Twin Fraction Research Articles

Analysis of deformation twinning in tantalum single crystals under shock loading conditions

The competition between dislocation slip and twinning in tantalum single crystals has been investigated utilizing a crystal level twinning model and the results from gas gun recovery experiments conducted at peak normal stresses of 25 and 55 GPa. The recovered samples were characterized using electron back scattered diffraction, and the observed twinning fractions were compared with the model. The experimental results show very low twin fractions in all orientations at 25 GPa, and that among (100), (110), (111), and (123) crystals, the (110) crystals had the largest amount of twinning at 55 GPa. The analysis shows that the general trends observed in the experimental data can be reproduced by the model when an orientation dependent dislocation evolution is used. This analysis gives insight into the possible influence of the dislocation density and its evolution on the observed twinning behavior.

Twinning in magnesium alloy AZ31B under different strain paths at moderately elevated temperatures

Twinning activity and deformation limits were investigated for wrought and cast magnesium at room and moderately elevated temperatures and varying strain paths. In situ compression and tension tests, combined with high-resolution electron backscatter diffraction (HR-EBSD) techniques, demonstrated that twin formation and geometrically necessary dislocation (GND) density strongly depend on initial microstructure and load directions. It was also shown that GND content was lower inside a twin than outside, in the parent grain. This explains the observation of a leveling off of GND density, while twin fraction continued to increase with plastic strain. Plane strain and biaxial tension experiments at room temperature resulted in compression twin shear bands, which began to appear at effective strains of about 4%, compared to the uniaxial tension case, where shear band formation had been shown to occur at effective strains closer to failure. This behavior was attributed to the strong basal texture of the AZ31B sheet, where thinning of the sheet requires c-axis compression. While thinning strains were accommodated by compression twinning at room temperature, compression twin shear bands were not observed in plane strain and biaxial tension specimens at 75 and 125°C, at effective strains between 5% and 12%. This provides evidence of a transition from compression twinning to 〈c+a〉 slip in order to accommodate thinning strains at these mildly elevated temperatures, resulting in significant formability increases, especially in biaxial tension.

Cis-Bis(2,2′-bipyridine-κ2N,N′)carbonylchloridoruthenium(II) hexafluoridophosphate

In the title compound, [RuCl(C10H8N2)2(CO)]PF6, the RuII atom is coordinated in a distorted octa­hedral geometry by four N atoms of the bipyridine ligands, a carbonyl C atom and a chloride ion. The carbonyl and chloride ligands in the cation adopt a mutually cis arrangement and these are disordered over two sets of sites with site occupancies of 0.721 (6) and 0.279 (6). The Ru—N bond length [2.117 (2) Å] trans to the carbonyl ligand is slightly longer than the average of the other Ru—N bond lengths (2.08 Å), which can be explained by the expected trans influence of the carbonyl group. In the crystal, weak C—H⋯F inter­actions are observed between the complex cation and the PF6 − anion, leading to the formation of a three-dimensional supramolecular structure. The crystal studied was an inversion twin with twin fractions of 0.78 (4) and 0.22 (4).

On the Relation of Microstructure and Texture Evolution in an Austenitic Fe-28Mn-0.28C TWIP Steel During Cold Rolling

In the present study, microstructure and texture evolution in an austenitic Fe-28 wt pct Mn-0.28 wt pct C TWIP steel in the range between 10 and 80 pct reduction by cold rolling were systematically analyzed. The formation of the observed microstructural features occurred in three different stages: I (10 to 20 pct)—mainly slip lines, grain elongation, and formation of few twin-matrix lamellae; II (30 to 50 pct)—severe increase of the volume fraction of twins, alignment of twins with the rolling plane, and formation of microshear bands; and III (60 to 80 pct)—further alignment of twins, evolution of a herring bone structure, and macroshear bands. In contrast to most f.c.c. metals, the transition from Copper- to Brass-type texture occurred at low strain levels (30 pct). This behavior is attributed to the early formation of deformation twins in the material and can be related to the SFE of this high manganese steel. At higher reduction levels, microscopic (≥40 pct) and macroscopic shear band formation (≥60 pct) contributed to the increase of randomly oriented grains, mainly at the expense of the Brass component. Furthermore, the formation of the Goss component and of the 〈111〉//ND fiber (γ) is attributed to severe twin formation.

