Twin Growth Research Articles

High-Rate Deformation of Titanium in Shock Waves at Normal and Elevated Temperatures

The elastoplastic shock compression, unloading, and steplike shock compression of titanium at room temperature and in the vicinity of 600°C are measured. The dependences of the dynamic yield strength on the pressure of shock compression and temperature are determined, and the dependences of the strain rate in shock waves in the range 105–107 s–1 on the shear stress and temperature are obtained. In contrast to the data obtained for fcc metals, the yield strength of titanium behind a shock wave is found to decrease. The decrease in the yield strength is thought to be caused by a decrease in the plastic flow stress during the growth of twins and their reversibility.

Temperature dependence of deformation behavior, microstructure evolution and fracture mechanism of Inconel 625 superalloy

The temperature effect on deformation behavior, microstructural evolution and fracture mechanism of Inconel 625 superalloy sheet were systematically investigated in a wide temperature range from room temperature (RT) to 950 °C. Tensile temperature was varied in increments of 50 °C after reaching 500 °C. Obvious serrations occurred at the medium temperature range and evolved in the sequence of B→B+C→C with increasing temperature. Microscopic observations from transmission electron microscopy (TEM) indicated that the interaction between C14-Ni 2 Nb Laves phases and mobile dislocations was found to be responsible for the type B serration, and the type C serration was associated with nucleation and growth of deformation twins. Recrystallized grains were observed above 850 °C, and results from electron backscattered diffraction and TEM showed that both continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) take place, while CDRX was the secondary nucleation mechanism. The stress concentration caused by Nb-rich phases was responsible for crack nucleation at tensile temperatures below 650 °C. The combined action of slip bands impingement on grain boundaries and stress concentration caused by Nb-rich phases led to crack initiation at the tensile temperature of 750 °C and 850 °C. Void nucleation at triple junctions of grain boundaries resulted from grain boundary sliding should be responsible for the ductile fracture at 950 °C. All data are available from the corresponding author on reasonable request. • Tensile behavior and fracture mechanism were closely dependent on temperature. • Serrated flow was related to Laves phases-dislocations interaction and deformation twins. • Dynamic recrystallization characteristic during tensile deformation was revealed.

Understanding the interaction of extension twinning and basal-plate precipitates in Mg-9Al using precession electron diffraction

Precession electron diffraction is used to characterize the interaction between {101¯2} tensile twins and basal plate-like precipitates in a post-deformed, precipitate-dispersed Mg-9Al micropillar. We observed a heterogeneous distribution of precipitates in the micropillar sample, which enabled the study of the different stages involved in twin-precipitate interactions. We show that twin nucleation was promoted, taking place on the surface, as well as from the interior of the micropillar. Twin tip propagation and twin growth were hindered by the precipitates. Twin tips either were arrested by precipitates and new twins formed on the other side of the precipitate, or continued to grow around the precipitate without the re-nucleation events. During twin thickening, the precipitates did not significantly alter the shear-dominant twin boundary migration mechanism, as evidenced by the relatively flat twin boundaries around the partially embedded precipitates. However, the twin boundary migration was retarded by the precipitates, especially in regions confined by closely-spaced precipitates.

Tribological behavior of Ni–SiC composite coatings produced by circulating-solution electrodeposition technique

Ni–SiC composite coatings were successfully deposited on interior surfaces of cylinders by circulating-solution electrodeposition (CSD) technique on a newly-designed composite electroplating apparatus. The circulating-solution agitation was effective to improve the dispersibility of SiC particles. The morphology, microstructure, hardness and tribological performances of Ni–SiC composite coatings varying SiC content were studied. Results showed that the dominant wear mechamism was oxidative wear, accompanied by abrasive wear. The friction coefficient and wear rate decreased with increasing SiC content. The excellent wear resistance was attributed to the existence of (111) growth twins, the improved hardness, the protections from NiO layer and SiC reinforcements, and the friction-induced microstructure changes underneath the tribo-surface. The protective capability of NiO layer and micro-SiC particles were discussed in detail.

