Double Twin Research Articles

Double twinning mechanisms in magnesium alloys via dissociation of lattice dislocations

In this work, we propose dislocation mechanisms for the formation of double twin structures in hexagonal close packed (HCP) crystals through the nucleation of secondary twins within primary twin domains. The model considers that secondary twins associated with the most commonly observed double twin variants (i.e. type 1 and type 2) nucleate and thicken by a sequence of three distinct dissociation reactions of mixed basal dislocations. Provided that the less frequently observed double twin variants (i.e. type 3 and type 4) also form by a dislocation-based mechanism, we show that their development must proceed by a separate set of dissociation reactions involving pyramidal 〈c+a〉 slip dislocations. Mechanistic, crystallographic and energetic considerations indicate that the type 1 variant should be the most prevalent. The mechanisms proposed here would also apply to the analysis of compound twins and HCP metals other than Mg that exhibit double twinning, such as titanium.

INFLUENCE OF ROLLING TEMPERATURE ON THE {1011}-{1012} TWINNING IN ROLLED AZ31 MAGNESIUM ALLOY SHEETS

The most common manufacturing process for wrought magnesium alloy sheet is warm-rolling and the microstructure and properties of the product are strongly related with the rolling temperature.In particular,{1011} {1012} double twinning is an important supplemental mechanism for the deformation of magnesium alloys during rolling at low or moderate temperatures.Therefore, the present work studies the microstructural variation of magnesium alloy sheets AZ31 rolled to 9%at different temperatures in the range of 150—350℃,and discusses the influence of rolling temperature on the quantity and variety of {1011}-{1012} double twins,as well as recrystallization related to the double twins.The mechanical properties of the rolled sheets and the influences of twins on the mechanical properties have also been discussed.The results show that {1011}-{1012} double twins are observed in the sheets rolled at temperature ranging from 150 to 300℃.The fraction of double twin per unit area,and the number of variant types within each primary twin,decreases with the increasing rolling temperature.At the temperature above 250℃,only co-planar double twins are observed in each primary twin.Recrystallized grains are observed to nucleate at the twin boundaries in the sheets rolled above 250℃,these can consume the twin boundaries and resulting in merging of the twin grains.No {1011}-{1012} twins are found in the AZ31 magnesium alloy sheet rolled at 350℃.With the increase of rolling temperature,the yield strength decreases and the ductile elongation increase for the rolled magnesium alloy sheet.

Improved quality of fungicide deposition and coverage of potato leaves using flat fan air-injector nozzle IDK

The aim of our experiment was to improve fungicide spray deposition and coverage of potato leaves by using air-injector nozzle types. We used two standard nozzle types a flat fan nozzle ST and a hollow cone nozzle TR, as well as a couple of air-injector nozzle types an air-injector compact nozzle IDK and a symmetric double flat fan air-injector nozzle TWIN (with a 30 ° forward and a 30 ° backward spray jet angle). Water-sensitive paper was placed on the upper, middle and lower part of the plant in order to determine the quality of fungicide deposition and the ability of droplets to penetrate lower parts of the plant. When using the air-injector compact nozzle IDK, potato leaves were covered well at all three levels of the plant. The use of the above-mentioned nozzle resulted in the lowest reduction in coverage value and droplet impression area from the top towards the lowest part of the plant. Furthermore, the nozzle created large enough droplets with sufficient mass and speed to penetrate the dense canopy all the way to the lowest part of the plant. When using the symmetric double flat fan air-injector nozzle TWIN, the angle of both spray jets was excessively wide in order for the droplets to reach the lower part of the plant despite a very large droplet impression area. The use of both standard nozzle types resulted in a poor spray mixture coverage of the middle and lower part of the plant. This predominantly occurred due to insufficient droplet size and the subsequent lack of kinetic energy. Results show that the use of a newer air-injector compact nozzle IDK improves the deposition and coverage of potato leaves with spray mixture.

