TWIP钢在不同应变量下的亚结构

采用静态拉伸、金相、TEM等方法,对两种不同锰含量Fe-Mn-Si-Al系TWIP钢进行表征,研究了成分、应变速率以及应变量对孪晶、位错、层错等亚结构的影响.结果表明:孪晶形成的临界应变量εc与层错能、原始晶粒尺寸以及应变速率有关;两种钢在以10-4s-1的应变速率加载时孪晶体积分数的增长曲线符合Boltzmann模型S曲线关系,孪晶增长速度先增大后减小,Fe25Mn3Si3Al钢在ε＝0.2时孪晶增长速率达到最大,Fe30Mn3Si3Al钢在ε＝0.27时孪晶增长速率最大.Fe25Mn3Si3Al钢以10-2s-1的应变速率加载时,孪晶体积分数呈线性上升;TEM观察发现孪晶总是在高密度位错与层错区产生,位错、层错的塞积,位错、层错以及孪晶间的交互作用共同引起材料强度的提高.     

TWIP钢在不同温度变形的加工硬化行为

通过温控拉伸试验、光学显微镜、X射线衍射技术和透射电镜分析了在298、373、473、573 K温度下变形时,20Mn24Cr5Al2Ni2TWIP钢的力学性能和显微组织变化规律。结果表明,TWIP钢的强度随变形温度的升高而降低,伸长率在373 K变形时比298 K变形显著下降;在373～573 K变形时伸长率有上升趋势;温度升高,组织中形变孪晶的数量减少,孪晶交叉现象减弱。研究TWIP钢的加工硬化行为表明,TWIP钢在拉伸过程中的加工硬化指数n值随真应变的增加而增加,在低应变区温度升高n值增加。在298～373 K变形时,形变孪晶占主导作用,在473～573 K变形时,形变孪晶和动态应变时效共同作用。     

TWIP钢的显微组织与变形机制研究

采用金相、X射线衍射、扫描电镜和透射电镜等方法对两种不同Mn含量应变诱发孪晶(TWIP)钢拉伸前后的显微组织进行了研究.结果表明,Fe-15Mn-3Si-3Al钢的塑性增长机理主要是),γrcc→εhcp,γfcc→εhcp→αbcc相变诱发的TRIP效应;Fe-25Mn-3Si-3Al钢主要的塑性增长机制是孪晶诱发的TWIP效应.Fe-25Mn-3Si-3Al钢拉伸后有些奥氏体晶粒内存在两个或多个孪晶系统,孪晶界与原始奥氏体晶界都会阻碍孪晶的长大.层错能强烈影响TWIP钢的变形机制,随着Mn含量的增加,层错能不断增加,孪晶强化逐渐起主导作用.     

TWIP钢的组织与力学性能研究

采用金相、静态拉伸试验方法研究了5种不同锰含量的TWIP钢的组织和力学性能。结果表明,5种钢的屈服强度随应变率的增大而提高,最大屈服强度可达280MPa;抗拉强度随应变率的增大而略有降低,最高抗拉强度超过1000MPa;1#钢的断裂延伸率和强塑积随应变率的增大而提高,其它4种成分钢的断裂延伸率和强塑积随应变率的增大而减小。1#钢只具形变诱发马氏体相变,不出现孪晶;而2#-5#钢具TWIP效应,其中3#钢的最大延伸率可达75%,强塑积最高可达45000MPa（%）。TWIP钢拉伸前组织中有退火孪晶,变形过程中产生大量的形变孪晶,孪晶诱导塑性,从而推迟了颈缩的产生,导致很高的均匀变形能力。     

高速冲击拉伸条件下TWIP钢的力学性能

采用气动式间接杆杆型冲击拉伸试验装置对5种不同成分的TWIP钢在10^2～10^3 s^-1应变速率范围内的动态拉伸变形行为进行了研究,并和静态拉伸性能作了比较。结果表明：随应变速率的提高,材料动态条件下的抗拉强度、断裂延伸率和能量吸收值均显著增加,均匀延伸率略有提高。TWIP钢在形变过程中产生形变孪晶显著改善了材料的塑性,因此在高应变速率下的延伸率仍较好。     

不同热处理工艺对Fe-30Mn-3Si-4AlTWIP钢力学组织性能的影响

采用拉伸性能测试、金相观察、SEM和EDS等方法研究了不同热处理工艺对Fe-30Mn-3Si-4AlTWIP钢微观组织、拉伸力学性能及断口形貌的影响,并采用X射线衍射仪测定材料的物相组成。结果表明,冷却速度越快,TWIP钢的延伸率和强度越高;热处理后其室温组织为含有退火孪晶的单一奥氏体,冷却速度越小,奥氏体晶粒和退火孪晶的尺寸越大。拉伸时发生典型的延性断裂,在拉伸过程中退火孪晶转变成形变孪晶,使材料的塑性提高。     