Structure and fatigue properties of 08Kh18N10T steel after equal-channel angular pressing and heating

The structure of corrosion-resistant austenitic 08Kh18N10T steel is studied after equal-channel angular pressing (ECAP), heating, and subsequent cyclic tests. After ECAP, an oriented mainly subgrain structure with a structural element size of 100–250 nm and a high fraction of deformation twins forms in the austenite of the steel, and 42 vol % of lath martensite appears. Dynamic twinning, martensitic transformation, dynamic recovery, and even recrystallization take place in the 08Kh18N10T steel during cyclic deformation in the course of fatigue tests according to the scheme of repeated tension. The fatigue strength increases after ECAP due to the refinement and twinning of an austenite structure and the appearance of martensite. The fatigue limit is maximal after ECAP and heating at 550°C for 20 h due to a high annealing twin density in a predominantly austenitic recrystallized matrix, intense dynamic twinning, and martensitic transformation during cyclic deformation.

1-[1-(2,1,3-Benzoxadiazol-5-ylmethyl)-1H-1,2,3-triazol-4-yl]hexan-1-one

The title compound, C15H17N5O2, was synthesized as part of a series of benzoxadiazole analogs which were examined for fluorescent properties by Cu-catalysed azide–alkyne cyclo­addition (CuAAC) of a 4-azido­methyl-benzoxadiazole substrate. The structure shows a nearly coplanar orientation of the hexa­none keto group and the 1,2,3-triazole ring [dihedral angle = 4.3 (3)°], while the benzoxadiazole and triazole groups are much more severely inclined [dihedral angle = 70.87 (4)°]. In the crystal, weak C—H⋯N inter­actions connect translationally-related triazole rings, while another set of C—H⋯N inter­actions is formed between inversion-related benzoxadiazole units, forming a three-dimensional network. The crystal studied was a non-merohedral twin with refined value of the twin fraction of 0.2289 (16).

3,6-Dibromophenanthrene

The phenanthrene ring in the title compound, C14H8Br2, is approximately planar [maximum deviation = 0.039 (3) Å]. In contrast, the two bromo atoms are displaced slightly from the phenanthrene plane [maximum deviation = 0.1637 (3) Å]. In the crystal, the mol­ecules adopt a herringbone-like arrangement and form face-to-face slipped π–π stacking inter­actions along the b axis, with an inter­planar distance of 3.544 (3) Å and slippage of 1.81 Å. The crystal studied was a racemic twin with a minor twin fraction of 0.390 (10).

Dibutylammonium bis(hydrogen methylphosphonato-κO)triphenylstannate(IV)

The asymmetric unit of the title organotin salt, (C8H20N)[Sn(C6H5)3(CH4O3P)2], contains two dibutyl­ammonium cations and two stannate(IV) anions consisting each of two monodentately bonding methyl hydrogenphosphate groups attached to an Sn(C6H5) unit. The overall coordination environment of the two SnIV atoms is trigonal–bipyramidal defined by three phenyl C atoms in equatorial positions and two methyl hydrogenphosphate O atoms at the apical sites. In the crystal, the stannate(IV) anions are linked to each other via pairs of short O—H⋯O hydrogen bonds, leading to an infinite chain extending parallel to the b-axis direction. Neighbouring chains are linked by N—H⋯O hydrogen bonds involving the butyl­ammonium cations, giving a two-dimensional structure parallel to the ab plane. The crystal under investigation was found to be twinned by reticular merohedry with twin fractions of 0.5342 (7):0.4658 (7).

Vascular Bioactivation of Nitroglycerin by Aldehyde Dehydrogenase-2

Aldehyde dehydrogenase-2 (ALDH2) catalyzes the bioactivation of nitroglycerin (glyceryl trinitrate, GTN) in blood vessels, resulting in vasodilation by nitric oxide (NO) or a related species. Because the mechanism of this reaction is still unclear we determined the three-dimensional structures of wild-type (WT) ALDH2 and of a triple mutant of the protein that exhibits low denitration activity (E268Q/C301S/C303S) in complex with GTN. The structure of the triple mutant showed that GTN binds to the active site via polar contacts to the oxyanion hole and to residues 268 and 301 as well as by van der Waals interactions to hydrophobic residues of the catalytic pocket. The structure of the GTN-soaked wild-type protein revealed a thionitrate adduct to Cys-302 as the first reaction intermediate, which was also found by mass spectrometry (MS) experiments. In addition, the MS data identified sulfinic acid as the irreversibly inactivated enzyme species. Assuming that the structures of the triple mutant and wild-type ALDH2 reflect binding of GTN to the catalytic site and the first reaction step, respectively, superposition of the two structures indicates that denitration of GTN is initiated by nucleophilic attack of Cys-302 at one of the terminal nitrate groups, resulting in formation of the observed thionitrate intermediate and release of 1,2-glyceryl dinitrate. Our results shed light on the molecular mechanism of the GTN denitration reaction and provide useful information on the structural requirements for high affinity binding of organic nitrates to the catalytic site of ALDH2.

Cloning, expression, purification, crystallization and preliminary X-ray diffraction analysis of succinyl-diaminopimelate desuccinylase (Rv1202, DapE) from Mycobacterium tuberculosis.