THE EFFECT OF {10¯11}–{10¯12} DOUBLE TWINNING ON THE MICROSTRUCTURE, TEXTURE AND MECHANICAL PROPERTIES OF AZ31 MAGNESIUM ALLOY SHEET DURING ROLLING DEFORMATION

{1011}—{1012} double twin is the most common twin type being observed in the rolled AZ31 magnesium alloy,especially at low or moderate rolling temperature.Four types of rolled AZ31 magnesium alloy sheets were produced by rolling strong basal textured AZ31 plates at 150℃and 300℃respectively through different rolling paths by two passes till total reduction 17%. The rolling paths are normal and cross rolling respectively:the rolling directions of the two passes for normal rolling are mentally paralleled while perpendicular to each other for the cross rolling.Sheets are characterized in terms of microstructure and texture using optical microscope and scanning-electron microscope equipped with EBSD detector,and their mechanical performances are measured by uniaxial tensile tests at room temperature and a strain rate of 0.001 s~(-1).Tensile samples are cut parallel or perpendicular to the final rolling direction to obtain the mechanical anisotropics.The effects of the {1011} {1012} double twins occurred during rolling on the microstructure.texture and mechanical properties of the rolled sheets are discussed and related to each other.The results show that the fraction of {1011}-{1012} double twins varies widely in different rolled sheets,which are determined by the rolling temperature.The twins in different rolled sheets are arranged in different ways.In the normal rolled sheets,the traces of twins are approximately perpendicular to the final rolling direction,while in the cross rolled sheets there are both traces of twins parallel and to the final directions.This phenomenon is related to the variant selection of twinning during rolling deformation. And the variant selection law is related to the rolling direction.For the same reason,the textures of twins in the rolled sheets produced through different paths are different.The textures of twins will add onto the bulks,and increases the textural difference between the bulks.The differences between mechanical properties of the rolled sheets are also related to the different textures and microstructures of the sheets caused by twins.

Extension of bridging capacity of cable-supported bridges using double main spans or twin parallel decks solutions

Technical investigation of current bridging capacity of cable-supported bridges shows that its single main span is up to 2 kilometres for suspension bridge and 1 kilometre for cable-stayed bridge respectively with the total deck width of about 40 metres. Approaches to extend bridging capacity by utilising double main span suspension structure and twin parallel deck cable-stayed structure are introduced. Special emphasis is placed on longitudinal stiffness design of central pylon for double main span suspension bridge. Both inversed Y-shaped pylons in Jiangsu Taizhou Bridge and I-shaped pylon with fixed connection of deck and central pylon in Anhui Maanshan Bridge can provide proper longitudinal stiffness for the central pylon. Two types of pylon and three kinds of deck section have been checked in the early design stage for a twin parallel deck cable-stayed bridge called Ningbo Yongjiang Bridge. Finally, a composite cross section with two diamond pylon has been decided on when considering structural durability.

Mechanical Properties and Microstructure Evolution of AZ31 Magnesium Alloy Sheet Subjected to High Strain Rates Loading of Different Orientations

Mechanical property tests of AZ31magnesium alloy sheet were carried out with compressive axis 0°，45° and 90° from normal direction respectively using Split Hopkinson Pressure Bar (SHPB). The results showed that the strain hardening rate for both 0° and 90° specimen decreases with increasing strain, and remain a constant approximately for 45° specimen at a strain rate of 2.8×103s-1. The flow stress of 45° specimen is the lowest compare with the other two specimens. Twined area fractions (TAF), dislocation multiplication and twinning system were characterized using optical microscopy (OM) and transmission electron microscopy (TEM). The results revealed that TAF for both 0° and 45° specimens are far less than 90° specimen at the same strain. Microstructure observation for specimens at a strain of 0.003 shows that dislocation multiplications for 0º specimen are far less than 45° specimen, and that {10-12}<10-11>twinning，{10-11}-{10-12}double twinning and twin intersection of vast amounts are activated for 90° specimen. Dynamic mechanical property anisotropy can be explained based on different microstructure evolution and plastic deformation mechanisms.

Deformation behavior of as-rolled and strip-cast AZ31 magnesium alloy sheets

Deformation behavior of AZ31 Mg alloy has been studied in relation to test temperature and grain size. Tensile specimens were prepared in the rolling direction of an as-rolled as well as a strip-cast sheet with 1 mm thickness. The as-rolled and strip-cast sheets had the mean grain sizes of 75 μm and 5 μm, respectively. Tensile tests were then carried out under the strain rate of 10 −2/s at room temperature, 100 °C, and 200 °C. Biaxial tests were also performed to obtain flow curves in terms of stress and strain as well as to estimate the biaxial forming limit. Plastic deformation was found due to the usual dislocation slip and dynamic recrystallization (DRX) together with some twinning. A number of twins were observed especially in large grained specimens at low homologous temperatures. The strip-cast sheet exhibited many small grains generated by the DRX at higher temperature. These fine grains due to DRX appeared to lead into an enhanced formability, evidenced by a sudden formability increase of a strip-cast sheet at 200 °C. The double twin mode was observed under biaxial loading, contrary to extension twins observed under uniaxial loading.