Nanostructured metal oxide@carbon dots through sequential chitosan templating and carbonisation route

Pyrolytic conversion of biomass stands as one of the most promising sustainable routes to carbon-based nanostructures, including carbon dots (CDs). Yet, molecular composition of functionalized carbon dots is limited to heteroatom doping and the incorporation of few single metal precursors. Besides, most of these modifications were undertaken by strenuous post-grafting procedures, requiring organic solvents for carbon dispersion and excluding a large library of potential reactants because of incompatible experimental conditions. In a significant divergence from the well-established literature, we herein disclose an innovative and highly versatile approach for enriching carbon dot functionalities. This straightforward method merges chitosan as carbon precursor and metal alkoxide as ceramic precursor and explores a twin growth of two dissimilar "metal oxide@carbon dots" phases through hydrothermal conversion of the carbohydrate solution. Owing to the structure-directing effect of chitosan toward metal alkoxide precursors, a set of crystalline metal oxides including titanium dioxide, germanium oxide, and iron oxide clusters were intimately fused within the in situ formed nitrogen-containing carbon framework. Distinctively, the following approach uses water as solvent and renewable biomass as carbon source and is expected to shed light on the hidden talent of discarded bio-waste for engineering functional nanomaterials.

Chemical fluctuation enabling strength-plasticity synergy in metastable single-phase high entropy alloy film with gigapascal yield strength

In this work, we synthesized a FeCoNiCrCu high entropy alloy film (HEAF) via magnetron sputtering, which exhibited a metastable single FCC phase with random chemical distribution. Owing to the metastable nature, the as-deposited atomic structure affords nano-scale chemical fluctuation induced by moderate thermal annealing without causing phase transition. Compared to the single-phase FCC metals hitherto reported, our as-deposited and thermally annealed HEAFs exhibited the highest yield strength (3.4–4.2 GPa) under microcompression due to the combined action of nano-grains and growth twins. Furthermore, according to our experiments and atomistic simulations, plasticity-strength synergy was achieved as the nano-scale chemical fluctuation was able to promote deformation twinning in the HEAFs. The outcome of our research provides not only an atomistic understanding of the strength-plasticity synergy in the metastable FCC HEAFs with gigapascal yield strength, but also a facile method to design high performance metastable chemically complex metallic films.

Basal stacking fault induced twin boundary gliding, twinning disconnection and twin growth in hcp Ti from the first-principles

First-principles calculations were performed to investigate the structures and energetics of coherent twin boundaries (CTBs) and glide twin boundaries (GTBs) in hexagonal close-packed (hcp) Ti. The formation mechanism of GTBs and their correlation with twin growth were fundamentally explored. Results suggested that GTBs can form from the gliding of CTBs, through their interaction with basal stacking fault. The gliding eventually restored the CTB structures by forming a pair of single-layer twinning disconnections. The pile-up of twinning disconnections should be responsible for the wide steps at twin boundaries as observed in high-resolution transmission electron microscopy, which can further promote twin growth. Possible effects of various alloying elements on pinning twin boundaries were also evaluated, to guide the strengthening design of Ti alloys.

Predictive Accuracy of Singleton Versus Customized Twin Growth Chart for Adverse Perinatal Outcome: A Cohort Study.

Background: Fetal growth of twins differs from singletons. The objective was to assess the fetal growth in twin gestations in relation to singleton charts and customized twin charts, respectively, followed by a comparison of the frequency of neonatal complications in small-for-gestational-age (SGA) twins. Methods: We performed an analysis of twin pregnancies with established chorionicity with particular emphasis on postnatal adverse outcomes in newborns classified as SGA. Neonatal birth weight was comparatively assessed using both singleton and twin growth charts with following percentile estimation. Using a statistical model, we established the prediction strength of neonatal complications in SGA twins for both methods. Results: The dataset included 322 twin pairs (247 cases of dichorionic and 75 cases of monochorionic diamniotic gestations). Utilization of twin-specific normograms was less likely to label twins as SGA—nevertheless, this diagnosis strongly correlated with risk of observing adverse outcomes. Using a chart dedicated for twin pregnancies predicted newborn complications in the SGA group with higher sensitivity and had better positive predictive value regarding postnatal morbidity. Conclusions: Estimating twin growth with customized charts provides better prognosis of undesirable neonatal events in the SGA group comparing to singleton nomograms and consequently might determine neonatal intensive care prenatal approach.

