Residual Twins Research Articles

Twinning–Detwinning-Dominated Cyclic Deformation Behavior of a High-Strength Mg-Al-Sn-Zn Alloy during Loading Reversals: Experiment and Modeling

The deformation behavior of a high-strength Mg-Al-Sn-Zn alloy under loading reversals has been thoroughly examined through a combination of experimental measurements and crystal plasticity modeling. We focused on an age-treated alloy fortified by distributed Mg2Sn particles and Mg17Al12 precipitates, which underwent two distinct loading cycles: tension-compression-tension (TCT) and compression-tension-compression (CTC), aligned with the extrusion direction (ED). The initial and deformed microstructures of the alloy were analyzed using the electron backscattering diffraction (EBSD) technique. Notably, the alloy displays tensile and compressive yield strengths (YS) of 215 MPa and 160 MPa, respectively, with pronounced anelastic behavior observed during unloading and reverse loading phases. Utilizing the elasto-viscoplastic self-consistent model incorporating a twinning–detwinning scheme (EVPSC-TDT), the cyclic stress–strain responses and resultant textures of the alloy were accurately captured. The predicted alternation between various slip and twinning modes during plastic deformation was used to interpret the observed behaviors. It was found that prismatic <a> slip plays an important role during the plastic deformation of the studied alloy, and its relative activity in tensile loading processes accounts for up to ~66% and ~67% in the TCT and CTC cases, respectively. Moreover, it was discerned that detwinning and twinning behaviors are predominantly governed by stresses within the parent grain, and they can concurrently manifest during the reverse tensile loading phase in the TCT case. After cyclic deformation, the area fractions of residual twins were determined to be 7.51% and 0.93% in the TCT and CTC cases, respectively, which is a result of the varied twinning–detwinning behavior of the alloy in different loading paths.

Effect of extrusion ratio on microstructures, mechanical properties, and high cycle fatigue behavior of Mg–5Zn–1Mn alloy

The effects of extrusion ratio (ER) on the microstructures, static mechanical properties, and tension-compression high cycle fatigue (HCF) behavior of the as-extruded Mg–5Zn–1Mn (ZM51) alloy was investigated systematically, and the HCF mechanism was discussed. The results show that texture weakening and grain refinement caused by increasing the ER can effectively enhance the static mechanical properties and alleviate the tensile-compressive asymmetry of the alloy. The fatigue strength at 107 cycles is 124 ± 5, 138 ± 4, and 142 ± 5 MPa for ER8, ER11.5, and ER23, respectively, and the corresponding fatigue ratio is approximately 0.45, 0.49, and 0.50, which means that the as-extruded ZM51 alloy possesses outstanding fatigue resistance. The failure analysis and microstructure evolution of post-fatigued samples demonstrated that the stress amplitudes and microstructure characteristics strongly influence the HCF mechanism of the alloy. At high-stress load, close to the tensile yield strength 160–180 MPa, abundant {101‾2} residual twin bands were observed near the fracture surface. Twinning/detwinning deformation is the dominant fatigue mechanism due to the plastic deformation incompatibility between the matrix and the residual twins. At low-stress load, close to the fatigue strength 125–145 MPa, the fatigue crack initiation mechanism transits from twinning/detwinning to dislocation slip. Grain refinement and texture weakening inhibit the activation of {101‾2} twins, alleviate the twin-dislocation interaction, and promote dislocation slip to dominate the fatigue deformation, thereby enhancing the fatigue life of the alloy. This work provides new insight into the design and development for enhancing the fatigue resistance of wrought Mg alloy to ensure the long-term service safety of structural materials.

