Tensile Twinning Research Articles

Saturation of Deformation Twinning in Magnesium Alloys

The saturation of primary tensile twins in heavily textured Mg-alloy AZ31 is investigated, and their strain accommodation limit is evaluated. EBSD results suggest that the mean number of twins per grain saturate rapidly, followed by the stop of twin growth. Twinning saturation is included in a physical model of twin evolution.

Numerical analysis of twin thickening process in magnesium alloys

A finite element study of the stress field evolution around the {101¯2} tensile twin of different thickness in magnesium alloy is performed. The system is represented by the 2D models of one and three elastic elliptical inclusions with different aspect ratios surrounded by the plastic matrix. Anisotropic elasticity and crystal plasticity theory are used to describe the material behavior. Numerical results for single inclusion overestimate the initial stress for the stable twin aspect ratio by a factor of two compared to the experimental results. Multiple inclusions case obtains values closer to the experimental ones. Therefore it is concluded that the process of twin thickening cannot be solved within the analysis of the single inclusion and an interaction between twins in the same and neighboring grains plays a crucial role during the twin thickening process.

A measure of plastic anisotropy for hexagonal close packed metals: Application to alloying effects on the formability of Mg

Mg is inherently plastically anisotropic and, over the years, alloying development efforts have sought to reduce the plastic anisotropy in order to enhance formability. To understand the relationship between alloy type and plastic anisotropy, we use a visco-plastic self-consistent (VPSC) polycrystal plasticity model to relate the macroscopic constitutive response to the underlying slip and twinning mechanisms in pure Mg and several Mg alloys. In the calculations, the influence of alloy type is represented by the differences in the CRSS values among the basal, prismatic, pyramidal slip and tensile twin systems. We show that for the same initial texture, this microscopic-level CRSS anisotropy can have a significant effect on the macroscopic indicators of formability, namely the anisotropy of the post-deformation polycrystal yield surface, tension-compression yield asymmetry, and Lankford coefficients. A plastic anisotropy (PA) measure is formulated to quantify the degree of single crystal plastic anisotropy acquired by the dissimilarities in the CRSS values of the slip and twinning modes for a given alloy. We demonstrate a strong correlation between the PA measure with the formability indicators mentioned above for multiple initial textures commonly enountered in processing. We find that alloys can be classified into two groups, those with a PA value below 2, which are more formable, less twinnable, and less sensitive to initial texture, where PA ∼2 for pure Mg, and those with a PA value above 2, which possess the opposite deformation response.

On the rapid hardening and exhaustion of twinning in magnesium alloy

We numerically study the rapid hardening and possible role of exhaustion of twinning in magnesium alloys under twinning dominated conditions, based on the large strain elastic visco-plastic self-consistent model. It is found that upon the exhaustion of the tensile twinning, further deformation requires the material to activate Prismatic and Pyramidal slips, which have relative high critical resolved shear stress (CRSS). However, the stress level at the exhaustion of the tensile twinning is not high enough to activate Prismatic and Pyramidal slips. Consequently, the imposed strain increments must mainly be accommodated by elastic deformations, and the strain hardening rate becomes a large fraction of the elastic modulus. Therefore, it can be concluded that the rapid hardening is a composite response more associated with elasticity of a large volume fraction of the material, than the dislocation-dislocation interactions normally held responsible for strain hardening of metals.

The Microstructure, Texture and Mechanical Properties of the Rolled Al/Mg/Al Clad Sheets

