Modeling the room temperature deformation of a two-phase zirconium alloy

A new modeling approach, which combines the finite element method (FEM) and elasto-plastic self-consistent model (EPSC), was used to model the deformation behavior of hot-rolled Zr–2.5Nb plate material and was shown to be an effective method for describing the deformation of multiphase materials. The macroscopic mechanical properties and the phase interaction stresses were well captured by a unit cell FEM model. The average phase stress–strain histories simulated by FEM were used as boundary conditions in an EPSC model to calculate the evolution of intergranular strains in both phases. Taking the asymmetric character of the <c + a> pyramidal slip during tension and compression into account in EPSC, the agreement between the experimental and the modeling results was considerably improved.     

Study of internal lattice strain distributions in stainless steel using a full-field elasto-viscoplastic formulation based on fast Fourier transforms

In this work, the evolution of internal lattice strains in face-centered cubic stainless steel under uniaxial tension is studied using a recently developed full-field elasto-viscoplastic formulation based on fast Fourier transforms. The shape of the diffraction peaks is simulated, and the predicted lattice strains (peak shift and broadening) are compared with the experimental measurements obtained by in situ tensile neutron diffraction. Detailed analysis of the lattice strain distributions reveal that {1 0 0} and {1 1 0} transverse families exhibit a bimodal nature, and that transverse lattice strains are more sensitive to local grain interactions compared with longitudinal lattice strains. A comparison with the results of a mean-field formulation indicates that type III (intragranular) stresses play a much larger role than type II (intergranular) stresses in diffraction peak broadening.     

Influence of the thermomechanical treatment on the microplastic behaviour of a wrought Al–Zn–Mg–Cu alloy

For plastic deformations smaller than the conventional limit of 0.2% for the yield stress, the so-called microplastic behaviour of a rolled Al–Zn–Mg–Cu alloy is investigated experimentally. Tension and compression responses are compared along the rolling and the transverse directions. The alloy shows an asymmetric response in tension and compression (i.e., compressive stress minus tensile stress for a given absolute plastic strain is non-zero). Moreover, this asymmetry changes sign between the rolling and the transverse directions. The difference between tension to compression is observed to decrease as the conventional limit is approached.The influence of the heterogeneous microstructure of the alloy and the fabrication process on these asymmetries is discussed. Modelling of the material response based on a self-consistent scheme is used to estimate the internal stresses resulting from the thermomechanical treatment, and also to investigate the influence of the heterogeneous elastoplastic behaviour of two types of constitutive grains (recrystallised and unrecrystallised). For a plastic strain smaller than 0.02%, the microplastic behaviour is not well described with the adopted model since the underlying assumption of uniform stress and strain fields per phase is questionable in this initial level of plasticity. However, the model shows that the asymmetries observed at plastic strains ranging from 0.02% to 0.2% are consistent with the intragranular stresses developed during the stretching step.     

On the origin of the tensile flow stress in the stainless steel AISI 316L at 300 K: back stress and effective stress

The tensile behaviour of a polycrystal austenitic stainless steel at 0.2T_m is discussed in terms of back and effective stresses with the help of qualitative and quantitative TEM observations. Particular attention is given to the transition between stages I and II which occurs at a plastic strain equal to 1.5%. The effective stress evolution can be interpreted as a competition process between the increase of mobile dislocation density and dislocation interactions and an annihilation process. The main purpose of this work is to provide a basis for separating the two different contributions of the back stress, namely the intragranular back stress X_intra arising from the heterogeneous dislocation distribution inside the grains and the intergranular back stress component X_inter resulting from plastic strain incompatibilities between grains. Moreover, it is shown that the latter contribution is dominant at small strains (stage I), whereas the former one is more important subsequently (stages II and III), when cross-slip and multiple slip occur.     

