Deformation behavior of thin copper films on deformable substrates

The role of strain hardening for the deformation of thin Cu films was investigated quantitatively by conducting specialized tensile testing allowing the simultaneous characterization of the film stress and the dislocation density as a function of plastic strain. The stress–strain behavior was studied as a function of microstructural parameters of the films, such as film thickness (0.4–3.2 μm), grain size and texture. It was found that the stress–strain behavior can be divided into three regimes, i.e. elastic, plastic with strong strain hardening and plastic with weak hardening. The flow stresses and the hardening rate increase with decreasing film thickness and/or grain size, and are about two times higher in (111)-grains compared to the (100)-grains. These effects will be discussed in the light of existing models for plastic deformation of thin films or fine grained metals.     

Plasticity and fracture modeling of quench-hardenable boron steel with tailored properties

In this article, a constitutive model for quench-hardenable boron steel is presented. Three sets of boron steel blanks are heat treated such that their as-treated microstructures are close to fully martensitic, bainitic and ferritic/pearlitic, respectively. Hardness measurements show that the resulting blanks cover the full scope of possible hardness values, from 165 HV in the ferritic/pearlitic range to 477 HV in the fully hardened state. These three main grades provide the input data for a constitutive model consisting of an extended Swift hardening law and a stress triaxiality and Lode angle dependent fracture criterion. The hardening behavior of each grade is determined using standard tensile tests at quasi-static strain rates. The strain-based fracture criterion is calibrated using four different flat fracture samples. The behavior of intermediate hardness grades is approximated by piecewise linear combination of the three calibrated constitutive models. A newly developed tapered tensile test specimen featuring a hardness transition zone in the gauge section is used to verify the model at hand. A four point bending test of a top hat section of intermediate hardness is used to verify the model for complex loading conditions.     

Deformation behavior of Zr60.0Al15.0Ni25.0 metallic glass

The effect of thermal crystallization on viscous flow of supercooled liquid in Zr₆₀Al₁₅Ni₂₅ metallic glass was investigated. Zr₆Al₂Ni crystalline precipitates with ellipsoidal morphology appeared during thermal annealing and deformation at high temperatures. No significant difference in phase selection or morphology of crystalline precipitates was observed between non-deformed and deformed specimens. The viscous flow behavior is very sensitive to the strain rate and the stress–strain behavior can be classified into three types depending on the strain rate: stress overshoot mode, stable viscous flow mode with constant flow stress, and strain hardening mode. The strain hardening is caused by the precipitation of Zr₆Al₂Ni phase from supercooled liquid. The flow stress increased with increase in the crystallization ratio for specimens containing a volume fraction of Zr₆Al₂Ni phase higher than 10% although the stress showed no significant change with slight crystallization.     

In situ flow stress of pure aluminium constrained by tightly packed alumina fibres

The in situ matrix flow stress of continuous fibre-reinforced aluminium is measured in tension along the fibre axis. We use a new, tighter, estimate for the effect of differential Poisson contraction between fibres and matrix and take into consideration nonlinear elastic fibre behaviour; these improvements remove inconsistencies found in earlier work. Resulting in situ matrix flow stress curves are characterized by a substantial gain in hardness of the matrix as compared to the unreinforced alloy, and a strong Bauschinger effect. These effects are caused by dislocation emission during cooldown by matrix/fibre thermal strain mismatch. The surprising insensitivity of hardening to the prior rate of composite cooldown suggests that thermal dislocational hardening starts already at temperatures where unreinforced pure aluminium would creep rapidly. The absence of significant recovery during furnace cooldown is attributed to a small amount of iron in supersaturated solution, and/or to subgrain boundary pinning at the fibres.     

ZK60镁合金热变形过程中的应变强化与动态再结晶行为(英文)

研究ZK60合金的高温流变应力行为。分别采用Kocks-Mecking模型和Avrami方程对合金的应变强化和动态再结晶过程进行模拟,在此基础上,构建一个考虑合金动态再结晶软化的流变应力方程并对流变应力进行预测。结果表明:预测曲线与实验结果具有很高的相关系数,所构建的流变应力方程能准确地描述热变形过程中合金的流变应力行为。微观组织观察表明在变形初期合金组织主要为动态回复组织,随着应变增加,逐渐转变为再结晶组织。     

Determination of mechanical properties of pure zirconium processed by surface severe plastic deformation through nanoindentation

Commercially pure zirconium was processed by the surface mechanical attrition treatment(SMAT),and the microstructure observation showed that a gradient structure was induced.Nanoindentation measurements were taken to obtain the load-displacement curves at different depths below the treated surface.Using dimensional analysis,the local yield stress,hardness,strain hardening exponent,and elastic modulus at the corresponding depths were derived.The results showed that the yield stress and hardness varied with depth,while the strain hardening exponent and elastic modulus were approximately invariable.The finite element method was used to simulate nanoindentation at different depths below the treated surface to verify the derivation of the local elastic-plastic constitutive relationship.Stressstrain curves were computed for the treated samples through the rule of mixtures,and they agreed well with the experimental results.The analysis showed that the surface and subsurface hardening layers as well as the transition layer shared a high load applied to the samples,even though their volume fraction was small.     

