Query: Magnesium twinning

title: Role of {101 2} twinning in the anisotropy and asymmetry of AZ31 magnesium alloy under high strain rate deformation
abstract: In order to investigate the role of 10 1 2 extension twinning in anisotropy and asymmetry of highly textured magnesium alloy under high strain rate deformation, tensile and compression deformation were conducted by Split Hopkinson Tension and Pressure Bar under the strain rate of 1800s 1. With the help of theoretical calculated Schmid Factors (SFs) the dynamic tensile and compression behavior and deformation mechanism of extruded AZ31 magnesium alloy along extrusion direction (ED) and transverse direction (TD) were analyzed in the present paper. The results demonstrated that extruded AZ31 magnesium alloy shows obvious anisotropic and asymmetric behaviors under high strain rate deformation and the anisotropic and asymmetric behaviors are arisen from the different role of 10 1 2 extension twinning. The Schmid Factor calculation results of six extension twinning variants revealed that the six variants have the same opportunity to activate under tensile loading along TD, while for compression along ED, only two variants have the opportunity of operating due to the highest SF among the six variants. When high strain rate tensile loading was applied along ED and compression loading along TD, 10 1 2 extension twinning is not favored.

title: Effect of pre straining on twinning, texture and mechanical behavior of magnesium alloys A review
abstract: 1 1 extension twin and their variants in magnesium alloys is one of the cost effective practical approaches for re assigning the crystallographic axes distributions in wrought magnesium alloys. The twin lamellae and variants can significantly refine the grain size through twin boundaries. Besides, the synergetic effect of twin boundaries, twining morphology, twinning variants, and textural changes can considerably increase the formability, yielding behavior, and anisotropic mechanical behavior of the magnesium alloys. Therefore, this review article emphasized the different pre straining techniques and the underlying mechanisms which were responsible for the changes in the crystallographic texture, enhancing the mechanical strength and exacerbation of anisotropic mechanical behavior. In the end, some scientific problems have been proposed for the further development of the magnesium alloys.

title: Multi scale mechanisms of twinning detwinning in magnesium alloy AZ31B simulated by crystal plasticity modeling and validated via in situ synchrotron XRD and in situ SEM EBSD
abstract: We present a combined experimental and numerical study that provides the understanding of deformation mechanisms and stresses in Mg AZ31B alloy across scales, from macro scale (Type I) to micro (inter granular, Type II) and nano scale (intra granular, Type III) The combination of in situ synchrotron X ray diffraction (XRD) in situ electron backscattered diffraction (EBSD) and crystal plasticity finite element (CPFE) modeling of crystal slip and twinning/detwinning was employed. The crystal rotation observed directly in the XRD and EBSD experiments revealed the onset and completion of the twinning/detwinning processes during in situ cyclic compression tension loading. It also allowed reliable calibration of the key model parameters, in particular critical resolved shear stress (CRSS) of detwinning. The validation of the model was performed at distinct different scales corresponding to all stress types. Direct comparison with the data from the loading device provided the confirmation of the model validity in terms of correct description of the macroscopic stress strain response (Type I stresses) The calibration led to the CRSS detwinning value of 23 MPa, and twinning of 46.5 MPa. At the inter granular micro scale (Type II stresses) the model satisfactorily predicted the transition between different plastic deformation modes (slip, twinning and detwinning) as confirmed by the comparison with peak intensities in XRD experiments. For the intra granular (nano scale) Type III stresses, it was concluded that the model was also valid at the level of statistical description (rather than local behavior) Namely, it has not been possible to predict correctly the real morphology of the twins observed in EBSD experiments.

