Experimental characterization of the intragranular strain field in columnar ice during transient creep

A digital image correlation (DIC) technique has been adapted to polycrystalline ice specimens in order to characterize the development of strain heterogeneities at an intragranular scale during transient creep deformation (compression tests). Specimens exhibit a columnar microstructure so that plastic deformation is essentially two-dimensional, with few in-depth gradients, and therefore surface DIC analyses are representative of the whole specimen volume. Local misorientations at the intragranular scale were also extracted from microstructure analyses carried out with an automatic texture analyzer before and after deformation. Highly localized strain patterns are evidenced by the DIC technique. Local equivalent strain can reach values as much as an order of magnitude larger than the macroscopic average. The structure of the strain pattern does not evolve with strain in the transient creep regime. Almost no correlation between the measured local strain and the Schmid factor of the slip plane of the underlying grain is observed, highlighting the importance of the mechanical interactions between neighboring grains resulting from the very large viscoplastic anisotropy of ice crystals. Finally, the experimental microstructure was introduced in a full-field fast Fourier transform polycrystal model; simulated strain fields are a good match with experimental ones.     

Fracture in strain gradient elasticity

Recent experiments have shown that the microscale material behavior is very different from that of bulk materials, and displays strong size effects when the characteristic length associated with the deformation is on the order of microns. Conventional continuum theories, however, can not predict this size dependence because they do not have an intrinsic length in their constitutive models. A new continuum theory, namely the strain gradient theory, has been proposed to investigate the deformation of solids at the microscale. For materials undergoing plastic deformation, the basis of strain gradient theory is the dislocation theory in materials science, and strain gradient plasticity has agreed remarkably well with experiments. For elastic materials with microstructures, it has also been established that the material behavior can be represented by an elastic strain gradient theory. A general approach to investigate fracture of materials with strain gradient effects is established. Both the near-tip asymptotic fields and the elastic full-field solutions are obtained in closed form. Due to stain gradient effects, stresses ahead of a crack tip are significantly higher than those in the classical K field. The plastic zone size surronunding a crack tip is estimated by elastic near-tip fields, as well as by the Dugdale model. It is established that the plastic zone is, in general, much more round and larger than that estimated from the classical K field.     

Effect of particles on microstructural evolution during cold rolling of the aluminum alloy AA3104

The effect of second-phase particles on the evolution of the deformation microstructure during cold rolling of the particle-containing aluminum alloy AA3104 has been investigated using electron channeling contrast imaging and electron backscattered diffraction (EBSD). The results show that the influence of second-phase particles on the deformation microstructure depends on the particle size. Fine dispersoids present in the microstructure have no clear effect on the grain orientation dependence of the dislocation structures formed in the strain range examined. However, large scale structural heterogeneities, in the form of deformation zones, are formed near coarse constituent particles, leading to significant local distortions of the deformed microstructure. Analysis of EBSD data shows that significant orientation gradients are found in the vicinity of the coarse particles. Within the deformation zones the largest lattice rotations occur at the tips of plate-shaped constituent particles. A symmetrical pattern of TD-rotations of alternating sign is found in the deformation zones, with the magnitude of the lattice rotations increasing with increasing strain.     

基于滑移/孪生耦合模型的镁合金多晶体塑性成形分析

镁合金的各向异性是由滑移和孪生共同作用产生,而孪生是协调镁合金塑性变形的重要机制。尤其在低温变形条件下,滑移和孪生相互协调并影响其成形性能。为准确描述镁合金成形过程的变形机制,文章建立了一种基于滑移和孪生相互耦合的多晶体塑性模型,并引入到有限元分析过程中。其中金属塑性流动,由每个晶粒内滑移面上沿滑移方向产生的剪切变形及孪生面上的孪生变形共同组成,进而采用率无关晶体塑性模型模拟了AZ31镁合金压缩过程,并给出了孪晶体积分数随时间的变化曲线。研究表明,孪晶体积分数随变形应力产生相应变化,并且基面滑移和孪生是影响镁合金室温成形性能及加工硬化的主要因素。     

Prediction of flow stresses at high temperatures with artificial neural networks

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Strain heterogeneity and damage nucleation at grain boundaries during monotonic deformation in commercial purity titanium

