Twin Volume Fraction Research Articles

Strain-rate-dependent behaviors of CoCrNiSi0.2 medium-entropy alloy: Deformation mechanism and microstructural-based constitutive modeling

Abundant strengthening mechanisms in medium-entropy alloys (MEAs) endow them with excellent comprehensive properties, rendering them promising candidates for use in extreme service conditions, such as low/high temperatures and strong impact environment. Investigation of the mechanical behaviors and micro deformation mechanisms of MEAs under intermediate/high strain rate loadings should therefore help to explore their application potential in impact-resistant structures. In this study, the tensile mechanical properties of a single-phase face-centered cubic (FCC) CoCrNiSi0.2 MEA were tested over low, intermediate, and high strain rates, and the micro deformation mechanisms at different strain rates were analyzed systematically. Dislocation evolution, deformation twinning, and phase transformation (from single FCC to FCC + hexagonal close-packed (HCP)) were observed in the three groups of tensile specimens loaded at low, intermediate, and high strain rates. The dislocation densities and twin volume fractions in the three groups were comparable, while the volume fraction of phase transformation increased significantly with the increasing strain rate. Therefore, more HCP phases generated during high-strain-rate deformation should lead to a stronger precipitation-strengthening effect on the material, which made the strain-rate-strengthening effect more significant to the extent that it could not be ignored in constitutive modeling. Finally, the contribution of the phase transformation was introduced for the first time into a constitutive model in which dislocation slip and deformation twinning were also considered to describe the viscoplastic flow behavior of the CoCrNiSi0.2 MEA over a wide range of strain rates.

The hardening effect of deformation twinning based on visco-plastic self consistent model and a multi-scale grain refinement prediction model during machining of titanium alloy

Deformation twins act as one of the deformation characteristics of hexagonal close packed (HCP) materials and occur during machining of Ti–6Al–4V alloy. To clarify the hardening mechanisms of deformation twins in Ti–6Al–4V machined surface, work-hardening effects induced by grain refinement in meso scale as well as deformation twins in micro scale were investigated. A 2D cutting finite element (FE) model was established to predict grain refinement in multi-scales and its hardening behavior during machining of Ti–6Al–4V. A visco-plastic self consistent model (VPSC) coupled with predominant twin reorientation (PTR) scheme was also used to simulate stress variation with considering the activations of twin systems. The results show that different grain refinement degrees in meso scale are observed with the increasing of cutting speeds (100–500 m/min). {101¯1} compression nano twins generate at higher cutting speeds, and the twin volume fraction is relatively small (20%). The hardening behaviors of grain refinement in meso scale as well as nano twins predicted with FE model and VPSC model are similar, and the generation of nano twins cause inhomogeneous hardening in Ti–6Al–4V machined surface. Nano-indentation tests further prove the accuracy of the prediction results for the hardening effect induced by nano twins in Ti–6Al–4V machined surface. The results provide two explanations and quantitative evaluations for the hardening effects of deformation twins from the macroscopic stress and the microscopic deformation of slip and twinning modes.

Modelling mechanical behaviour of a gradient-microstructured material obtained by surface mechanical attrition treatment accounting for residual stresses

In this work, an austenitic 316 L steel was processed by surface mechanical attrition treatment (SMAT), and the induced gradient microstructure was highlighted by experimental measurements. A combined dislocation density-based and grain size-dependent constitutive model is developed to describe the mechanical behaviour of the gradient-microstructured material. In addition, this model also incorporates the grain size-dependent initial twin distribution and evolution of deformation twinning. Residual stress, initial dislocation density and twins are considered to reconstruct the depth-dependent residual fields induced by SMAT. Furthermore, the evolution of dislocation density and twin volume fraction is described during uniaxial tensile loading. Particular attention is devoted to investigate the effect of residual stress and deformation twinning on mechanical behaviour. Results of finite element simulation showed that the gradient microstructure enhances the yield strength, which is in agreement with previous experimental observations. It was revealed that residual stress significantly weakens the yield strength at the initial deformation stage, whereas it has little influence on the plastic behaviour at large deformation. The assumed grain size-dependent deformation twinning model allows describing the evolution of deformation twinning on gradient microstructure. The deformation twinning increases in relation to increased strain hardening during tensile loading. However, the evolution of twin volume fraction shows almost no twin and no variation of twinning within the nanocrystalline grain region.

