Twin Fraction Research Articles

Quantitative measurements of strain-induced twinning in Mg, Mg-Al and Mg-Zn solid solutions

The area fraction and number density of twins were determined in pre-polished tensile specimens of cast pure Mg, Mg-Al [0.5, 2 and 9 at.% Al] and Mg-Zn [0.8 and 2.3 at.% Zn] solid solutions, for different applied tensile strains. Profuse twinning developed rapidly in pure Mg, while it occurred very gradually in the Mg-Zn alloys, with Mg-Al showing an intermediate behavior. In the more dilute Mg-Al alloys the area fraction and the number density decreased with the concentration, increasing again for the 9 Al, reaching a level above that of pure Mg at large strains. In Mg-Zn both parameters decreased monotonically with the Zn concentration up to the solubility limit (2.5 at.%). For given alloy and solute content, the number density increased (the twins became smaller) for finer grain sizes in all three materials. It is argued that the availability of nucleating sites created by the microplasticity preceding profuse twinning, which is greater for the alloys due to the delayed onset of twinning, or for fine grained polycrystals due to the increased specific grain boundary area, accounts for the increased number density with the solute concentration or reduced grain size. Solute specific effects are rationalized using concepts of solid solution softening of the prismatic planes in all alloys, and the stronger solid solution hardening due to short range order in the concentrated Mg-Zn alloys.

Gradient heterostructured laser-powder bed fusion processed CoCrFeMnNi high entropy alloy

Developing metal additive parts with a synergy in strength and ductility is a demanding need for many critical applications. Recently, gradient structures have been of research interest as it facilitates intrinsic synergetic strengthening. In the current work, gradient microstructures are obtained on laser-powder bed fusion processed (LPBF) CoCrFeMnNi high entropy alloy using ultrasonic nanocrystal surface modification (UNSM). The UNSM treatment resulted in a thin layer of gradient microstructures with a gradient in dislocation density, twin fraction, and grain size on the surface of the LPBF samples. The gradient microstructures led to gradient properties and contributed significantly to hetero deformation-induced (HDI) strengthening. Further, a comparison with the wrought counterparts, which showed higher HDI stresses and strain hardening than the LPBF samples, establishes the need for a substantial strength difference between the hard UNSM-affected region and the soft unaffected region for significant HDI strengthening. Moreover, in the current work, a dislocation-based constitutive model is developed to represent the deformation mechanism of the gradient structured sample and is validated with the experimental results.

Effect of Al addition and annealing on the microstructure evolution of Mg-3Sn Mg alloy subjected to high-speed rolling

The microstructure evolution of Mg-3Sn, Mg-3Sn-3Al-Zn (ATZ331), and Mg-3Sn-6Al-Zn (ATZ631) alloys during high-speed rolling (HSR) and annealing was studied. High-speed rolling was carried out on the three alloys at a rolling speed of 1100 m/min, a rolling temperature of 400 °C, and a reduction of 40% during a single pass. The results showed that the microstructure of the samples after HSR contained many twins and was dominated by {10−12} extension twins (ETs) and supplemented by {10−11} contraction twins (CTs) and {10−11}-{10−12} double twins (DTs). After adding Al, the content of these different types of twins was reduced. The twinning fraction decreased significantly with 3 wt% Al addition, while the twinning fraction decreased slightly with 6 wt% Al addition. The high-speed rolled alloys were annealed at 370 °C for 1 min, 2 min, 3 min, 5 min, and 8 min. When the annealing time was 1 min, the recrystallization fraction of the ATZ631 alloy was low, indicating that increasing the Al content to 6 wt% inhibited recrystallization. When the annealing time increased to 3 min, the recrystallized grains in the ATZ631 alloy grew, indicating that the grain growth progressed faster in this alloy. When the annealing time was 8 min, the grain size of the three alloys grew. The electron backscattered diffraction (EBSD) results of the alloy annealed for 8 min showed that the twin fraction in the annealed state was lower than that after high-speed rolling. In addition with the increase in Al content, the twin fraction decreased. During high-speed rolling, the maximum intensity also increased with an increase in Al content, were 22.72, 25.09, and 26.35, and the texture of the annealed alloy was weakened. The Al addition also increased the maximum intensity.