Succinyl-diaminopimelate desuccinylase from Mycobacterium tuberculosis (DapE, Rv1202) has been cloned, heterologously expressed in Escherichia coli and purified using standard chromatographic techniques. Diffraction-quality crystals were obtained at acidic pH from ammonium sulfate and PEG and diffraction data were collected from two crystals to resolutions of 2.40 and 2.58 Å, respectively. The crystals belonged to the monoclinic space group P2(1), with unit-cell parameters a = 79.7, b = 76.0, c = 82.9 Å, β = 119°. The most probable content of the asymmetric unit was two molecules of DapE, which would correspond to a solvent content of 56%. Both examined crystals turned out to be pseudo-merohedrally twinned, with twin operator -h, -k, h + l and twin fractions of approximately 0.46 and 0.16, respectively.

On the role of residual strain in controlling sensitisation of twin-boundary engineered type 304 stainless steel

Sensitisation in twin boundary engineered type 304 austenitic stainless steel (SS) was investigated using electrochemical potentiokinetic reactivation technique. As-received (AR) specimens of type 304 SS were given two solution-annealing treatments at 1000°C for 0.5 and 2h. Solution-annealed specimens were given a small strain of 5% using cold-rolling followed by strain–annealing (strain–annealing) at 927°C for 24, 48, and 72h. Electron backscatter diffraction analyses have shown increase in fraction of special boundaries (Σ⩽29) in two solution-annealed specimens. Twin-boundary engineered specimens were given two potential sensitisation treatments at 575 and 675°C for 1h. The increase in twin fraction and decrease in grain average misorientation (GAM) always resulted in reducing the extent of sensitisation. An exponential correlation between GAM and twin-fraction was established and increase in GAM always associated with increase in twin-fraction.

Twinning in pure Ti subjected to monotonic simple shear deformation

The aim of this paper is to provide a thorough study on the occurrence and importance of deformation twinning in simple shear deformed pure α-Ti. A statistically relevant inspection of the morphology of the deformation twins in relation to the applied strain/deformation is performed. The investigated microstructural aspects are the twin volume fraction, the twin thickness distribution and the resolved shear stress distribution on the twin plane. All these aspects are examined as a function of the twin types and two initial textures. Monotonic simple shear experiments are carried out for three different loading directions with respect to a direction linked to the initial crystallographic texture. EBSD and TEM observations reveal the presence of 101¯2 and 112¯2 twins. The statistical analysis reveals that 101¯2 and 112¯2 twins have a similar average thickness around 1.9 nm, but the 101¯2 twins show a far larger spread on their thickness and can grow to almost the size of the original parent grain. Correlation of the twin fractions with the RSS analysis shows that RSS is an acceptable method explaining the difference in twin fractions for different textures and orientations. A detailed analysis shows that 112¯2 twins occur in average with a smaller volume fraction but with a higher RSS, indicating they are more difficult to nucleate or grow compared to 101¯2 twinning. In general a higher RSS value on the twin plane is not connected to a higher twin thickness; only in the case of 101¯2 twins the highest RSS values show clearly thicker twins.

2-(4-Bromophenyl)-N-[3-(1H-imidazol-1-yl)propyl]quinazolin-4-amine

In the title compound, C20H18BrN5, the bromo­phenyl-substituted quinazoline unit is essentially planar [maximum deviation = 0.098 (3) Å] and makes a dihedral angle of 56.04 (14)° with the imidazole ring. In the crystal, mol­ecules are associated by pairs of N—H⋯N hydrogen bonds to form inversion dimers. All the quinazoline planar systems are oriented almost perpendicular to the [110] direction, making π–π inter­actions possible between adjacent dimers [centroid–centroid distances = 3.7674 (16) and 3.7612 (17) Å]. There are also a number of C—H⋯π inter­actions present. The crystal is a nonmerohedral twin, with a minor twin fraction of 0.47.

Formula omitted] Twinning in magnesium-based lamellar microstructures

The magnesium-based alloy Mg–9Al–1Zn has been extruded and heat treated to produce a dense population of lamellar plate-shaped particles. In compression with a testing orientation well aligned for prolific { 1 0 1 ¯ 2 } twinning, precipitation resulted in a significant increase in the yield point, but there was no change in the volume fraction of twins that were produced. It is proposed that the larger number of smaller twins observed in the aged condition is the result of inhibition of twin growth by the particles.

Ion beam sputter deposition of epitaxial Ag films on native oxide covered Si(1 0 0) substrates

Epitaxial Ag films were grown on native oxide covered Si(100) substrates by an ion beam sputter deposition process at elevated deposition temperatures. At RT, films were observed to be non-epitaxial but with preferred (111) orientation. However, elevated substrate temperatures and under highly energetic sputter deposition process assist the growth of Ag films, that exhibit an epitaxial relationship with the underlying Si(100) substrates. With increasing deposition temperature an increase in the crystalline quality was observed with a narrowing mosaic distribution of crystallites and a decrease in the fraction of 1st order twins. The lowest epitaxial growth temperature was observed to be as low as 100°C.