Study on static and fatigue behaviors of the vertical and horizontal studs at lower concrete slab in double composite twin I-girder bridge

Study on static and fatigue behaviors of the vertical and horizontal studs at lower concrete slab in double composite twin I-girder bridge

Effect of initial texture on dynamic recrystallization of AZ31 Mg alloy during hot rolling

Wedge-shaped AZ31 plates with two kinds of initial textures were rolled at 573K to investigate the effect of initial texture on dynamic recrystallization (DRX). The results indicated that the initiation and nucleation of DRX were closely related to the initial texture. The initiation and completion of DRX in the TD-plate were significantly retarded compared with that in the ND-plate. Twin related DRX nucleation was mainly observed in the ND-plate samples; while gain boundary related DRX nucleation was mainly observed in the TD-plate samples. The different DRX behavior between the TD- and ND-plates was attributed to the different deformation mechanism occurring before DRX initiation. For the ND-plate, dislocation glide was considered as the main deformation mechanism accompanied with {10−11}–{10−12} double twin, which led to the increment of a faster increasing stored energy within the grains. And {10−11}–{10−12} double twin was mainly found to be DRX nucleation site for the ND-plate. For the TD-plate, {10−12} extension twin was the dominant deformation mechanism which resulted in a basal texture with the c-axis nearly parallel to ND. The stored energy caused by dislocation motion was relatively small in the TD-plate before a basal texture was formed, which was considered as the main reason of that DRX was retarded in the TD-plate compared with that in the ND-plate. Based on the difference in deformation mechanism and DRX mechanism caused by the different initial texture, the variation in grain size, micro-texture and misorientation angle distribution in the ND and TD plates were discussed.

Deformation twinning in AZ31: Influence on strain hardening and texture evolution

This paper describes the main results from an experimental investigation into the consequences of deformation twinning in AZ31 on various aspects of plastic deformation, including the anisotropic strain-hardening rates, the tension/compression yield asymmetry, and the evolution of crystallographic texture. It was seen that AZ31 exhibited unusually high normalized strain-hardening rates compared to α-Ti that occurred beyond the strain levels where extension twins have completely altered the underlying texture. This observation challenges the validity of the generally accepted notion in the current literature that the high strain-hardening rates in AZ31 are directly caused by extension twins. It is postulated here that the thin contraction twins are very effective in strain hardening of the alloy by restricting the slip length associated with pyramidal 〈c+a〉 slip. This new hypothesis is able to explain the major experimental observations made in this study and in the prior literature. We have also presented a new hypothesis for the physical origin of the observed differences in the thicknesses of the extension and contraction twins. The stress fields in selected matrix–twin configurations were modeled using crystal plasticity finite element models. The contraction twin (01¯11)[01¯12¯] was predicted to form an internal extension twin (011¯2)[01¯11], resulting in the commonly observed “double twin” sequence. The extension twin is suggested to inhibit thickening of this double twin by loss of twin–matrix coherency. Extension twins were predicted to retain their coherency and thus thicken.

Deformation Behavior of Magnesium Single Crystal in <i>c</i>-Axis Compression and <i>a</i>-Axis Tension

In general, deformation behavior of magnesium in compression is different from tensile. To investigate deformation behavior of magnesium single crystals by non-basal slips and twins, c-axis compression and a-axis tension tests were performed in the range of 77K-573K. The crystals were yielded by second order pyramidal slip, and the yield stress shows anomalous temperature dependence (increased with increasing temperature) between 203K and 293K. Yield stress of c-axis compression was bigger than that of a-axis tensile. In compression, fracture surface were (11 4) under 293K and were {30 4} above 373K, and fracture strain was smaller than the case of tension test. {10 1}-{10 2} double twin were activated at higher temperature and the crystal, therefore, fractured along the twin interface.

Relationship between deformation twinning and surface step formation in AZ31 magnesium alloys

Rolled sheets of AZ31 Mg alloys were deformed at room temperature under tension along the rolling direction. A number of large surface steps were observed in the region near the fractured edge. Transmission electron microscopy showed the presence of twins under these steps. Crystallographic analysis indicated that the twins are of the { 1 0 1 ¯ 1 } – { 1 0 1 ¯ 2 } double twin type with their basal planes tilted by 37° with respect to the basal planes of the matrix, which is considered to make basal dislocation slip highly active within the twin. The localized deformation within the twins may lead to the formation of crack-like faults and to failure in a macroscopically brittle manner.

