Deformation Behavior Of Magnesium Alloy Research Articles

Unusual Spreading of Strain Neutral Layer in AZ31 Magnesium Alloy Sheet during Bending.

In this work, we reported an unusual phenomenon of strain neutral layer (SNL) spreading in an as-rolled AZ31B magnesium alloy sheet during V-bending. The SNL on the middle symmetrical surface perpendicular to the transverse direction (TD) of the sheet extended to the compression region and was accompanied by a mound-like feature. However, the SNL on the side surface perpendicular to the TD was distributed with a parallel band feature. The underlying mechanism was revealed by the finite element (FE) analysis. The results indicate that the three-dimensional compressive stresses in the compression region of the bending samples were responsible for the above phenomenon. Moreover, the area of the SNL in the middle position gradually decreased as the bending test progressed. The findings in this study provide some new insights into the bending deformation behavior of magnesium alloy.

Advanced in-situ experimental techniques for characterization of deformation mechanisms in magnesium alloys

A combination of advanced in-situ experimental techniques, comprised of neutron diffraction, acoustic emission (AE), and in-situ electron backscattered diffraction (EBSD) was selected to provide extensive insight into the deformation behavior of magnesium alloys based on their mutual complementarity. The potential and limitations of these techniques were shown and discussed in the scope of the study of the influence of the crystallographic texture influence on the activity of individual deformation mechanisms in a hot-rolled sheet of the AZ31 magnesium alloy. The neutron diffraction experiments coupled with a recording of the AE signal allowed monitoring of the twinning activity and the evolution of its dynamics from nucleation toward twin growth. The AE suggested microplastic behavior, which was confirmed by the lattice strain evolution analysis for the sample compressed in the sheet normal direction. The in-situ EBSD experiments provided direct observation of the deformed microstructure, including the formation of twinning bands and the transition from twin nucleation to growth with progressing strain, being in very good agreement with the indirect neutron diffraction and AE measurements. Furthermore, the subsequent analysis of the EBSD maps enabled the quantification of the twinned volume fraction and its qualitative comparison with the evolution measured by neutron diffraction. The EBSD-based slip trace analysis revealed a notable non-Schmid basal activity and offered insight into the mechanisms accommodating strain once the twinning was exhausted.

بررسی اثر شدت تکسچر بر رفتار تغییر شکل آلیاژ Mg-0.8wt%Y اکسترود شده تحت بارگذاری کشش و فشار با استفاده از روش المان محدود کریستال پلاستیسیته

Magnesium alloys due to their compact hexagonal structure (HCP) usually have a yield asymmetry in tensile and compressive loadings. This is due to various active plastic deformation mechanisms under tensile and compressive loadings. Therefore, it is necessary to study the deformation behavior of magnesium alloys on the grain scale. This study focuses on the 3D Representative Volume Element (RVE) simulations of extruded magnesium alloy to investigate the effect of basal texture intensity on tensile–compressive yield asymmetry of Mg-0.8wt% Y. Polycrystalline aggregate models are built based on the theory of Voronoi diagram; then, the Slip-based crystal plasticity formulation is applied as a User Material Subroutine in ABAQUS (UMAT) to study the relationship of microstructures and stress-strain responses in Mg-0.8wt% Y alloy during tension and compression test and the results are presented. The results show that when the orientation of the grains is considered random (zero texture intensity), the activities of different deformation modes are almost the same and minimum tensile-compressive yield asymmetry happens. Also, the results reveal when all the grains are oriented in a certain direction (texture intensity 1), maximum tensile–compressive yield asymmetry occurs. Indeed, according to the Relative Activity (RA) results, in agreement with Schmid Factor (SF) results, in a strong basal texture with the intensity of 1, maximum contribution of accumulated plastic deformation is related to tensile twinning while in compressive loading, prismatic slip and contraction twinning are responsible for major plastic deformation. Therefore, due to the activation of various deformation modes in strong basal texture, the highest amount of tensile–compressive yield asymmetry occurs. The S-shape strain hardening curve which is the main characteristic of twin-dominated deformation shows a strong dependency on texture in the extruded material so that during loading at the weaker texture, s-shape hardening is no longer present due to the dominated non-basal slips in weaker texture.