A Constitutive Model for Asymmetric Cyclic Hysteresis of Wrought Magnesium Alloys under Variable Amplitude Loading

A cyclic plasticity constitutive model was developed for materials with asymmetric cyclic behavior to explain the stabilized stress–strain response under variable amplitude loading. The proposed constitutive model incorporated the von Mises yield function with an adjustment to accommodate asymmetric yielding under tension and compression. A combined isotropic–kinematic hardening model was proposed to describe the evolution of the yield surface in the reference uniaxial frame and the actual frame. The history of plastic deformation is memorized by introducing internal variables, accumulated slip, and residual twins, which govern the cyclic flow behavior in the subsequent reversal. The additional conditions required to predict the stabilized hysteresis response of a material under variable amplitude loading were set out and incorporated into the constitutive model. The model was numerically implemented and programmed into a user material (UMAT) subroutine to run with the commercial finite element program, Abaqus/Standard 2019. The model was calibrated using the stabilized hysteresis response of ZEK100 and AZ31B sheets under constant amplitude strain-controlled cyclic loading for different strain amplitudes. To verify the model, constant amplitude and four different variable amplitude load spectra tests were performed and the stabilized stress–strain hysteresis response predicted by the model was compared with test results. It was demonstrated that the results are in very good agreement.

Despeckling of SAR Images Using Residual Twin CNN and Multi-Resolution Attention Mechanism

The despeckling of synthetic aperture radar images using two different convolutional neural network architectures is presented in this paper. The first method presents a novel Siamese convolutional neural network with a dilated convolutional network in each branch. Recently, attention mechanisms have been introduced to convolutional networks to better model and recognize features. Therefore, we propose a novel design for a convolutional neural network using an attention mechanism for an encoder–decoder-type network. The framework consists of a multiscale spatial attention network to improve the modeling of semantic information at different spatial levels and an additional attention mechanism to optimize feature propagation. Both proposed methods are different in design but they provide comparable despeckling results in subjective and objective measurements in terms of correlated speckle noise. The experimental results are evaluated on both synthetically generated speckled images and real SAR images. The methods proposed in this paper are able to despeckle SAR images and preserve SAR features.

In-situ observation of twinning and detwinning in AZ31 alloy

Twinning and detwinning behavior of a commercial AZ31 magnesium alloy during cyclic compression–tension deformation with a total strain amplitude of 4% (±2%) was evaluated using the complementary techniques of in-situ neutron diffraction, identical area electron backscatter diffraction, and transmission electron microscopy. In-situ neutron diffraction demonstrates that the compressive deformation was dominated by twin nucleation, twin growth, and basal slip, while detwinning dominated the unloading of compressive stresses and subsequent tension stage. With increasing number of cycles from one to eight: the volume fraction of twins at -2% strain gradually increased from 26.3% to 43.5%; the residual twins were present after 2% tension stage and their volume fraction increased from zero to 3.7% as well as a significant increase in their number; and the twinning spread from coarse grains to fine grains involving more grains for twinning. The increase in volume fraction and number of residual twins led to a transition from twin nucleation to twin growth, resulting in a decrease in yield strength of compression deformation with increasing cycles. A large number of < c >-component dislocations observed in twins and the detwinned regions were attributed to the dislocation transmutation during the twinning and detwinning. The accumulation of barriers including twin boundaries and various types of dislocations enhanced the interactions of migrating twin boundary with these barriers during twinning and detwinning, which is considered to be the origin for increasing the work hardening rate in cyclic deformation of the AZ31 alloy.

Effect of twinning and precipitation on low-cycle fatigue damage behaviour of extruded AZ80 magnesium alloy

In this paper, the fatigue behaviour and failure mechanisms of extruded AZ80 magnesium alloy samples were studied with different initial structures. The results show that the twinning-twinning behavior was inhibited to a certain extent by fine grain and precipitates, but the initiation of fatigue cracks was promoted. Studies of the fatigue life and fracture mechanism show that both residual twins and precipitated phases can cause fatigue crack initiation, but less fatigue damage is caused by the twin-induced cracks than by second phase-induced.