The Effects of Free Surfaces on Deformation Twinning in HCP Metals

Deformation twinning is a predominant mode of plastic deformation for hexagonal close packed metals, like Mg and Ti. The heterogenous microstructure and the local stresses associated with twinning play a key role in their mechanical response and fracture. Surface analyses, like electron microscopy, are frequently employed to spatially map microstructure and micromechanical fields in order to study twinning behavior. However, these measurements are inherently influenced by the vicinity of the free surface. Here, an elasto-visco-plastic fast-Fourier-transform (EVP-FFT) polycrystal modeling approach is employed to investigate the effects of free surfaces on twin development before and after loading. We compare calculated micromechanical fields on free surfaces with those calculated inside the bulk and in some cases, experimental surface measurements. The results indicate that the creation of free surfaces can promote twin propagation and growth and can influence twin morphology by causing a twin lamella to become larger, more blunted and skewed. The structure along the twin boundaries are also affected, due to the higher driving stresses that extend prismatic-basal and basal-prismatic facets. Furthermore, free surfaces invoke different slip activities in the twin and the surrounding parent crystal by enhancing basal, prismatic and pyramidal slip in some localized regions, while reducing slip in others. We demonstrate that the simulated free-surface effects lead to better qualitative and quantitative agreement with experimental measurements from scanning electron microscopy and digital image correlation.

Patterns of discordant growth and adverse neonatal outcomes in twins

Intertwin size discordance is an independent risk factor for adverse neonatal outcomes in twin pregnancies. However, size discordance at a given point in gestation fails to take into consideration information, such as the timing of onset and the rate of progression of discordance, that may be of prognostic value. In this study, we aimed to identify distinct patterns of discordant fetal growth in twin pregnancies and to determine whether these patterns are predictive of adverse pregnancy outcomes. This was a retrospective cohort study of women with twin pregnancies in a single tertiary referral center between January 2011 and April 2020, who had at least 3 ultrasound examinations during pregnancy that included assessment of fetal biometry. Size discordance was calculated at each ultrasound examination, and pregnancies were classified into 1 of 4 predetermined patterns based on the timing of onset and the progression of discordance: pattern 1, no significant discordance group (referent); pattern 2, early (<24 weeks' gestation) progressive discordance group; pattern 3, early discordance with plateau group; or pattern 4, late (≥24 weeks' gestation) discordance group. The associations of discordance pattern (using pattern 1 as referent) with preterm birth, preeclampsia, size discordance at birth, and birthweight<10th percentile were expressed as adjusted relative risk with 95% confidence intervals and were compared with those observed for a single measurement of size discordance at 32 weeks' gestation. Of 2075 women with a twin gestation who were identified during the study period, 1059 met the study criteria. Of the 1059 women, 599 (57%) were classified as no significant discordance (pattern 1), 23 (2%) as early progressive discordance (pattern 2), 160 (15%) as early discordance with plateau (pattern 3), and 277 (26%) as late discordance (pattern 4). The associations of discordance pattern with preterm birth at <34 weeks' gestation and preeclampsia were strongest for pattern 2 (rates of 43% [adjusted relative risk, 3.43; 95% confidence interval, 2.10-5.62] and 17% [adjusted relative risk, 5.81; 95% confidence interval, 2.31-14.60], respectively), intermediate for pattern 3 (rates of 23% [adjusted relative risk, 1.82; 95% confidence interval, 1.28-2.59] and 6% [adjusted relative risk, 2.08; 95% confidence interval, 1.01-4.43], respectively), and weakest for pattern 4 (rates of 12% [adjusted relative risk, 0.96; 95% confidence interval, 0.65-1.42] and 4% [adjusted relative risk, 1.41; 0.68-2.92], respectively). In contrast, a single measurement of size discordance at 32 weeks' gestation showed no association with preeclampsia and only a weak association with preterm birth at <34 weeks' gestation. We identified 4 distinct discordance growth patterns among twins that demonstrated a dose-response relationship with adverse outcomes and seemed to be more informative than a single measurement of size discordance.

