Twin Fraction Research Articles

2-[N-(2,4-Difluorophenyl)carbamoyl]-3,4,5,6-tetrafluorobenzoic acid

The title compound, C14H5F6NO3, was synthesized by condensation of tetra­fluoro­phthalic anhydride and 2,4-difluoro­aniline. It was then recrystallized from hexane to give a nonmerohedral twin with two crystallographically unique mol­ecules in the asymmetric unit. The refined twin fraction is 0.460 (3). Torsional differences between the aryl rings and the central amide group account for the presence of two unique mol­ecules. The compound packs as double tapes formed by O—H⋯O and N—H⋯O hydrogen-bonding inter­actions between each unique mol­ecule and its symmetry equivalents.

Microstructure evolution of Mg–Al–Zn alloys during compression test at low strain and temperature

The microstructure evolutions and texture changes during the compression test were investigated using an extruded magnesium alloy with average grain sizes of 11.4 and 49.6 μm. The deformation twins were formed in all the samples; however, a comparison of the fraction of deformation twins on the effect of grain size and initial texture, i.e., the cutting position (normal or parallel to the extrusion), showed that the fine-grained alloy and/or the sample with the normal-cut to the extrusion had a lower fraction of deformation twins. On the other hand, the texture change showed different tendencies depending on the grain size and/or the initial texture. In the coarse-grained alloy, since the dominant deformation mechanism was the deformation twins, the lattice was rotated without relation to the initial texture. However, in the fine-grained alloy, even the applied strain of 0.20, the intensity peaks existed at 〈10-10〉 and the basal texture remained in the sample with the parallel- and normal-cut to the extrusion, respectively. This resulted from the difference in the fraction of deformation twins and the occurrence of partial grain boundary sliding.

1,2-Diphenyl-2-[4-(4-pyridyl)benzylidenehydrazono]ethan-1-one

In the title compound, C26H19N3O, the dimethyl­ene hydrazine (—C=N—N=C—) unit is approximately planar, the torsion angle around the N—N bond being 162.2 (6)°. The phenyl and benzoyl­phenyl rings at one end of the hydrazine unit are aligned at angles of 9.5 (5) and 88.5 (4)°, respectively, with respect to the hydrazine unit, whereas the benzene ring at the other end is twisted by an angle of 14.4 (4)°. In the crystal structure, mol­ecules are linked into centrosymmetric dimers by inter­molecular C—H⋯O hydrogen bonds. The monoclinic crystal under investigation shows pseudo-merohedral twinning with twin fractions of 0.63 and 0.37.

First discovery and structural characterization of a new compound in Al–Si–O–C system

A quaternary oxycarbide, [ Al 16.77 ( 5 ) Si 1.23 ( 5 ) ] Σ 18 [ O 3.04 ( 9 ) C 10.96 ( 9 ) ] Σ 14 , has been for the first time discovered in the Al–Si–O–C system. The crystal structure was characterized by X-ray powder diffraction, transmission electron microscopy and energy dispersive X-ray spectroscopy (EDX). The atom ratios [Al:Si] were determined by EDX, and the initial structural model was derived by the direct methods. The structural parameters as well as the atom ratios [O:C] were determined by the Rietveld method. The crystal is monoclinic (space group C2/ m, Z=1) with lattice dimensions a=0.57404(1) nm, b=0.331435(5) nm, c=1.92410(2) nm, β=90.036(1)° and V=0.366076(9) nm 3. The final structural model showed the positional disordering of Al/Si sites. The validity of the split-atom model was verified by the three-dimensional electron density distribution, the structural bias of which was reduced as much as possible using the maximum-entropy methods-based pattern fitting (MPF). The reliability indices calculated from the MPF were R wp=4.20% ( S=1.14), R p=3.09%, R B=0.92% and R F =1.05%. The crystal was an inversion twin with nearly the same twin fraction.

Nonmerohedrally twinned 6-amino-3-methyluracil-5-carbaldehyde: a hydrogen-bonded ribbon containing four types of ring

In the title compound [systematic name: 6-amino-5-formyl-3-methylpyrimidine-2,4(1H,3H)-dione], C(6)H(7)N(3)O(3), the intramolecular dimensions provide evidence for some polarization of the electronic structure. There is an intramolecular N-H...O hydrogen bond; this and a combination of three intermolecular N-H...O hydrogen bonds generate an almost planar ribbon containing S(6), R(2)(2)(4), R(2)(1)(6) and R(4)(4)(16) rings. These ribbons are linked into sheets by a dipolar carbonyl-carbonyl interaction. The structure was refined as a nonmerohedral twin, with twin fractions 0.7924 (1) and 0.2076 (10).

