Twin Growth Research Articles

Effects of pre-aging on microstructure evolution and deformation mechanisms of hot extruded Mg–6Zn–1Gd–1Er Mg alloys

Abstract The influence of pre-aging treatment on the microstructure, texture and mechanical properties of the Mg–6Zn–1Gd–1Er (wt.%) alloy was investigated. The microstructure analysis shows that the presence of pre-aging is beneficial to { 10 1 ¯ 2 } twin nucleation at the early stage of extrusion and inhibits the growth of twins and promotes the formation of [ 10 1 ¯ 0 ] -fiber texture components, thus accelerating the complex process of recrystallization. In the middle stage of extrusion, the extruded samples under the condition of solid solution were subjected to dynamic precipitation during severe shear deformation. The precipitation of the second phase particles followed the particle stimulating nucleation (PSN) mechanism, which increased the volume fraction of DRX grains during extrusion. In the extruded samples under the peak-aged condition, the particles appear dissolved during the severe shear deformation strain, which slows down the DRX process. In the later stage of extrusion, the small rod-shaped particles followed the PSN mechanism, and finally formed the strong fiber texture. The extruded alloy exhibits the strongest mechanical properties under the peak-aged state, with ultimate tensile strength (UTS) of 346 MPa, tensile yield strength (TYS) of 217 MPa, and elongation to failure (EL) of 13.6%. The improvement of mechanical properties is mainly attributed to the existence of strong fiber texture, small rod-shaped and block-shaped phases.

Effects of pre-existing twins on the mechanical properties of Laves phase V2Hf-based alloy

Twins have attracted much attention due to the significant effects on strengthening and toughening metallic and ceramic materials, while relevant research on twins in intermetallic Laves phases is rare despite their wide applications as structural and functional materials. Herein, a Laves phase V2Hf-based alloy V-14Hf-22Nb (at.%) was designed and twins were incorporated by precompression for probing the effects of pre-existing twins on the mechanical properties of Laves phase alloy. The results showed that the pre-existing twins not only prompted the twin growth during the secondary compression but also reacted with perfect dislocation in the V2Hf. Consequently, the compression strength of the alloy decreased from 2.13 GPa to 1.95 GPa, and the plastic strain of the alloy dropped from 13.75% to 12.20%. The results obtained were helpful for better understanding of the effects of pre-existing twins on the mechanical properties of Laves phase alloys.

Local hardening and asymmetric twin growth by twin-twin interactions in a Mg alloy

Local hardening and asymmetric twin growth by twin-twin interactions in a Mg alloy

Second-trimester growth velocities in twin and singleton pregnancies.

Previous small studies used individualized growth assessment (IGA) to characterize prenatal growth velocities of singletons and twins. We aimed to compare second-trimester growth velocities of individual anatomical parameters between monochorionic diamniotic (MCDA) twins, dichorionic diamniotic (DCDA) twins and singleton fetuses in a larger study. This was a study of a novel cohort of 222 MCDA twins and previously published cohorts of 40 DCDA twins and 118 singletons with serial ultrasound data. Fetal biometric measurements of biparietal diameter, head circumference, abdominal circumference and femur diaphysis length from prenatal ultrasound examinations were used to calculate second-trimester growth velocities using direct calculation or linear regression analysis. Linear fit was assessed based on the coefficient of determination (R2 ). Mean growth velocities and variances were compared among the three groups. The majority of cases underwent three second-trimester ultrasound examinations with fetal biometry available. All fetuses had linear growth, with R2 > 99% for all parameters. Only 1-2% of all MCDA and DCDA anatomical parameters had abnormal growth velocity scores outside the 95% reference range for singletons. There were no significant differences in mean growth velocity for any parameter between MCDA twins and singletons. Femur diaphysis length growth velocity was significantly lower in DCDA twins than in both MCDA twins and singletons. There were no other significant differences among the groups. Expanding on prior work using IGA, we found that second-trimester growth velocity of the four major anatomical parameters overall was similar between twins and singletons and between MCDA and DCDA twins, supporting the use of singleton-derived growth standards for IGA in twins. Twin growth potential appears to be similar to that of singletons in the second trimester, suggesting that subsequent growth divergence may be due to third-trimester physiological or pathological changes in twin pregnancies. © 2022 International Society of Ultrasound in Obstetrics and Gynecology.