Crystal plasticity modeling of low-cycle fatigue behavior of an Mg-3Al-1Zn alloy based on a model, including twinning and detwinning mechanisms

The low-cycle fatigue of strongly-textured Mg alloys is dominated mainly by an alternation of twinning-detwinning process, which poses a great challenge to conventional constitutive models. As a result, a good reproduction of both mechanical response and deformation behavior often fails in previous simulations, in particular for high strain amplitudes. In the present study, the numerical simulations of low-cycle fatigue behavior of an Mg AZ31 plate with a ± 2% axial strain amplitude along the rolling direction were conducted, using a modified crystal-plasticity based on the finite strain elastic-viscoplastic self-consistent model containing twinning and detwinning mechanisms (EVPSC-TDT), and the results were compared with the mechanical response and deformation behavior determined by a real-time in-situ neutron diffraction. The simulations well predict the hysteresis loops and cyclic-hardening responses throughout the whole fatigue life (80 cycles), along with the evolution of the maximum twin volume fraction and residual twins. For the first time, the evolution of lattice strain throughout the whole fatigue life was calculated, and the results are in good accordance with that determined by neutron diffraction. The present study strongly demonstrates that the EVPSC-TDT model is very effective for modeling the low-cycle fatigue behavior of Mg alloys. More details about the evolution of the maximum twin volume fraction with fatigue cycles are found, an increase from 1st to 2nd cycle, followed by a decrease from 2nd to 20th cycle and a secondary increase after 20th cycle. This finding corrects the previous opinion that the maximum twin volume fraction increases at the initial stage and tends to be saturated at the late stage. Combining the results of simulations and neutron diffractions, the corresponding reasons are explained. Afterward, the mechanisms of the cyclic-hardening behavior at tension stage and compression stage are revisited, and some new insights are provided.

Deformation twinning and detwinning in extruded Mg-4Al: In-situ experiment and crystal plasticity simulation

Deformation twinning and detwinning in extruded Mg-4Al were investigated using in-situ SEM-DIC experiments and crystal plasticity finite element (CPFE) simulation. In this study, the in-situ SEM-DIC method was used to provide a unique set of data including twin/detwin characteristics and twin area fraction in addition to strain maps. A statistical analysis of the activation of twin variants and twin area fraction during both twinning and detwinning was conducted. A strong correlation was found between twin growth/shrinkage and the Schmid Factor (SF) for individual twin variants Higher twin SF during loading and unloading led to higher twin growth and shrinkage, respectively. However, after the applied compressive strain was removed, the pattern of the twin area fractions of the residual twin variants versus their nominal SFs did not follow the trend observed at the maximum compressive strain. Using a systematic methodology and an advanced twin/detwin model, the PRISMS-Plasticity CPFE simulation was calibrated using experimentally determined stress versus strain and twin area fraction versus strain information. A comprehensive evaluation of the CPFE model was conducted to determine its ability to capture the statistics of twin variants activation and twin area fraction. CPFE accurately captured the statistical aspects of both twinning and detwinning. It also predicted the first dominant twin variant for 47.5% of the grains and at least one of the two dominant twin variants for 80% of the grains at maximum compressive strain.

Low-cycle fatigue behaviour of Mg-9Gd-4Y-2Zn-0.5Zr alloys with different structures

The strain-controlled cyclic deformation behaviour of Mg-9Gd-4Y-2Zn-0.5Zr with different structures was investigated. Alloys were prepared by solution, extrusion and pre-ageing extrusion, and the microstructures before and after the fatigue tests were characterized. Experimental results indicated that the bimodal structure owned the better performance in fatigue test, which was attributed to the higher yield strength. For the equiaxed structure, cyclic hardening induced stress concentration until the failure. Stable cyclic deformation and persistent cyclic softening played an important role at the low and high strain amplitudes, respectively. This was attributed to the formation of fine grains relieving the stress concentration during cyclic loading. In addition, residual twins were observed in equiaxed structure to induce crack, and the bimodal structure effectively restrain it.

