Strain Path Change Research Articles

An alternative method for manufacturing Al/B4C/SiC hybrid composite strips by cross accumulative roll bonding (CARB) process

For the first time, the cross accumulative roll bonding (CARB) process was used as an effective alternative method for manufacturing Al/B4C/SiC hybrid composite strips and compared with the accumulative roll bonding (ARB) processed hybrid composite. The XRD results showed that nanostructured Al/B4C/SiC hybrid composites have successfully been fabricated by using CARB process without any additional phase. During CARB, the ceramic reinforcement particles (B4C and SiC) were more uniformly dispersed within the matrix compared with ARB process, due to changes in strain path. In addition, the tensile strength, elongation, and microhardness of CARB composites were measured to be higher than those in the ARB product. Finally, scanning electron microscopy (SEM) observations of fractured surfaces showed that fracture mode both hybrid composites was shear ductile rupture, dimples, and shear zones.

Effect of cryogenic temperature and change of strain path on grain refinement during rolling of Cu–30Zn brass

Abstract The effect of cryogenic temperature and change of strain path on grain refinement during the rolling of Cu–30Zn brass was determined. To this end, the material was unidirectionally rolled or cross-rolled to 90% thickness reduction at either ambient or liquid-nitrogen temperatures, and the resulting grain structures and crystallographic textures were determined via electron backscatter diffraction (EBSD) technique. In all cases, grain refinement was found to be governed primarily by twinning and shear banding. Lowering of the rolling temperature to the cryogenic range was found to provide only a minor effect. Cryogenic rolling was thus concluded to impart no practical benefit with regard to grain refinement or property improvement for this material. In contrast, a change of strain path via cross rolling was shown to enhance twinning and shear banding and thus to promote the formation of a relatively homogeneous ultrafine-grain microstructure.

Rate-Independent Deformation Patterning in Crystal Plasticity

Metal forming processes involve continuous strain path changes inducing plastic anisotropywhich could result in the failure of the material. It has been often observed that the formation andevolution of meso-scale dislocation microstructures under monotonic and non-proportional loading have substantial effect on the induced anisotropy. It is therefore quite crucial to study the microstructureevolution to understand the underlying physics of the macroscopic transient plastic behavior. In thiscontext the deformation patterning induced by the non-convex plastic energies is investigated in amulti-slip crystal plasticity framework. An incremental variational approach is followed, which resultsin a rate-independent model exhibiting a number of similarities to the rate-dependent formulationproposed in [Yalcinkaya, Brekelmans, Geers, Int. J. of Solids and Structures, 49, 2625-2636, 2012].However there is a pronounced difference in the dissipative character of the models. The influenceof the plastic potential on the evolution of dislocation microstructures is studied through a Landau-Devonshire double-well plastic potential. Numerical simulations are performed and the results arediscussed with respect to the observed microstructure evolution in metals.

Cyclic behavior of AZ31B Mg: Experiments and non-isothermal forming simulations

Magnesium (Mg) alloys have unique room temperature mechanical properties such as yielding asymmetry, anisotropy, and unusual hardening response under strain path change. Mg alloy sheets often represent inferior formability at room temperature due to their limited active slip systems induced from specific microstructure and texture. This low formability is known to be mitigated as temperature increases, which is the result of active non-basal slip systems. Considering these unique properties, Mg alloy sheets has been often formed at 200 °C or higher. For optimizing the forming process using Mg alloy sheets, accurate constitutive models describing the unique behavior of the materials under non-proportional loading and non-isothermal temperature conditions are vital. In this paper, the mechanical behavior of AZ31B Mg alloy sheets under in-plane tension–compression (or compression-tension) cyclic loading was experimentally measured for various pre-strains and temperatures. Then, a practical hardening model modified from existing advanced hardening law was applied to calculate the stress–strain responses and formability in the cross shape die forming under non-isothermal condition.