Influence of lowering basal stacking fault energy on twinning behaviours

Deformation twinning plays a crucial role in the strong anisotropic strength, poor deformability, and texture and microstructure evolutions of hexagonal metals. Modifying twinning behaviour thus may tailor mechanical behaviour of hexagonal metals. Since propagation and growth of deformation twins inevitably interact with other plastic deformation carriers, such as dislocations, stacking faults and phase transformation bands if they can be activated simultaneously, we hypothesize that lowering basal stacking faults energy (SFE) may promote the activity of partial dislocations, thus generating basal stacking faults (BSFs) and even face centred cubic (fcc) phase. Based on first-principles density functional theory calculations, Al solutes can reduce basal SFE and the cohesive energy difference between hexagonal close packed (hcp) and fcc phases of Ti alloys. Microscopy characterizations of deformed Ti-10at.%Al alloy demonstrated that lowering basal SFE actually promotes the formation of profuse BSFs and fcc nanobands in hcp-phase Ti-10at.%Al alloy during mechanical deformation. Consequently, these defects constrain the propagation and growth of {101¯2} deformation twins. More importantly, BSFs and fcc nanobands are activated from twin boundaries during twinning, pinning deformation twins. As a result, deformation twins in Ti-10at.%Al alloy have an average thickness of ∼130 nm, much thinner than twins in pure Ti and other Ti alloys with the similar grain size and under similar strains. In addition, BSFs and fcc nanobands are activated during twin-twin interactions, releasing local stress/strain concentration. Using topological model and atomistic simulations, we further explored interaction mechanisms between deformation twinning and BSFs or fcc nanobands that formed prior to twinning, and the mechanisms of the emission of BSFs and fcc nanobands from twin boundaries during twinning. This work provides insights into understanding the influence of lowering basal SFE on twinning behaviours in hexagonal metals.

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.

Study on Positions of Twin Scales and Growth Regulators on Propagation of Narcissus (Daffodil) cv. Stamen Cream

Study was carried out to evaluate the influence of position of twin scales and growth regulators on propagation of Narcissus (Daffodil) cv. Stamen creamx. Results of the study reveals that out of eight treatment combinations tested in the experiment minimum days for callus initiation (26.00 days),root initiation (36.50 days) and bud initiation (44.50 days) after incubation resulted with combination of outer scales treated with IBA 100 ppm. Further maximum number of buds per chip (2.00), bud diameter (11.95 mm), bud length (2.76 cm) and bud weight (0.41g) were recorded with the same treatment. Inner scales treated with IBA 100PPM resulted with maximum days for callus initiation (37.50 days), root initiation (48.00 days) and bud initiation (60.50 days) after incubation. Least number of buds per chip (1.00), minimum bud diameter (7.45 mm), bud length (1.30 cm) and bud weight (0.19 g) were also recorded with same treatment. Most of the differences among the treatments for almost all recorded parameters were statistically significant.

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.

Fetal brain injury in survivors of twin pregnancies in single fetal intrauterine death – a short literature review

Background. The incidence of multiple pregnancies is increasing in the last 10 years, along with the associated complications. Brain injuries and neurological complications associated with single fetal intrauterine death (sIUFD) are the most serious and have a negative impact on the surviving twin. Methods. This review was performed by a single individual who searched via Google Scholar and Pubmed clinical studies which included sIUFD in monochorionic (MC) twin pregnancies. This research included studies from the last 10 years and the keywords used were: “single intrauterine fetal death”, “monochorionic”, “fetal brain lesions”, and “twin pregnancies”. Results. After analyzing the clinical studies according to the keywords, only 15 studies (462 pregnancies) met the inclusion criteria. These included: monochorionic pregnancies and sIUF that occurred after 14 weeks. The results of these trials showed a strong statistical association between single fetal intrauterine death and co-twin fetal brain lesions and neonatal death. Also, many of these recent studies mentioned the relationship between monochorionic city and preterm delivery, fetal growth restriction and twin–twin transfusion syndrome (TTTS). Conclusion. Analyzing all these clinical studies, we can conclude that the intrauterine death of a twin in monochorionic pregnancies after the age of 14 weeks of gestation significantly affects the neurological development of the surviving twin.

Pregnancy outcomes in twin pregnancies over 10 years.