Crystal plasticity modeling of the multiaxial ratchetting of extruded AZ31 Mg alloy

Macroscopic experimental observations on the non-proportional multiaxial ratchetting of extruded AZ31 Mg alloy is firstly summarized, and corresponding microscopic investigations are conducted by electron backscatter diffraction (EBSD) technology on the Mg alloy before and after the multiaxial ratchetting. Then, a constitutive model was developed in the framework of crystal plasticity for the Mg alloy single crystal at finite strain. Various plastic deformation mechanisms including twinning, detwinning and dislocation glide are considered. Meanwhile, three different kinematic hardening laws are presented to capture the evolutions of back stresses associated with different deformation mechanisms, respectively, from the macroscopic and microscopic experimental results. Also, the residual twins generated in the cyclic deformation and their accumulations are considered. The developed single crystal model is extended to the polycrystalline version by means of a newly developed elasto-viscoplastic self-consistent homogenization scheme at finite strain. Finally, the non-proportional multiaxial ratchetting of the AZ31 Mg alloy is reproduced by the developed model. Conclusions can be drawn that the developed model can effectively describe the multiaxial ratchetting and its dependences on the stress level and stress path as well as the relative texture evolution during the multiaxial ratchetting of the AZ31 Mg alloy.

Twin quark dark matter from cogenesis

We extend the fraternal twin Higgs scenario to include a novel dark matter candidate as well as a mechanism for generating a matter/antimatter asymmetry in both sectors. A spontaneous breaking of twin color results in quark degrees of freedom that are singlets under the residual twin color group. These twin-color-singlet quarks, along with a subdominant component of twin leptons, constitute the asymmetric dark matter. The asymmetry between matter in antimatter in both sectors is co-generated from the decay of singlet fermions that provide an additional portal between the visible and twin sectors. We discuss the phenomenological aspects of this model, evaluating constraints on the parameter space and highlighting promising discovery channels in future experiments. We briefly discuss how the discovery of signals in multiple experiments may help establish the connection between the mechanisms that address the naturalness, dark matter and matter/antimatter asymmetry puzzles.

Modeling crystal plasticity with an enhanced twinning–detwinning model to simulate cyclic behavior of AZ31B magnesium alloy at various temperatures

In this study, a multiscale model within crystal plasticity finite element (CPFE) framework was developed to predict the cyclic loading behaviors of a wrought magnesium alloy, AZ31B, at various testing temperatures. The temperature dependency was systematically modeled by employing a modified strain-hardening model, implemented in the CPFE. The developed model was further extended to cyclic loading scenarios, under which abnormal mechanical responses were observed, using an enhanced twinning–detwinning model based on the well-known predominant twinning reorientation scheme. For the first time, the concept of variable residual twin fraction was introduced in the twinning–detwinning model as a criterion for the inactivation of detwinning. The newly developed model was employed to predict the mechanical responses under monotonic in-plane tension and compression at testing temperatures ranging from room temperature to 200°C. In addition, the mechanical behaviors of the alloy under cyclic loadings at these testing temperatures were also predicted using the CPFE model. These predictions were then compared with the experimentally measured data. The micromechanical responses, such as the evolution of twin volume fraction and activation of slip/twin/detwin systems, under various loading and temperature conditions were also predicted and compared with the reported data.

An Intelligent Hybrid Scheme for Identification of Faults in Industrial Ball Screw Linear Motion Systems

Reliability of high precision linear motion system is one of the main concerns in industrial and military systems. The performance and repeatability of these systems are influenced by their respective linear drives and load bearings. A fault in these members severely affects the safe working of overall system. This paper gives a reliable intelligent approach to detect and classify faults for linear motion systems based on deep learning methods. Accuracy in faults identification is highly dependent on improved features extraction. For this purpose, a novel Residual Twin CNN (ResT-CNN) is proposed that uses combination of 1-D and 2-D CNN in parallel learning which improves features extraction performance; followed by knowledge base-Remnant-PCA (Kb-Rem-PCA) architecture in combination with multi-class support vector machine (Mc-SVM). This novel hybrid combination proved very effective in accurate faults identification. The performance of proposed methodology was also validated by IMS-UC (Intelligent Maintenance Systems - University of Cincinnati) public bearing dataset. The results confirm the effectiveness of proposed scheme in comparison to existing state of the art techniques.

Statistical Analysis of Grain-Scale Effects of Twinning Deformation for Magnesium Alloys under Cyclic Strain Loading.