Twinning-mediated texture weakening in a basal-textured Mg-6Al-1Zn (mass%) alloy sheet by a novel cold-sample rolling method

In this paper, we report the use of cold-sample rolling (CSR) processing, which induced a high fraction of double twins and a substantial texture-weakening effect in an Al-concentrated commercial Mg-6Al-1Zn (mass%) alloy sheet. A CSR-processed and subsequently annealed sheet showed a weak basal texture with a broad distribution of the (0001) poles toward the rolling and transverse directions. This led to moderate stretch formability with a limiting dome height of 5.5 mm. High-temperature rolling (HTR) processing has been reported to weaken the basal texture in Mg-Al-based alloy sheets because of the activation of non-basal <c+a> dislocations, resulting in texture weakening. However, the processing induced a relatively low fraction of double twins. Consequently, CSR processing led to a relatively high texture-weakening effect. The results showed that twinning-mediated static recrystallization is essential for texture weakening in Al-concentrated Mg-alloy sheets.

Effects of deformation twins on microstructure evolution, mechanical properties and corrosion behaviors in magnesium alloys - A review

Due to lattice reorientation, grain segmentation, induced recrystallization, twins play a very important role in regulating texture, refining grains, improving mechanical properties and corrosion resistance, and has received more extensive attention. Numerous studies have shown that {10-12}<10-11> tensile twins (TTWs) are easily activated in large quantities due to the lower critical resolve shear stresses (CRSS). Introduction of TTWs under uniaxial compression improved the strength, ductility, and formability of magnesium (Mg) alloys. Moreover, TTWs produced by multi-directional impact forging (MDIF) can optimize the microstructure by dividing grains and promoting recrystallization, resulting in significant improvement of mechanical properties. Although {10-11}<10-12> compressive twins (CTWs) and {10-11}-{10-12} double twins (DTWs) can promote dynamic recrystallization (DRX), they are also favorable nucleation sites for cracks. In addition, the type and volume fraction of twins can affect the corrosion resistance, and they also play different roles in the corrosion process of different Mg alloys. Twins have shown great potential for improving structure and properties, but a comprehensive and critical discussion of twins in Mg alloys is still lacking. Therefore, based on previous studies, this article reviews the common types and variants of twins in Mg alloys, influencing factors, and their effects on the microstructure, mechanical properties and corrosion resistance. In addition, some interesting ideas are being proposed for further research.

Revealing the deformation behavior and resultant texture evolution in extruded dilute Mg–Bi–Sn–Ca alloy during hot compression

In this paper, the deformation behavior and resultant texture evolution of an extruded dilute Mg-0.5Bi-0.5Sn-0.5Ca (BTX000) alloy during hot compression with different strain rates were studied. The results indicated that the activation of twins and dynamic recrystallization (DRX) are greatly dependent on the strain rates. The critical shear stress (CRSS) of twinning ranged from 66 to 197 MPa at different strain rates. In the case of strain rate of 3.3 s −1, the largest CRSS value led to the lowest area fraction of twins and the strongest texture intensity. In addition, the DRX extent increased with the increased strain rate. This phenomenon was caused by the variation in local stress concentration due to the different degrees of dislocation accumulation generated at different strain rates. More specifically, the dominant DRX mechanism at low strain rates was CDRX, while at high strain rates, the dominant DRX mechanisms were CDRX and twin-induced dynamic recrystallization (TDRX). Furthermore, the activation of twinning and DRX process altered the crystal orientation from favorable for basal slip to non-basal slip. This shows that, in addition to twinning, non-basal dislocation slip could be activated, and it plays an important role in plastic deformation behavior of extruded BTX000 alloy. The experimental results of this study provide new insight for further improving the plastic deformation ability of dilute Mg alloy.