The aim of this study was to evaluate the changes in twin pregnancy outcomes between 2007 and 2016 in a Korean population. The data for this nationwide population-based study was obtained from the national birth registry of the Korean National Statistical Office and the Health Insurance Review & Assessment Service of Korea. Women with twin pregnancies who gave birth between 2007 and 2016 were included. From 2007 to 2016, the rate of twin pregnancies increased (1.37% vs. 1.91%, respectively, P<0.0001). The risk of preterm birth (adjusted odds ratio [aOR], 1.77; 95% confidence interval [CI], 1.66-1.89) also increased; however, the risk of twin growth discordance (aOR, 0.90; 95% CI, 0.82-0.99) decreased. The risks of cesarean section (aOR, 1.16; 95% CI, 1.03-1.29), gestational diabetes mellitus (aOR, 2.10; 95% CI, 1.83-2.39), and postpartum hemorrhage (aOR, 1.27; 95% CI, 1.14-1.41) all increased from 2007 to 2016. Twin pregnancy outcomes have changed significantly in Korea over a recent 10-year period.

Generation of fine extension twins in rolled magnesium alloy AZ31B at room temperature by simple shear

A textured AZ31B plate was subject to simple shear along 45° to its normal direction at room temperature, and extension twinning under biaxial stress was investigated by statistical analyses of electron backscatter diffraction maps. Extension twinning is the primary deformation mechanism up to a plastic shear strain of 15% (8.66% equivalent plastic strain), and the most frequent twin thickness remains to be 1.5 μm. The simple shear suppresses twin growth, yet brings sustained twin nucleation. The critical resolved shear stress of extension twinning is determined as 25 MPa and remains virtually constant up to a plastic shear strain of 10%. The present study suggests that by using biaxial stress state that promotes multi-twin nucleation, a microstructure with dense and fine twins can be produced at room temperature in a conventional Mg alloy, without jeopardizing the contribution of twinning to plastic straining.

Merohedral Mechanism Twining Growth of Natural Cation-Ordered Tetragonal Grossular

Garnet supergroup minerals are in the interest of different applications in geology, mineralogy, and petrology and as optical material for material science. The growth twins of natural tetragonal grossular from the Wiluy River, Yakutia, Russia, were investigated using single-crystal X-ray diffraction, optical studies, Raman spectroscopy, microprobe, and scanning electron microscopy. The studied grossular is pseudo-cubic (a = 11.9390 (4), c = 11.9469 (6) Å) and birefringent (0.01). Its structure was refined in the Ia3¯d, I41/acd, I41/a, and I4¯2d space groups. The I41/a space group was chosen as the most possible one due to the absence of violating reflections and ordering of Mg2+ and Fe3+ in two independent octahedral sites, which cause the symmetry breaking according to the group–subgroup relation Ia3¯d → I41/a. Octahedral crystals of (H4O4)4−-substituted grossular are merohedrally twinned by twofold axis along [110]. The mechanism of twining growth led to the generation of stacking faults on the (110) plane and results in the formation of crystals with a long prismatic habit.

Pregnancy outcomes and risk factors for preeclampsia in dichorionic twin pregnancies after in vitro fertilization: a five-year retrospective study

BackgroundBoth in vitro fertilization (IVF) and preeclampsia (PE) were associated with placental dysfunction. Although IVF can increase the incidence of PE, the pregnancy outcomes and risk factors for preeclampsia in dichorionic twin pregnancies conceived via IVF remain unclear. This study aimed to investigate the pregnancy outcomes and the risk factors for preeclampsia in dichorionic twin pregnancies conceived through IVF compared to those conceived after natural conception (NC).MethodsThis retrospective observational study enrolled 181 dichorionic twin pregnancy women with preeclampsia from 2016 to 2020. According to the mode of conception, they were allocated into IVF (n = 117) and NC groups (n = 64). The clinical characteristics of preeclampsia and pregnancy outcomes between the two groups were compared by using Student’s t test, chi-square test, and Fisher’s exact test, and logistic regression models were used to obtain adjusted odds ratios (aOR) with 95% confidence intervals (CI) for risk factors of early-onset preeclampsia.ResultsThe incidence of early-onset PE and growth discordance in dichorionic twin pregnancies with PE is significantly higher in IVF-PE group than in NC group (78.60% vs 43.80%, P < 0.001, 11.10% vs 25.00%, P = 0.015). We found that IVF (aOR = 4.635, 95% CI: 2.130–10.084, P < 0.001) and growth discordance (aOR = 3.288; 95% CI: 1.090–9.749, P < 0.05) increased the incidence of early-onset PE.ConclusionsIn preeclamptic dichorionic twin pregnancies, IVF and growth discordance were associated with the increased incidence of early-onset PE. The underlying mechanism for the relationship between IVF and early-onset PE or growth discordance may be placental dysfunction.