Two types of three-layered Al/Mg/Al clad sheets were fabricated by hot rolling. The first (sheet A) underwent a single pass with a small rolling reduction of 33% and the second (sheet B) underwent four passes with a large rolling reduction of 71%, and both were subsequently annealed at 200 °C for 1 h. Microstructural examination and tensile tests on the fabricated sheets revealed that 17.8-μm-thick intermetallic compound layers (IMCLs) appeared at AZ31/5052 interfaces in sheet B while none were observed in sheet A. The AZ31 layers in sheets A and B exhibited basal textures with intensities of 15.1 and 9.8, respectively, and only sheet A exhibited tensile twins (TTs) in the AZ31 layer. Recrystallization resulting in grains was preferred near the AZ31/5052 interface and the intersections between TTs. Owing to its larger rolling reduction, more extensive recrystallization was observed in the sheet B component layers than in sheet A. Sheet B exhibited better mechanical properties with a much higher ultimate tensile strength (UTS) than sheet A (230 versus 102 MPa) and a slightly larger elongation (19 versus 17%).This indicates that texture intensities and the extent of recrystallization of component layers have a significant effect upon the mechanical properties of clad sheets.

The Texture Effect on the Dynamic Fracture Properties of Magnesium Alloy AZ31B

In this work, Mode I dynamic fracture experiments are conducted on pre-cracked three point bending specimens by using modified split-Hopkinson pressure bar. Two sets of specimens with different initial textures are considered here: one set of the specimens are machined from a hot rolled AZ31B Mg alloy plate with a bigger grain size. The others are treated by four pass of equal channel angular pressing (ECAP) after they are cut from the initial material. They are with the finer grain size. Digital image correlation (DIC) technique is used to determine the strain contours around the crack tip and electron back scatter diffraction (EBSD) is employed to analyze the texture evolution after tests. It is found that the dynamic fracture toughness of finer grain specimen is higher than that of coarse grain specimen. The fracture toughness of both sets of specimens is enhanced by increasing the loading rates. Texture analysis shows the formation of tensile twinning in the ligament ahead of the crack tip in the coarse grain specimen but no sign in fine grain specimen. The brittle features e. g. cleavage planes and twinning lamellas are observed on the fracture surface of coarse grain specimen by scanning electron microscope (SEM). However, the relative ductile features such as micro-voids surrounding by tear ridges present on the fracture surface of fine grain specimen.

Modeling of twin formation, propagation and growth in a Mg single crystal based on crystal plasticity finite element method

A crystal plasticity finite element method (CPFEM) with a stress relaxation effect for twinning in the constitutive description is employed to simulate twin formation, propagation and growth in Mg single crystals subjected to plane-strain compression along the direction perpendicular to the c-axis of its HCP lattice. Tensile twinning dominates the early stage of plastic deformation and numerical results explicitly show the twinning process including twin formation, propagation and growth. After a twin band (or a narrow twin) is formed, a large gradient of effective stress is found across the band front. Also, the resolved shear stress (RSS) on the twin plane along the twin direction is highly non-homogeneous across the band front. It has been demonstrated that softening facilitates the formation of a twin band and work hardening is contributive to the growth of the twin band. In addition, numerical results indicate that softening plays an important role in the propagation of a twin across a grain boundary with a large misorientation.

Deep drawing of ZK60 magnesium sheets fabricated using ingot and twin-roll casting methods

Deep drawing was carried out on two different ZK60 (Mg-Zn-Zr) magnesium sheets fabricated via ingot and twin-roll casting methods. The effects of the microstructure and texture on the deep drawability and the associated microstructural evolution of ZK60 alloys were examined. Twin-roll casted sheets with refined particles were stronger and showed greater elongation than ingot-casted sheets. Moreover, they were fully deep-drawn at lower temperatures than ingot-casted sheets, and the deep drawability of the ingot-casted sheets was better at a high deformation rate compared to that of the twin-roll casted sheets. During the deep drawing process, the ingot-casted sheets with large grains experienced more tensile twinning than the twin-roll casted sheets, and shear bands were observed. The twin-roll casted sheets revealed peak broadening of the basal texture along the TD and peak splitting along the RD. Thickness variations of the drawn cups showed that the twin-roll casted sheets resisted wall thinning more effectively than the ingot-casted sheets.