To reveal the relationship between grain size and twinning deformation of magnesium alloys under cyclic strain, this study carried out a group of strain-controlled low-cycle fatigue experiments and statistical analysis of microstructures. Experimental results show that the shape of the hysteresis loop exhibits significant asymmetry at different strain amplitudes, and the accumulation of residual twins plays an important role in subsequent cyclic deformation. For the different strain amplitudes, the statistical distribution of the grain size of magnesium alloy approximately follows the Weibull probability function distribution, while the statistical distribution of twin thickness is closer to that of Gaussian probability function. The twin nucleation number (TNN) increases with the increase of grain size, but there is no obvious function relationship between twin thickness and grain size. Twin volume fraction (TVF) increases with the increase of grain size, which is mainly due to the increase of TNN. This work can provide experimental evidence for a more accurate description of the twinning deformation mechanism.

A predictive mirror twin Higgs with small Z2 breaking

The twin Higgs mechanism is a solution to the little hierarchy problem in which the top partner is neutral under the Standard Model (SM) gauge group. The simplest mirror twin Higgs (MTH) model — where a Z2 symmetry copies each SM particle — has too many relativistic degrees of freedom to be consistent with cosmological observations. We demonstrate that MTH models can have an observationally viable cosmology if the twin mass spectrum leads to twin neutrino decoupling before the SM and twin QCD phase transitions. Our solution requires the twin photon to have a mass of ∼ 20 MeV and kinetically mix with the SM photon to mediate entropy transfer from the twin sector to the SM. This twin photon can be robustly discovered or excluded by future experiments. Additionally, the residual twin degrees of freedom present in the early Universe in this scenario would be detectable by future observations of the cosmic microwave background.

Fatigue characteristics of the extruded AZ80 automotive wheel

In order to provide a reliable theoretical foundation for fatigue resistant design of the magnesium alloy automobile wheel, specimens machined from rim and disc of the extruded AZ80 wheel were subjected to strain-controlled fatigue tests. Various characterizations (such as microstructure and texture analysis, stress-strain response monitoring, twinning/detwinning behavior evolution, and fatigue fracture morphology analysis) have been carried out to investigate the static and fatigue performance of the AZ80 wheel. The obtained microstructural analysis revealed that the rim sample exhibited finer and more homogeneous dynamic recrystallized grains with the average grain size of ~17.2 μm, while ~30.5 μm for the disc sample. The rim specimen achieved the ultimate tensile strength of ~339 MPa with a noticeable elongation to fracture of ~14.6%. In addition, the rim sample also showed superior fatigue properties as compared with the disc sample, which was related to the smaller grains and the weaker texture intensity. The twinning/detwinning behavior participated in the cyclic deformation above the strain amplitude of 0.4% for both samples, resulting in asymmetric hysteresis loops and residual twins. The fatigue crack initiation site was observed with second phase particles and secondary cracks near the surface for all specimens, and in particular, cleavage-like faceted area with twins was identified at strain amplitude of 1%.

Discontinuous and inhomogeneous strain distributions under monotonic and cyclic loading in textured wrought magnesium alloys

The present work investigates the deformation behaviour of strongly textured AZ31B wrought magnesium sheets under monotonic as well as cyclic mechanical loading. Of special focus is the distribution of strain, which is investigated employing optical strain field measurements on the specimen surfaces. Microstructural investigations of the material after loading provide detailed information on the underlying deformation characteristics. The investigations show that bands of twinned grains (BTGs) form up once twinning occurs. These BTGs consist of solely {101‾2} tension twins and are clearly delimited to the adjacent absolutely twin-free regions. Most importantly, strain fields are strongly inhomogeneous and discontinuous due to these BTGs with considerably higher compressive strain inside BTGs, both after monotonic and cyclic loading. During the first few load cycles, BTGs are observed to grow and the formation of new BTGs can occur. Additionally, a significant hardening is found during the first load cycles, most likely caused by residual twins and unretwinnable twins. However, material behaviour stabilizes after a few load cycles and remains approximately constant for most of a specimen's lifetime. The presented specific deformation characteristics are important for understanding and describing the material. Modelling the investigated material behaviour is not yet completely feasible with phenomenological constitutive models, which is why important modelling aspects are discussed.