{101̄2} twinning in single-crystal titanium under shock loading

ABSTRACT We employ molecular dynamics simulations to investigate the evolution dynamics of twinning in single-crystal Ti under shock loading. The shock compression applied perpendicular to the c-axis leads to the activation of twins in single-crystal Ti. We find the twin variant activation for each case of the applied loading conditions follows Schmid criterion. However, the time for the activation of twin variants is not the same even with equal Schmid factor. The evolution and dominance of twin variants in each case of the applied loading conditions do not depend on the Schmid factor. High nucleation events and low overall twin volume fraction occur for the case where two conjugate pairs of twin variants activate while low nucleation events and high overall twin volume fraction occur for the case where only one conjugate pair of twin variants activate. Twin nucleation takes place throughout the simulation time for each case of the applied loading conditions. High twin growth occurs for the case where only one conjugate pair of twin variants activate in comparison to the case where two conjugate pairs of twin variants activate. This indicates that the number of activated twin variants affects the growth rate of the twins.

Interaction of Migrating Twin Boundaries with Obstacles in Magnesium

Interaction of migrating {101¯2} twin boundary with obstacles was analyzed by atomistic and finite elements computer simulations of magnesium. Two types of obstacles were considered: one is a non-shearable obstacle and another one is the void inside bulk material. It is shown that both types of obstacles inhibit twin growth and increased stress is necessary to engulf the obstacle in both cases. However, the increase of critical resolved shear stress is higher for the passage of the twin boundary through raw of voids than for interaction with non-shearable obstacles.

Theory of&amp;nbsp;History-Dependent Multi-Layer Generalized Stacking Fault Energy:&amp;nbsp;A&amp;nbsp;Modeling&amp;nbsp;of&amp;nbsp;the Micro-Substructure Evolution Kinetics&amp;nbsp;in&amp;nbsp;Chemically Ordered Medium-Entropy Alloys

In this study, a chemical order related concept “history-dependent multi-layer generalized stacking fault energy” (HDML-GSFE) was proposed, and it was then demonstrated by employing the recent, very interesting multi-principal element alloy (CoCrNi medium-entropy alloys; MEA) with different chemical short-range order (CSRO) levels using a density functional theory (DFT)-based neural network interatomic potential. To demonstrate the impacts of the history dependency and interlayer (atomic interlayers of the slip system) coupling effect on the GSFE of CSRO MEAs, HDML-GSFEs were computed for different shear deformation pathways of the MEAs with different CSRO levels, such as interlayer multiple-time slipping, twin growth, and γ - e (FCC- HCP) phase transformation. It was demonstrated that multiple-time slipping induces CSRO collapse, leading to local shear softening due to the history dependency of GSFE. In addition, it was found that the slipping of neighboring atomic interlayers is affected by the slipping resulting from the induced CSRO collapse of present interlayers because of the interlayer coupling effect of GSFE. Eventually, by employing a novel kinetic Monte Carlo (kMC) simulation method based on dislocation/disconnection loop nucleation events and using the HDML-GSFE with the history dependency and interlayer coupling effect, we proposed a laminated micro-substructure evolution that involves twinning and γ - e phase transformations subject to a finite shear strain rate and finite temperature.