BT25钛合金β相区动态再结晶行为及数值模拟

利用Gleeble-3500 型热模拟试验机对BT25 钛合金进行单道次等温压缩实验,分别采用Najafizadeh-Jonas 加工硬化率模型和Cingara-McQueen 流变应力模型研究了合金在变形温度1040～1100℃,应变速率0.001～1 s-1 和最大压下率为60％的条件下动态再结晶的临界条件,分析真...     

基于温度变化的AA5083合金拉伸变形行为描述

在温度为100℃~525℃,应变速率为0.008s-1、0.013s-1条件下,采用恒应变速率法研究AA5083合金板的流变行为,以及流变应力、变形温度与应变速率之间的关系。结果表明,在该条件下,AA5083合金受应变速率硬化与应变硬化共同作用;其应变速率敏感性指数随温度的升高逐渐增大,应变硬化指数随温度的升高逐渐减弱至零,而后略有增大。建立了材料基于温度变化的修正Fields-Backofen本构模型,其值与实验值吻合良好。     

Uniform Elongation and the Stress-Strain Flow Curve of Steels Calculated from Hardness Using Empirical Correlations

An empirical relationship between the hardness and uniform elongation of non-austenitic hypoeutectoid steels has been developed. This new hardness-elongation relationship was combined with previously developed correlations of hardness and strength (yield and ultimate tensile strength) to predict the stress-strain flow curve from a single hardness test. The current study considers both power law hardening behavior and exponential hardening behavior. Reasonable agreement was observed between the experimental and predicted flow curves of a high strength, low alloy steel. Additionally, an empirical correlation of the flow strength at instability with hardness is provided.     

流动应力-应变关系对5754O铝合金板成形极限的影响(英文)

为了合理描述单向拉伸试验曲线,给出了一种修正的Swift型流动应力—应变关系。基于两种流动应力—应变关系,采用Yld2000-2d屈服准则计算5754O铝合金板的成形极限应变图(FLD-strain)。通过对比理论和实验结果,发现基于修正的Swift型的应力—应变关系所计算的FLD-strain能够合理地描述实验结果。虽然常用的Voce型应力—应变关系能够精确地描述均匀变形阶段的变形行为,但基于该应力—应变关系计算的FLD-strain明显低于实验结果。结果表明,板料的强化率越高则相应的成形极限也越高。为了描述板料在非均匀变形阶段的变形行为和成形极限,建议了一种用于确定合理的流动应力—应变关系的方法。     

管件液压成形技术及其进展

管件液压成形技术属先进制造技术,因其具有众多优点,已在汽车行业得到广泛应用。简要介绍该技术的成形原理、优缺点及应用,从设备及模具、材料及成型性、工艺参数及失败模式、数值模拟及优化方法、预成形、摩擦与润滑等方面分析国内外的研究和应用状况,并指出该技术的发展趋势。     

Observations of dynamic strain aging in polycrystalline NiAl

Dynamic strain aging has been investigated at temperatures between 77 and 1100 K in eight polycrystalline NiAl alloys. The 0.2% offset yield stress and work hardening rates for these alloys generally decreased with increasing temperature. However, local plateaus or maxima were observed in conventional purity and carbon doped alloys at intermediate temperatures (600–900 K). This anomalous behavior was not observed in low interstitial high-purity, nitrogen doped, or in titanium doped materials. Low or negative strain rate sensitivities (SRS) were also observed in all eight alloys in this intermediate temperature range. Coincident with the occurrence of negative SRS was the occurrence of serrated flow in conventional purity alloys containing high concentrations of Si in addition to C. These phenomena have been attributed to dynamic strain aging (DSA). Chemical analysis of the alloys used in this study suggests that the main species causing strain aging in polycrystalline NiAl is C but indicate that residual Si impurities can enhance the strain aging effect.     

CONSTITUTIVE RELATIONSHIP OF SUPERALLOY IN718

ＣＯＮＳＴＩＴＵＴＩＶＥＲＥＬＡＴＩＯＮＳＨＩＰＯＦＳＵＰＥＲＡＬＬＯＹＩＮ７１８Ｊ．Ｍ．Ｚｈａｎｇ;Ｌ．Ｚ．Ｍａ;Ｊ．Ｙ．Ｚｈｕａｎｇ;Ｑ．Ｄｅｎｇ;Ｊ．ＨＤｕａｎｄＺ．ＹＺｈｏｎｇ（ＤｅｐａｒｔｍｅｎｔｏｆＳｕｐｅｒａｌｌｏｙｓ,ＣｅｎｔｒａｌＩ...     

时效处理对超高锰钢组织及力学性能的影响

研究了时效处理工艺对超高锰钢的组织及小变形轴向压缩情况下的形变硬化能力的影响.结果表明,适当的时效处理可以提高超高锰钢的形变硬化能力,硬化速率与起始硬度无关.经时效处理的超高锰钢,真应力一真应变分段符合Hollomon方程,具有双n力学行为,小变形即可获得较高形变硬化.     