Effect of copper addition on tensile behavior in Fe-Cr-Ni stable austenitic stainless steel

Effects of Cu on tensile properties and deformation mechanisms were investigated in the stable Fe-Cr-Ni austenitic stainless steel with different wt% of Cu. Addition of Cu slightly decreased the yield strength and negligibly affected the ultimate tensile strength, while it induced a considerably large reduction of fracture elongation. Deformation twins were generated during the tensile deformation, and fraction of deformation twins decreased with increasing Cu content. However, degree of twinning was similar, regardless of Cu content, at the same tensile strain, which indicated that development of twins was interfered by earlier fracture triggered by Cu addition and hardly contributed to different hardening behavior at the early stage of deformation. Cu significantly altered the motion of dislocations. The low-Cu-containing alloy mainly developed tangled dislocations and cellular structures, whereas the high-Cu-containing alloy did planar slip and high density of dislocation walls. Although the addition of Cu decreased the shear modulus and increased the stacking fault energy, dislocation substructure changed from tangled to planar slip with increasing Cu content. This indicated that slip planarity of high-Cu-containing alloys might be originated from the formation of short-range ordering or clustering, thereby leading to the local stress concentration and reduction of the elongation.

Tensile behavior at various temperatures of the Mg-Gd-Y-Zn-Zr alloys with different initial morphologies of LPSO phases prior to extrusion

The tensile behavior at (−100)-300 °C of the extruded and peak-aged Mg-5Gd-3Y-xZn-0.5Zr (x = 1 and 2 wt%, designated as VWZ531K and VWZ532K) alloys was investigated in the aspects of microstructural evolution, deformation modes, mechanical properties, etc. The higher Zn content endows the VWZ532K alloy with more intragranular lamellar LPSO phases and larger interdendritic ones at the initial homogenized status, which induces the lower DRX ratio, finer DRXed grain size, and negligible aging hardening response subsequently. During extrusion, precipitation of intragranular LPSO phases occurred in the VWZ531K alloy and partial DRXed grains formed a special texture component with the c-axis aligning parallel to the extrusion direction. However, the VWZ532K alloy experienced the dissolution of intragranular LPSO and dynamic precipitation of β phases. During tensile tests, deformation modes (including twinning, basal slip and non-basal slip) were modified by the testing temperature, grain orientation, LPSO morphology, etc. Compared with the β′ precipitates, LPSO phases are more thermostable to maintain the high-temperature strength, which induce the higher tensile yield strength of the peak-aged VWZ532K alloy (182 MPa) at 300 °C. The larger volume fraction of tension twins and lamellar LPSO phases result in the abnormal high elongation (17.9%) at −100 °C for the VWZ531K alloy.

Damage evolution of extruded magnesium alloy from deformation twinning and dislocation slipping in uniaxial stress-controlled low cycle fatigue

The damage evolution of an extruded AZ31 magnesium alloy in uniaxial stress-controlled low cycle fatigue is investigated using a quasi-in-situ method. In the three studied samples, that is, slipping dominated (SD), twinning/detwinning/basal slipping (TDBS), and twinning/detwinning/slipping (TDS), the fractions of slip bands and/or twins keep increasing till a critical cyclic number, and are essentially unchanged in the subsequent cycles. Inter-granular micro-crack initiation is found to be favorable in the TDS and SD samples. While both the inter-granular and intra-granular micro-cracks are detected in the TDBS sample. Moreover, the non-Schmid behavior in the crack trans-granular propagation path is investigated.