Intergranular strain accumulation in a near-alpha titanium alloy during plastic deformation

Neutron diffractometry has been used to characterise the evolution of intergranular lattice strains during the tensile loading of the near-α titanium alloy Ti-834. The measurements were made on tensile testpieces in directions both parallel and perpendicular to the applied load. To rationalise the observations, an elastic–plastic self-consistent model for the α phase is used, in which the three basal and two non-basal slip systems observed in titanium alloys are invoked; it is found that it is possible to reproduce successfully the observed trends and magnitudes of the microstrains in both the longitudinal and transverse directions. Unfortunately, no reflection was found for which intergranular strain accumulation is small, i.e. less than 1×10^-4. This situation should be recognised when making measurements of residual strains and stresses in large-scale engineering components, since intergranular strains must be distinguished from the macro-scale engineering strains. To circumvent this difficulty a novel method is proposed whereby the results of two or more peaks are combined to form a synthetic ‘actor’ which accumulates near-zero intergranular strain in both the parallel and perpendicular directions.     

Intergranular Strain Evolution During Biaxial Loading: A Multiscale FE-FFT Approach

Predicting the macroscopic and microscopic mechanical response of metals and alloys subjected to complex loading conditions necessarily requires a synergistic combination of multiscale material models and characterization techniques. This article focuses on the use of a multiscale approach to study the difference between intergranular lattice strain evolution for various grain families measured during in situ neutron diffraction on dog bone and cruciform 316L samples. At the macroscale, finite element simulations capture the complex coupling between applied forces and gauge stresses in cruciform geometries. The predicted gauge stresses are used as macroscopic boundary conditions to drive a mesoscale full-field elasto-viscoplastic fast Fourier transform crystal plasticity model. The results highlight the role of grain neighborhood on the intergranular strain evolution under uniaxial and equibiaxial loading.     

EBSD studies of the stress-induced B2–B19′ martensitic transformation in NiTi tubes under uniaxial tension and compression

In situ electron backscattering diffraction (EBSD) investigations were conducted on polycrystalline NiTi tube specimens during tensile and compressive deformation. The long-range cooperative and catalytic martensitic transformation under tension induces the transformation to proceed in the form of helical Lüders band. Propagation of the band is closely related to the spatial distribution of the orientations of individual grains. In uniaxial compression, the larger variation in Schmid factors, and consequently the larger variation in the critical transformation stresses among grains, leads to a homogeneous martensitic transformation, and therefore the absence of the Lüders band. To interpret the observed tension–compression asymmetry, a crystallographic model of the critical transformation stress and transformation strain for polycrystalline NiTi under tension and compression is proposed. The model defines three crystallographic regions: tension-favorable, compression-favorable and neutral zones. The orientation population in which tensile strains are larger than compressive strains is much higher than that of orientations with higher compressive strains. For resolved shear stress, orientation populations favoring tension and compression do not show any great difference.     

Effect of deformation mode and grain orientation on misorientation development in a body-centered cubic steel

Strain-induced misorientation development was studied in an IF steel as a function of strain for two deformation modes, plane strain compression and simple shear. Using electron back-scattered diffraction, orientation maps of “large” areas were obtained, from which several individual grains associated with the principal texture components could be extracted so that only intragranular misorientations could be estimated for these orientations. It was observed that the increase of the misorientation angle was more prominent in simple shear than in plane strain compression and that the orientation influence was different for each mode. Considering texture evolution as a possible source of misorientation development, the lattice spin tensor was estimated with the Taylor model for the two deformation modes; both reorientation axis and angle were compared with misorientation angle and axis. The striking concordance of both quantities allows us to conclude that there is a direct contribution of texture evolution to misorientation accumulation with strain.     

The effects of N on the microstructures and tensile properties of Fe–15Mn–0.6C–2Cr–xN twinning-induced plasticity steels

Three Fe–15Mn–0.6C–2Cr–xN (wt.%) twinning-induced plasticity (TWIP) steels with N concentrations of 0.02%, 0.09% and 0.21% were fabricated using a pressure-induction furnace. The variations in tensile properties and deformed microstructure as a function of N concentration were investigated. The yield and tensile strengths of the steels increased without a loss of total elongation as the concentration of N increased. The strain hardening rates (SHR) of the TWIP steels with 0.02% and 0.09% N decreased gradually as the true strain increased until failure. The SHR of the TWIP steel with 0.21% N decreased sharply until a strain of approximately 0.07 and increased with further strain, exhibiting the highest value at a strain of over 0.15. The addition of N delayed the kinetics of mechanical twinning, particularly secondary twinning. The TWIP steel with 0.02% N had many intersections between primary and secondary twins, at which α′ martensite formed. The TWIP steel with 0.09% N possessed few intersections because primary twins obstructed the growth of secondary twins. The TWIP steel with 0.21% N had primary twins with few secondary twins and intersections. The addition of N increased the critical strain for triggering serrations on the tensile stress–strain curves, indicating a reduction in dynamic strain aging (DSA). The TWIP steel with 0.21% N exhibited the highest SHR at large strains, despite the reduced twinning and DSA, because of both the thinning of the mechanical twins and the hardening of the twin boundaries as the N concentration increased.     