Nonmetallic inclusion characterizations and effects on impact toughness anisotropy of TWIP steel with different Ce additions

The effects of cerium (Ce) addition (0, 0.015% and 0.045%) on the microstructure, inclusions and impact toughness of Fe-18%Mn-0.6%C twinning-induced plasticity (TWIP) steel were investigated. The results show that the addition of Ce has no significant effect on the texture of TWIP steel. The grain size of TWIP steel decreases, and the volume fraction of twins increases slightly with Ce addition. Typical MnO-MgO-MnS complex inclusions and MnS were observed in TWIP steel without Ce. The inclusions gradually transform from the long strip MnS-Ce oxysulfide complex inclusions to ellipsoidal Ce oxysulfides with the increase in Ce addition from 0.015% to 0.045%. Notably, the transverse impact toughness of TWIP steel with 0.015% Ce addition is much lower than that of the other samples and exhibits serious anisotropy, and these differences are attributed to the stronger deformation ability of the complex inclusions with MnS and Ce oxysulfides compared with Ce oxysulfides. Fine grain size and ellipsoidal inclusions lead to excellent isotropic properties in TWIP steel with 0.045% Ce addition.

On the cyclic torsion behavior of extruded AZ61A magnesium alloy tube

This work investigated the cyclic torsional behaviors of extruded AZ61A Mg alloy tubes through experiments and simulations. To capture the Bauschinger effect, the elastic viscoplastic self-consistent model with the twinning and detwinning scheme, together with a torsion-specific finite element approach was improved by adding a back-stress. A unique Swift effect with the “butterfly” shape was observed and reasonably reproduced by our model. Furthermore, the relative activities of deformation mechanisms, internal stress distributions, and twin volume fraction distributions under cyclic torsion were discussed. The findings could be beneficial to the design, manufacturing, and service of Mg alloys, particularly under cyclic torsion.

Finite element analysis using elasto-visco-plastic self-consistent polycrystal model for E-form Mg sheet subjected to bending

The mechanical behavior of a magnesium alloy E-form under bending was investigated using the elasto-visco-plastic polycrystal model (ΔEVPSC) and its finite element (FE) implementation (ΔEVPSC-FE) developed in Jeong et al. and Jeong and Tomé. The crystallographic orientation distribution (COD) obtained from X-ray diffraction was used to represent the initial texture, and the Voce hardening parameters were calibrated by fitting the uniaxial tension and the compression flow stress curves. A quasi-static FE analysis of a miniaturized V-bending operation was conducted using the ΔEVPSC-FE model. The bending induced an inhomogeneous stress response along the through-thickness and the lateral directions, which was well captured by the model. Moreover, the predictive capability of the model was validated by comparing with various experimental results such as (1) force vs. displacement curves; (2) the through-thickness variations in the twin volume fraction; and (3) the changes in crystallographic texture as a function of displacement. Additional bending simulation was performed using an isotropic texture, the result of which suggests that the potential improvement in bendability of the magnesium alloy is attainable by weakening the initial texture. Moreover, the simulation results imply that the crystallographic texture, which may affect the twin activation across the thickness direction, plays a significant role in the shifting direction of the neutral layer.

Comparison of the Strain Rate Sensitivity in AZ31 and WE43 Magnesium Alloys under Different Loading Conditions

The initial texture effect of strain rate sensitivity response in magnesium alloys was studied using the quasi in situ electron backscatter diffraction technique. The strain accumulations, under uniaxial loading of both tensile and compression along the rolling direction, showed strain rate dependency. For the AZ31 magnesium sheets with strong basal texture (max intensity to 11.2), twinning first occurs at the onset of plastic deformation, resulting in macroscopic strain rate insensitivity. With further loading, the strain rate sensitivity significantly increases (with the strain rate sensitivity coefficient m increased from 0.0025 to 0.012), arising from the initiation of slip. For the WE43 alloy sheets with weak basal texture (max intensity to 2.6), the overall deformation is sensitive to strain rate under both tensile and compression. The twinning growth rate and twinned volume fraction have been used to identify whether the process is twin-dominated or slip-dominated during the plastic deformation of a magnesium alloy.