Crystal plasticity analysis of plane strain deformation behavior and texture evolution for pure magnesium

A viscoplastic self-consistent model is implemented to study the plane strain deformation behavior and texture evolution of pure magnesium single crystals and polycrystals. The numerical simulations of single crystals are carried out based on seven channel-die compression tests. Due to the different deformation mechanisms of tensile twinning and compressive twinning, the hardenings caused by the twins are distinguished to better describe their effects on the whole deformation. The uniform parameters are calibrated with the channel-die experimental data in single crystal cases and then used to predict the polycrystalline deformation. The polycrystalline aggregates with 100 and 300 random orientations are developed for their plastic deformation. Five various self-consistent schemes are employed to investigate the influence of their linear hypotheses on stress–strain responses. A trend of hardening rate from weak to intense is shown for the five self-consistent schemes: Tangent, neff=10, Affine, Secant and Taylor. The same trend is also obtained in their corresponding texture evolutions and yield surfaces. At low plastic strains, a high asymmetry and a strong anisotropy between compressive and tensile yield strengths are exhibited by the yield surfaces.

Texture Evolution of Single-Pass Hot-Rolled 5052/AZ31/5052 Clad Sheets

Three-layered 5052/AZ31/5052 clad sheets with maximum rolling reductions of 33% and 48% were prepared, using single-pass hot rolling followed by thermal annealing at 200°C for 1 h. The evolutions of microstructures and textures were analyzed. The experimental results show that the AZ31 layer exhibited a typical deformation microstructure with rolling-induced twins. The AZ31 layer with the 33% rolling reduction possessed a texture with the basal pole tilting about 35° away from normal direction to transverse direction and the majority of twins consists of {10\( \bar{1} \)1}–{10\( \bar{1} \)2} double twins and {10\( \bar{1} \)2} tensile twins. The AZ31 layer with the 48% rolling reduction possessed a typical basal texture because {10\( \bar{1} \)1} compression twins were activated by c-axis strain to compete with the tensile twins. No intermetallics were observed after annealing, and recrystallization occurred preferentially at the interface between AZ31 and 5052. The typical rolling texture of the 5052 layer disappeared, and the stable {001} 〈110〉 rotation cube component was dominant. The tensile test of the rolled three-layered 5052/AZ31/5052 clad sheets was performed. The tensile experimental results show that the annealed clad sheets with 33% rolling reduction and smaller degree of recrystallization have the largest elongation of 22.5% and larger ultimate tensile strength (UTS) than the annealed clad sheets with 48% rolling reduction.

Deformation mode transition of Mg[sbnd]3Li alloy: An in situ neutron diffraction study

Deformation mode transition of Mg3Li alloy was investigated using in situ neutron diffraction along with texture characterization. Lattice strain and texture evolution were used as an indicator to tailor the deformation mode under uniaxial tension. It is shown that basal slip, tensile twinning and prismatic slip contribute to the plastic deformation at different stages. A clear hysteresis loop at plastic strain of about 1% is observed during loading/unloading, which is ascribed to the anelasticity resulting from {101¯2} twins upon unloading. Load redistribution continuously occurs between soft and hard grain orientations and the prismatic slip is dominated around the yield point.

Crystal plasticity modeling of damage accumulation in dissimilar Mg alloy bi-crystals under high-cycle fatigue

Damage accumulation in Mg AZ31–AZ80 alloy bi-crystals under fatigue loading at room temperature is studied using a modified version of the crystal plasticity finite element model of Abdolvand and Daymond. The model accounts for strain accommodation by both slip and tensile twinning, and is first shown to reasonably describe monotonic single crystal Mg experimental data from the literature. The high cycle fatigue behavior was then investigated in misoriented dissimilar alloy bi-crystals through stress-controlled simulations up to 1000cycles. Nine different orientation combinations were simulated and the fatigue damage evolution, defined as the cumulative shear strain amplitude, were compared and analyzed. The bi-crystal geometry was used to simulate possible microstructure combinations occurring, for instance within an idealized friction stir weld. Findings suggest that when either of the alloy bi-crystal grains is oriented for basal slip, poor fatigue performance can occur by twinning or slip localization depending upon the neighboring orientation.