title: Mechanical response, twinning, and texture evolution of WE43 magnesium rare earth alloy as a function of strain rate: Experiments and multi level crystal plasticity modeling
abstract: This work adapts a recently developed multi level constitutive model for polycrystalline metals that deform by a combination of elasticity, crystallographic slip, and deformation twinning to interpret the deformation behavior of alloy WE43 as a function of strain rate. The model involves a two level homogenization scheme. First, to relate the grain level to the level of a polycrystalline aggregate, a Taylor type model is used. Second, to relate the aggregate level response at each finite element (FE) integration point to the macro level, an implicit FE approach is employed. The model features a dislocation based hardening law governing the activation stress at the slip and twin system level, taking into account the effects of temperature and strain rate through thermally activated recovery, dislocation debris formation, and slip twin interactions. The twinning model employs a composite grain approach for multiple twin variants and considers double twinning. The alloy is tested in simple compression and tension at a quasi static deformation rate and in compression under high strain rates at room temperature. Microstructure evolution of the alloy is characterized using electron backscattered diffraction and neutron diffraction. Taking the measured initial texture as inputs, it is shown that the model successfully captures mechanical responses, twinning, and texture evolution using a single set of hardening parameters, which are associated with the thermally activated rate law for dislocation density across strain rates. The model internally adjusts relative amounts of active deformation modes based on evolution of slip and twin resistances during the imposed loadings to predict the deformation characteristics. We observe that WE43 exhibits much higher strain hardening rates under high strain rate deformation than under quasi static deformation. The observation is rationalized as primarily originating from the pronounced activation of twins and especially contraction and double twins during high strain rate deformation. These twins are effective in strain hardening of the alloy through the texture and barrier hardening effects.

title: Numerical investigation of the influence of twinning/detwinning on fatigue crack initiation in AZ31 magnesium alloy
abstract: An analysis of the fatigue behavior of rolled AZ31 magnesium alloy through numerical simulations of polycrystalline aggregates using the crystal plasticity finite element method is presented. A phenomenological pseudo slip model including twinning, detwinning and subsequent slip in the twinned region was considered in this study. It was calibrated against uniaxial monotonic and cyclic experiments on rolled AZ31 Mg alloy. Multiple periodic two dimensional synthetic microstructures were generated based on morphological and crystallographic data obtained from electron backscattering diffraction (EBSD) measurements. Fatigue simulations were conducted at different stress amplitudes and mesoscopic non local fatigue criteria for slip induced and twin induced crack initiation were investigated. It was observed that basal slip and twinning detwinning are the main deformation mechanisms taking place during cyclic loading but that these deformation modes generally occur in different grains. The comparison of the investigated criteria with experimental data also revealed that the twin induced criterion exhibit better trends in terms of mean behavior, relative dispersion and stress amplitude sensitivity compared with the slip induced criterion. Finally, the predictive capabilities and limitations of the current modeling approach are discussed.

title: The Effect of Precipitates on Twinning in Magnesium Alloys

title: Unveiling the formation of basal texture variations based on twinning and dynamic recrystallization in AZ31 magnesium alloy during extrusion
abstract: Commercial Mg extrusions usually develop various types of basal textures, affecting the formability and mechanical properties. The mechanisms for the formation of basal texture variations were systematically investigated for the first time based on the twinning and dynamic recrystallization (DRX) during extrusion using electron back scatter diffraction (EBSD) characterization. The results indicate that {10 1 2} extension twins with various variants played a key role in the formation of [10 1 0] fiber texture, while twin induced DRX mechanism associated with {10 1 1} twins made a limited contribution to overall texture evolution. The microstructure developed consecutively as a result of continuous DRX (CDRX) and discontinuous DRX (DDRX) after twinning with different nucleation of new orientations. In the unDRXed region, new DRXed grains with 30deg [0001] GBs preferentially nucleated via CDRX, forming preferred selection of [2 1 1 0] fiber orientation. Consequently, [10 1 0] fiber texture became weakened with [2 1 1 0] fiber component strengthening, progressively leading to the development of [10 1 0] [2 1 1 0] double fiber texture. In the DRXed region, DDRX further occurred along the serrated grain boundaries and at the triple junctions of the parent grains by bulging, forming fresh DRXed grains without preferred orientation selection. Thus, the [10 1 0] [2 1 1 0] double fiber texture became randomized and gradually transformed into non fiber texture at the late stage of extrusion. Besides, typical nucleation sites for DRX and subsequent grain growth were also discussed in this study. These findings unveil that basal texture variations are attributed to the twinning and DRX mechanisms during extrusion, which leads to new insight to design new wrought Mg alloys with high performance by grain refinement and texture modification.