DEVELOPMENT OF CONSTITUTE EQUATIONS FOR Ti-6Al-4V ALLOY UNDER HOT-WORKING CONDITION

1.~nonNumericalmodelingofindustrialplasticdeformationprocesseshasbecomeafieldofveryactiveresearchinthepastfewyears.ForthefullpotentialeXPloitingofthismethod,itisessentialtogetthepreciseknowledgeofconstitutivebehaviorofthematerial.Severalpapershavedevotedtotheestablishingofgeneralconstitutiveeqllationsfordescribingtheflowstressofthematerialasafunctionoftheprocessparameterssuchasstrain,strainrate,andtemperature['--7).Theseparametersareoftencalculatedforpeakstressvalueonly,becausemanymodelsassum…     

Hot Deformation and Corrosion Resistance of High-Strength Low-Alloy Steel

The hot deformation characteristics and the corrosion behavior of a high-strength low-alloy(HSLA) steel were investigated at deformation temperatures ranging from 800 to 1100 ℃ and strain rates ranging from 0.1 to 10 s-1 using an MMS-200 thermal simulation testing machine. Based on the flow curves from the experiment, the effects of temperature and strain rate on the dynamic recrystallization behavior were analyzed. The flow stress decreased with increasing deformation temperature and decreasing strain rate. With the assistance of the process parameters, constitutive equations were used to obtain the activation energy and hot working equation. The hot deformation activation energy of HSLA steel in this work was 351.87 kJ/mol. The work hardening rate was used to determine the critical stress(strain) or the peak stress(strain). The dependence of these characteristic values on the Zener-Hollomon parameter was found. A dynamic recrystallization kinetics model of the tested HSLA steel was constructed, and the validity of the model was confirmed by the experimental results. Observation of the microstructures indicated that the grain size increased with increasing deformation temperature,which led to a lowered corrosion resistance of the specimens.     

On the effects of predeformation on work hardening of ultra-low-carbon sheet steel

Predeformation affects the work-hardening behavior of sheet metals in sequential forming operations by producing various strain histories in different parts of the sheet. Several investigators have reported the effects of two-stage deformation on the behavior of sheet metals, particularly justification has been presented on face-centered cubic (fcc) alloys. However, the works on low-carbon ferritic steels are not conclusive. This article reports some new findings of the effects of two modes of predeformation on the subsequent stress-strain relationship in ultra-low-carbon sheet steels. The details of a laboratory test system are presented along with methods used to reduce the data. The effect of the stability ratio, a measure of the degree to which the interstitial atoms are free, on the hardening rate at second-stage of deformation was studied. For stabilized sheet steels, it was found that changes in strain path from equibiaxial stretching to uniaxial tension cause an increase in the flow stress relative to the flow stress at similar effective strain in continued monotonic. For unstabilized sheet steels, a significant increase in the flow stress was not observed with either equibiaxial prestraining or cold rolling and equibiaxial stretching.     

Hot Deformation Behavior of 6061 and 7108 Al-SiCp Composites

Hot deformation behavior of Al 6061- and Al 7108-SiC particulated composites (Al-PMMCs), prepared by stir casting with SiC particulates (SiCp) size of 8 and 15 μm and volume fraction from 0 to 20% is studied by uniaxial compression test carried out at temperature range from room temperature to 500 °C. The flow stress, work hardening behavior, and Young’s modulus are determined. Dynamic recrystallization is also studied. Work hardening and Young’s modulus are directly correlated with composite constituents, whereas the flow stress is greatly influenced by the porosity and SiCp agglomeration. The role of the SiCp in increasing the flow stress decreases by increasing the deformation temperature. The dynamic recrystallization process is stimulated by refining the SiCp and increasing their fraction in soft Al matrix. On the other hand, the PMMCs with Al6061 matrix has more potential for strain hardening than that with Al 7108 matrix. The strain hardening rate is influenced by the matrix type more than the SiCp volume fraction and size.     

Internal void closure during the forging of large cast ingots using a simulation approach

Large cast ingots often contain defects or undesirable microstructural features, such as voids and zones related to casting. Some of these features can remain after hot open die forging, which is an important process for converting large cast ingots into wrought components. During the initial cogging and deformation steps prior to the detailed open-die-forging operations, any internal voids should be eliminated. The present work focuses on the closure of internal voids during open die forging so as to produce a sound component. Hot compression tests were conducted to obtain the flow strength of the cast microstructure at different temperatures and strain rates. The measured flow strength data together with other appropriate material properties were used to simulate the forging steps for a large cast ingot. The numerical simulations for the forging deformation and for the internal void behavior were performed using DEFORM-3D™. Actual defects were measured in commercial ingots with an X-ray scanner. The simulation results for the void deformation behavior are compared with voids measured before and after forging. Through the comparison of experimental results and numerical simulation, a criterion for void closure is proposed. The criterion is that a local effective strain value of 0.6 or greater must be achieved for void closure during forging. Such a criterion can be used in conjunction with simulations to insure that a sound component is produced during the hot open die forging of large cast ingots.