Characterization of microplasticity in TiAl-based alloys

Three different characteristic microstructures of Ti–44Al–8Nb–1B (at.%) have been studied by in situ loading, with acoustic emission and image correlation and ex situ by electron backscatter diffraction and transmission electron microscopy. Microyielding and pre-yield cracking occur at different applied stress levels; nearly fully lamellar material yielding at the lowest and duplex material at the highest stress. The early microyielding in the lamellar microstructures is explained by strain heterogeneity seen in the early stages during loading; the microyielding is higher in lamellae at 45° than in lamellae parallel or perpendicular to the loading direction. Duplex microstructures show no detectable strain heterogeneity until macroscopic yield occurs. The lower bound of the internal stress present in undeformed near lamellar and fully lamellar samples was estimated by observations of dislocation loops at alpha-2/gamma interfaces. The observations are discussed in terms of the influence of internal stress on the tensile and fatigue properties of the different microstructures.     

Dynamic Deformation Behaviors of an In Situ Ti-Based Metallic Glass Matrix Composite

Quasi-static and dynamic deformation behaviors, fracture characteristics, and microstructural evolution of an in situ dendrite-reinforced metallic glass matrix composite: Ti₅₀Zr₂₀V₁₀Cu₅Be₁₅ within a wide range of strain rates are investigated. Compared with the quasi-static compression, the yielding stress increases, but the macroscopic plasticity significantly decreases upon dynamic compression. The effects of the strain rate on strain hardening upon quasi-static loading and flow stress upon dynamic loading are evaluated, respectively. The Zerilli-Armstrong (Z-A) model based on dendrite-dominated mechanism is employed to further uncover the dependence of the yielding stress on the strain rate.     

Wrinkling behavior of magnesium alloy tube in warm hydroforming

In tube hydroforming with axial feeding, under the effect of coupled internal pressure and axial stress, wrinkles often occur and affect the forming results. Wrinkling behavior of an AZ31B magnesium alloy tube was experimentally investigated with different loading paths at different temperatures. Features of wrinkles, including shape, radius and width, were acquired from the experiments, as well as the thickness distribution. Numerical simulations were carried out to reveal the stress state during warm hydroforming, and then the strain history of material at the top and bottom of the wrinkles were analyzed according to the stress tracks and yielding ellipse. Finally, effects of loading paths on expansion ratio limit of warm hydroforming were analyzed. It is verified that at a certain temperature, expansion ratio limit can be increased obviously by applying a proper loading path and realizing enough axial feeding.     

Compressive formability of 7075 aluminum alloy rings under hydrostatic pressure

1 Introduction In order to meet the demand of light-mass structure for aerospace and automobile industry, many new kinds of light alloys and composites are emerging day by day. But most of the high-strength alloys and the metal matrix composites used in t…     

粉末多孔材料等径角挤压热力耦合有限元数值分析

等径角挤压是获得块体超细晶材料的一种重要工艺方法。针对粉末多孔材料,采用体积可压缩刚粘塑性热力耦合有限元法对其等径角挤压过程进行模拟分析。结果显示,等径角挤压工艺对粉末多孔材料具有强烈的致密效果,变形温度和接触摩擦状况对变形及致密存在明显的影响。研究表明,材料所处的静水压力状态对多孔材料的挤压效果影响显著,高的静水压力状态有利于提高粉末材料的变形能力及致密效果,改进的带背压工艺有利于提高变形的均匀性,扩大工艺的应用范围。     

Yield surface of polyurethane and aluminium replicated foam

It has been proposed in the literature, based on theoretical considerations and on finite-element calculations, that all three stress tensor invariants govern the yield surface of cellular materials. Recent experiments on 75 μm pore size aluminium replicated foams (Combaz E, Bacciarini C, Charvet R, Dufour W, Mortensen A. Multiaxial yield behaviour of Al replicated foam, submitted for publication) showed such a dependence of the yield surface in axisymmetric tests. This study explores the yield behaviour of 400 μm pore size aluminium replicated foams: experiments confirm the influence of the third invariant on the yield surface shape, together with the observations that (i) the yield surface shape does not depend on relative density and (ii) measured flow vectors conform with normality. A simple parabolic model fitting data in the previous study also captures well the present data under all tested stress states (biaxial, axisymmetric and Π-planes in stress space). Biaxial and axisymmetric tests are also performed on 400 μm pore size polyurethane (PU) replicated foams with a similar mesostructure. Results show yield to occur at a value lower than predicted by micromechanical models for both matrix materials (aluminium and PU). This suggests that the “knock-down” factor usually observed between predicted and observed stress values probably cannot be explained by a lowered yield stress in the material making the foam. The data also suggest an influence of the matrix nature on the yield surface geometry.     