Revealing grain coarsening and detwinning in bimodal Cu under tension

Abstract Metals with a bimodal grain size distribution have been found to have both high strength and good ductility. However, the coordinated deformation mechanisms underneath the ultrafine-grains (UFGs) and coarse grains (CGs) still remain undiscovered yet. In present work, a bimodal Cu with 80% volume fraction of recrystallized micro-grains was prepared by the annealing of equal-channel angular pressing (ECAP) processed ultrafine grained Cu at 473 K for 40 min. The bimodal Cu has an optimal strength-ductility combination (yield strength of 220 MPa and ductility of 34%), a larger shear fracture angle of 83° and a larger area reduction of 78% compared with the as-ECAPed UFG Cu (yield strength of 410 MPa, ductility of 16%, shear fracture angle of 70°, area reduction of 69%). Grain refinement of recrystallized micro-grains and detwinning of annealing growth twins were observed in the fractured bimodal Cu tensile specimen. The underlying deformation mechanisms for grain refinement and detwinning were analyzed and discussed.

The effects of stress, temperature and facet structure on growth of [formula omitted] twins in Mg: A molecular dynamics and phase field study

The present study focuses on the roles of structure, stress and temperature on the mobility of facets bounding twin domains and on the overall impact on the growth kinetics of {101¯2} twins in Mg. We first use molecular dynamics simulations (MD) to study the relationship between interface structures, stress, temperature and mobility. As facets can exhibit distinct atomic scale structures, we also quantify the effect of stress relaxation mediated by misfit dislocations on the mobility of the basal-prismatic interface. The information gathered for ten different facets is used to calibrate an anisotropic phase field model predicting the effects of temperature and stress on the kinetics and shape of a growing {101¯2} 3D twin, by comparison with MD data. The comparative analysis suggests that, as compared to simulations of individual facets, the effective mobility of facets can be significantly different during growth of a 3D twin. This can be attributed to the complex three-dimensional internal stress state of the twin and to coupling in facet motion triggered by facet junctions. As such, by directly fitting both the twin growth kinetics and morphology as predicted by MD for different stresses and temperatures, we can identify a limited set of rate-limiting facets that control twin growth and morphology, and measure their effective mobility.

Effects of the Application of Electric Fields on the Growth of SrB4O7 Crystals by the Micro-Pulling-Down Method

SrB4O7 (SBO) has received much attention as a solid-state ultraviolet laser crystal for a wide variety of applications such as micromachining and spectroscopy. Here we demonstrate the effects of electric fields E on the growth of SBO crystals along the b axis by the μ-pulling-down method to fabricate SBO with periodic twins, which increases the conversion light intensity. During the growth, the direction of electric fields was alternated for 30 min periods. The crystal fibers were evaluated by polarized optical microscopy, X-ray diffraction, and transmission electron microscopy. Without an external voltage, single SBO crystals grew. In contrast, at E ≥ 400 V/cm needles of SrB2O4 appeared inside SBO fibers. At E ≥ 1000 V/cm, new domains of SBO grew from the seed crystals, and interestingly, growth twins were formed perpendicular to growth fronts. Thus, we found that the growth of SBO by the μ-PD method via alternating the application of opposing electric fields exhibits two mechanisms. In contrast, when the electrodes in melt and growth fronts switched to + and −, respectively, growth fronts changed from concave into convex shapes, irrespective of E. We could explain the relation between the two mechanisms and the inversion of surface shapes, using the convection.

Autonomous growth of NbN nanostructures on atomically flat AlN surfaces

Integrating the functions of superconductors and semiconductors by epitaxial growth can lead to the fabrication of quantum devices such as on-chip quantum communication systems with single-photon emitters and detectors. Furthermore, a combination of nitride superconductors and nitride semiconductors is one of the most suitable candidates for application in these quantum devices. However, the structure of superconducting NbN films grown on nitride semiconductors needs to be elucidated. In this study, we report the self-organization of NbN nanostructures that were epitaxially grown on an atomically flat AlN surface. Structural investigation of the NbN/AlN heterostructure revealed that the growth of NbN twins on the AlN surface leads to the autonomous formation of nanostructures. These results significantly contribute to the materials science of cubic transition metal nitride heteroepitaxy.