Assessment of fetal growth in twins: Which method to use?

Fetal growth restriction contributes to the excess perinatal mortality and morbidity associated with twin pregnancies. Regular ultrasound monitoring for fetal growth restriction is an essential component of antenatal care of twin gestations. It is accepted that twins have divergent growth trajectories around 28-30 weeks' gestation and are born smaller compared to singletons. Despite this well-established difference in fetal growth, twin pregnancies have been traditionally managed using growth standards developed for singleton pregnancies. Numerous recent studies have demonstrated a strong case supporting the use of twin-specific growth standards, but clinical implementation has been lacking. In this paper, we will review the evidence on factors affecting fetal growth, the rationale for twin-specific reference charts, clinical evidence for their use, and future direction of research. Applying singleton growth standards to twin pregnancies inflates the abnormal growth rate, and recent clinical evidence from several studies suggests that they are too stringent for classification of twins. The association of adverse perinatal and maternal outcomes such as perinatal death, preterm birth, neonatal care unit admission, hypertensive disorders of pregnancy, and composite neonatal morbidity is stronger when classification is made using twin-specific standards compared to singletons.

Establishment of a measurement system for similarity and dissimilarity of twins

Using previous twins study methods, it is thought that assessments of a given level may certainly be derived by analysis of similarity. However, there is the drawback that analysis cannot be done without a large population of twin pairs. In this study, author attempted the combined use of WIM and cross correlation function as a method that can evaluate similarity even in small populations of twin pairs. An evaluation chart was constructed for similarity from cross correlation function statistics with random pairs in the height growth of general boys, and from that chart a cross correlation function, r of 0.9 or higher, became a standard indicating high similarity. When cross correlation functions for height growth in monozygotic and dizygotic twins are applied to this evaluation chart, the correlation in monozygotic twins is high and that in dizygotic twins is somewhat high. Higher similarity is thus seen in monozygotic twins.

The Role of Digital Twins in Connected and Automated Vehicles

Digital twins found their genesis in the halls of NASA and the methods of product lifecycle management. Rapidly evolving trends around the proliferation of sensors, the Internet of Things, Industry 4.0, and cyber-physical systems have spurred the growth of digital twins. This paper reviews digital twins and their use in connected and automated vehicles (CAVs). Strictly speaking, digital twins must have communication between a physical system and its model, as opposed to similar methodologies that achieve indirect communication through iteration, or that substitute different parts of a system simulation with bits of hardware or software for testing. In practice, new methodologies for testing CAVs are sufficiently complex and difficult to apply simple labels. This is seen in our review of vehicular digital twins. Several gaps and challenges are apparent for the continued advancement of digital twin applications. We note some developing areas as traffic management centers, digital maps, onboard diagnostics, and logistics. Digital twins foster model reuse and encourage the use of multiple models at different scales of resolution. The role of digital twins will continue to grow as models become more tightly integrated to the physical systems they represent. This will drive such models towards uniqueness (matching a particular vehicle or road), adaptability (evolving with changing conditions and subject to wear and tear), and interpretability (conveying useful information to a human user). A maturing connected infrastructure and the development of smart cities will cause the number of new digital twin services to explode in a myriad of unforeseen ways.