Heterogeneous strain was analyzed in polycrystalline, commercial-purity titanium using many experimental techniques that provide information about microstructure, dislocation arrangement, grain orientation, orientation gradients, surface topography, and local strain gradients. The recrystallized microstructure with 50–200 μm grains was extensively characterized before and after deformation using 4-point bending to strains between 2% and 15%. Extremely heterogeneous deformation occurred along some grain boundaries, leading to orientation gradients exceeding 10° over 10–20 μm. Patches of highly characterized micro-structure were modeled using crystal plasticity finite element (CPFE) analysis to simulate the deformation to evaluate the ability of the CPFE model to capture local deformation processes. Damage nucleation events were identified that are associated with twin interactions with grain boundaries. Progress toward identifying fracture initiation criteria based upon slip and twin interactions with grain boundaries is illustrated with related CPFE simulations of deformation in a TiAl alloy.     

Geometrically necessary dislocations and strain gradient plasticity––a dislocation dynamics point of view

The relations between mesoscopic plastic strain gradients, ‘geometrically necessary’ dislocations (GND), and dislocation dynamics are discussed. It is argued that the connection between GND and size effects in crystal plasticity should be established on the basis of dislocation dynamics, taking into account the specific deformation conditions. It is demonstrated that dislocation dynamics based models for size effects lead to different phenomenological forms of gradient plasticity ‘laws’ proposed in the literature.     

Experimental and numerical determination of texture evolution during deep drawing tests

This paper deals with the study of texture evolution during deep drawing tests. The aim of these tests is to obtain the whole range of deformation paths encountered in real industrial processes. Nakazima tests are carried out in order to obtain deformation states ranging from simple tension to equibiaxial stretching. Shear tests are also achieved in a simple shear setup developed for this work. The experimental strain fields are determined accurately with an optical measurement system based on the correlation of digital images. Texture measurements were carried out for the undeformed material and after each test. The texture evolution is compared to the texture obtained from an elastic–plastic rate-independent polycrystalline material model recently developed. Boundary conditions applied in the model reproduce the in-plane stretching of the material with an out-of-plane stress relaxation procedure in order to reach plane stress conditions.     

Determination of micro-scale plastic strain caused by orthogonal cutting

An electron beam lithography technique has been used to produce microgrids in order to measure local plastic strains, induced during an orthogonal cutting process, at the microscopic scale in the shear zone and under the machined surface. Microgrids with a 10 μm pitch and a line width less than 1 μm have been printed on the polished surface of an aluminium alloy AA 5182 to test the applicability of the technique in metal cutting operations. Orthogonal cutting tests were carried out at 40 mm/s. Results show that the distortion of the grids could successfully be used to compute plastic strains due to orthogonal cutting with higher accuracy compared to other techniques reported in the literature. Strain maps of the machined specimens have been produced and show high-strain gradients very close to the machined surface with local values reaching 2.2. High-resolution strain measurements carried out in the primary deformation zone also provide new insight into the material deformation during the chip formation process.     

Prediction of damage in small curvature bending processes of high strength steels using continuum damage mechanics model in 3D simulation

Sheet metal bending of modern lightweight materials like high-strength low-alloyed steels (HSLA) is one major challenge in metal forming, because conventional methods of predicting failure in numerical simulation, like the forming limit diagram (FLD), can generally not be applied to bending processes. Furthermore, the damage and failure behaviour of HSLA steels are changing as the fracture mechanisms are mainly depending on the microstructure, which is very fine-grained in HSLA steels composed with different alloying elements compared to established mild steels. Especially for high gradients of strain and stress over the sheet thickness, as they occur in small curvature bending processes, other damage models than the FLD have to be utilised. Within this paper a finite element (FE) 3D model of small curvature bending processes is created. The model includes continuum damage mechanics model in order to predict and study occurring failure by means of ductile coherence loss of the material and crack formation with respect to influencing process parameters. Damage parameters are determined by inverse numerical identification method. The FE-model is strain based validated considering the deformation field at the outer bending edge of the specimen by using an optical strain measurement system. The Lemaitre based damage model is calibrated against the experimental results within metallographic analysis adapting the identified damage parameters to the bending process und thus adjusting the crack occurrence in experiment and simulation. Using this model the bendability of common HSLA steel, used for structural components, is evaluated with respect to occurring damage and failure by numerical analysis.     