Texture evolution of rolled Mg–3Al–1Zn alloy undergoing a {10-12} twinning dominant strain path change

The texture modification, caused by {10-12}–{10-12} double twinning, in rolled Mg–3Al–1Zn alloy was investigated by carrying out consecutive in-plane compressions along two orthogonal directions in combination with an electron backscatter diffraction measurement and a Schmid factor analysis. The results revealed that the Schmid law still governs twin variant selection, thereby controlling texture evolution, however, the specific crystallographic feature of {10-12} twinned region in which the c-axis orients to the compression direction with a variation of ±30° on the rolling plane and one prismatic plane including the rotation axis of {10-12} twinning, corresponding to an a-axis, is aligned almost parallel to the rolling plane gives rise to a significantly different texture evolution of {10-12}–{10-12} double twinned region as compared to {10-12} twinned region.

Investigations of the effect of strain path changes on forming limit curves using an in-plane biaxial tensile test

Optimization of sheet metal forming processes requires a very good knowledge of material forming ability, more especially for aluminum alloys which generally exhibit a poor formability at ambient temperature. During the forming of industrial parts, very complex strain paths are usually observed and can affect the formability of the sheet. In this work, in order to investigate strain path effects on formability, an innovative one-step procedure is proposed to control the strain path changes with a single test, without unloading. The test is based on the use of a cruciform shape loaded with a planar biaxial tensile device. Strain path is controlled by the displacements in the two main directions of the cruciform specimen. For a given non-linear strain path type, experimental forming limit points are greatly influenced by the level of prestrain which can either improve or reduce formability. The same tendency is observed when using a predictive tool based on a finite element model of the same cruciform shape and a rather good correlation is observed between experimental and numerical results.

Effect of natural ageing and pre-straining on strength and anisotropy in aluminium alloy AA 6016

During processing of age-hardenable AA 6xxx series alloys for automotive applications the sheets may experience significant time spans between solution heat treatment at the aluminium supplier and age hardening upon the final paint bake cycle at the carmaker. Natural ageing during these pause times and intermediate deformations by sheet levelling or forming operations both affect materials properties of autobody sheet. In the present study we explore the impact of a combination of natural ageing and pre-straining on the tensile properties and the in-plane anisotropy of alloy AA 6016. Materials properties are modelled with the help of a nano-scale material model called NaMo, which consists of a precipitation model simulating the formation of clusters and phases upon natural ageing as input to a mechanical model simulating the evolution of yield strength and work hardening as a function of natural ageing and pre-straining. Plastic anisotropy is modelled by polycrystal-plasticity simulations under consideration of latent hardening to account for strain path change effects introduced by the pre-straining.

An elasto-plastic self-consistent model with hardening based on dislocation density, twinning and de-twinning: Application to strain path changes in HCP metals

In this work, we develop a polycrystal mean-field constitutive model based on an elastic–plastic self-consistent (EPSC) framework. In this model, we incorporate recently developed subgrain models for dislocation density evolution with thermally activated slip, twin activation via statistical stress fluctuations, reoriented twin domains within the grain and associated stress relaxation, twin boundary hardening, and de-twinning. The model is applied to a systematic set of strain path change tests on pure beryllium (Be). Under the applied deformation conditions, Be deforms by multiple slip modes and deformation twinning and thereby provides a challenging test for model validation. With a single set of material parameters, determined using the flow-stress vs. strain responses during monotonic testing, the model predicts well the evolution of texture, lattice strains, and twinning. With further analysis, we demonstrate the significant influence of internal residual stresses on (1) the flow stress drop when reloading from one path to another, (2) deformation twin activation, (3) de-twinning during a reversal strain path change, and (4) the formation of additional twin variants during a cross-loading sequence. The model presented here can, in principle, be applied to other metals, deforming by multiple slip and twinning modes under a wide range of temperature, strain rate, and strain path conditions.

Hardening behavior and texture evolution of TWIP steel during strain path change

Polycrystal materials exhibit large changes in the flow stress and hardening behavior during the strain path change. Such changes are related with the crystallographic texture anisotropy and the rearrangement of dislocation structure during the pre-loading. These effects have been captured by a dislocation hardening model embedded in the visco-plastic selfconsistent (VPSC) model. In this work, the texture evolution and mechanical behavior of TWIP steel during the strain path change are investigated. The experimental studies are carried out on rolled TWIP steel sheet. The mechanical responses are obtained under tensile tests along rolling direction, followed by tension along the directions with 0° and 90° from the pre-loading direction. The simulated results of strain-stress curves and the texture evolution are in good agreement with the experimental data.

