Influence of pre-deformation on age-hardening response and mechanical properties of extruded Mg-6%Zn-1%Mn alloy

The effect of cold plastic deformation between solution treatment and artificial aging on the age-hardening response and mechanical properties of alloy was investigated by micro-hardness test,tensile test,optical microscopy（OM） and TEM observation.After solution treatment at 420 ℃ for 1 h,three kinds of pre-deformation strains,i.e.0,5% and 10%,were applied to extruded ZM61 bars.Age-hardening curves show that pre-deformation can significantly accelerate the precipitation kinetics and increase peak-hardness value;however,as pre-deformation strain rises from 5% to 10%,there is no gain in peak hardness value.The room temperature（RT） tensile properties demonstrate that increasing the pre-deformation degree can enhance the yield strength（YS） and ultimate tensile strength（UTS） but moderately reduce elongation（EL）;furthermore,the enhancement of YS is larger than that of UTS.No twin can be observed in 5% pre-deformed microstructure;however,a large number of twins are activated after 10% pre-deformation.The peak-aged TEM microstructure shows that pre-deformation can increase the number density of rod-shaped β 1 ＇ precipitates which play a key role in strengthening ZM61 alloy.

Cu-Cr-Zr-Y合金动态再结晶临界机制

利用Gleeble-1500D热模拟试验机,采用高温等温压缩试验,对Cu-0.4Cr-0.15Zr-0.04Y合金在应变速率为0.001~10 s-1、变形温度为650~850℃、最大变形程度为50%条件下的动态再结晶行为以及组织转变进行了研究。利用加工硬化率和应变（θ-ε）的关系曲线确定了该合金发生动态再结晶的形变条件为T≥750℃,应变速率小于0.1 s-1;根据θ-σ模型,确立了合金变形特征参数之间的关系：σc/σp=0.86,εc/εp=0.30;同时建立了合金变形特征参数与Z参数的关系：εp=2.61×10-3Z0.14,εc=7.83×10-4Z0.14。Cu-0.4Cr-0.15Zr-0.04Y合金在热变形过程中的动态再结晶机制受变形温度和应变速率的控制。当温度达到850℃,应变速率为0.001 s-1时,合金发生完全的动态再结晶。

Cryogenic forming behaviour of AW-6016-T4 sheet

The objective of this work is to study the cryogenic sheet metal forming behaviour of precipitation hardening AW-6016-T4.In this regard,the flow curves and forming limit curves were obtained by tension and Nakazima experimental testing methods in thetemperature ranges from-196to25°C.It was found that strength and elongation increase with decreasing temperature.Small butperceived differences between microstructure of the material deformed at the room and cryogenic temperatures respectively wereidentified by electron backscatter diffraction(EBSD)analysis.However,no significant difference in the precipitation kinetics duringcontinuous heating in the DSC has been observed.This study has demonstrated the potential of cryogenic forming by manufacturinga B-pillar part with8mm depth of side design element as compared to6mm at room temperature.

Effect of strain hardening and strain softening on welding distortion and residual stress of A7N01-T4 aluminum alloy by simulation analysis

The effect of strain hardening and strain softening behavior of flow stress changing with temperature on welding residual stress, plastic strain and welding distortion of ATN0 1-T4 aluminum alloy was studied by finite simulation method. The simulation results show that the weld seam undergoes strain hardening in the temperature range of 180-250 ℃, however, it exhibits strain softening at temperature above 250 ℃ during welding heating and cooling process. As a result, the strain hardening and strain softening effects counteract each other, introducing slightly influence on the welding residual stress, residual plastic strain and distortion. The welding longitudinal residual stress was determined by ultrasonic stress measurement method for the flat plates of A7N01-T4 aluminum alloy. The simulation results are well accordant with test ones.

Tensile properties of strain-hardening cementitious composites containing polyvinyl-alcohol fibers hybridized with polypropylene fibers

Partially replacing polyvinyl-alcohol(PVA)fibers with polypropylene(PP)fibers in strain-hardening cementitious composites(fiber hybridization)modify certain mechanical properties of these materials.The hybridization based on the introduction of low-modulus hydrophobic polypropylene fibers improves the ductility and the strain-hardening behavior of the cementitious composites containing polyvinyl-alcohol fibers of different types(PVA-SHCC).Pull-out tests indicate that adding PP fibers increases the energy capacity of the hybrid composite with respect to the material containing only PVA fibers under tensile loading,and PP-fiber geometry(i.e.,section shape and length)is a key factor in enhancing the strain capacity.