Refinement of αs phase and formation of nano-twins of Ti–7Mo–4Al–3Nb–2Cr alloyed by Zr element

To refine the αs phase and form the nano twins, different contents of Zr were added to as-cast Ti–7Mo–4Al–3Nb–2Cr-xZr alloys. Phase and microstructure evolution, mechanical properties and related mechanisms were systematically studied. Results show that as the Zr increases from 0 to 8 wt%, only α and β phases exist and the peak of β (110) shifts from 39.5 to 39.1° by XRD. The volume fraction of the αs phase changes from 75.4 to 73.9%, and its length decreases from 0.62 to 0.22 μm. More dislocations appear in the α/β interface and the growth twin of {10 1¯ 1}α < 11 2¯ 0> type with a width of ∼20 nm forms in α phase as observed by TEM. The refinement of the αs phase is due to the addition of Zr to promote nucleation and inhibit the growth rate. With increasing Zr content from 0 to 8 wt%, the strength increases from 961 to 1303 MPa, while the toughness decreases from 77 to 62 MPa m1/2. The fracture toughness decreases by 19.5%, while the strength increases by 35.6%. The refinement of αs phase, the solubilization of the Zr in the matrix, the increase of dislocation in the α/β interface, and the formation of nano twins are the main reasons for improving the strength and toughness of the studied alloys.

An experimental study of anisotropic fatigue behavior of rolled ZK60 magnesium alloy

The strong anisotropy of wrought magnesium alloy significantly affects its application in structural components bearing fatigue loads. This study aims to explore the effects of grain orientations (RN-N versus RN-R) and sampling surfaces (RT-R versus RN-R) on the fatigue behavior of the ZK60 Mg alloy. Stress-controlled fatigue tests and quasi-in-situ EBSD tests were carried out on three different specimens. The sampling surface and grain orientation both have significant effects on the fatigue life of samples, especially at low-stress amplitude. Although the sampling methods are different, the three samples show similar cyclic evolution patterns. The consumption of available twins dominates the cyclic hardening behavior in the fatigue early stage. When loaded along ND, abundant intersecting twins induced by various variants can produce large plastic deformation, which is detrimental to the fatigue resistance of RN-N samples. When loaded along RD, the activation of specific twin variants leads to the reduction of the twinning areas, thus decreasing the plastic deformation of RN-R and RT-R samples. In addition, the constraint force along the width direction and sandwich-like metallographic structure of RN-R samples will hinder the activation and growth of twins, which results in smaller twinning areas and higher fatigue life than RT-R samples.

Investigation of twin growth mechanisms in precipitate hardened AZ91

In this work, an elasto-viscoplastic fast-Fourier-transform (EVP-FFT) model with a dislocation-density (DD) based hardening law is employed to study the growth of a {101¯2} tensile twin that is blocked by basal-precipitates in precipitate-hardened AZ91 magnesium alloy. It is frequently reported that twin growth is hindered in precipitate-hardened Mg alloys; however, thick twin domains are often observed experimentally in these material systems. Detailed numerical investigation of deformation twinning starting from an early propagation stage, before twin growth, reveals that the stress fields that result from two sequentially propagated twins co-impinging on a precipitate relaxes the twin back stress locally and promotes twin growth at the twin-precipitate junction. Based on these findings, a two-step growth mechanism is proposed for twins arrested by precipitates. In the first step, the interaction of a twin tip with a precipitate develops a stress concentration on the other side of the precipitate, prompting the formation of a second twin. Subsequently, the back stresses associated with the first twin are relaxed by the formation of the second twin, allowing the first twin to grow at the twin-precipitate junction and eventually engulf the precipitate. This mechanism suggests that twin growth can be achieved locally with minimal additional external forces, explaining how relatively large twin domains can develop even in the presence of arrays of precipitates.

Improved strength and plasticity of magnesium matrix nanocomposites reinforced by carbonaceous nanoplatelets and micro-clusters

The uniform distribution of carbonaceous nanomaterials in a magnesium matrix composite can dramatically improve the strength without sacrificing ductility. In this work, graphene oxide (GO) reinforced AZ31 magnesium alloy matrix composites were fabricated using friction stir processing to achieve higher strength, toughness, and plasticity compared with the unreinforced condition. The composite microstructure consisted of individual GO nanoplatelets and GO micro-clusters, having different effects on the deformation behavior of the matrix. Texture analysis revealed that adding GO led to the activation of different twin types and variants during tensile deformation. In contrast, only one extension twin variant was dominant in the case of the unreinforced condition. In spite of the GO micro-clusters, the individual GO nanoplatelets cannot hinder the growth of twins. The GO micro-clusters inhibit twin growth, change the stress state in the parent grains, and provide a driving force for twin nucleation. The extensive twinning occurrence was followed by twin intersections and significantly enhanced the plasticity of the alloy by providing easy orientations for further basal slip.