is found that the values of relative permittivity and polarization sheet charge density of AlN/GaN diode are both much smaller than the ones of AlGaN/GaN diode, and also much lower than the theoretical values. Moreover, by fitting the measured forward 1-V curves, the extracted dislocations existing in the barrier layer of the AlN/GaN diode are found to be much more than those of the AlGaN/GaN diode. As a result, the conclusion can be made that compared with AlGaN/GaN diode the Schottky metal has an enhanced influence on the strain of the extremely thinner AlN barrier layer, which is attributed to the more dislocations.

Instantaneous Creep in Face-centered Cubic Metals at Ultra- low Strain Rates by a High-resolution Strain Measurement

Instantaneous creep in face-centered cubic metals, 5N Al（99.999%）, 2N Al （99%） and 4N Cu （99.99%） with different grain sizes, was firstly investigated by sudden stress-change experiments at ultra- low strain rates ε ≤10-10 s-1 and temperature T 〈 0.32 Tn. The experimental results indicate that the observed instantaneous creep is strongly dependent on grain size, the concentration of impurity, and stacking fault energy. Creep in high-purity aluminum, 5N Al, with a very large grain size, d 〉 1600μm, shows non-viscous behavior, and is controlled by the recovery of dislocations in the boundary of dislocation cells. On the other hand, for 5N A1 with a small grain size, d=30μm, and low-purity aluminum, 2N A1, with d8= 25μm, creep shows viscous behavior and may be related to ＇low temperature grain boundary sliding＇. For high-purity copper, 4N Cu, with d= 40 grn and lower stacking fault energy, creep shows a non-viscous behavior, and is controlled by the recovery process of dislocations. For all of the samples, creep shows anelastic behavior.

基于应变梯度理论的切削加工SiCp/Al表面创成机理分析

为了深入研究颗粒增强铝基复合材料的切削过程,对45%体积分数的SiCp/Al复合材料通过多相建模的方法建立了基于应变梯度的本构模型,并对软件ABAQUS进行了二次开发,用程序语言将所建本构模型以及描述断裂的B-W断裂准则编入用户子程序VUMAT,并导入ABAQUS进行仿真分析。结合切削实验和仿真分析结果,研究了在相同切削速度及不同切削深度下SiCp/Al复合材料加工表面温度、材料内部应力应变和位错的变化情况。研究结果表明,相较于传统Johnson-Cook模型,修正后的本构模型所得到的结果与实验结果更加吻合;切削深度增大会引发高温梯度;材料内部应力会在颗粒附近形成应力集中;在加工表面应变较大的区域出现大量位错,导致加工颗粒增强金属基复合材料的难度上升。

The Effect of Unloading Positions and Times on Sheet Metal’s Stress-Strain Curve

The loading-unloading-reloading process could affect the tensile deformation of metals with the combined function of stress relaxation and work hardening, which has been reported in multiple experiments. Nevertheless, the effects of different unloading positions and unloading times have not been investigated. In this study, unloading-reloading tests on three materials (AL6061, HSLA and Q195) were conducted. The stress exhibits a rapid rise momentarily upon reloading and stabilizes afterward while the post stress-strain curve deviates up or down from the monotonic tensile curve. The ductility is enhanced by the unloading-reloading process in general. Different unloading positions and unloading times have different degrees of influence on the stretching of these metals. The effect of loading conditions on a medium manganese steel was further studied. The functions to modify the post stress-strain relationship after unloading-reloading were established.

多层金属复合材料的应变局部化延迟和抗断裂失稳机理

多层金属复合材料在航天航空、汽车、船舶、核能电力领域中起着非常重要的作用。相比于传统单一金属,多层金属复合材料具有超高的塑性变形能力和断裂韧性。综述了多层金属复合材料在塑性变形过程中的应变局域化延迟和抗断裂失稳的特征和机理,归纳出周期性颈缩、脱层断裂、隧道裂纹、弥散剪切带对抑制多层金属复合材料塑性失稳的作用机理,并阐明脱层断裂、裂纹分叉、隧道裂纹对多层金属复合材料的增韧机理,和对韧脆转变行为的作用规律,可为金属材料强韧化提供新的设计思路和技术支撑。

基于响应面法的汽车翼子板冲压成形工艺多目标优化

为解决汽车翼子板冲压成形时出现的破裂、表面凹陷及滑移线等质量问题,利用Autoform软件对拉延成形过程进行数值模拟,基于响应面法的中心复合设计,选取压边力、摩擦因数、拉延筋阻力系数为优化变量,以最大减薄率、主次应变、表面滑移线最大滑移距离为优化目标,建立优化变量对优化目标影响的响应面模型,确定最优工艺参数组合为:压边力700 kN,摩擦因数0.15,拉延筋1、拉延筋2、拉延筋3和拉延筋4的阻力系数分别为0.45、0.45、0.35和0.45 N·mm^(-1)。基于最优工艺参数的模拟结果,通过设计加强拉延筋将表面滑移线最大滑移距离降为4.94 mm。对比模拟结果和实际试模样件的检测结果发现,两者基本一致,制件的最大减薄率、主次应变、表面滑移线最大滑移距离均满足技术要求,从而验证了响应面法优化冲压工艺的可行性和有效性。