Twinning and martensite in a 304 austenitic stainless steel

The microstructure characteristics and deformation behavior of 304L stainless steel during tensile deformation at two different strain rates have been investigated by means of interrupted tensile tests, electron-backscatter-diffraction (EBSD) and transmission electron microscopy (TEM) techniques. The volume fractions of transformed martensite and deformation twins at different stages of the deformation process were measured using X-ray diffraction method and TEM observations. It is found that the volume fraction of martensite monotonically increases with increasing strain but decreases with increasing strain rate. On the other hand, the volume fraction of twins increases with increasing strain for strain level less than 57%. Beyond that, the volume fraction of twins decreases with increasing strain. Careful TEM observations show that stacking faults (SFs) and twins preferentially occur before the nucleation of martensite. Meanwhile, both ɛ-martensite and α′-martensite are observed in the deformation microstructures, indicating the co-existence of stress-induced-transformation and strain-induced-transformation. We also discussed the effects of twinning and martensite transformation on work-hardening as well as the relationship between stacking faults, twinning and martensite transformation.

Twinning-Slip Transitions in Mg AZ31B

Many metals, particularly ones with hexagonal close-packed (HCP) crystal structures, undergo deformation by combinations of twinning and slip, the proportion of which depends on variables such as the temperature and strain rate. The typical techniques to reveal such mechanisms rely on metallography, X-ray diffraction, or electron optics. Simpler, faster, less expensive mechanical tests were developed in the current work and applied to Mg AZ31B. The curvature of compressive stress–strain plots over a fixed strain range was found to be a consistent indicator of twinning magnitude, independent of the temperature and strain rate. The relationship between the curvature and areal fraction of twins was determined. The transition temperatures determined based on stress–strain curvature were consistent with the ones determined by metallographic analysis and flow stresses, and the results depended on the strain rate according to the Zener-Hollomon parameter, a critical value for which was measured. The transition temperature was found to depend significantly on the grain size, a relationship for which was established. Finally, it was shown that the transition temperature can be determined consistently, and much faster, using a single, novel “step-temperature” test.

Extension twin variant selection during uniaxial compression of a magnesium alloy

► Activation of extension twin variants during compression of AZ31 investigated. ► Significant number of variants cannot be predicted using the Schmid factor (SF). ► At low strains twinning is delayed in the smallest grains. ► Wide scatter in relationship between twin fraction and SF in individual grains. ► Envelope for the maximum twinned fraction as function of SF can, however, be defined. Samples of the magnesium alloy AZ31 have been deformed by compression to strains of 5% and 10% and microstructural observations made to investigate the activation of specific { 1 0 1 ¯ 2 } extension twin variants. The twinning has been analyzed on a grain-by-grain basis for more than 260 grains to determine both the number of extension twin variants in each grain, and the volume fraction of each. At 5% strain approx. 30% of the grains contain twins corresponding to variants with the third or lower ranked Schmid factor, with the fraction increasing to 40% after 10% compression. A grain size effect is also observed in that smaller grains are less likely to contain lower ranked twin variants. For both 5% and 10% compression no clear relationship exists between the volume fraction of each twin variant in a given grain population and the Schmid factor for the twin variant. A positive linear relationship can be defined, however, between the maximum twinning fraction that a twin variant can reach and its Schmid factor.

Processing twining induced plasticity steel through simple shear extrusion

Microstructure and deformation behavior of twinning induced plasticity steel processed by simple shear extrusion are investigated. Two appropriate dies with the maximum distortion angle of 30° and 45° are designed and constructed. Results show that the twin fraction increases while the twin spacing decreases as the equivalent strain rises in the deformation channel. The hardness reaches to its maximum at the middle of the deformation channel and decreases in the second half of the channel due to reversal straining. A similar trend is observed for dislocation density. The crystallite size is determined as 710nm and 500nm after one pass of simple shear extrusion with maximum distortion angle of 30° and 45°, respectively.

A formation mechanism for ultra-thin nanotwins in highly textured Cu/Ni multilayers

High density nanotwins with average twin thickness varying from 3 to 6 nm are formed in sputtered highly (111) textured Cu/Ni multilayers, when individual layer thickness is 25 nm or less. Twin interfaces are normal to growth direction. Both maximum twin thickness and volume fraction of twins vary with the individual layer thickness. Coherency stress plays an important role in tailoring the formation of nanotwins. Nanotwins compete with misfit dislocations in accommodating elastic strain energy in epitaxial Cu/Ni multilayers.