Effect of ECAP processing temperature on an austenitic TWIP steel's microstructure, texture and mechanical properties

The development of microstructure and texture of twinning-induced plasticity (TWIP) steel sheet during ECAP at two temperatures (250° and 350°) up to four passes were investigated. The high chromium content is the differential of this TWIP steel, a chemical element that reduces the stacking-fault energy and increases corrosion resistance. By imposing ECAP, the volume fraction of twins, dislocation density, and yield strength significantly increase while ductility decreases. This study compared the effect of temperature during applied severe plastic deformation (SPD). Thin parallel slip bands’ high density was observed, and in addition to an intense micro shear, banding took place when pressing was continued by increasing the number of passes. The martensitic transformation was not detected in the deformation-twinning microstructure after more passes. The relationship between the microstructure, strain-hardening behavior, and mechanical properties was investigated after two different deformation schedules of ECAP. The best balance between high strength and good ductility (1915 MPa and 7%) was obtained after four passes at 250 °C and 350 °C. It was found that the quantity of ECAP pass affects the displacement substructure with the formation of shear bands, subgrains, and various variants of twins. These also influenced strain-hardening behavior, microhardness, texture, and ultimate strength.

Enhanced strength-ductility synergy of medium-entropy alloys via multiple level gradient structures

The microstructures, mechanical properties, and deformation substructures of gradient Mo0.3NiCoCr medium-entropy alloys (MEAs) with very coarse grain size created by pre-torsion have been investigated. The strength of MEAs increases with the increase of torsion angle, while the tensile elongation nearly remains the same, suggesting the enhanced strength-ductility synergy. The initial dislocation density gradient structure after torsion and the following deformation substructure under tension are uncovered by means of electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI), and transmission electron microscopy (TEM). The crystal plasticity finite element method (CPFEM) is employed to quantitively evaluate the evolution of dislocation densities and mechanical twinning volume fraction. The combination of experimental characterization and theoretical modeling enables to clarify the underlying strengthening and strain hardening mechanisms. The gradient distribution of dislocations created by the torsion leads to the rise of yield strength. Moreover, the high order of microbands, which arise from the activation of multiple slip systems during torsion, and additional mechanical twinning form in the gradient MEAs upon loading, constituting multiple level gradient structures. As the plastic strain goes on, the microbands can propagate and refine continuously, along with the interactions with the nano twins, in these MEAs with very coarse grain size up to ∼500 µm, which produce progressively high strain hardening and stabilize the plastic deformation over the whole deformation regime. This study thus offers guidance for optimizing the mechanical performance of structural materials via tuning the design of gradient structure.

Texture-dependent bending behaviors of extruded AZ31 magnesium alloy plates

The relatively insufficient knowledge of the deformation behavior has limited the wide application of the lightest structure material-Mg alloys. Among others, bending behavior is of great importance because it is unavoidably involved in various forming processes, such as folding, stamping, etc. The hexagonal close-packed structure makes it even a strong texture-dependent behavior and even hard to capture and predict. In this regard, the bending behaviors are investigated in terms of both experiments and simulations in the current work. Bending samples with longitudinal directions inclined from the transverse direction by different angles have been prepared from an extruded AZ31 plate, respectively. The moment-curvature curves and strain distribution have been recorded in the four-point bending tests assisted with an in-situ digital image correlation (DIC) system. A crystal-plasticity-based bending-specific approach named EVPSC-BEND was applied to bridge the mechanical response to the microstructure evolution and underlying deformation mechanisms. The flow stress, texture, twin volume fraction, stress distribution, and strain distribution evolve differently from sample to sample, manifesting strong texture-dependent bending behaviors. The underlying mechanisms associated with this texture dependency, especially the occurrence of both twinning and detwinning during the monotonic bending, are carefully discussed. Besides, the simulation has been conducted to reveal the moment-inclination angle relation of the investigated AZ31 extruded plate in terms of the polar coordinate, which intuitively shows the texture-dependent behaviors. Specifically, the samples with longitudinal directions parallel to the extruded direction bear the biggest initial yielding moment.

Multi-mechanism constitutive model for uniaxial ratchetting of extruded AZ31 magnesium alloy at room temperature

A multi-mechanism macroscopic phenomenological constitutive model is established to characterize the uniaxial ratchetting of extruded AZ31 magnesium (Mg) alloy at room temperature in the framework of small deformation. Because the uniaxial ratchetting of the alloy depends greatly on the strong basal texture and different deformation mechanisms (i.e., dislocation slipping, twinning and detwinning) and presents apparent tension-compression asymmetry, two separated yield functions and different hardening rules are used in the proposed model regarding to different deformation mechanisms. That is, the traditional von Mises yield criterion and a flow rule in power-law form are used for the plasticity contributed by dislocation slipping, while the Cazacu–Barlat–Plunkett (CPB) yield criterion extended with a back stress tensor and an internal variable f (i.e., twin volume fraction) is employed to express the plastic deformation induced by twinning/detwinning. In addition, the modified Armstrong-Frederick (A-F) kinematic hardening models are adopted to reflect the strain hardening features contributed by different plastic mechanisms, and the interaction among different plastic mechanisms is considered in the developed constitutive model. The reproduced uniaxial ratchetting of extruded AZ31 Mg alloy by the proposed multi-mechanism constitutive model is in a good agreement with the corresponding experimental results obtained at room temperature, which validates the reasonability and capability of the proposed model.