A comprehensive study on the mechanical behavior, deformation mechanism and texture evolution of Mg alloys with multi texture components

In this study, the deformation behaviors of Mg-3Al-1Zn alloy with multi texture components have been investigated using a combined experimental measurement and crystal plasticity modeling. Six types of multi textured samples were successfully prepared by an easily operated processing containing cold rolling, compression loadings, and subsequent annealing. The corresponding processing parameters to control multi texture components were also proposed. The effect of multi texture distribution on macroscopic mechanical response, activation of deformation modes, texture evolution, as well as the 101̅2 and 101̅1 twin fractions were systematically investigated. It’s a challenge work to model several texture components and random textures using EVPSC-TDT model for the first time. The simulation results generally match well with the experimental data. It has been found that the mechanical performance are strongly dependent on the multi texture components distribution and it was revealed by detailing the quantitative contribution of different deformation mechanisms to the plastic deformation. It’s the more disperse (0002) distribution of hard orientations that matters in enhancing the yield strength and ductility of samples with a given fraction of soft orientations. The findings in the present study are crucial for enriching the texture engineering for Mg alloys and could assist in designing better materials in term of mechanical properties and material performance.

Friction stir welding of CoCrNi medium-entropy alloy: Recrystallization behaviour and strengthening mechanism

Dynamic recrystallization (DRX), deformation-induced mechanical twins, and strengthening mechanism of CoCrNi equi-atomic medium-entropy alloy (MEA), welded by friction stir welding (FSW) at different welding speeds, were systematically investigated in this study. The results indicated that FSW led to grain refinement in the stir zones (SZs) with diameters fluctuating in the range of 2.1–9.6 μm. The hardness was improved from ∼154HV (base material, BM) to 238–263HV (SZ) by FSW. The yield strength and ultimate tensile strength of the optimal SZs were 601 MPa and 844 MPa, respectively, which were 260% and 134% higher than those of the as-received material, respectively. Analyses of DRX behaviour indicated that discontinuous DRX (DDRX), continuous DRX (CDRX), and geometric DRX (GDRX) were successively activated with increasing strain during FSW to facilitate grain refinement. The larger the grain size, the higher the fraction of mechanical twins. However, the thickness of the twins had a negative correlation with the grain size. Although a few HCP structures were formed on the twin boundaries, grain refinement and dislocation hardening mechanisms remained dominant in enhancing the mechanical properties of the SZs. The strength-ductility synergy by FSW, under low heat input conditions, was caused by the formation of thin mechanical twins in fine equiaxed grains. Thinner twins were more effective in transferring and homogenising plastic deformation, thereby contributing to the postponing of plastic instability and further promoting an excellent combination of strength and ductility. These findings confirm that FSW is a promising approach for strengthening the MEA for superior performance.

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.

Effect of CaO on structure and properties of AZ61 magnesium alloy

In this study, the effects of CaO additions on microstructure, texture, and mechanical properties of AZ61 magnesium (Mg) alloy were investigated. For this target, AZ61 Mg alloy samples containing 0.5 and 1 wt% CaO were investigated. The results showed that the addition of CaO leads to formation Ca-rich intermetallic compounds, which in turn, influenced the structure evolution of the alloy. In addition, it was observed the formation of twined structures in the alloys containing CaO, where the fraction of tension twins increased by increasing the amount of CaO. This was conjugated with the evolution of non-basal texture components, as found by the orientation distribution functions recorded for the investigated samples. Mechanical properties showed that the ductility of the AZ61 Mg alloy reduced by the CaO addition, whereas the yield strength was enhanced in the AZ61-1CaO sample.

The effect of grain refinement on the deformation and cracking resistance in Zn–Al–Mg coatings

The present study is dedicated to explore the effect of grain refinement on cracking resistance of hot-dip galvanized Zn–Al–Mg coatings on steel substrate. In this work, we demonstrate the enhancement of plastic deformation and cracking resistance by refining the microstructure (primary zinc grains) of the Zn–Al–Mg coatings. For this purpose, two types of Zn–Al–Mg coatings namely, fine grained and coarse grained microstructures are investigated utilizing in-situ scanning electron microscopy tensile tests. Electron backscatter diffraction technique is used to illuminate the deformation behavior at the scale of grains (and/or within grains). The results reveal that the coating with fine grained microstructure possesses higher ductility and cracking resistance, whereas the coating with coarse grain microstructure induces more transgranular cracking during deformation. Moreover, primary zinc grain refinement has been shown to decrease the fraction of coarse deformation twins that serve as undesirable sites of micro-cracking. In particular, both deformation mechanisms and cracking behavior are found to be grain size-dependent in these coatings.