Optimization of deformation parameters of dynamic recrystallization for 7055 aluminum alloy by cellular automaton

In order to simulate the microstructure evolution during hot compressive deformation, models of dynamic recrystallization (DRX) by cellular automaton (CA) method for 7055 aluminum alloy were established. The hot compression tests were conducted to obtain material constants, and models of dislocation density, nucleation rate and recrystallized grain growth were fitted by least square method. The effects of strain, strain rate, deformation temperature and initial grain size on microstructure variation were studied. The results show that the DRX plays a vital role in grain refinement in hot deformation. Large strain, high temperature and small strain rate are beneficial to grain refinement. The stable size of recrystallized grain is not concerned with initial grain size, but depends on strain rate and temperature. Kinetic characteristic of DRX process was analyzed. By comparison of simulated and experimental flow stress–strain curves and metallographs, it is found that the established CA models can accurately predict the microstructure evolution of 7055 aluminum alloy during hot compressive deformation.     

Correlation Between Crystal Rotation and Redundant Shear Strain in Rolled Single Crystals: A Crystal Plasticity FE Analysis

The correlation between crystal rotation and redundant shear strain in rolled single crystals was investigated by using the crystal plasticity finite element(CPFE) model in this paper. The deformation in aluminium single crystals of four representative orientations(rotated-Cube, Goss, Copper, and Brass) after rolling and plain strain compression was simulated, and the predictions have been validated by the experimental observations. In the rotated-Cube and Goss, the redundant shear strain and crystal rotation were in the same pattern, alternating along the thickness, while the relation between them was not obvious for the Copper and Brass due to their asymmetrical distributions of activated slip systems. The relations between slip system activation, crystal rotation, and shear strain were investigated based on the CPFE model, and the correlation between shear strain and crystal rotation has been built.     

外拘束角接头旋转电弧焊接应力应变分析

为探究旋转电弧角焊焊接残余应变分布规律,控制工件变形,分析对比了三维多体耦合模型与外拘束力模型两种拘束模型下工件残余应力与变形.首先,设定模型间的接触关系,建立了由焊件、工作台和夹具组成的三维多体耦合模型;然后,通过设定工件上节点区域法向压力和位移约束,建立了外拘束力模型;再采用热—力直接耦合法,基于上述两种模型,运用ANSYS软件模拟了旋转电弧焊接变形,获得了两种模型下残余应变曲线,并进行试验验证.结果表明,三维多体耦合模型的焊件应变曲线更接近试验结果,并且其焊接残余应变范围及峰值均较平缓,为优化焊件装夹方式来控制旋转电弧角焊变形提供参考依据.     

Al-4.6Zn-2.58Mg合金热变形工艺研究

在工业化生产条件下,采用半连续铸造、自由锻造、固溶和时效处理技术制备A l-4.6Zn-2.58Mg合金锻件。采用热加工模拟方法优化该合金的热加工工艺。试验结果表明:该合金高温压缩变形时的流变应力随变形温度的升高而减小,随变形速率的提高而增大。合金在420℃以下热变形,热变形组织主要为动态回复组织;在420℃以上热变形,热变形组织有动态再结晶发生。在400℃~420℃之间热压缩变形,变形抗力比较小;380℃~420℃时铸态塑性最好。该合金较适宜的热加工温度范围为400℃~420℃。     

Influence of in-grain mesh resolution on the prediction of deformation textures in fcc polycrystals by crystal plasticity FEM

The ability of three different crystal plasticity finite element models to predict deformation textures in face-centered cubic metals observed in experiments is assessed. These methods are: (i) Taylor averaging, in which the interactions of the grains are considered in a homogenized manner; (ii) low-resolution simulation (LRS), in which grain interactions are considered explicitly albeit with low resolution; and (iii) direct numerical simulation (DNS), which provides high-resolution details of the deformation fields inside the grains and of the grain interactions. A quantitative comparison of the numerical results provided by these three methods against experimental plane-strain compression textures is performed via orientation distribution functions and fiber line analysis. It is found that some details of the texture which are inaccessible to either Taylor averaging and LRS approaches are captured by the DNS approach. This can be explained by the ability of the high-resolution DNS method to describe details of the grain interactions, including heterogeneous deformation under homogeneous macroscopic strain and smooth gradients of lattice rotations inside the grains which are missing in low-resolution models.     