Mechanical Characterization of The Plastic Deformation Behavior of AZ31 Magnesium Alloy Processed Through Spinning Using Nanoindentation

The nanomechanical properties of an AZ31 magnesium alloy processed through spinning was systematically characterized with nanoindentation experiments using a Berkvoich indenter. The plastic deformation behavior of magnesium alloy was revealed by quantitatively studying microstructure evolution through spinning. Indentation depth, elastic recovery rate, yield strength, and hardening exponent were evaluated at the maximum loads. It was found that the grains observed by optical microscope were gradually refined. The nanoindentation plots revealed that the nanomechanical properties improved with the increase of spinning passes. Additionally, the calculated yield strength and hardening exponent of the cylinders were in the range of 42.0±0.5–125.5±0.6 MPa and 0.182–0.017 respectively with the increase of spinning passes. In summary, it was possible to evaluate the elastoplastic behavior of AZ31 magnesium alloy after spinning using the results of the nanoindentation tests.

Assessment in predictability of visco-plastic self-consistent model with a minimum parameter approach: Numerical investigation of plastic deformation behavior of AZ31 magnesium alloy for various loading conditions

By predicting the complex deformation mechanisms, visco-plastic self-consistent (VPSC) model has been found to be a useful tool for investigating plastic deformation behavior of magnesium alloy. The standard version of VPSC model involves a large number of adjustable parameters. Using independent arguments obtained from single tests, VPSC model with a minimum parameter approach immensely reduces the number of adjustable parameters, hence shows huge potential in simulating plastic deformation behavior of thin magnesium alloy sheet. The predictability of this new approach is thoroughly assessed in the present study. Although only the mechanical response of in-plane tension (IPT) in AZ31 magnesium alloy is applied to calibrated the corresponding material parameters, simulated results in terms of texture evolution using VPSC model with a minimum parameter approach show a relatively small difference by comparison with the predicted ones applying the standard version of VPSC model during IPT, in-plane compression (IPC), through-thickness compression (TTC), and plane strain compression (PSC), respectively. Furthermore, the corresponding activated deformation mechanisms during various deformation processes are further analyzed. With the exception of pyramidal <c+a> slip, the predicted activities of remaining deformation mechanisms are generally consistent with each other. This phenomenon is identified to be the root of minor difference in texture evolution. Moreover, the predicted activities in pyramidal <c+a> slip using VPSC model with a minimum parameter approach are relatively higher than the corresponding ones in the case with the standard version of VPSC model. However, these reported results are not beyond the maximum of published literature, and hence are acceptable when simulating various plastic deformation behavior of AZ31 magnesium alloy.

Modeling twinning, detwinning, and dynamic recrystallization of magnesium alloys

Abstract

In-situ quantitative TEM investigation on the dynamic evolution of individual twin boundary in magnesium under cyclic loading

Quantification of dynamics of individual twin boundary (TB) migration such as the velocities and corresponding stresses, is of critical importance for understanding the deformation behavior of magnesium alloys. By conducting in-situ cyclic loading experiments on submicron magnesium pillars inside transmission electron microscope (TEM), the dynamics of individual TB migration and the associated twinning-detwinning phenomena are systematically investigated. It is found that the TB can migrate forward and backward under each cyclic loading paths, corresponding to the twinning-detwinning cycles. The TB morphology changes constantly during its migration. Surprisingly, the stress required for TB migration is found to be higher in compression than in tension, and the TB migration velocity in compression is slower than in tension. Such asymmetry is proposed to be associated with different defect environment on either side of TB and the TB structure per se. The considerable amount of energy absorbed during the TB migration is believed to account for at least part of the good damping properties of Mg. Our results are also expected to benefit the modeling of deformation twinning behavior in Mg and other HCP metals.

Finite Element Simulation of Rotating Compression Forming

In this study, finite element analyses of Rotating Compression Forming (RCF) of magnesium alloy AZ31 with 20mm in diameter are carried out. A commercial software DEFORM-3D is used to simulate the plastic deformation behavior of magnesium alloy during rotating compression forming processes. Various forming parameters, such as the compression pressures (55, 60, 62 MPa), rotation speeds (10, 20, 30 rpm), rotation numbers (10, 20, 30 revolutions) and forming temperatures (280°C, 320°C, 360°C), are used in the FE simulation to obtain different effective strain distributions. Observation of the gradient microstructure and hardness measurement in the specimens of Magnesium alloy are carried out. From the comparisons of effective strain distributions and the obtained grain size distributions. It is known that a larger effective strain corresponds to a smaller grain size and the maximum effective strain is occurred at the middle of the contact plane of the specimen. Larger effective strain gradients generated with compression pressure of 62 MPa and rotation number of 30 revolutions at the radius of 4 mm.