Effect of particles on the formation of deformation twins in a magnesium-based alloy

The deformation behaviour of the age hardenable alloy Mg–5%Zn after different precipitation treatments has been examined. It has been found that during compressive deformation, fine particles increase the number of twins that form, but reduce the size and total volume fraction of twins. Visco-plastic self-consistent modelling has been used to show that the presence of precipitates hardens the twin and prismatic slip systems more than the basal slip system. It is proposed that because the { 1 0 1 ¯ 2 } twin requires basal slip to accommodate the twinning shear, this twin type will always be hardened equal to, or more than, the basal slip system in response to precipitation.

A continuum model for initiation and evolution of deformation twinning

Within continuum dislocation theory the plastic deformation of a single crystal with one active slip system under plane-strain constrained shear is investigated. By introducing a twinning shear into the energy of the crystal, we show that in a certain range of straining the formation of deformation twins becomes energetically preferable. An energetic threshold for the onset of twinning is determined. A rough analysis qualitatively describes not only the evolving volume fractions of twins but also their number during straining. Finally, we analyze the evolution of deformation twins and of the dislocation network at non-zero dissipation. We present the corresponding stress–strain hysteresis, the evolution of the plastic distortion, the twin volume fractions and the dislocation densities.

Aspects of Dynamic Recrystallization in Cobalt at High Temperatures

Deformation behavior of cobalt in the fcc phase was studied over a wide range of strain rates (0.001 to 1 s–1) and temperatures (600 °C to 950 °C). At the lower temperatures and higher strain rates, a single peak in flow stress followed by softening to steady state was observed, whereas at higher temperatures and lower strain rates, an oscillatory flow stress behavior finally converging to a steady state was observed. Both these represent typical flow stress responses during dynamic recrystallization (DRX). The critical condition for the initiation of DRX was determined by the inflection point in the work-hardening vs stress plot. This critical condition in terms of the work done was found to increase with temperature-compensated strain rate. Electron backscattered diffraction (EBSD) revealed that, on quenching after deformation, a certain fraction of the fcc phase was always retained, the extent of retention being higher for smaller grain sizes. Misorientation measurements across boundaries revealed a significant amount of twins in the retained fcc phase. The grain average misorientation (GAM), signifying the misorientation within grains, was found to be inversely related to the twin boundary fractions in the fcc phase.

Determining the DUF55-domain structure of human thymocyte nuclear protein 1 from crystals partially twinned by tetartohedry

Human thymocyte nuclear protein 1 contains a unique DUF55 domain consisting of 167 residues (55-221), but its cellular function remains unclear. Crystals of DUF55 belonged to the trigonal space group P3(1), but twinning caused the data to approach apparent 622 symmetry. Two data sets were collected to 2.3 A resolution. Statistical analysis confirmed that both data sets were partially twinned by tetartohedry. Tetartohedral twin fractions were estimated. After the structure had been determined, only one twofold axis of rotational pseudosymmetry was found in the crystal structure. Using the DALI program, a YTH domain, which is a potential RNA-binding domain from human YTH-domain-containing protein 2, was identified as having the most similar three-dimensional fold to that of DUF55. It is thus implied that DUF55 might be a potential RNA-related domain.

9R structure in drawn industrial single crystal copper wires

By using transmission electron microscopy, the microstructures of drawn industrial single crystal copper wires produced by Ohno Continuous Casting(OCC) process were analyzed. The results show that the typical microstructures in the wires mainly include extended planar dislocation boundaries, a small fraction of twins and some dislocation cells sharing boundaries parallel to drawn direction. Besides the typical microstructures, 9R structure configurations were observed in the wires. The formation of 9R polytypes may be caused by the coupled emission of Shockley dislocations from a boundary.

Detwinning of High-Purity Zirconium: In-Situ Neutron Diffraction Experiments

Twinning is an important deformation mode in hexagonal metals to accommodate deformation along the c-axis. It differs from slip in that it accommodates shear by means of crystallographic reorientation of domains within the grain. Such reorientation has been shown to be reversible (detwinning) in magnesium alloy aggregates. In this paper we perform in-situ neutron diffraction reversal experiments on high-purity Zr at room temperature and liquid nitrogen temper- ature, and follow the evolution of twin fraction. The experi- ments were motivated by previous studies done on clock-rolled Zr, subjected to deformation history changes (direction and temperature), in the quasi-static regime, for temperatures ranging from 76 K to 450 K. We demonstrate here for the first time that detwinning of 1012 1011 tensile twins is favored over the activation of a different twin variant in grains of high-purity polycrystalline Zr. Avisco-plastic self-consistent (VPSC) model developed previously, which includes com- bined slip and twin deformation, was used here to simulate the reversal behavior of the material and to interpret the experi- mental results in terms of slip and twinning activities.