Twin suppression by atomic scale engineering of precipitate-matrix interfaces

This work aims to investigate the influence of solute segregation at particle-matrix interfaces on the twin activity of aged Mg alloys. With that purpose a binary Mg-8Al alloy (wt%) and two ternary Mg-8Al-1 Zn and Mg-8Al-1Ag alloys (wt%) were first homogenized and then aged in order to obtain comparable precipitate distributions. Particle-matrix interfaces in the binary alloy were observed to be clean, while Zn and Ag atomic segregation was clearly present at the interfaces of the ternary alloys. Room temperature micropillar compression tests were carried out in grains with favorable orientations for tensile twinning in the homogenized and the aged conditions for the three materials under investigation. The activation of twinning and slip was then characterized by electron microscopy. This study shows that while in the binary alloy precipitation did not lead to any major changes in the twinning activity and, in particular, it did not prevent the formation of thick twin lamellae encompassing the entire width of the tested micropillars, a drastic reduction of the twin activity took place in the aged ternary alloys. In the latter, straining led to the formation of narrow twin lamellae belonging to a single variant, and occasionally arrested at the interior of the micropillars. We propose that the drastic reduction of the twin volume fraction in the aged ternary alloys may be attributed to the decrease of the particle-matrix interface energy due to Zn and Ag segregation, which would hinder re-nucleation at such interfaces, thus suppressing twin propagation.

The effect of temperature on load partitioning evolution in magnesium metal matrix composite reinforced with Ti particles using in-situ synchrotron radiation diffraction experiments

The load partitioning between the magnesium and titanium phases in an extruded Mg-15%Ti (vol.%) composite from room temperature up to 300 °C using synchrotron radiation diffraction during in-situ compression tests. During compression, the magnesium matrix composite deforms mainly by the activation of the extension twinning system up to 200 °C. The volume fraction of twins increases with the plastic strain but decrease with the compression temperature. Hard titanium particles bear an additional load transferred by the soft magnesium matrix from room temperature up to 300 °C. This effect is amplified after yield stress during plastic deformation. Additionally, twins within magnesium grains behaves as an additional reinforcement at low temperature (below 200 °C) inducing an increase in the work hardening of the composite.

Quantitative characterization of mechanical twins based on new digital image processing method

To measure the volume fraction of mechanical twins in transmission electron microscopy (TEM) images, a new image processing method named “Oriented-Linear Object Segmentation (OLOS) method” was proposed. The processing results showed that the twin volume fractions in TEM images could be measured more directly, rapidly, and effectively. This OLOS method is potentially developed for quantifying the twinning-induced plasticity (TWIP) effect in metals, e.g. TWIP steels, high entropy alloys, and copper. • A new image processing method to characterize mechanical twins was proposed. • Mechanical twins in TEM images could be measured quantitatively by this method. • This new method is effective and fast to quantify mechanical twins automatically.

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.

Influence of pre-twinning on adiabatic shear sensitivity of AM30 magnesium alloy

The effects of volume fraction of pre-introduced {10−12} extension twins on adiabatic shear sensitivity of AM30 magnesium (Mg) alloys were studied via dynamic compression experiments using a split Hopkinson pressure bar (SHPB). The relationship between adiabatic shear sensitivity and volume fraction of twins was analyzed by combining mechanical responses and microscopic analysis. The recrystallization mechanism in the adiabatic shear bands (ASBs) was investigated as well. According to the results, the adiabatic shear sensitivity of AM30 Mg alloys increased with the increase in the volume fraction of twins, while the degree of recrystallization in the shear band decreased. The recrystallization mechanism in the shear band was found to obey rotational dynamic recrystallization (RDRX). • The effect of volume fractions of {10–12} extension twins on the adiabatic shear sensitivity of AM30 magnesium alloys under high strain rate impact was investigated in the first time. • The adiabatic shear sensitivity of the AM30 alloy increased with the increasing volume fraction of extension twins. • The recrystallization in shear bands occurred by rotational dynamic recrystallization based on the calculation of dynamic equation.