Influence of pre-strain on the aging hardening effect of the Mg-9.02Zn-1.68Y alloy

The solid-solution state Mg-7.68Zn-1.68Y alloy was pre-compressed before the isothermal aging treatment at 200°C, then the pre-induced twinning and its influence on the aging microstructure and aging hardening effect were investigated using EBSD, TEM and SEM. Results showed that: about 15% (area fraction) lenticular (101¯2) tensile twinning were induced into the matrix when the alloy was compressed by 3% pre-strain; and when the amount of pre-strain increased, area fraction of twinning would increase, average size of twinning would decrease, morphology and structure of twinning would change: when the pre-strain reached 15%, about 28% (area fraction) of narrow double twinning (101¯1)-(101¯2)would be induced, while the anamorphosis of double twinning were mostly inter-crossing. In addition, twinning could become an effective substrate for nucleation. And different from precipitates in the matrix, which is rod-shaped β′1-MgZn2 growing along a certain direction, precipitates in the twin boundaries were granular icosahedral quasi-crystal phase with 5-times rotational symmetric diffraction pattern. And when the pre-strain was in the range of 3–15%, aging hardening effect increased with pre-strain; when pre-strain were 10–15%, the peak hardness arrived 110HV, 28% improved compared to that of the not-pre-deformed one (86HV); when the pre-strain exceeded 20%, the aging hardening effect would decrease.

Characterization and modeling of mechanical behavior of single crystal titanium deformed by split-Hopkinson pressure bar

Single crystal titanium samples were dynamically loaded using split-Hopkinson pressure bar (SHPB) and the resulting microstructures were examined. Characterization of the twins and dislocations present in the microstructure was conducted to understand the pathway for observed mechanical behavior. Electron backscatter diffraction (EBSD) was used to measure textures and quantify twinning. Microstructures were profusely twinned after loading, and twin variants and corresponding textures were different as a function of initial orientation. Focused ion beam (FIB) foils were created to analyze dislocation content using transmission electron microscopy (TEM). Large amounts of dislocations were present, indicating that plasticity was achieved through slip and twinning together. Viscoplastic self-consistent (VPSC) modeling was used to confirm the complex order of operations during deformation. The activation of different mechanisms was highly dependent upon crystal orientation. For [0001] and [101¯1]-oriented crystals, compressive twinning was observed, followed by secondary tensile twinning. Dislocations, though prevalent in the microstructure, contributed to final texture far less than twinning.

Effect of rolling speed on microstructure and mechanical properties of as-cast AZ31B alloy under different reduction schedules

AZ31B alloy was severely rolled at speeds between 18 and 102m/min by two different reduction schedules, respectively. The recrystallization behaviors, rolling feasibility, temperature distribution, texture and mechanical properties were examined. The {101¯2} tensile twinning and subsequent secondary twinning and continuous dynamic recrystallization dominated the deformation. For 4-pass rolling, as the speed increased, the twin density gradually decreased and the grain size slightly rose due to the suppressed temperature reduction at higher speed. However the effective grain size sharply increased and the recrystallization fraction dropped with increased rolling speed for 2-pass rolling. Full recrystallization mainly occurred in grains with beneficial orientations for {101¯2} twinning, rather than dislocation, during 2-pass rolling at speeds higher than 72m/min. The rolling feasibility, texture and mechanical properties were largely affected by rolling speed. However, their trends were significantly different, sometimes even on the contrary, under different reduction schedules.

Twinning-mediated formability in Mg alloys.

Mg alloys are promising candidates for automotive applications due to their low density and high specific strength. However, their widespread applications have not been realized mainly because of poor formability at room temperature, arising from limited number of active deformation systems and strong basal texture. It has been recently shown that Mg-Zn-Ca alloys have excellent stretch formability, which has been ascribed to their weak basal texture. However, the distribution of basal poles is orthotropic, which might result in anisotropy during deformation and have adverse effect on formability. Here, we show that tension twinning is mainly responsible for enhanced formability of Mg-Zn-Ca alloys. We found that tension twinning is quite active during both uniaxial deformation and biaxial deformation of Mg-Zn-Ca alloy even under the stress conditions unfavourable for the formation of tensile twins. Our results provide new insights into the development of Mg alloys having high formability.