On tension–compression yield asymmetry in an extruded Mg–3Al–1Zn alloy

The tension–compression yield asymmetry of an extruded Mg–3Al–1Zn alloy was examined by changing load directions and grain sizes in room-temperature mechanical tests. A criterion for the activation of deformation modes was proposed to analyze the effect of load direction on twinning activity. When the load angle was 45°, the twin area fractions after tension and compression resembled each other, and the corresponding ratio between compression yield stress and tension yield stress equaled 1.02. As the load direction was parallel or perpendicular to the extrusion axis, the yield stress and twinning activity in tension differed obviously from those in compression, resulting in marked tension–compression yield asymmetry. Although grain-coarsening promotes twinning in tension along extrusion axis, it cannot reduce the yield asymmetry. Further, the contributions of twinning to strain during yield deformation were evaluated based on the quantitative statistic of twin area fraction.

Expression, purification and crystallization of the ecto-enzymatic domain of rat E-NTPDase1 CD39

CD39 is a prototype member of the ecto-nucleoside triphosphate diphosphohydrolase family that hydrolyzes extracellular nucleoside diphosphates and triphosphates in the presence of divalent cations. Here, the expression, purification and crystallization of the ecto-enzymatic domain of rat CD39, sCD39, are described. The 67 kDa secreted soluble glycoprotein was recombinantly overexpressed in a glycosylation mutant CHO line, Lec.3.2.8.1, and purified from conditioned media. Diffraction-quality crystals of sCD39 were produced by the vapor-diffusion method using PEG 3350 and ammonium dihydrogen phosphate as precipitants. The enzyme crystallized in a primitive trigonal form in space group P3(2), with unit-cell parameters a = b = 118.1, c = 81.6 A and with two sCD39 copies in the asymmetric unit. Several low- to medium-resolution diffraction data sets were collected using an in-house X-ray source. Analysis of the intensity statistics showed that the crystals were invariably merohedrally twinned with a high twin fraction. For initial phasing, a molecular-replacement search was performed against the complete 3.2 A data set using a maximum-likelihood molecular-replacement method as implemented in Phaser. The initial model of the two sCD39 monomers was placed into the P3(2) lattice and rigid-body refined and position-minimized with PHENIX.

Equations for determining tetartohedral twin fractions

In most cases of merohedral twinning, two different twin-domain orientations are present. A rarer type of merohedral twinning exists in which there are four different twin-domain orientations. The former case is referred to as hemihedral twinning, while the latter more complex type is referred to as tetartohedral twinning. In tetartohedral twinning, each observed reflection is the weighted sum of four twin-related but otherwise independent reflection intensities. The weights that determine how the true crystallographic intensities combine to give the observed intensities are described by four twin fractions representing the fractional volumes of the four different domain orientations within the specimen. Here, equations are developed to determine values for the four tetartohedral twin fractions based on a statistical comparison of quadruplets of twin-related reflections. Knowledge of the twin fractions is important in working backwards to obtain values for the true crystallographic intensities. Use of the equations is demonstrated with synthetic intensity data simulated according to given values of the twin fractions.

Crystallization and Preliminary Crystallographic Analysis of Laminarinase from Rhodothermus marinus: A Case of Pseudomerohedral Twinning

Thermophilic endo-1,3(4)-beta glucanase (laminarinase) from Rhodothermus marinus was crystallized by the hanging-drop vapor diffusion method. The needle-like crystals belong to space group P2(1) and contain two protein molecules in the asymmetric unit with a solvent content of 51.75 %. Diffraction data were collected to a resolution of 1.95A and resulted in a dataset with an overall R(merge) of 10.4% and a completeness of 97.8%. Analysis of the structure factors revealed pseudomerohedral twinning of the crystals with a twin fraction of approximately 42%.

Pseudo-merohedral twinning in crystals of the dihaemc-type cytochrome DHC2 fromGeobacter sulfurreducens

The dihaem cytochrome DHC2 from Geobacter sulfurreducens has been described as a model system for the study of one of the conserved haem-packing motifs typically observed in multihaem cytochromes c, in which the covalent attachment of haem to the protein allows effective spatial clustering of redox centres and a high haem:protein ratio. Heterologously expressed DHC2 has been crystallized previously and the structure has been solved by multiple-wavelength anomalous dispersion (MAD) and refined to a resolution of 1.5 A (PDB code 2czs). Crystals belonged to the monoclinic space group P2(1), with unit-cell parameters a = 39.65, b = 55.68, c = 39.64 A, beta = 105.91 degrees . Close inspection of the data indicated the presence of pseudo-merohedral twinning; while not hindering structure solution by MAD, this resulted in high refinement R factors. A re-assessment of the diffraction data for DHC2 is presented, together with a refined structure that includes the twinning present with a twinning fraction alpha of 0.32.