Ocular dimensions and cone photoreceptor topography in adult Nile Tilapia Oreochromis niloticus

Information on the anatomy of the eye and the topography of cone photoreceptor cells in the retina is presented for the Nile Tilapia (Oreochromis niloticus). In adults, the shape and proportions of the ocular components of the prominent eye conform to the general form of fish eyes, as determined using cryo-sectioned eyes. The lens is approximately spherical and there is little variation in the distance from the centre of the lens to the border between the choroid and retina at a range of angles about the optical axis. The average ratio of the distance from the centre of the lens to the retina: lens radius (Matthiessen’s ratio) is 2.44:1. In retinal wholemounts, single and double (twin) cone photoreceptors, forming a square mosaic, are present. Peak photoreceptor densities for both morphological cone types are found in the temporal retina. Using peak cone densities and estimates of focal length from cryo-sectioned eyes, visual acuity is calculated to be 5.44 cycles per deg. The lack of apparent specific ocular or retinal specializations and the relatively low visual acuity reflect the lifestyle of the Nile Tilapia and may allow it to adapt to changes in visual environment in its highly variable natural habitat as well as contributing to the ‘ecological flexibility’ of this species.

Recrystallization mechanism and the relationship between grain size and Zener–Hollomon parameter of Mg–Al–Zn–Ca alloys during hot compression

Continuous dynamic recrystallization (DRX) associating with { 1 0 1 ¯ 1 } – { 1 0 1 ¯ 2 } double twin, { 1 0 1 ¯ 1 } compression twin and { 1 0 1 ¯ 2 } tensile twin is observe during the hot compression of Mg–9, 10 wt.% Al–0.8 wt.% Zn–1 wt.% Ca alloys. The pinning effect of Al–Ca compounds, eutectic and precipitated Mg17Al12 phase is observed at the DRXed grain boundaries. The pinning effect of Mg17Al12 precipitates is greatly influenced by compression temperature, leading to a stronger influence of compression temperature on DRXed grain size than strain rate. Addition of 1 wt.% Ca to Mg–Al–Zn alloys reduces these influences.

Torsional behavior of double composite twin I-girder bridge

Torsional behavior of double composite twin I-girder bridge

The 10B(n,α0)7Li and 10B(n,αlγ)7Li alpha-particle angular distributions, branching ratios and cross-sections measurements for En < 3 MeV

The 10B(n,α0)7Li and 10B(n,αiγ)7Li angular distributions have been measured at the GELINA time-of-flight spectrometer in the incident neutron energy range from 0.1 keV to 1 MeV by means of a twin Frisch-grid ionization chamber. With this type of detector it is possible to measure the angular distribution of the charged reaction fragments in a close to 2×2π solid angle with ~100% efficiency and a clear separation of both reaction channels: emission to the 7Li ground state (α0) or to its first excited state (α1). A strong angular anisotropy was observed at ~ 520 keV. In order to extend the energy range up to 2.5−3 MeV and to measure, also, the reaction cross sections, a double twin Frisch-grid ionization chamber was constructed. It is loaded with two very thin 94% 10B-enriched samples, mounted back-to-back with 235U samples on the common cathodes. New data acquisition, visualization and analysis software is used in a new set of long-term measurements, which are still going on.

THE EFFECT OF WATER AND POTASSIUM SUPPLEMENT ON YIELD AND LYCOPENE CONTENT OF PROCESSING TOMATO

THE EFFECT OF WATER AND POTASSIUM SUPPLEMENT ON YIELD AND LYCOPENE CONTENT OF PROCESSING TOMATO

Design method of stud at lower concrete slab in double composite twin I-girder bridge

Design method of stud at lower concrete slab in double composite twin I-girder bridge

Childhood infection experience and risk of Young Adult Hodgkin Lymphoma.