Theoretical study on the [formula omitted] deformation twinning and cracking in coarse-grained magnesium alloys

This paper theoretically and systematically investigates: (1) the effect of local transformed strains within deformation twinning on twin intersection; (2) the fracture mode based on type I co-zone tensile twin intersection in coarse-grained magnesium alloys, as well as the impacts of twin intersection and grain diameter on interfacial crack nucleation along twin boundaries; and (3) the influence of the local stresses arising from the encountered twin bands on crack growth. A novel dislocation-based strain nucleus model and a Green's function method, which are applicable to any material with local transformations in which elastic properties are reasonably approximated as isotropic, are specifically employed to model the concentrated transformed strain and calculate the local stress field resulting from deformation twinning and the stress intensity factors at crack tips in the magnesium alloys, respectively. In addition, an electron backscatter diffraction (EBSD) measurement is provided for crack nucleation originating from Type I co-zone tensile twin intersection. The theoretical modeling indicates: (i) the local strains within barrier twins strongly dictate the growth of incident twins and enhance the twin propagation stress; (ii) larger grains favor brittle fracture. More specifically, the dislocation reactions and pile-ups at the junctions between tensile twins can result in interfacial crack nucleation and growth along the twin boundaries, which is a brittle fracture mode based on lower twinning stress and stress concentration in the coarse-grained magnesium alloys; and (iii) the direction of crack propagation is easily changed by high-density twin bands and twin intersections owing to the local strains.

Deformation mechanisms of pure Mg materials fabricated by using pre-rolled powders

In the present work, a powder rolling process was utilized in the fabrication of fine grained pure Mg via powder metallurgy. Mg flakes were obtained after each rolling process, and broken into pieces for further rolling or sintering. Mg samples of experiencing 0, 5 and 10 rolling passes were obtained following spark plasma sintering (SPS) and hot extrusion. Microstructural results from electron backscatter diffraction (EBSD) revealed that, without experiencing powder rolling, the specimen contained a great number of residual tensile twins; in contrast, after powder rolling the specimen showed uniform and equiaxed grain structures. In addition, the average grain size was measured to be around 9.2, 2.9 and 2.1µm for the samples subjected to 0, 5 and 10 rolling passes. The powder rolled specimens were found superior in mechanical properties. Post-loading microstructure examinations were also performed for the samples and a discussion regarding the relationship between their mechanical behavior and microstructures was provided.

Analysis of the microstructure and deformation mechanisms by compression along normal direction in a rolled AZ31 magnesium alloy

The twinning and slipping modes of a rolled AZ31 magnesium alloy sheet were investigated through in situ compressive tests. A method based on scanning electron microscopy and electron backscattered diffraction measurements was used to identify activated twinning and slip systems. {10-12} tensile twins were present, but only occasionally. Contraction twins were not observed. Post deformation examination revealed the dominance of pyramidal <c+a> slip.

Microstructure evolution of Mg–11Gd–4.5Y–1Nd–1.5Zn–0.5Zr (wt%) alloy during deformation and its effect on strengthening

Microstructure and texture evolutions during tensile and compression deformation of an as-extruded Mg–11Gd–4.5Y–1Nd–1.5Zn–0.5Zr (wt%) alloy have been investigated using in-situ synchrotron radiation diffraction and subsequent microscopy. The alloy consists of 〈101¯0〉 fiber texture, {112¯0}[0001] and {112¯0}〈101¯0〉 texture components prior to deformation. The texture evolves from [0001] to 〈101¯0〉 in tension, but from 〈101¯0〉 to [0001] in compression. The evolution of texture is attributed to the activity of the tensile twinning and non-basal 〈a〉 type slip. The tendency of texture evolution depends on the favorable texture component for the activation of above deformation modes. The grain refinement, Mg5(Gd, Y, Nd) and LPSO phases, and the texture contribute to the improvement in strength.