Effect of twin-roll casting parameters on microstructure and mechanical properties of AA5083-H321 sheet

Thepurpose of the present paper is to study the mechanical propertiesand microstructureof the twin-roll cast and cold rolled AA5083 aluminum alloy sheet in strain-hardened H321 temper. To reach this goal, first, a sound surface slab of 8.90 mm thick and 1260 mm wide was cast by a 15°; tilt back twin roll caster at a casting speed of 490 mm/min. After homogenization at 520 ℃, the product was cold rolled to two thicknesses of 6.30 mm and 3.85 mm with an intermediate annealing at 370 ℃ and final stabilization at 180 ℃. Opticalmicroscopyand scanning electron microscopy （SEM） investigations of the as-cast state depicted the segregation of intermetallic particles mainly in grain boundaries which wasthe cause of grain removal observed in the fracture surface of tensile test samples. In addition, mechanical properties indicated an increase in total elongation after homogenization heat treatment dueto the elimination of the grain boundary segregations. Finally, it was observed that the properties of the 3.85 mmthick sheet were consistent with the H321 temper requirements according to ASTM B 290M standard due to applying sufficient cold reduction during cold rolling stage.

Influence of the Main Technological Variables of Cyclical Mechanic-Thermal Processing on the Strain Hardening of Steel Parts

The main objective of the present work was to determine the influence of the most important technological variables of CMTP （cyclical mechanic-thermal processing） on the strain hardening in the surface layers of steel parts. For this, it was designed a full factorial plan at two levels of five independent variables that include the whole processing in two and three cycles, the cold-forming degree and force during the plastic deformation （burnishing）, and the temperature and time at the given temperature during the aging. Each cycle is composed of plastic deformation at room temperature plus aging. As dependent variables, the degree and penetration depth of strain hardening were evaluated. Based on the appropriately used set of experimental data, it had been fitted an exponential model for each dependent variables and also a two-degree polynomial fitting of in-depth evolution of microhardness profile was obtained. The amount of cycles and the cold-forming degree are the technological variables of CMTP that influence the most on strain hardening, although other variables also are significant. The microhardness profile highlights that during the CMTP, the strain hardening decreases from the outer bound to the transition zone of the surface layers, where it disappears.

ECC材料的研究进展与应用

ECC是Engineered Cementitious Composites的简称,是一种具有超强韧性的乱向分布短纤维增强水泥基复合材料。不同于普通的纤维增强混凝土(FRC),ECC是一种经细观力学设计的先进材料,具有应变-硬化特性,在纤维体积掺量小于2%的情况下,其极限拉应变通常在3%～7%的范围内。ECC具有多缝稳态开裂的特点,在安全性、耐久性、适用性等方面有着优异的性能,可以很好地解决传统混凝土由于易脆性、弱拉伸性而导致的种种缺陷,在水泥基制品开发、桥梁道路施工、结构加固补强等领域有着广阔的应用前景。综述了ECC目前在国际上的研究进展和工程应用情况。

设计PVA纤维水泥基复合材料的研究进展

本文综述了设计聚乙烯醇纤维水泥基复合材料(PolyvinylAlcohol-EngineeredCementitiousComposite,简称PVA-ECC)目前在国际上的研究进展。文中简要介绍了ECC的概念及其基本性能,详细介绍了具有超高韧性和应变-硬化特性的PVA-ECC的理论研究情况,特别是基于性能设计的ECC的微观机理和结构与材料相结合的综合设计方法(ISMD)的概念。并介绍了PVA-ECC和构件的界面特性试验、弯曲试验、拉/压周期、抗剪试验所取得的成果。在此基础上提出了一些需要进一步研究的问题。

高延性永久模板在建造耐久混凝土结构中的应用

介绍了高延性水泥基复合材料(PDCC)永久性模板在建造耐久混凝土结构中的应用.在预浇筑的PDCC永久性模板上浇筑普通混凝土,两者共同组成结构构件.低水灰比永久性模板具有的低渗透性和抗开裂能力使其成为防止钢筋锈蚀的有效保护层.在初步应用上有2种途径:使用素PDCC基体制作永久性模板和使用玻璃纤维增强塑料筋(GFRP reinforcement)加固PDCC模板从而为实际构件提供有效的承载力.进一步的应用可在构件中配合使用GFRP Bar和钢筋从而同时获得足够的承载力和延性破坏形式.介绍了PDCC永久性模板的制作过程和力学性能,比较了在不同表面处理方法下模板与普通混凝土之间的粘结力,并且分析了使用GFRP Bar加固模板制作构件的4点弯曲的理论计算和实验结果.初步的实验结果显示了该技术在实际应用上的广阔前景.

