Twin Fraction Research Articles

Evolution equations of deformation twins in metals—Evolution of deformation twins in pure titanium

The evolution equations of the volume fractions of deformation twins are obtained in this article by using the theory of inclusions in micromechanics and analyzing the Gibbs free energy and dissipation of a system. The evolution process of the volume fractions of twins is got by using the Runge–Kutta method in this article. The computational results of the evolution equations (the critical twinning stress and the families of twins appeared under different loading conditions) are consistent with the experiment results.

Crystal structure of the leucine zipper domain of small‐conductance Ca2+‐activated K+ (SKCa) channel from Rattus norvegicus

Crystal structure of the leucine zipper domain of small‐conductance Ca²⁺‐activated K⁺ (SK_Ca) channel from <i>Rattus norvegicus</i>

Fracture toughness in direct extruded Mg–Al–Zn alloys

Fracture toughness and its deformed structures were investigated on a Mg–3Al–1Zn (AZ31) alloy processed by direct extrusion. The average grain sizes of the alloy after extrusion at temperatures of 473 and 573 K were 4.0 and 14.5 μm, respectively (i.e., fine-grained and coarse-grained alloys). The plane-strain fracture toughness,KIC, as determined by stretched zone analysis in fine-grained and coarse-grained alloys was estimated to be 24.9 and 22.7 MPam1/2, respectively. Microstructural observations of the fine-grained alloy after fracture toughness testing showed that non-basal slip and grain-boundary sliding was activated even at room temperature. In addition, the {10-12} deformation twins were observed despite the fine-grained structures. However, the fraction of deformation twins was reduced with grain refinement, which related to the enhancement of fracture toughness with grain refinement.

Preliminary X-ray analysis of cellobiohydrolase Cel7B from Melanocarpus albomyces.

Cellobiohydrolases are enzymes that cleave off cellobiose units from cellulose chains in a processive manner. Melanocarpus albomyces Cel7B is a thermostable single-module cellobiohydrolase that has relatively low activity on small soluble substrates at room temperature. It belongs to glycoside hydrolase family 7, which includes endo-beta-1,4-glucanases and cellobiohydrolases. Cel7B was crystallized using the hanging-drop vapour-diffusion method and streak-seeding. The crystals belonged to space group P2(1), with unit-cell parameters a = 50.9, b = 94.5, c = 189.8 A, beta = 90.0 degrees and four monomers in the asymmetric unit. Analysis of the intensity statistics showed that the crystals were pseudo-merohedrally twinned, with a twinning fraction of 0.37. X-ray diffraction data were collected at 1.6 A resolution using synchrotron radiation.

2-Phenyl-4H-thiazolo[3,2-a][1,3,5]triazine-4-thione

The title compound, C11H7N3S2, was synthesized from benzoyl chloride, ammonium thio­cyanate and 2-amino­thia­zole in dry acetone. The whole molecule is essentially planar. The structure was determined using data from a non-merohedrally twinned crystal with a refined twin fraction of 0.523 (1). The structure is stabilized by short inter­molecular S⋯S [3.491 (1) Å] and π–π stacking inter­actions, as well as weak C—H⋯N and C—H⋯S hydrogen bonds.

Influence on Annealing Treatments on Twins in Magnesium Alloy AZ31

This research looks at the influence of annealing treatments on twins in magnesium alloy AZ31. Samples containing different twin fractions are created in order to examine the influence of the initial structure on the annealing behavior. It is seen that twins can both expand, taking over an entire grain, or shrink, leaving completely untwinned grains. The grains, containing expanding twins, are found to contain, in general, only one twin variant and limited slip deformation after twin formation. The grains containing shrinking twins, on the contrary, are seen to contain different twin variants and have undergone more slip deformation after twinning.

Pseudo-merohedral twinning in monoclinic crystals of human orotidine-5′-monophosphate decarboxylase

Human UMP synthase is a bifunctional enzyme that catalyzes the penultimate and last steps in the de novo biosynthesis of UMP. In contrast to prokaryotes, UMP synthase from higher eukaryotes combines the orotate phosphoribosyltransferase and the orotidine-5'-monophosphate (OMP) decarboxylase activities on a single polypeptide chain. The decarboxylase activity is unusual in that it represents the fastest rate acceleration of any enzyme studied to date. Although several crystal structures of OMP decarboxylases have been described, the precise decarboxylation mechanism remains elusive. The crystal structure of the OMP decarboxylase domain from human UMP synthase was determined by molecular replacement using data from a highly twinned monoclinic crystal. The space group is P2(1), with unit-cell parameters a = 69.18, b = 61.70, c = 69.17 A, beta = 113.06 degrees. Self-rotation function analysis and various intensity statistics revealed the presence of pseudo-merohedral twinning, but these tests underestimated the true twin fraction of alpha approximately = 0.44. Data analysis, the origin of the twinning and structure determination are discussed.