AZ31B镁合金板材中晶界滑移的晶体塑性仿真

建立一种耦合滑移、动态再结晶以及晶界滑移的晶体塑性模型以仿真镁合金的高温变形行为及织构演化.首先,通过实验测量单轴拉伸、压缩后的织构以及显微组织演化,研究AZ31B镁合金在300°C的变形机制.结果发现,动态再结晶在应变小于0.2时起到细化晶粒的作用,之后晶界滑移在变形过程中起显著作用.此外,建立晶界滑移模型来评估由晶...     

The generation of intergranular strains in 309H stainless steel under uniaxial loading

The generation of intergranular strains in 309H austenitic steel has been measured in situ up to 13.0% deformation by neutron diffraction. Experimental data have been obtained in the directions parallel and perpendicular to the tensile axis. Large tensile strains are observed for the (002) reflection along the tensile axis while the other reflections show relatively small intergranular effects. Both texture and residual strain states of the as-received and deformed samples have also been measured. The number of grains with [111] parallel to the tensile axis increases from ×2 to ×4 random texture and is coupled with a depletion of [220] grains in the same direction. The in situ experiments have been simulated with the EPSC model of Turner and Tomé. The model tends to overestimate the intergranular strains in the directions perpendicular to the tensile axis.     

On the stress state dependence of the twinning rate and work hardening in twinning-induced plasticity steels

The influence of the stress state on the twinning rate and work hardening is studied in the case of an Fe–Mn–C TWIP steel strained in uniaxial tension, simple shear and rolling. The resulting stress–strain responses exhibit marked differences. The twinning rate, number of activated twinning systems in each grain, twin thickness and transmission of twins across grain boundaries are dependent on the imposed stress state during straining. Relationships between twin features and macroscopic work hardening rate are established.     

45钢在应变循环与棘轮变形下的随动硬化演化实验研究

对调质处理的45碳钢进行了不同应变幅值的对称应变和具有平均应变控制下的屈服面半径和背应力演化分析,及具有不同平均应力的低应力、较高应力幅值控制下的屈服面半径和背应力演化分析．研究表明：屈服面半径在给定应变幅值下随循环周次的增加而变小,且随循环塑性应变幅值增大而减小,在单调加载时增大;循环背应力幅值随单调应变而减小,随循环塑性应变幅值增大而增大．     

TWIP钢的动态力学性能及变形机制研究

利用Zwick/Roell Z100万能材料试验机和Hopkinson拉杆对TWIP钢进行了准静态及动态力学性能的研究。基于力学实验结果,修正了Johnson-Cook动态本构模型中应变硬化项以及应变强化项。采用X射线衍射(XRD)、扫描电镜(SEM)、电子背散射衍射(EBSD)技术对TWIP钢拉伸变形前后的组织进行了观察与分析。结果表明：TWIP钢在准静态加载下表现为负应变率敏感性,动态加载时表现为正应变率敏感性。拉伸过程中,孪生诱发塑性是TWIP钢的主要变形机制,同时滑移也起到重要作用;动态加载下TWIP钢中形变孪晶的起始应变和孪晶体积分数均小于准静态加载过程;形变孪晶的生成以及孪晶相互作用导致的晶粒细化,使TWIP钢兼具高强度、高塑性及高动态吸能性能,在抗冲击、抗爆领域具有广泛的应用前景。     

AN EXPERIMENTAL STUDY ON THE KINEMATIC HARDENING RULES FOR NONPROPORTIONAL CYCLIC PLASTICITY

1.IntroductionIthasveryimp0rtantsignificancetodescribeaccuratelythesolidmaterialplasticbe-haviorinpracticalapplication.F0rgeneralclassicalplasticitymodels,materialplasticbehavi0risgenerallydescribedbytheis0tropicl']andthekine.ati.[2]hardeningrules.Inthen0nproporti0nalcyclicloading,theprediction0fthedirectionofplasticstrainratevec-torismainlybasedonthekinematicrule-Therefore,astudyontheev0lutionsofplasticstrainratevectorandkinematichardeningduringnonprop0rtionalcyclinghasextremelyimp0rtantrole0…     