Effect of strain path change on precipitation behaviour of Al-Cu-Mg-Si alloy

The effect of strain path change on precipitation behaviour of Al- Cu-Mg-Si alloy was investigated. Two different types of crystallographic textures were produced by changing the strain path during rolling. The deformed samples were subjected to a short recrystallization treatment and ageing to identify the effect of strain path change manifested in terms of crystallographic texture on precipitation behaviour. Preliminary characterization indicates that ageing kinetics as well as precipitate morphology vary depending upon the mode of rolling. The coherency strains associated with a coherent interface is relieved in a unlike manner for differently rolled samples.

Analysis of Microstructure and Texture Evolution in Mg-3Al-1Zn Alloy Processed Through Groove Rolling

The mechanism of grain refinement in a AZ31 Mg alloy subjected to hot groove rolling is investigated up to large strain (et ~ 2.5). The alloy shows enhanced yield strength without compromising ductility. The change in strain path during rolling has resulted in significant weakening of basal texture. The microstructure analyses show that dynamic recrystallization (DRX) contributed significantly to grain refinement and hence to the observed mechanical properties. The combined effects of DRX and texture evolution on mechanical properties have been addressed.

Large strain cyclic behavior of metastable austenic stainless steel

Metastable austenitic stainless steel will transform to martensite when subjected to mechanical working. In this research an austenitic stainless steel has been subjected to large amplitude strain paths containing a strain reversal. During the tests, apart from the stress and the strain also magnetic induction was measured. From the in situ magnetic induction measurements an estimate of the stress partitioning among the phases is determined.When the strain path reversal is applied at low strains, a classical Bauschinger effect is observed. When the strain reversal is applied at higher strains, a higher flow stress is measured after the reversal compared to the flow stress before reversal. Also a stagnation of the transformation is observed, meaning that a higher strain as well as a higher stress than before the strain path change is required to restart the transformation after reversal.The observed behavior can be explained by a model in which for the martensitic transformation a stress induced transformation model is used. The constitutive behavior of both the austenite phase and the martensite is described by a Chaboche model to account for the Bauschinger effect. Mean-field homogenization of the material behavior of the individual phases is employed to obtain a constitutive behavior of the two-phase composite. The overall applied stress, the stress in the martensite phase and the observed transformation behavior during cyclic shear are very well reproduced by the model simulations.

Influences of simple strain path changes on mechanical behaviours of pearlitic steel wire

The mechanical behaviours of the pearlitic steel wires under simple strain path changes (SSPCs) are investigated, including monotonic, cross and reversal loadings. Both the cross and reversal loadings lead to an apparent softening. Especially during the whole reversal loading, the stress is much lower than before unloading. And it is a function of the pre-strain. Additionally, it is found that the maximum strain increases significantly in reversal loading. The microstructures under different SSPCs are investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), with an aim to reveal the micro-mechanisms of the SSPC effects on the mechanical behaviours.

Strain-path change induced transients in flow stress, work hardening and r-values in aluminum

Commercially pure aluminum with random texture was prestrained either by rolling or by uniaxial compression, and then tested in uniaxial tension to study the transients in flow stress, work hardening and r-value induced by the strain-path change. New experimental results are reported on the variation of the r-value and the permanently reduced work hardening subsequent to the strain-path change. A continuum plasticity model was developed that can reproduce the observed behavior. The model applies a second-order “delayed pointer” tensor to represent the microstructural anisotropy and was implemented into the finite element software LS-DYNA. The model was calibrated to the experimental data, and a simulation of early strain localization subsequent to an orthogonal strain-path change was compared to strain fields measured by a digital image correlation technique.