聚丙烯纤维混凝土梁裂缝发展的试验研究与模型计算

为探究PP-ECC梁在弯曲荷载作用下的裂缝发展规律,制作了8根PP-ECC梁和2根RC梁,首先通过四点弯曲加载研究了其裂缝发展形态、裂缝宽度和延伸高度,并与RC梁进行对比;然后,在考虑梁构件受弯时ECC的抗拉能力的基础上,基于粘结滑移理论和平截面假定建立了裂缝平均间距模型、裂缝最大宽度模型及裂缝最大延伸高度模型,并通过模型计算结果与试验结果的对比对各模型的合理性和准确性进行了验证。结果表明:PP-ECC梁在单轴拉伸时具有明显的应变-硬化特性,呈现多条微裂缝发展的破坏模式;PP-ECC梁在弯曲荷载下的裂缝数量显著多于RC梁,且裂缝发展贯穿整个试验过程;PP-ECC梁主裂缝宽度和延伸高度均小于RC梁,且随荷载增加而呈现较均匀的发展状态;基于粘结滑移理论建立的PP-ECC梁裂缝平均间距模型的精度系数在0.95~1.05之间,裂缝最大宽度模型的精度系数在0.85~1.2之间,基于平截面假定建立的裂缝最大延伸高度模型的精度系数在0.95~1.1之间。

Effect of strain rate and deformation temperature on strain hardening and softening behavior of pure copper

The effects of the deformation temperature and the strain rate on the hot deformation behavior of pure copper were investigated based on compression tests. The expressions of strain hardening rate, dynamic recrystallization critical stress, saturated stress, dynamic recovery volume fraction and dynamic recrystallization volume fraction were determined. According to the processing map, the instability regions occur in regions of 400?450 °C, 0.001?0.05 s?1 and 450?750 °C, 0.05?1 s?1. The deformation mechanism in the stability region is dynamic recrystallization. The flow stress was predicted. The results also show that the true stress–true strain curves predicted by the extracted model are in good agreement with the experimental results.

Analysis of plane strain bending of a strain hardening curved beam based on unified yield criterion

The analysis of plane strain elastic-plastic bending of a linear strain hardening curved beam with a narrow rectangular cross section subjected to couples at its end is conducted based on a unified yield criterion. The solutions for the mechanical properties of plane strain bending are derived, which are adapted for various kinds of non-strength differential materials and can be degenerated to those based on the Tresca, von Mises, and twin-shear yield criteria. The dependences of the two critical bending moments, the radii of the interfaces between the elastic and plastic regions and the radial displacements of the points at the symmetrical plane on different yield criteria and Poisson’s ratios are discussed. The results show that the influences of different yield criteria and Poisson’s ratio on the two critical bending moments, the radii of the interfaces between the elastic and plastic regions and the radial displacements of the points at the symmetrical plane of the curved beam are significant. Once the value of bis obtained by experiments, the yield criterion and the corresponding solution for the materials of interest are then determined.

钢中动态再结晶力学测定及其数学模型

采用物理模拟的方法 ,对 W1 8Cr4V高速钢和 H1 3热作模具钢热变形过程中的动态再结晶规律进行了研究 ,提出采用加工硬化率 -应变图可以判断动态软化的开始和结束所对应的应变 .与传统的采用应力 -应变图的方法相比 ,该方法具有准确、直观的优点 .采用该方法 ,可以判断动态软化的类型 .建立了 W1 8Cr4V高速钢和 H1 3热作模具钢的动态再结晶的动力学。

Construction and solution of strain model along thickness of aluminum alloy plate under plastic deformation

A thickness strain model of aluminium alloy plate under plastic deformation,based on thin plate assumption was proposed.It is found that when ratio of stress fractions is constant during in-plane loading,ratios of strain components under various loading conditions are linearly related and these points of ratios form a η-η line.Under these simple loadings,strains in thickness direction can be easily calculated by the η-η line equation without integral and differential work.When the plate is under more complicated loading conditions,the thickness can be computed by the proposed optimization and piecewise calculation model.Validation computations indicate that the relative error of the results of the presented model is less than 0.75% compared with the proven theories and FE simulation.Therefore,the developed model can be applied to engineering calculation,e.g.pre-stretching analysis of aerospace aluminium thick plate,with acceptable accuracy.