Distortional hardening concept for modeling anisotropic/asymmetric plastic behavior of AZ31B magnesium alloy sheets

A continuum-based approach for describing the plastic hardening behavior of magnesium alloy sheets subjected to non-proportional strain path changes at room temperature is presented. The constitutive model is based on an anisotropic distortional yield function that combines a stable component and a fluctuating component. The stable component initiates the yield criterion that characterizes the typical strength differential (SD) effect between tension and compression in magnesium alloys at room temperature. The evolution of the fluctuating component is reformulated based on its cubic metal counterpart to represent the deformation behavior of magnesium alloys, which consists of slip- and twinning-dominant deformation modes. To validate the proposed model, a predictor–corrector numerical algorithm was implemented in a finite element (FE) program by using a user material subroutine, and its numerical accuracy was evaluated by iso-error map analysis. Comparison of the experimental and simulated results obtained for various loading path changes showed that, although the model was not formulated using the kinematic hardening concept, the model could reproduce complex features of the stress–strain responses of an AZ31B magnesium alloy sheet under non-proportional loading paths, i.e., asymmetric hardening behavior under tension and compression, the sigmoidal nature of the hardening curve during monotonic compression and compression followed by tension.

Deformation Behavior of Magnesium Alloys under Multi-axial Stress States

Deformation Behavior of Magnesium Alloys under Multi-axial Stress States

Material design for magnesium alloys with high deformability

Magnesium alloys are very promising structural material, especially because of their low density, but their poor deformability at room temperature is making their commercial application impractical. Some alloying elements have been shown to improve the ductility dramatically, and a search for the yet better alloying elements is now a pressing task. In this study, we investigated the effects of several alloying elements on the mechanical and deformation behaviour of magnesium alloys using both first-principles calculations as well as experiments. The first-principles calculations indicate that the influential key factor for the activation of non-basal slips is the electronegativity and atomic radius. The experimental result proves that the alloys with a similar electronegativity as magnesium and a little larger atomic radius than that of magnesium, such as Ca, Sr and some rare earth elements, show superior ductility due to activation of non-basal dislocation slips. These results propose a promising design principle for the alloys with the improved deformability at room temperature ranges.

Modelling the role of slips and twins in magnesium alloys under cyclic shear

Magnesium alloys under cyclic shear show different deformation behaviour from that under cyclic tension–compression, for example, symmetry vs. asymmetry of stress–strain curves. Using the Twinning–DeTwinning (TDT) model, we studied the role of different deformation mechanisms, slip systems, twinning and detwinning, in shear deformation behaviour of magnesium alloys. The results indicate that the nearly symmetrical shear stress-shear strain curve unlike the asymmetrical stress–strain curve under cyclic tension–compression is ascribed to the dominated prismatic slips and the simultaneous activation of deformation twinning and detwinning under reverse shear. The typical shear texture observed in experiments is ascribed to the activation of extension twin that induces the extension strain along normal direction of the shear plane under simple shear.

3D finite element modeling of grains refinement for magnesium alloys by extrusion-shear and experimental verification

To explore the deformation mechanisms of a new composite extrusion including extrusion and successive shear subsequently which is shorten ES, Three dimensional finite element modeling of grain refinements for magnesium alloys by ES process has been researched. The ES die have been designed and manufactured and installed to the horizontal extruder. Finite element software DEFORMTM-3D to investigate the plastic deformation behaviors of magnesium alloy during extrusion-shear has been employed. The extrusion loads and temperatures distribution of billets and maximum extrusion forces have been obtained from simulation results. From the simulation results it is clear that evolutions of extrusion loads curve and effective stresses and temperatures can be divided into three stages. ES process has been applied to fabricate AZ31 magnesium alloy rod at preheat temperature of 420oC with extrusion speed of 20 mm/s. The results proved that the ES process is a formality method for magnesium suitable for large scale industrial application. The microstructures of AZ31 magnesium alloy along the longitudinal section of rods have been sampled and examined and observed. Fine grained microstructures can be observed throughout longitudinal section of extruded rod. The researches results show that ES process would cause severe plastic deformation and improve the dynamic recrystallization of AZ31 magnesium alloy. The simulation results and calculated Zener-Hollomon parameters showed that the grains of magnesium would be refined gradually during ES process.

Yield asymmetry design of magnesium alloys by integrated computational materials engineering

Deformation asymmetry of magnesium alloys is an important factor on machine design in the automobile industry. Represented by the ratio of compressive yield stress (CYS) against tensile yield stress (TYS), deformation asymmetry is strongly related to texture and grain size. A polycrystalline viscoplasticity model, modified intermediate ϕ-model, is used to predict the deformation behavior of magnesium alloys with different grain sizes. Validated with experimental results, integrated computational materials engineering is applied to find out the route in achieving desired asymmetry via thermomechanical processing. For example, CYS/TYS in rolled texture is smaller than 1 under different loading directions. In other textures, such as extruded texture, CYS/TYS is large along the normal direction. Starting from rolled texture, asymmetry will increase to close to 1 along the rolling direction after being compressed to a strain of 0.2. Our modified ϕ-model also shows that grain refinement increases CYS/TYS. Along with texture control, grain refinement also can optimize the yield asymmetry. After the grain size decreases to a critical value, CYS/TYS reaches to 1 because CYS increases much faster than TYS. By tailoring the microstructure using texture control and grain refinement, it is achievable to optimize yield asymmetry in wrought magnesium alloys.