挤压态AZ31合金小应变变形时的孪生行为

在室温下对挤压态AZ31合金沿棒材径向进行拉伸变形（RDT试样）和沿挤压方向进行压缩变形（EDC试样）,2种变形应变速率均为10^-4 s^-1。采用金相显微镜（OM）和背散射电子衍射（EBSD）研究了变形过程中合金的孪生行为。结果表明：拉伸孪晶影响了合金的屈服点,EDC试样的屈服点为139 MPa,高于RDT试样的屈服点88 MPa。2种变形应力状态下,随应变增加,合金的应变硬化速率都是先快速下降,但EDC试样的硬化速率随后明显上升,并一直持续到断裂,而RDT试样则几乎保持稳定的硬化速率。EDC试样硬化速率的升高与合金中产生大量的拉伸孪晶以及孪晶织构诱导的滑移行为有关。基于EBSD测试结果,给出了一种计算晶粒内孪晶体积分数的方法,得出RDT试样在应变为0.04时,（0002）晶粒中拉伸孪晶体积分数约为45%。     

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)。     

应变速率对低C高Mn TRIP/TWIP钢组织演变和力学行为的影响

研究了Fe-18Mn低C高Mn TRIP/TWIP钢在应变速率范围为1.67×10^-4-10³s^-1的室温拉伸实验过程中力学性能和组织的变化.在准静态拉伸应变速率范围内(1.67×10^-4-1.67×10^-1s^-1),应变速率对高Mn TRIP/TWIP钢的抗拉强度产生逆效应,随着应变速率的加快,抗拉强度和延伸率都降低;而在动态拉伸应变速率范围内(10¹-10³s^-1),应变速率对高Mn TRIP/TWIP钢的延伸率产生逆效应,抗拉强度和延伸率都随着应变速率的加快而增加;在应变速率为10³s^-1时,高Mn TRIP/TWIP钢抗拉强度可达到957 MPa,延伸率达到55.8%,具有较好的综合力学性能;随着应变速率的提高,马氏体转变量减少,孪生变形向多个方向发展.采用SEM,TEM和XRD等方法对变形前后的组织进行了分析,在所有应变速率范围内的拉伸变形过程中都产生了奥氏体向马氏体转变和形变孪晶,并且在应变速率为10³s^-1的高速拉伸过程中产生绝热温升效应,使得基体软化.     

TWIP钢中晶粒尺寸对TWIP效应的影响

冷轧TWIP钢经1073,1173,1273和1373K固溶处理10min后,得到了晶粒尺寸分别为7,13,30和63μm的奥氏体组织.拉伸实验表明,随着晶粒尺寸的增加,加工硬化速率(dσ/dε)与真应变(ε)的变化关系由2阶段变为3阶段.当晶粒尺寸大于30μm时,加工硬化速率与真应变关系中的第2阶段对应的应变长度随着晶粒尺寸的增加而迅速增加.当真应变为0—0.2时,加工硬化指数随真应变的增加而迅速增加;在随后的变形中,与上述4个晶粒尺寸对应的试样的加工硬化指数分别稳定在0.47,0.53,0.56和0.68.OM和TEM观察显示,随晶粒尺寸的增大,变形过程中形变孪晶数量增多.对于较大晶粒尺寸的试样,形变孪晶在拉伸变形过程中形核的临界应力较低,随变形量增加,形变孪晶可持续形成,使其加工硬化能力增加,从而增大了TWIP效应;相反,晶粒尺寸减小使变形过程中的形变孪晶形核临界应力增大,抑制形变孪晶的产生,从而减小了TWIP效应.     

Effects of plastic strain and strain path on youngs modulus of sheet metals

The effects of plastic strain and strain path on Young’s modulus of sheet metals are experimentally investigated using low carbon steel, stainless steel, aluminium, copper and brass sheets of 1 mm thickness. These sheets are firstly deformed to different plastic strains under a few strain paths from balanced biaxial stretching to uniaxial tension. Then, a small uniaxial tension test specimen is cut from each deformed sheet and Young’s modulus is measured using electrical-resistance strain gauges glued to both surfaces of the specimen. The experimental results show that Young’s moduli of the low carbon steel and stainless steel sheets decrease with increasing plastic strain, while those of aluminium, copper and brass sheets hardly change with the plastic strain. In all materials, however, the effect of the strain path on Young’s modulus is not necessarily evident. It is confirmed that Young’s modulus of the low carbon steel sheet can be recovered to the initial value of undeformed sheet by a subsequent annealing. In addition to Young’s modulus, the effects of the plastic strain and the strain paths on Poisson’s ratio of these materials are also shown.     