The phase transformation residual stress in granular structure

在假设马氏体岛形态和分布的基础上, 采用有限元法计算了粒状组织形成过程中的相变残余应力, 并讨论 了相变残余应力对宏观变形行为的影响. 结果表明: 粒状组织形成过程中, 奥氏体转变为马氏体后将在铁素体基体中产生静水张应力, 而马氏体岛承受压应力作用; 残余应力随马氏体体积分数和铁素体基体强度的增加而增大, 存在临界马氏体体积分数, 此时铁素体完全屈服; 残余应力是导致空冷粒状组织钢出现连续屈服现象的原因之一, 但对屈服强度的影响较小.     

Consideration of grain-size effect and kinetics in the plastic deformation of metal polycrystals

This work extends the constitutive model for the prediction of grain-size dependent hardening in f.c.c. polycrystalline metals proposed by Acharya and Beaudoin [1] (Grain-size effect in fcc viscoplastic polycrystals at moderate strains, 1999, in press) to include effects of temperature and strain rate dependence. A comparison is made between model predictions and compression data, taken at varying temperature and strain rate, for pure Ag having two different grain sizes. It is shown that an initial increase in yield stress and concomitant decrease in hardening rate for a fine-grained material, relative to a coarse-grained counterpart, can be captured through initialization of a state variable serving to describe stress response at prescribed reference conditions of temperature and strain rate. A grain-size dependence of hardening rate during parabolic (stage III) hardening is characterized by the evolution of net dislocation density in a finite element model of a polycrystal aggregate. Finally, observations from simulations of deformation of the polycrystal aggregate are introduced into an existing macroscopic constitutive model for metal plasticity based on the mechanical threshold.     

Back Matter

Abstract

This paper is concerned with the experimental determination of thermomechanical properties of a cold box sand, which have a strong influence during the solidification in a sand casting process. To this end, the uniaxial behaviour for different temperatures up to the casting temperature of an aluminium alloy is investigated. For the tests at room temperature, the authors use optical measurement equipment, which is ideally suited to measure inhomogeneous deformations and strains within the material specimens. Furthermore, different strain rates are applied to analyse rate dependent behaviour. The storage time of the sand is also varied, since it influences the material characteristic of the binder in the cold box. An effective heating procedure is developed to characterise the mechanical material behaviour due to different isothermal loadings. The pressure dependence of multiaxial stress states is investigated in a pressure cell. The experimental data are intended to develop and identify a material model for sand, taking into account the strength–differential effect (S–D effect), rate and temperature dependence and the effect of hydrostatic stress states.     

不锈钢0Cr18Ni10Ti焊接头高温、高应变率下的动态力学性能木

利用带有加热装置和同步组装系统的Hopkinson压杆系统对反应堆工程管道材料0Cr18Ni10Ti焊接头的母材和焊缝进行了高温、高应变率下的动态力学性能测试．实验的应变率范围为20m-3800s^-1,温度范围为25—600℃,得到了材料在不同温度和应变率耦合作用下的应力-应变曲线．着重考察了两种材料塑性流变应力的温度和应变率敏感性,并得到了它们的Johson—Cook（J-C）模型．实验表明,母材和焊缝材料均具有较强的热软化效应及应变强化效应,而应变率强化效应相对较弱,并且这些效应本身也受到温度的影响．温度较高时,材料的塑性流变应力受应变和应变率强化的程度减弱,在一定变形量下甚至出现降低趋势．根据热激活位错运动理论对上述现象的内在机理进行了解释和探讨,并对试样的金相组织进行了观察和分析．     