基于SHPB技术测试典型金属动态压缩性能的尺寸效应分析

为探究金属材料在SHPB动态测试中的尺寸效应问题,该文选用3种具有典型晶体结构的金属材料2A12铝合金(面心立方,FCC)、45钢(体心立方,BBC)以及TC4钛合金(密排六方,HCP),分别加工成具有相同长径比(0.8)、不同尺寸(Φ10 mm×8 mm,Φ5 mm×4 mm,Φ2 mm×1.6 mm)的圆柱形压缩试样,并采用三组不同杆径的SHPB装置(杆径分别为Φ19 mm、Φ12.7 mm、Φ5 mm)进行动态力学性能测试。结果表明:在金属材料的动态压缩力学性能测试中,所用试样的尺寸在满足一定要求时方可获得其稳定的塑性流动应力,对于绝大多数金属建议采用直径大于5 mm的试样进行测试;采用微型SHPB装置配合小尺寸试样所测得材料的塑性流动应力偏低;对应变率敏感的材料,其尺寸效应也更为显著;被测试样的塑性流动应力或与试样的应力三轴度(即一维应力程度)相关。该文意在分析SHPB测试中的尺寸问题以期对相关学者和工程技术人员准确地获取金属材料的动态实验数据提供参考。

基于取向纤维基底和图案化双相金属层的高灵敏应变传感器

为了提高液态金属基应变传感器的灵敏度,研究了一种高灵敏度、可拉伸的应变传感器,该传感器由取向的静电纺热塑性聚氨酯纤维基底和图案化双相金属传感层构成。以液态金属Galinstan为“岛”,固体金属银为“海”,构建了图案化双相金属传感层。银“海”可阻止连续的液态金属导电路径的形成,基于银导电区域的裂纹扩展机制,传感器的灵敏度显著提高。此外,纤维取向(水平或垂直)削弱了在受力时纤维的重新排列,增强了传感层的变形。结果表明,对于单一液态金属层或Ag层组成的传感器,基底纤维垂直取向与随机排列比较,灵敏度分别提高1.09倍和33.19倍。最终制成的基于垂直取向纤维基底和图案化双相金属层的应变传感器具有超高的灵敏度(灵敏度系数高达952.20)和宽传感范围(高达59.33%)。这种设计有望在可穿戴设备中显示良好的应用潜力。

液态金属导电水凝胶纤维的制备及性能

以海藻酸钠(SA)和丙烯酰胺(AM)为原料,掺杂碳纳米管(CNT)和液态金属(LM),N,N′-亚甲基双丙烯酰胺为交联剂,N,N,N′,N′-四甲基乙二胺为催化剂,过硫酸钾为引发剂,采用模板法和自由基聚合策略成功制备了聚丙烯酰胺/海藻酸钠/碳纳米管/液态金属(PAM/SA/CNT/LM)双网络导电水凝胶纤维。表征了水凝胶纤维的化学结构、力学性能和导电性能,并测试了其作为传感器的应变传感性能。结果表明:制备的PAM/SA/CNT/LM水凝胶纤维具有良好的力学性能(拉伸强度达257 kPa,断裂伸长率为1148%)和导电性能(电导率为0.51 S/m),在0~200%和200%~600%应变范围内的灵敏度分别为2.56和7.53,具有良好的应变循环稳定性和可靠性,可应用于监测手指、手腕和膝盖等人体运动信号。

湿法纺丝制备拉伸电阻不敏感液态金属导电纤维

液态金属导电纤维在拉伸、折叠等状态下仍可保持导电性,在智能纺织品领域具有广阔的应用前景。然而,大多数液态金属导电纤维在拉伸状态下导电性能大幅下降。以液态金属(LM)/聚氨酯弹性体(TPU)为芯层材料,TPU为皮层材料,通过同轴湿法纺丝工艺制备拉伸电阻不敏感的液态金属导电纤维,探究液态金属含量及芯层内径对纤维结构和性能的影响。结果表明:液态金属导电纤维具有良好的力学性能,拉伸模量为17.7 MPa,断裂伸长率约为475%,断裂强度约为29.5 MPa。此外,所制备的导电纤维具有明显的皮芯结构,芯层直径越大,液态金属含量越高,拉伸电阻稳定性越好。当芯层直径为0.6 mm,液态金属质量分数为97%时,在0~300%的应变范围内纤维电阻几乎保持不变。