Interactions between twins and dislocations in ZK60 alloy under dynamic compression

The interactions between tension twins and dislocations of ZK60 alloy under dynamic compression were investigated by electron backscattering diffraction and transmission electron microscopy. The samples were divided into two groups, where group-1 experienced primary deformation with different strains (4, 6, 9, and 12%) and group-2 experienced secondary deformation based on the primary deformation (4% + 8%, 6% + 6%, 9% + 3%) to reach the final deformation of 12%. The results reveal that the tension twins are the dominant deformation mechanism during the early stages of deformation, and the volume fraction of tension twins and dislocation density increase with the increase in deformation. Moreover, the twin boundaries and dislocations generated during the primary deformation become the resistance to the dislocations motion during the secondary deformation.

Constitutive and transformation kinetics modeling of [formula omitted]-, [formula omitted]-Martensite and mechanical twinning in steels containing austenite

In the present study, a physically based constitutive and transformation kinetics model of stress assisted (SA) and strain induced (SI) ε-, α′-Martensite and twinning in steels containing austenite, is presented. The deformation behavior of γ-Austenite, α′-Martensite, α- and δ-Ferrite is modeled in terms of the dislocation density. The macroscopic elastoplastic response of the material in uniaxial tension is calculated with respect to the partitioned stress and strain in each phase using a homogenization method. Inelastic strain accommodation below the yield strength of austenite is considered due to the SA transformation. The kinetics of SA and SI ε-, α′-Martensite and twinning in austenite, are calculated in terms of the ε-, α′- and twin embryo nucleation rate and the subsequent growth of shear bands, promoted by the applied stress and strain, respectively. Heterogeneous nucleation of α′-Martensite only on ε and twin shear band intersections is considered, presenting a stress state dependence. Austenite stability against TRIP and TWIP is modeled as a function of the austenite stacking fault energy and the separation distance of Shockley partial dislocations. Fitting the model to experimental stress-strain data allows for the prediction of α′-, ε-Martensite and mechanical twin fractions, without a prior notion of the transformation kinetics. A MATLAB implementation and an Extended Methodology section are provided as Supplementary Material. The model could be used to aid in the design of novel alloys with exceptional properties, like medium Mn steels.

Effects of cold-rolling and subsequent annealing on the nano-mechanical and creep behaviors of CrCoNi medium-entropy alloy

The influences of thermomechanical treatment on the microstructural evolution and nano-mechanical behaviors of CrCoNi medium-entropy alloy (MEA) were systematically studied through a series of nanoindentation experiments. Cold-rolling induced substantial grain refinement and during subsequent annealing, the CrCoNi MEA exhibited a highly temperature-dependent recrystallized microstructure characterized by significant variations in grain size, dislocation density, and twin fraction with increasing annealing temperature. The hardness of the cold-rolled sample increased with annealing temperature, reaching its highest value at 600 °C and then decreasing gradually. Annealing was also found to influence the creep behaviors of the CrCoNi MEAs. A cold-rolled sample annealed at 700 °C exhibited the highest resistance to creep. The results of a detailed microstructure examination, calculations, and theoretical analysis revealed that annealing twins play a key role in increasing both the strength and the creep resistance of the CrCoNi MEAs. Short-range order clusters also contributed to the strength and creep resistance.

Electroplastic Effects on the Mechanical Responses and Deformation Mechanisms of AZ31 Mg Foils.