Assessment of forming limit stress curves as failure criterion for non-proportional forming processes

The forming limit stress curve (FLSC) is often recommended as failure criterion for the virtual tryout of forming processes which include non-proportional loading. However, parameters influencing position and shape of the forming limit stress curve are not fully known. Up to today it has not been proved if the forming limit curve is strain path-independent, or at least approximately path-independent. In this study the influence of the parameters yield criterion, flow curve extrapolation and level of pre-stretching on the applicability of the FLSC as failure criterion are assessed. The work is performed using the aluminum alloy AA6014 based on forming limit curves for proportional and non-proportional loading published in Werber et al. Key Eng Mater 502–506:71–76, (2012). FLSCs are generated for yield criteria according to von Mises and Hill’48, for flow curve extrapolations according to Ghosh and Hockett-Sherby as well as for an experimentally measured flow curve. In order to be able to assess the influence of the described parameters on the failure criterion based on the FLSC the application of a mean forming limit stress curve (MFLSC) is used. This method is based on the assumption that all FLSCs gained for proportional as well as non-proportional loading map to one single curve. The influence of yield criterion and flow curve approximation on the FLSCs is addressed; the strain path dependency of FLSCs is proved and the forming limit curves for non-linear loading calculated with an assumed mean forming limit stress curve are compared to the experimentally gained forming limit curves.     

Failure of Zr61Ti2Cu25Al12 bulk metallic glass under torsional loading

The torsional properties of the Zr₆₁Ti₂Cu₂₅Al₁₂ BMG have been tested using cylinder samples, including the shear yield strength, shear elastic strain limit and shear modulus. Under torsional loading, the BMG fails via a major shear band, without obvious macroscopic plasticity on the specimen surface. The shear band maintained stable propagation by a distance of ∼300 μm (∼20% of cylinder radius) before final catastrophic failure, owing to the constraint of stress gradient along the radial direction. The combined tensile, compressive and torsional properties of the Zr₆₁Ti₂Cu₂₅Al₁₂ BMG suggest that recent ellipse criterion and eccentric ellipse criterion are more appropriate than other well-known ones in describing the yield behavior of this BMG. The cooperative shear model underestimates the shear elastic strain limit, because of its default assumption that the yielding behavior follows the Tresca yield criterion.     

Effect of thermal residual stresses on yielding behavior under tensile or compressive loading of short fiber reinforced metal matrix composite

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DISTRIBUTION OF HYDROGEN NEAR NOTCH TIP UNDER MODE Ⅰ LOADING

Distribution of hydrogen near crack tiphas long been recognized as a major factor inhydrogen induced cracking.It governs manyaspects,such as the location where the prima-ry crack occurs,the way that crack propa-gates,etc.Therefore understanding of distri-bution of hydrogen near crack tip is of im-portant significance.The first relationship be-     

Effect of strain rate on compressive behavior of a Pd40Ni40P20 bulk metallic glass

Mechanical deformation of Pd₄₀Ni₄₀P₂₀ was characterized in compression over a wide strain rate range (3.3×10⁻⁵ to 2×10³ s⁻¹) at room temperature. The compression sample fractured with a shear plane inclined 42 degree with respect to the loading axis, in contrast to 56 degree for the case of tension. This suggests the yielding of the material deviates from the classical von Mises yield criterion, but follows the Mohr-Coulomb yield criterion. Fracture stress as well as strain was found to decrease with increasing applied strain rate. The compressive stress (1.74 GPa) was also found to be higher than the tensile fracture stress at a quasi-static strain rate. Close examination of the stress–strain curves revealed that localized shear might have occurred at a compressive stress of about 1.4 GPa, much lower than the “apparent” yield stress of 1.74 GPa. However, the stress of 1.4 GPa for shear band initiation is almost the same as the fracture stress measured at a dynamic strain rate of 5×10² s⁻¹. These results suggested that the fracture of a bulk metallic glass is sensitive to the applied loading rate.     