title: On the role of twinning and stacking faults on the crystal plasticity and grain refinement in magnesium alloys
abstract: We have investigated the detailed microstructural mechanisms associated with the excellent crystal plasticity and ultra fine grain refinement observed under high strain rate deformation of Mg alloys, focusing on ZK60. Firstly, we have identified the clear formation of stacking faults in deformation induced twinned crystal segments. Specifically, we have found that intrinsic I1 and I2 stacking faults bounded by 1 6 2 023> and 1 3 1 0> partial dislocations, respectively, were found to occur in very high number densities within the twins. This was due to the high Schmid factor for stacking fault shearing in twins and the critical role that twin boundaries played in emitting partial dislocations. Secondly, we have clarified the interplay between twinning and stacking faults on the enhanced crystal plasticity. Apart from the strain accommodated by the extensive twinning itself, we propose that the improved plasticity during high strain rate deformation is mainly due to the nucleation of 1 3 11 23>{11 2 2} dislocation within twins, which provides enough independent slip systems to achieve a homogeneous deformation in the material. Finally, we have demonstrated the interplay between twinning and stacking fault formation on the nucleation of new grains via dynamic recrystallisation. The twin boundaries and stacking faults, especially those of the I1 type, facilitate the formation of low angle grain boundaries that can subsequently transition into high angle grain boundaries, and form ultra fine dynamically recrystallised grains.

title: Precipitation hardening effects on extension twinning in magnesium alloys
abstract: Precipitation is an efficient method to strengthen metallic materials. While precipitation hardening effects on dislocation slip have been studied extensively in the past, the influence of precipitates on twinning mediated plasticity and the development of corresponding hardening models that account for twin precipitate interactions have received less attention. Here, the interaction of {10 12} extension twin boundaries (TBs) in pure magnesium with precipitates of plate sphere and rod like shapes is studied using molecular dynamics (MD) simulations. We find that TBs that engulf precipitates are absorbed by the precipitate matrix interfaces, and the precipitates are neither twinned nor sheared but deform elastically leading to their rotation. TBs can pass small precipitates (length 20 nm) and remain intact. In contrast when TBs are interacting with large precipitates (length 50 nm) basal dislocations or stacking faults nucleate from the interfaces, causing local plastic relaxation. The stress field around a plate like precipitate as calculated in the MD simulations suggests that a strong back stress is imposed on the TBs. We then coarse grain these mechanisms into an analytical mean field model of precipitation hardening on twinning in magnesium alloys, which is based on the energy conservation during the TB precipitate interaction. The model is in good agreement with the current MD simulations and published experimental observations. The hardening model shows that spherical precipitates have the strongest hardening effect on twinning, basal and prismatic plate like precipitates have a medium effect while rod like precipitates exert the weakest influence. We also find that most types of precipitates show a stronger hardening effect on twinning mediated plasticity than on basal dislocation slip. Finally, prismatic plate like precipitates are predicted to have reasonable hardening effects on both twinning and basal slip. These results can help guiding the development of magnesium alloys with enhanced strength and ductility.

title: Twinning, dynamic recrystallization, and crack in AZ31 magnesium alloy during high strain rate plane strain compression across a wide temperature
abstract: Plane strain compression (PSC) tests were carried out on as cast AZ31 magnesium alloy samples at temperatures from 250 to 400 degC with different strain rates 10 s 1) The microstructures along the crack boundaries and secondary cracks were detected. The electron backscatter diffraction (EBSD) results indicated that twin induced dynamic recrystallization (DRX) The flow curves were analyzed using the double differentiation method and two or three minima were detected. The first set of minima is shown to identify the critical strain for twinning, while the second set indicates the critical strain for the initiation of DRX or micro cracks, and the third set is related to the onset of micro cracks. Twinning affects the nucleation and propagation of crack. The crack in the matrix grain is inhibited due to the preferred initiation of twinning to impede competitive crack extension. With increasing temperature, the competitive relationship between crack and DRX exists on the twin boundaries. The nucleation, growth and coalescence of voids on the grain boundaries and at grain triple junctions contributes to the formation of cracks in the fine grained area.