Three-dimensional observation and image-based modelling of thermal strains in polycrystalline alumina

Diffraction contrast tomography (DCT) with synchrotron X-rays was used to map the three-dimensional microstructure of alumina. Each grain boundary (GB) of this coarse-grained ceramic was characterized by its orientation and the crystal misorientation of the adjacent grains. The microstructure of alumina was sufficiently well described by DCT to produce a microstructurally representative image-based finite-element model comprising ～400 grains. Grain boundary cohesive elements were used to calculate the local thermal stresses acting on each GB arising from the crystal anisotropy. Digital volume correlation of the CT images was used to gauge the degree of bending induced during loading and to extract polycrystalline elastic properties. The model simulations showed the average intergranular stress to be influenced by the orientation of the GB plane relative to the basal planes of the adjacent grains. Boundaries to which at least one of the basal planes was closely aligned tended to develop higher tensile stress; these boundaries were predicted to have a tendency for intergranular fracture. Under compressive loading, the normal stresses of the boundaries that cracked were slight more tensile relative to the general population due to grain-to- grain interactions. The predicted effect of crystal lattice strains and rotations on diffraction, due to the modelled thermal stresses, showed general agreement with the observed X-ray diffraction images of individual grains.     

考虑单向拉伸横向应变演化的本构模型材料参数测定

有限元数值模拟的精度不但与本构模型的描述能力有关,而且与其材料参数的测定方法密切相关。该文以由Hill1948屈服函数和Swift等向强化模型组成的本构模型为例,比较了不同的材料参数测定方法对单向拉伸的轴向应力-轴向应变、横向应变-轴向应变试验数据的预测能力。参数测定方法采用了两种,一种是传统方法,即使用r值计算屈服函数系数,采用轧制方向单向拉伸应力-应变数据拟合强化模型参数;另一种是考虑单向拉伸横向应变演化的反向优化法,即使用不同方向单向拉伸轴向应力-轴向应变和横向应变-轴向应变试验数据,同时求解屈服函数和强化模型的材料参数。结果表明,当使用传统方法时,所得材料参数不能很好描述与轧制方向成45°方向的单向拉伸数据;当使用考虑单向拉伸横向应变演化的反向优化法时,所得材料参数能够较准确描述各个方向的单向拉伸力学性能。     

Revealing the strain-hardening behavior of twinning-induced plasticity steels: Theory,simulations, experiments

We present a multiscale dislocation density-based constitutive model for the strain-hardening behavior in twinning-induced plasticity (TWIP) steels. The approach is a physics-based strain rate- and temperature-sensitive model which reflects microstructural investigations of twins and dislocation structures in TWIP steels. One distinct advantage of the approach is that the model parameters, some of which are derived by ab initio predictions, are physics-based and known within an order of magnitude. This allows more complex microstructural information to be included in the model without losing the ability to identify reasonable initial values and bounds for all parameters. Dislocation cells, grain size and twin volume fraction evolution are included. Particular attention is placed on the mechanism by which new deformation twins are nucleated, and a new formulation for the critical twinning stress is presented. Various temperatures were included in the parameter optimization process. Dissipative heating is also considered. The use of physically justified parameters enables the identification of a universal parameter set for the example of an Fe–22Mn–0.6C TWIP steel.     

Plastic heterogeneities of a copper multicrystal deformed in uniaxial tension: experimental study and finite element simulations

A copper sample made of a single layer of grains is plastically deformed by uniaxial tension at room temperature and low strain rate. The deformation field is measured by means of grids deposited on the polished surface of the undeformed specimen and local orientations are recorded using electron back scattering diagrams in a scanning electron microscope. These measures are compared with simulations made by a finite element code using a physically based model for the deformation and hardening of face centered cubic crystals. A good agreement is found between measured and computed values. The simulations give access to much more detail about the history of glide in each grain and help establish which systems are active at a local level. They also provide the evolution of internal variables such as dislocation densities. A new insight into intergranular accommodation as well as intragranular heterogeneities is provided.