Mode I crack tip fields in amorphous materials with application to metallic glasses

In this work, stationary crack tip fields in amorphous materials such as metallic glasses under mode I loading are studied to understand the factors that control crack tip plasticity and in turn impart toughness to those materials. For this purpose, finite element simulations under plane strain, small scale yielding conditions are performed. A continuum elastic–viscoplastic constitutive theory, which accounts for pressure sensitivity of plastic flow as well as the localization of plastic strain into discrete shear bands, is employed to represent the material behavior. The influence of internal friction and strain softening on the plastic zone, stress and deformation fields and notch opening profile is examined. It is found that higher internal friction leads to a larger plastic zone. Also, it enhances the plastic strain ahead of the notch tip but leads to a substantial decrease in the opening stress. Thus, it appears that a higher friction parameter promotes toughening of amorphous solids. The shear band patterns within the plastic zone and brittle crack trajectories around the notch root generated from the simulations match qualitatively with those observed in experiments.     

Mechanics of viscous sintering on the micro- and macro-scale

The macroscopic quantities in a continuum model of viscous sintering are determined from analysis of the dynamic evolution of pore structures on the microscopic scale that is governed by the principle of fluid mechanics. A single ellipsoidal pore shrinks to be more anisotropic due to local viscosity, even though the surface tension acts to make it more spherical. The constitutive equation is derived by defining macroscopic quantities as volume averages over a volume element which contains many pores. The presence of dispersed pores generates a bulk stress that affects the macroscopic bulk viscosity and the shear viscosity. The hydrostatic component of macroscopic sintering stress is simply calculated from the total pore volume and the total pore area without knowing their surface curvature. The sintering stress, and then the shrinkage rate, are predicted rigorously from the distribution function of pore size.     

Crystal plasticity model accounting for pressure dependence of yielding and plastic volume expansion

Using a strain rate-dependent crystal plasticity theory with non-Schmid effects, increase in yield (flow) stress under superimposed hydrostatic pressure, as well as the so-called strength-differential (S-D) effect, is numerically simulated. The assumption that the hydrostatic stress (mean stress) affects the slip behavior in crystal yields quantitatively correct predictions.     

一种建立板料成形极限应力图的新方法

基于塑性理论建立了比例加载条件下双向拉伸应力应变关系,结合Swift分散性失稳准则,提出了一种建立板料成形极限应力图的方法。分别应用Hill 48和Hosford屈服准则以及单向拉伸性能参数,建立了铝合金板(r<1)和薄钢板(r>1)两种材料的成形极限应力图(FLSD),分析表明,不同的屈服准则的选取对于成形极限应力曲线有不同的影响,对于不同类型的材料屈服准则的影响程度也不同。与由通常的成形极限图(FLD)转换所得到的成形极限应力图(FLSD)进行了对比分析,结果表明,所提出的方法计算过程更为简便,并能较为准确地建立成形极限应力图,可以作为复杂加载路径下的成形极限破裂判据。     

Abnormal Twinning Behavior and Constitutive Modeling of a Fine-Grained Extruded Mg-8.0Al-0.1Mn-2.0Ca Alloy under High-Speed Impact along Various Directions

To understand the mechanical and twinning behaviors of a fine-grained extruded Mg-8.0Al-0.1Mn-2.0Ca alloy under high-speed impact, impact tests were carried out using a split Hopkinson pressure bar, and microstructures at strains of 0.05, 0.10 and 0.20 were obtained using a series of stop rings manufactured by high-strength steel. The stress response and twinning behavior are closely related to loading direction and applied strain rate. The true stress-true strain curves are s-shaped in extrusion direction (ED) and c-shaped in transverse direction (TD), showing apparent anisotropy, while the yield strength is insensitive to loading direction. Almost identical strain-rate sensitivity is demonstrated by the stress in ED and TD. Interestingly, de-twinning is apparent as the applied strain increases to 0.20, and it is enhanced with increasing the applied strain rate. In contrast, the twin density in ED samples is clearly higher than that in TD samples. By modifying the terms of strain hardening and strain rate hardening in the classical JC model, an optimized model is built, which can accurately predict the stress response behavior of the studied alloy under high-speed impact along ED and TD. The correlation coefficient (R) and average absolute relative error (AARE) are 98.63% and 0.0199 for ED, and 96.88% and 0.0202 for TD, respectively.