Static recrystallization behavior of multi-directional impact forged Mg-Gd-Y-Zr alloy

The static recrystallization behavior of a (Mg-9.02Gd-4.21Y-0.48Zr wt.%) GW94 alloy during annealing treatment in the temperature range of 350–500 °C has been investigated by means of electron backscattered diffraction (EBSD). To investigate the role of twin boundaries in static recrystallization of GW94 alloy, a large number of {10–12} extension twins were introduced initially by multi-directional impact forging (MDIF) process. After the MDIF process, coarse grains containing parallel and intersected twins while small grains containing only parallel twins were observed. It has been observed that twin/grain boundaries (TBs/GBs) are stable at 350 °C, but static recrystallization governed by the increased twin/grain boundary mobilities begins at the 400–450 °C temperature range. Microstructure evolution during static annealing reveals that nearly all pre-existing {10–12} extension twins start to recrystallize at 400 °C and 60 min, yet few parallel twins plus residual twin traces with misorientations of 5°–10° were observed. In contrast to the parallel twins in both large and small grains, the intersected {10–12} extension twins in coarse grains were found to be unstable and more effective to induce recrystallization.

Quantification of cyclic twinning-detwinning behavior during low-cycle fatigue of pure magnesium using high energy X-ray diffraction

The cyclic twinning and detwinning behavior of extruded Mg was investigated using in-situ high energy X-ray diffraction (HEXD) under fully-reversed low cycle fatigue conditions. Measurements were conducted at three levels of applied strain. The initial texture was such that the c-axis in most grains was perpendicular to the loading direction, an orientation in which extension twinning is favored during compressive loading. At strain amplitudes greater than 0.5%, tension-compression asymmetry was observed during cyclic loading and related to cyclic twinning and detwinning. The twinning and detwinning behavior were characterized by monitoring the evolution of X-ray diffraction peaks associated with the basal {0 0 0 2} planes throughout selected cycle. At cyclic strains greater than 0.5%, in-situ HEXD results show that twinning occurs during the compression portion of the cycle and, at early stages of fatigue, most twins are detwinned under reversed loading during the tensile portion of the cycle. It was also observed that as the number of fatigue cycles increases the twin volume fraction increases. After 100–200 fatigue cycles, the detwinning process was observed to be incomplete and a significant fraction of residual twins remained throughout an entire cycle. Using electron back scatter diffraction imaging on the surface of interrupted fatigue tests, twinning and detwinning behavior was investigated and the presence of persistent twins, including residual twins, was observed. At a lower applied strain (0.4%), twinning and tension-compression yield asymmetries associated with twinning were not observed.

Effects of loading conditions on twin characteristics in an extruded Mg-Zr alloy

In the present study, compression and compression-tension tests of different strain levels (maximum 4%) were carried out on an extruded Mg-Zr alloy rod along the extrusion direction. The effects of loading conditions (strain level and strain path) on twin characteristics (twin aspect ratio and twin incidence) in the alloy were quantitatively investigated from a statistical viewpoint, with focus on seeking for an appropriate loading condition where dense and fine twin lamellae can be obtained. The results show that the twin aspect ratio strongly depends on the strain path when the accumulated compressive strain levels are relatively small (≤ 2%) and that compression-tension deformation can obtain finer twin lamellae than simple compression deformation. In addition, under compression-tension deformation with the same accumulated compression strain (0%, 1% and 2%), the higher the initial compression strain is, the finer the residual twins become. The twin incidence, which shows an upward tendency with the increase of the initial compression strain (from 0.09 at the initial compression strain of 2% to 0.57 at that of 6%), is found to be sensitive to the strain path only when the accumulated compression strain is 0%. When the accumulated compression strain is larger (4%), the strain path has little effect on both the twin aspect ratio and the twin incidence.