Expression, purification, crystallization and structure determination of two glutathioneS-transferase-like proteins fromShewanella oneidensis

Genome analysis of Shewanella oneidensis, a Gram-negative bacterium with an unusual repertoire of respiratory and redox capabilities, revealed the presence of six glutathione S-transferase-like genes (sogst1-sogst6). Glutathione S-transferases (GSTs; EC 2.5.1.18) are found in all kingdoms of life and are involved in phase II detoxification processes by catalyzing the nucleophilic attack of reduced glutathione on diverse electrophilic substrates, thereby decreasing their reactivity. Structure-function studies of prokaryotic GST-like proteins are surprisingly underrepresented in the scientific literature when compared with eukaryotic GSTs. Here, the production and purification of recombinant SoGST3 (SO_1576) and SoGST6 (SO_4697), two of the six GST-like proteins in S. oneidensis, are reported and preliminary crystallographic studies of crystals of the recombinant enzymes are presented. SoGST3 was crystallized in two different crystal forms in the presence of GSH and DTT that diffracted to high resolution: a primitive trigonal form in space group P3(1) that exhibited merohedral twinning with a high twin fraction and a primitive monoclinic form in space group P2(1). SoGST6 yielded primitive orthorhombic crystals in space group P2(1)2(1)2(1) from which diffraction data could be collected to medium resolution after application of cryo-annealing protocols. Crystal structures of both SoGST3 and SoGST6 have been determined based on marginal search models by maximum-likelihood molecular replacement as implemented in the program Phaser.

Quantitative analysis of deformation twinning in zirconium

We have used electron backscatter diffraction (EBSD) to quantify the contributions of first generation and second generation twinning to the total plastic strain of zirconium compressed at 76 K. For compression parallel to a primary c-axis texture, prismatic slip and first generation { 1 1 2 ¯ 2 } compression twinning are the dominant deformation mechanisms with twinning accommodating roughly one third of the plastic strain. Second generation { 1 0 1 ¯ 2 } and { 1 1 2 ¯ 1 } tensile twins increase with the third power of the first generation { 1 1 2 ¯ 2 } twin fraction. For compression perpendicular to the primary c-axis texture, prismatic slip and first generation { 1 0 1 ¯ 2 } tensile twinning are the dominant deformation mechanisms with a small contribution from first generation { 1 1 2 ¯ 1 } tensile twinning. Above approximately 17% strain, second generation { 1 1 2 ¯ 2 } compression twins begin to make a contribution to the overall strain. These observations are used to explain the measured mechanical responses and texture evolution during deformation of zirconium.

Effect of Sn on the microstructure and compressive deformation behavior of the AZ91D aging alloy

The effect of Sn on the microstructure and compressive deformation behavior of aging AZ91D magnesium alloy was investigated. The results show that the discontinuous precipitation occurs near the grain boundary in the form of lamellar precipitates of the Mg 17Al 12 phase. These precipitates alternatively lie in the α-Mg matrix perpendicular to the grain boundaries. With an increase in the Sn content, the cellular discontinuous precipitation can be efficiently suppressed, such that no discontinuous precipitation is observed when the Sn content is increased to 2 wt.%. The AZ91D aging alloy has a higher ultimate fracture strength than the base 2%Sn alloy due to the large numbers of grains containing the cellular discontinuous precipitation forming nearby the grain boundaries. In these regions the β phase can effectively hinder the dislocation movement. Conversely, the soft behavior induced by a high fraction of twins is the main reason that the strength of 2%Sn alloy is lower than that of the AZ91D alloy.

Twinning–detwinning behavior during the strain-controlled low-cycle fatigue testing of a wrought magnesium alloy, ZK60A

The twinning and detwinning behavior in a strongly textured magnesium alloy was investigated using in situ neutron diffraction during the cyclic deformation along the prior extrusion direction at the fully reversed total constant strain amplitude of 1.2% at room temperature. The initial preferred orientation places the c-axis in most grains perpendicular to the loading axis, and this favors extensive { 1 0 1 ¯ 2 } 〈 1 0 1 ¯ 1 〉 twinning under compressive loading. In contrast, the grains are not favorably oriented to undergo such twinning during monotonic tensile loading along the prior extrusion axis. This is the reason for the well-known tension–compression strength asymmetry of wrought magnesium alloys. The strength in compression is controlled by the stress required to activate twinning, while the strength in tension is controlled by the harder non-basal slip mechanisms. The unique orientation relationship between the parent grains and the twin grains favors detwinning during the subsequent loading reversal. In situ neutron-diffraction results indicate that such twinning and detwinning alternates with the cyclic loading, i.e. most of the twins formed during compression are removed when the load is reversed. However, a small volume fraction of residual twins gradually increases with increasing cycles, which may be an important factor in dictating the low-cycle fatigue behavior of the magnesium alloy.