AZ31镁合金铸轧和常规轧制板的变形组织及形变特征

在变形温度为150～400 ℃、应变速率为0.3～0.000 3 s~(-1)条件下,在Gleeble1500热模拟机上采用等温拉伸试验对AZ31镁合金铸轧和常规轧制板的高温塑性及组织演变进行研究.结果表明:两种AZ31镁合金板的峰值应力和峰值应变均随着变形温度的降低和应变速率的增加而逐渐增大.铸轧板的应变硬化指数和应变速率敏感系数均大于常规轧制板的.在高温低应变速率变形条件下,铸轧板的晶界滑移引起的空洞尺寸、体积分数和密度均大于常规轧制板的.低应变速率下拉伸变形后的动态再结晶晶粒尺寸随温度的升高逐渐增加;不同变形条件下铸轧板的晶粒尺寸均小于常规轧制板的;再结晶晶粒尺寸和Z参数呈幂律关系.     

非晶合金的相变韧塑化

块体非晶合金因其独特的原子结构而具有许多优异的力学性能,成为近年来材料领域的研究热点之一,但是由于其在变形过程中的室温脆性和应变软化等关键问题,一直制约其大规模工程应用。为解决此问题,块体非晶合金领域的研究者们提出了多种方案,其中利用“相变诱导塑性”概念制备块体非晶合金复合材料来韧塑化非晶合金成为卓有成效的方案之一,通过此方法成功地制备出同时具有拉伸塑性和加工硬化能力的非晶合金复合材料。然而,该类块体非晶合金复合材料要求的形成条件更严格,同时具有更复杂的多相协调变形过程和更独特的性能优化方案。从该类块体非晶合金复合材料的形成、性能特点、韧塑化机理及性能优化等方面进行综述,并对其未来发展进行了简要展望。     

Accounting for shear deformation in fast models for plate rolling

For on-line prediction of roll force and torque, fast models have been available for a long time, which are mainly based on the slab method or other solution methods that allow for computing times in the range of seconds. Such fast models typically treat the rolling process as a plane strain problem and neglect shear deformation, which is always present in real processes. The shear deformation results in inhomogeneous strain profiles and thus might lead to inhomogeneity in microstructure over plate thickness. In this paper, a novel method is presented that allows for superposition of shear strain onto the strain state obtained from the slab method. The shear strains are interpolated from an extensive finite element (FE) parameter study of rolling processes that covers the entire parameter range of today’s plate rolling. For the regimes of very thick and thin plates different interpolation functions are introduced. It is shown that when the proposed shear strain model is combined with the slab method, similar results are obtained as with a full-scale FE calculation of the rolling problem but with a calculation time in the range of seconds.     

Mechanisms of dynamic deformation and dynamic failure in aluminum nitride

Uniaxial quasi-static, uniaxial dynamic and confined dynamic compression experiments have been performed to characterize the failure and deformation mechanisms of a sintered polycrystalline aluminum nitride using a servohydraulic machine and a modified Kolsky bar. Scanning electron microscopy and transmission electron microscopy (TEM) are used to identify the fracture and deformation mechanisms under high rate and high pressure loading conditions. These results show that the fracture mechanisms are strong functions of confining stress and strain rate, with transgranular fracture becoming more common at high strain rates. Dynamic fracture mechanics and micromechanical models are used to analyze the observed fracture mechanisms. TEM characterization of fragments from the confined dynamic experiments shows that at higher pressures dislocation motion becomes a common dominant deformation mechanism in AlN. Prismatic slip is dominant, and pronounced microcrack–dislocation interactions are observed, suggesting that the dislocation plasticity affects the macroscopic fracture behavior in this material under high confining stresses.