Plastic strain and strain gradients at very small indentation depths

Plastic strains and their respective strain gradients produced by nanoindentation have been theoretically interpreted and experimentally measured at shallow indentation depths. Existing data for 〈100〉 tungsten with four different conical tip radii varying from 85 to 5000 nm and new data for four conical tips (R=0.5 to 20 μm) into 〈100〉 aluminum are presented. Theoretical results based on geometrically necessary dislocations and semi-empirical experimental continuum calculations are compared for spherical and wedge indenters. For a sharp wedge, both experimental continuum based and theoretical geometrical approaches suggest strain gradient decreasing with the increasing indentation depth, δ. In contrast, theoretical geometrical analysis for a spherical contact yields a depth independent strain gradient proportional to 1/R and continuum calculations suggest a slight increase of a strain gradient proportional to δ^1/4/R^3/4. Both single crystals exhibit about a factor of two decrease in hardness with increasing depth, irrespective of either increasing or decreasing average strain gradients. Implications to strain gradient plasticity and indentation size effect interpretations at very shallow depths are discussed.     

Localization of strain in friction

The kinetics of development of strain phenomena in surface layers of solid bodies is studied by the method of decorrelation of speckle images. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 38–42, May, 2006.     

Anisotropic plasticity model coupled with strain dependent plastic strain and stress ratios

It is necessary to describe properly anisotropic material behavior for realistic numerical analyses of sheet metal forming processes. The implementation of many yield criteria in finite element analysis is very complicated. Various material tests are also required to determine yield function coefficients. Stress ratios and anisotropy coefficients are not constant during forming processes due to deformation induced anisotropy. This paper introduces a yield function using strain dependent plastic strain ratios and stress ratios. The main advantage is to fully utilize the data of uniaxial tensile tests. The described material behavior shows a significantly improved agreement with experimental data.     

金属材料拉伸试验中基于应变网格的非接触实时测量

在金属材料拉伸试验中,传统的基于接触式引伸计测量存在诸如不能测量较软质材料,不能应用于高温环境,并且由于接触夹持力的影响会引进实验误差等缺陷.本文利用椭圆拟合技术,建立了基于应变网格的非接触实时检测系统,实现了金属材料在加载过程中长度和宽度应变的实时测量.试验结果表明,该测量方法能够在一定程度上取代接触式引伸计检测方法,其测量精度达到±0.5%,能够满足实际应用的要求.     

Geometrically induced strain hardening

The concept of geometrically induced strain hardening is presented in order to demonstrate how the strain hardening of materials can be improved by the use of architectured reinforcements with corrugated geometries embedded in a matrix. Theoretical computations are highlighted and examples of real materials obtained by different methods are given of this methodology in order to illustrate the application to the design of structural materials.     

Experimental evaluation of strain and strain rate during rapid cooling friction stir welding of pure copper

ABSTRACT

The strain and strain rate during friction stir welding was evaluated by measuring the distortion of the marker material. A thin Cu-40Zn foil as marker was inserted into the butting surface of two pure copper workpieces and the tool ‘stop action’ was employed. The results show that the strain in the shoulder-affected zone increases in a two stair-step shape as the material flows from the front to the rear of the tool, corresponding to the first accelerated and then decelerated flow stages. However, the strain at the two stages has opposite directions. In other words, a strain reversal occurs. Accordingly, the strain rate in the shoulder-affected zone varies in a sinusoidal shape. In the probe-affected zone, there is no obvious strain reverse occurring due to the formation of banded structures. The average strain rate during the band formation is significantly higher than the maximum strain rate in the shoulder-affected zone.     

Effect of strain,strain rate and temperature on workability of AZ80 wrought magnesium alloy

The hot deformation behavior of AZ80 wrought magnesium alloy was studied in the temperature range of 523-673 K and the strain rate range of 0.01-10 s-1 using hot compression tests.Through the flow stresses behavior,the processing maps were calculated and analyzed according to the dynamic materials model.The stable,metastable and unstable regimes were clarified.The optimum processing conditions were suggested as following:the DRX regions in Domain #1-0.25,Domain #2-0.25,Domain #1-0.45,Domain #2-0.45,Domain #3-0.45,Domain #1-0.65 and Domain #1-0.85,and the DRV regions in Domain #3-0.25 and Domain #4-0.45.In each "safe" DRX domain,it is preferable to conduct hot working in the small region around efficiency peak point.The strain has a great influence on the processing maps.The whole area of the "safe" domains increases with the increase of true strain from 0.25 to 0.65,while it decreases with the increase of true strain from 0.65 to 0.85.The results of kinetic analysis reveal that the values of apparent activation energy in all the domains are higher than that for self-diffusion in pure magnesium (135kJ/mol),and the deformation mechanism in all the domains is likely to be cross-slip.     

Control of strain aging in alpha-iron

The principles of strain aging are described in terms of the Cottrell theory. It is deduced that the practical control of strain aging must come through control of the effective amounts of carbon and nitrogen in solution. Existing control methods such as precipitation through alloying or heat treatment are reviewed and their deficiencies described. Experimental results for a series of alloys containing carbon and nitrogen show that only carbon strain aging can be eliminated by heat treatment. The use of titanium, aluminum, vanadium, and boron as alloying elements for the control of nitrogen strain aging is examined. All of these elements are effective but for economic reasons boron additions appear to offer the most promising practical solution to the problem of nitrogen strain aging in steels.