Thickness of Magnesium Alloy Cylindrical Cup in Pressure-Lubricating Deep Drawing

Forming process of AZ31B magnesium alloy cup parts in pressure-lubricating deep drawing was simulated by Dynaform at room temperature. The technological parameters which influence the wall thickness difference of cup parts were investigated in this paper, including internal pressure, blank holder force and punch corner radius, etc. Compared with the deformation behaviors of magnesium alloy in mechanical deep drawing and pressure-lubricating deep drawing, the wall thickness distribution of cup parts was discussed. The result shows that preferable deformation behaviors can be obtained in pressure-lubricating deep drawing when adopted adaptive technological parameters.

Extrusion of AZ31 Magnesium Sheet

Due to the increasing demand of deep drawing applications for magnesium alloys in the future magnesium sheets with good mechanical and forming properties are required. These properties depend on the processing route of the sheet material. The deformation behavior of magnesium alloys is strongly influenced by the texture. Extruded magnesium sheets exhibit a different texture than rolled magnesium sheets. Therefore, the forming properties of the extruded magnesium sheets are supposed to be different compared to rolled sheets. Thin extrusion of the magnesium alloy AZ31 with a thickness of 1.5 and 2 mm were performed. Adjacent the extruded sheets were tested for their microstructure, texture and mechanical properties. The texture stability and evolution after the rolling of extruded magnesium sheets were investigated. Thus some of the 1.5 mm sheets were rolled to 1.0 mm and analyzed by OIM, X-Ray and mechanical testing. Concluding the results were compared to the properties of the just extruded 1.5 mm sheet and conventionally rolled sheet of 1 mm thickness.

On the Fatigue Behaviour of Wrought Magnesium Alloys*

Abstract The influence of crystallographic texture on the mechanical properties of magnesium alloys is already well-known. However, it is not clear how the crystallographic texture affects the fatigue properties in the sense of cyclic deformation behaviour of magnesium alloys. The objective of this study is to investigate and compare the deformation processes of the extruded magnesium alloys AZ31 and AZ80 during static and cyclic tensile testing. The influence of the loading condition and chemical composition on the evolution of the microstructure and the crystallographic texture has been observed. Changes of the plastic strain have been observed and have been interpreted for both AZ alloys with cyclic hardening curves. However, the maximal stress during the fatigue test was higher than the TYS for both alloys. Both experimental materials exhibited significant lower plastic deformations and lower density of tensile twins compared to static tensile test. The microstructure and texture analyses have been performed before and after experiments.

The Effect of Grain Size on the Deformation Behaviour of Selected Mg Alloys

The mechanical properties and deformation behaviour of magnesium alloys are significantly influenced by the testing temperature, texture and grain size. The paper gives an overview on the influence of the grain size on the mechanical properties of Mg and Mg-2Al alloys at various temperatures. The yield stress and the fracture stress increase with decreasing grain size. It was shown that the elongation to fracture increases with inverse square root of grain size. The effect of grain size on the tensile strength and the elongation to fracture is influenced by the testing temperature. The observed grain size dependence of the elongation to fracture is explained by the activity of non-basal slip systems.

Cover Picture: The Effect of Grain Size on the Deformation Behaviour of Magnesium Alloys Investigated by the Acoustic Emission Technique (Adv. Eng. Mater. 5/2006)

AbstractThe cover shows Micrographs from longitudinal sections of extruded bars of ZEK100, extrusion direction horizontal. More information can be found in the paper by J. Bohlen et al. on page 422 in this issue.

The Effect of Grain Size on the Deformation Behaviour of Magnesium Alloys Investigated by the Acoustic Emission Technique

Extruded round bars from magnesium alloys ZM21, ZK30, ZE10 and ZEK100 with various grain sizes were tested in tension and compression. The effect of grain size on the deformation behaviour of the alloys was investigated through the Hall‐Petch relation. In‐situ acoustic emission measurements were conducted during testing to evaluate the mechanisms of plastic deformation. The results are discussed with respect to twinning as one important deformation mechanism in magnesium alloys.