Electrical-assisted (EA) forming technology is a promising technology to improve the formability of hard-deformable materials, such as Mg alloys. Herein, EA micro tensile tests and various microstructure characterizations were conducted to study the electroplastic effect (EPE) and size effect on the mechanical responses, deformation mechanisms, and fracture characteristics of AZ31 Mg foils. With the assistance of electric currents, the ductility of the foils was significantly improved, the size effects caused by grain size and sample thickness were weakened, and the sigmoidal shape of the flow stress curves during the early deformation stage became less obvious. The EBSD characterization results showed that the shape change of the flow stress curves was due to the EPE suppressing the activation of extension twinning at the early deformation stage, especially for the coarse grain samples. The suppression of extension twinning resulted in a quick increase in flow stress due to the dislocation-dominant work hardening, and the increased flow stress eventually promoted extensive deformation twins at large deformation. Thus, as the sample strained to 10% tensile deformation, the EA-tested samples showed a larger volume fraction of deformation twins than the non-EA samples. The reference orientation deviation analysis verified that the deformation twins in the EA samples were formed in the large deformation stage. Combined with the fractography, the EPE also improved the ductility by suppressing the expansion of cleavage surfaces.

Effect of Precipitates in Mg−Sm Alloys on Their Deformation Behavior and Yield Asymmetry

The strengthening effect of precipitates in a Mg−0.4Sm (wt%) alloy on dislocation slip and extension twinning (ET) is evaluated by experimental work and viscoplastic self‐consistent (VPSC) modeling. Experimental results reveal that the extruded Mg−Sm alloy consists of rare‐earth texture (&lt;−12−11&gt; // ED) and extrusion texture (&lt;−12−10&gt; // ED) components. After aging precipitation, tensile yield stress is more effectively enhanced than compressive yield stress. Transmission electron microscopy (TEM) analysis confirms that the precipitates in the aged Mg−Sm alloy are of β′ with an orthorhombic crystal structure. The introduction of β′ precipitates in the compression sample stimulates a higher fraction of extension twins. VPSC modeling indicates that the critical‐resolved shear stresses (CRSS) for prismatic slip, pyramidal &lt;c + a&gt; slip, and basal slip are increased by 75, 30, and 6 MPa, respectively, by aging precipitation, while the precipitation strengthening on ET is negligible. By preferentially hardening against prismatic slip, the compressive and tensile yield asymmetry are increased for the aged Mg−Sm alloy.

Strain rate effects on mechanical behavior and microstructure evolution with the sequential strains of TWIP steel

Dynamic and quasi-static mechanical behaviors and microstructural mechanisms of the TWIP steel were investigated at strain rates from 0.001 to 3000 s-1 by using a split Hopkinson tensile bar paired with interrupted strain fixtures and electron backscattered diffraction technology. The positive strain rate sensitivity of dynamic yield stress is significantly different from that of quasi-static stress at a transition strain rate of about 100 s-1 for the different yield mechanisms. Owing to the high stress from dynamic loading, the smaller twin onset strain and the higher twin fraction as well as the more intense twin boundary-dislocation interaction are responsible for the enhanced strain hardening behavior in the early stage of strain. Nevertheless, the rapid decrease of the strain hardening rate in the later stage and the resulting negative strain hardening rate sensitivity are due to the decreasing growth rate of the twin boundary fraction. It is closely associated with the softening effect of adiabatic temperature rise (∼117 °C) by raising the stacking fault energy and inhibiting twinning activity. In addition, the fuzzy regions caused by high-density dislocations within interfaces between twin and grain boundaries may lead to the lower elongation of the dynamic tensile specimen. The grains with a high Schmid factor are conducive to slip rather than twinning, which enables the twin region orientation for slip-mediated plastic deformation. This study thus advances the understanding of the strain and strain rate dependence of the mechanical behaviors and twinning mechanisms of TWIP steels for the safety performance of automobiles.