OS0102-133 二軸圧縮下におけるTi-Nb合金の塑性変形挙動に及ぼすひずみ経路変化の影響

OS0102-133 二軸圧縮下におけるTi-Nb合金の塑性変形挙動に及ぼすひずみ経路変化の影響

Stress update algorithm for enhanced homogeneous anisotropic hardening model

A stress integration algorithm is provided for a novel homogeneous-yield-function-based anisotropic hardening (HAH) model. The new model is an extension of the original HAH model that describes cross-hardening or softening of a sheet metal under an orthogonal strain path change. A semi-explicit integration scheme for the stress update is utilized to efficiently handle the gradient of the distorted yield surface during complex strain path changes, as originally proposed by Lee et al. (2012). Validations of the algorithm developed were conducted by comparing the predicted stress–strain curves of dual-phase (DP) 780 and extra-deep-drawing-quality (EDDQ) steels with experimental stress–strain responses observed under cross-loading conditions. Finally, the accuracy of the proposed finite element (FE) formulations was assessed by r-value prediction and preparation of iso-error maps.

Abnormal texture evolution of rolled Mg–3Al–1Zn alloy containing initial {10-12} twins

The importance of initial {10-12} twins in the formation of deformation texture was investigated by conducting strain path change tests of rolled Mg–3Al–1Zn alloy in which {10-12} twinning dominated the deformation. The existence of initial twins changed a twinning process by preferring the growth of initial twins rather than the nucleation of new twins, which led to a non-Schmid behavior of twinning characteristics and consequently caused a completely different deformation texture compared with the counterpart without initial twins.

Texture Evolution in Nanocrystalline Nickel: Critical Role of Strain Path

The effect of strain path change during rolling on the evolution of deformation texture has been studied for nanocrystalline (nc) nickel. An orthogonal change in strain path, as imparted by alternating rolling and transverse directions, leads to a texture with a strong Bs {110}〈112〉 component. The microstructural features, after large deformation, show distinct grain morphology for the cross-rolled material. Crystal plasticity simulations, based on viscoplastic self-consistent model, indicate that slip involving partial dislocation plays a vital role in accommodating plastic deformation during the initial stages of rolling. The brass-type texture evolved after cross rolling to large strains is attributed to change in strain path.

Microstructure evolution and mechanical behavior of a high strength dual-phase steel under monotonic loading

Transmission electron microscopy (TEM) microstructures of a high-strength dual-phase steel DP800 have been examined after moderate plastic deformations in simple shear and uniaxial tension. Special attention has been paid to the effect of the intergranular hard phase (martensite) on the microstructure evolution in the near-grain boundary regions. Quantitative parameters of dislocation patterning have been determined and compared with the similar characteristics of previously examined single-phase steels. The dislocation patterning in the interiors of the ferrite grains in DP800 steel is found to be similar to that already observed in the single-phase IF (Interstitial Free) steel whereas the martensite-affected zones present a delay in patterning and display very high gradients of continuous (gradual) disorientations associated with local internal stresses. The above stresses are shown to control the work-hardening of dual-phase materials at moderate strains for monotonic loading and are assumed to influence their microstructure evolution and mechanical behavior under strain-path changes.

Development of Ultrafine Grained Zircaloy-2 by Room Temperature Cross Rolling

Effect of change in strain path by cross rolling up to a true strain of 1.89 has been studied in the present work. The Zircaloy-2 was subjected to solutionising heat treatment at 800 °C in argon environment for 2 h and subsequently quenched in mercury prior to cross rolling at room temperature. The fragmentation of near basal grains due to change in strain path is evident from the EBSD micrographs. The dislocation density in the crossrolled alloy increases with true strain as calculated from the XRD and EBSD data and it is found to be 2.806453 × 1016/m2. $$ \left\{ {10\bar{1}2} \right\} $$ extension twins are observed initially up to 25% reduction, with the further reduction in thickness, near basal grains are oriented toward the normal direction. These basal grains undergone fragmentation due to changes in strain path upon cross-rolling as observed from KAM and EBSD images. TEM results of the cross-rolled sample confirm the formation of ultrafine and nanograins in the alloy due to orientation of incidental dislocation boundaries in the direction of macroscopic plastic flow and post-annealing treatment of the deformed alloy. A tensile strength of 991 MPa with 7.5% ductility is observed in the 85% cross-rolled alloy. The cross-rolled alloy upon annealing at 400 °C for 30 min improves ductility to 11%.