The Interaction Between $$ \{ 10\bar{1}2\} $$ Twinning and Long-Period Stacking Ordered (LPSO) Phase During Hot Rolling and Annealing Process of a Mg-Gd-Y-Zn-Zr Alloy

The interaction between $$ \{ 10\bar{1}2\} $$ twinning and the LPSO phase was explored in the microstructure of a hot-rolled Mg-5.5Gd-4.4Y-1.1Zn-0.5Zr (wt pct) sheet. The twinning behavior in the nanoscale matrix lamellae, which were sandwiched between LPSO plates, was analyzed using high-resolution transmission electron microscopy. Profuse $$ \{ 10\bar{1}2\} $$ twinning is activated within grains containing the LPSO phase. Twinning easily occurs in the matrix with sparsely distributed LPSO plates, and the twins continually “traverse” across multiple LPSO plates. The occurrence of $$ \{ 10\bar{1}2\} $$ twinning in the matrix lamellae sandwiched between LPSO plates has a significant nanosize effect. With the decrease of matrix lamellae thickness, the nucleation and growth of twins become more difficult. Non-twinned matrix lamellae are found when the lamellae thickness is smaller than 50 nm. Additionally, the LPSO plates, which have been engulfed by twins, exist stably inside the twin after 10 pct hot rolling reduction at the temperature of 450 °C, and decompose during the annealing process. For the multi-pass hot rolling, the decomposition of the LPSO plates during intermediate annealing is considered to be beneficial for the continuous twin growth in the subsequent rolling pass, due to the relaxation of back-stress.

Investigating the compressive strength and strain localization of nanotwinned nickel alloys

Sputter deposited nickel-molybdenum-tungsten (Ni-Mo-W) thin films possess a beneficial suite of properties that stem from the extremely fine growth twins that form during the deposition process. Previously these materials were only characterized in tension, however, in this study in situ micropillar compression and post-mortem microstructural analysis of nanotwinned Ni84Mo11W5 micropillars were employed to measure the compressive response and elucidate the attendant deformation mechanisms. The pillars exhibit Hookean behavior to compressive strengths of 3-3.5 GPa and the onset of non-linear plastic flow was manifest by discrete strain bursts and highly localized shear bands. Plastic deformation was concentrated at the top of the pillar, while the bulk of the micropillar was nominally unaffected. Post-mortem investigations indicate that at sufficiently high stresses shear banding is triggered, resulting in intense and highly localized plastic deformation that led to the formation of twin-free nanocrystalline grains within highly deformed shear bands. By contrast, the regions adjacent to the shear bands were unaffected. The absence of detwinning and dislocation glide mechanisms was unexpected and in direct contrast to what has been observed in nanotwinned Cu-Al. Post-mortem observations of the Ni-Mo-W micropillars suggest that the ultrafine twins create a unique form of dislocation starvation and source-limited plasticity. The ultrahigh compressive strength is governed by the triggering of shear bands rather than the activation of dislocation glide. The specialized nature of plasticity in nanotwinned Ni-Mo-W is clear, even though the precise trigger for shear band formation in nanotwinned Ni-Mo-W remains to be identified.

Twin reversed arterial perfusion sequence in a monochorionic monoamniotic twin pregnancy: a very rare condition

BackgroundTwin reversed arterial perfusion sequence (TRAP) is a very rare congenital anomaly. We present sonographic findings of TRAP sequence in the case of a multiparous woman with a monochorionic monoamniotic twin pregnancy who was referred to our unit for blood sugar control.Case presentationThe patient had a history of co-twin demise at 13 weeks of gestation without appropriate fetal surveillance afterwards. We found a monochorionic placentation with a normal appearing pump twin, an abnormal appearing co-twin without obvious cardiac activity and reversed arterial flow toward instead of away from the anomalous acardiac fetus. Therefore, the sonographic diagnosis of TRAP sequence was confirmed.ConclusionsWe recommend considering the potential rare complications of monochorionic twin pregnancy which necessitates proper surveillance and intervention to monitor suitable growth of pump twin.