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.

On the modeling of deformation mechanisms in a Mg-3Al-1Zn alloy under biaxial tension

Although the {10-12} twinning behavior of Mg alloys under uniaxial tension and compression has been extensively investigated, the simulations of {10-12} twinning behavior under biaxial tension have rarely been reported. In this work, the EVPSC-TDT model is first employed to systematically investigate the deformation behavior of a Mg alloy AZ31 plate under biaxial tension in the RD-TD and ND-TD planes. The RD, TD and ND refer to the rolling direction, transverse direction, and normal direction of the hot rolled plate. The measured stress-strain curves and texture evolutions are well predicted and the contours of plastic work under biaxial tension are also constructed for comparison with experiments. The plastic response has been interpreted in terms of relative activities of various deformation modes. For biaxial tension in the RD-TD plane, basal and pyramidal slips mainly contribute to the plastic deformation for stress ratios of σRD:σTD= 1:2 to 2:1. Prismatic slip becomes more active for σRD:σTD=1:4 and 4:1. Compression twinning could be activated and so cause texture reorientation at large strains, especially for σRD:σTD=1:1. The six-fold feature of {10-10} pole figure could still be observed forσRD:σTD=1:4 and 4:1 at large strain. For biaxial tension in the ND-TD plane, tensile twinning plays an important role for σND:σTD≥1:2, while prismatic slip contributes to plastic deformation for the other cases. With the increase of stress ratio from σND:σTD≥1:1 to 1:0, the predicted twin volume fractions (VFs) at a specific strain along the ND, εND, almost linearly decrease, however, it is seen that the experimental ones at given strains along the ND do not follow such a trend with the measured twin VFs within the range of stress ratios, 2:1≤σND:σTD≤6:1, clearly being overestimated, and the difference between experiments and simulations becomes most obvious at the relatively small strain of εND=0.015. The possible reasons for the observed difference are discussed.

Compressive deformation behavior of AZ31 alloy at 21K: an in-situ neutron diffraction study

Deformation behavior during uniaxial compression at 21 K and 298 K in a commercial AZ31 magnesium alloy was studied by in-situ neutron diffraction. Decreasing the deformation temperature resulted in a slight increase in yield stress (115 MPa →139 MPa), but a remarkable enhancement in both the fracture stress (365 MPa → 551 MPa) and fracture strain (12.8%→16.5%). The low temperature sensitivity of the {101¯2} extension twinning which governed the macroscopic yielding led to the slight increase in yield stress. At 21K, basal slip was suppressed, while the extension twinning was promoted resulted in higher twin volume fraction. In the late stage of deformation, the {101¯1}-{101¯2} double twinning was suppressed, which is considered to be the reason to delay the fracture at 21 K.

Dynamic tensile behavior of Al0.1CoCrFeNi high entropy alloy: Experiments, microstructure and modeling over a wide range of strain rates and temperatures

High entropy alloys (HEAs) are considered as new metallic materials with great application prospects due to their excellent mechanical properties. Massive experimental studies have revealed simultaneous strength-ductility enhancements in face-centered-cubic (FCC) HEAs resulting from the plasticity mechanisms transition under much harsher conditions. However, it still lacks quantitative understanding of such extraordinary mechanical behaviors. Aiming at precisely comprehending and describing the mechanical behaviors of FCC HEAs, a physical mechanism-based constitutive model is quite desirable to be established. As such, we first systematically investigated the uniaxial tension behavior of Al0.1CoCrFeNi HEA over wide strain rate (10−3-7 × 103s−1) and temperature (77–298K) ranges. Subsequent microstructural analyses indicated that its plasticity was dominated by dislocation slip at 298K or during small strain, while deformation twins acted as an additional plasticity mechanism with an increase of flow stress at higher strain rates or cryogenic temperatures. Moreover, combining theoretical calculations and experimental verification we further determined the critical twinning stress for this HEA. On basis of these experimental analyses, some specific models were employed to quantify the evolution of dislocation density or twin volume fraction during the plastic deformation. Finally, a physical-based constitutive model was developed to accurately describe the strain rate and/or temperature-dependent behaviors of this typical FCC HEA. This work not only helps quantitatively understand effects of various micro-mechanisms on the mechanical behaviors, but also provides the numerical simulation foundation for their future applications.