Plastic flow behavior of superalloy GH696 during hot deformation

In order to investigate the hot deformation behavior of superalloy GH696, isothermal compression experiments were carried out at deformation temperatures of 880?1120 °C and strain rates of 0.01?10 s?1. And the deformation amount of all the samples was 50%. The strain rate sensitivity exponent (m) and strain hardening exponent (n) under different deformation conditions were calculated, meanwhile the effects of the processing parameters on the values ofm andn were analyzed. The results show that the flow stress increases with the increase of strain rate and the decrease of deformation temperature. The value ofm increases with the increase of deformation temperature and decreases with the increase of strain rate, while the value ofn decreases with the increase of deformation temperature. A novel flow stress model during hot deformation of superalloy GH696 was also established. And the calculated flow stress of the alloy is in good agreement with the experimental one.

低碳钢薄板的塑性特征

本试验用单轴拉伸试验和平面应变拉伸试验研究了低碳钢薄板的三种塑性特征。用Wagoner法测量和计算了应变硬化特性。屈服行为和应变硬化特性主要依赖轧制工程。应力－应变曲线表明在连续退火薄板钢中存在明显的屈服应变，而在平整和间隙退火状态下却无明显屈服现象。用单轴拉伸和平面应变拉伸可确定薄板钢的形成应变极限。在平面应变拉伸中，切应力使试样中间萌生裂纹然后扩展。计算出了每种试样的成型极限图（FLD）的左半部。

Grain refinement and mechanical properties of pure aluminum processed by accumulative extrusion bonding

Ultrafine-grained aluminum processed by a new severe plastic deformation technique, accumulative extrusion bonding (AEB), was investigated. Microstructural characterization indicated good interfacial bonding and an average grain size of ~440 nm was obtained after six passes. Tensile testing revealed that the strength reached the maximum value of 195 MPa and the total elongation exceeded 16% after five passes. The hardness was also significantly improved and almost reached saturation after the first pass. SEM fractography of AEB-processed specimens after tensile test showed that failure mode was shear ductile fracture with elongated shallow dimples. Comparison with conventional accumulative roll bonding indicates that this new AEB technique is more effective in refining grain and improving mechanical properties of the specimens.

Plastic characterization of metals by combining nanoindentation test and finite element simulation

Materials with the same elastic modulus E and representative stress and strain （σr,εr） present similar indentation-loading curves, whatever the value of strain hardening exponent n. Based on this definition, a good approach was proposed to extract the plastic properties or constitutive equations of metals from nanoindentation test combining finite element simulation. Firstly, without consideration of strain hardening, the representative stress was determined by varying assumed representative stress over a wide range until a good agreement was reached between the computed and experimental loading curves. Similarly, the corresponding representative strain was determined with different hypothetical values of strain hardening exponent in the range of 0-0.6. Through modulating assumed strain hardening exponent values to make the computed unloading curve coincide with that of the experiment, the real strain hardening exponent was acquired. Once the strain hardening exponent was determined, the initial yield stress ay of metals could be obtained by the power law constitution. The validity of the proposed methodology was verified by three real metals： AISI 304 steel, Fe andA1 alloy.

Deformation behavior of high performance fiber reinforced cementitious composite prepared with asphalt emulsion

A novel engineered cementitious composite(ECC) was prepared with the complex binder of Portland cement and asphalt emulsion.By adjusting the amount of asphalt emulsion,different mixture proportions were adopted in experiments,including four-point bending test,compressive test,and scanning electric microscopy(SEM).The SEM observation was conducted to evaluate the contribution of polyvinyl alcohol(PVA) fiber and asphalt emulsion to the composite toughening mechanism.The tests results show that the most remarkable deflection-hardening behavior and saturated multiple cracking are achieved when the content of asphalt emulsion is 10%.However,excessive content of asphalt emulsion causes severe damages on the deformation behavior as well as loss in compressive strength of the mixture.SEM observation indicates that the influence of asphalt emulsion on the fiber/matrix interfacial property changes the dominant fiber failure type from rupture into pull-out mode,and thus causes beneficial effects for strain-hardening behavior.