The risk pattern of adolescent/young adult Hodgkin lymphoma (HL) is consistent with causation by a relatively late infection to a common childhood virus. However a causal virus has not yet been convincingly found. Susceptibility is known to be heritable, and a lower genetically determined interleukin-12 [IL-12] response is a risk factor. A lower IL-12 response can also be produced by diminished gastrointestinal exposure to the environmental microbiome, with resulting persistence of a Th2-skewed cytokine phenotype. We have therefore studied twin pairs discordant for HL to search for early life differences in sources of both viral and non-viral infection. HL-discordant twin pairs volunteered for the International Twin Study in ignorance of specific hypotheses. 70% of the questionnaires sent to individual twins were returned producing 188 informative pairs. The Kappa statistic was used to assess between-twin agreement. Designating the twin with HL as the case and the unaffected twin as the control, odds ratios (ORs) and 95% confidence intervals (CI) were calculated using conditional logistic regression for matched pairs. Tonsillectomy or appendectomy at least 5 years prior to diagnosis were associated respectively with a 2- and 3-fold statistically significant increase in risk. Infection with 3 or more childhood exanthems was associated with a 60% decreased risk (95% confidence intervals 0.2, 0.9). Behaviors likely to produce oral exposure to the environmental microbiome conveyed statistically significant decreases in risk (OR=0.2–0.5). A history of eczema increased risk (OR= 2.8, 95% CI= 1.0, 7.8). Kappa statistics were high (>0.8) for significant findings. Our evidence supports a role for early exposure to various infections in the etiology of adolescent/young adult Hodgkin lymphoma.Table 1. History of infections, immune-related surgeries and other childhood experience at least 5 years prior to diagnosis and risk of adolescent-young adult Hodgkin Lymphoma diagnosed between 13–50 years of age in disease-discordant twin pairs from the International Twin International Twin Registry (n = 188 total pairs).ExposuresKappa1Ratio of Exposure Discordant Pairs2Odds Ratio395% CI43 or more childhood exanthems-6/150.40.2, 0.9Infectious mononucleosis0.7222/191.20.6, 2.1Cold Sores (Herpes Simplex 1)0.7018/101.80.8, 3.9Tonsillectomy0.8625/122.11.1, 4.1Appendectomy0.8433/113.01.5, 5.9Eczema0.7914/52.81.0, 7.8Sucked pacifier/thumb/fingers more as an infant/young child?0.8718/340.50.3, 0.9Put more things in mouth as an infant/young child?0.8610/300.30.2, 0.71. Kappa calculated from the 93 double respondent twin pairs (pairs in whom both twins returned questionnaires)2 Total number of twin pairs in which case-twin was exposed and the unaffected co-twin was unexposed/Total number of twin pairs in which unaffected co-twin was exposed and the case-twin was unexposed3 Odds ratio estimated using conditional logistic regression using SAS Version 8.14 Confidence interval estimated using conditional logistic regression

Identification of double/twin bolled and bicolor unit ‘cluster’ in Gossypium hirsutum L. cotton

Cultivated tetraploid Gossypium hirsutum L. (Malvaceae) cotton has been characterized as having one flower at a primordia. In a strain (CSH-13) of Gossypium hirsutum cotton, four plants out of 103 had two to three double bolls or twin bolls/plant during 2003–2004 crop season at CICR regional station, Sirsa, Haryana, India. Progeny evaluation in 2004 and 2005 crop seasons indicated that plants raised from seeds harvested from double bolls/twin bolls produced plants bearing double bolls only and plant progeny raised from the seed of single bolls from these mutant plants produced two to three double bolls per plant similar to the parent. Progeny testing revealed that double boll formation is the result of spontaneous mutation and environment does not influence its expression. The mutant is early in maturity by 10–15 days, naked seeded and possesses comparable agronomic characters with normal plants. Another spontaneous mutant of Gossypium hirsutum having more than three appendages originating from primordia.: four bolls one leaf, three bolls one leaf, two bolls two leaves, one boll three leaves were obtained from the population of CISV-13 strain in crop season 2003. These appendage groups have been described as bicolor unit by Clemens Bayer, [Zur Infloreszenzmorphologie der Malvales. Dissertations Botany 212 (1994)] and cluster mutant by Russell and Luther [J Cott Sci 6 (2002) 115]. One to three bolls in a cluster or bicolor unit were observed without formation of seeds and lint. About one-third of the total bolls on the mutant plant were of this type. In progeny testing during 2004 and 2005 crop seasons, this mutant produced plants exclusively with more than three appendages revealing that plants identified in 2003 were due to spontaneous mutation. This mutant was also early and naked seeded. Many of the other characters of the mutant plants were comparable with the wild type plants of the strain. Both the mutants were observed having economic impact due to their better yielding ability as compare to respective parents.