A tripodal tris(thiophene) derivative of hexahydropyrimidine and its ladder-like extended structure

In the title compound, 2-(2-thien­yl)-1,3-bis­(2-thienyl­methyl)­perhydro­pyrimidine, C18H20N2S3, a new hexa­hydro­pyrimidine derivative with no formal crystallographic symmetry (but approximate Cs mol­ecular symmetry), the thio­phene rings are approximately orthogonal to the mean plane of the saturated pyrimidine `core' of the mol­ecule, with the three S atoms positioned on one side of the six-atom mean plane. The S atom of the directly attached thio­phene ring is involved in an unconventional (alk­yl)C—H⋯S hydrogen bond (H⋯S = 2.87 Å) with a thio­phene CH group of the closest neighbouring mol­ecule, leading to a ladder-like one-dimensional chain as the extended structure. The crystal specimen used for data collection was an inversion twin [twin fraction x = 0.41 (4)].

The effect of thermomechanical processing on the creep behavior of Alloy 690

The effect of thermomechanical processing on the microstructure and elevated-temperature creep behavior of Alloy 690 was investigated. Commercially available sheet was subjected to four cycles of cold rolling to 25% deformation followed by annealing at 1000 °C for 1 h. Both the resultant microstructure and the original microstructure were characterized using electron backscattered diffraction. The thermomechanically processed microstructure exhibited a slightly lower fraction of twins and a smaller average grain size than the original microstructure. Tensile–creep experiments were performed in an open-air environment at temperatures between 650 and 690 °C and stresses between 75 and 172 MPa. The measured creep stress exponents (4–5) activation energies (320–368 kJ/mol) suggested that dislocation creep with lattice self-diffusion was dominant. The thermomechanically processed microstructure exhibited significantly worse creep resistance than the original as-processed microstructure. Thus, cyclic strain and annealing processing, which has been shown to improve the ductility-dip cracking susceptibility of Alloy 690, is not recommended for enhancing the creep resistance.

Texture Evolution in Cu Films and Lines

ABSTRACTGrain growth in polycrystalline films is controlled by the energetics of the surface, interface and grain boundaries as well as strain energy. The unique character of damascene lines fabricated from electroplated Cu films introduces the additional considerations of bath chemistry and geometric constraints. The moderate stacking fault energy of Cu allows for the development of a substantial twin fraction for certain growth conditions. This paper discusses in-situ observation of grain growth in Cu films and lines under various processing conditions. It is shown that for thicker films and for structures constrained within damascene trenches the energetics of twin boundary formation play a large role in texture development of these structures.

Annealing twin formation and recrystallization study of cold-drawn copper wires from EBSD measurements

The crystallographic texture and microstructure of an electrolytic tough pitch copper have been investigated by Electron Back Scattered Diffraction (EBSD) after cold wire drawing (reduction in area between 52% and 94%) and after primary recrystallization. The material presents a deformation texture composed of major 〈111〉 and minor 〈100〉 fibers. The evolution of the quality index of the Kikuchi patterns shows that the stored energy is lower in the 〈100〉 fiber than in the 〈111〉 fiber. Then, after recrystallization, the volume fraction of the 〈100〉 fiber increases at the expense of the other texture components. The study of the grain boundary nature shows that the recrystallization twin fraction decreases with increasing strain. It is shown that this evolution is the consequence of the grain size reduction.

Structures of Ternary Complexes of BphK, a Bacterial Glutathione S-Transferase That Reductively Dechlorinates Polychlorinated Biphenyl Metabolites

Prokaryotic glutathione S-transferases are as diverse as their eukaryotic counterparts but are much less well characterized. BphK from Burkholderia xenovorans LB400 consumes two GSH molecules to reductively dehalogenate chlorinated 2-hydroxy-6-oxo-6-phenyl-2,4-dienoates (HOPDAs), inhibitory polychlorinated biphenyl metabolites. Crystallographic structures of two ternary complexes of BphK were solved to a resolution of 2.1A. In the BphK-GSH-HOPDA complex, GSH and HOPDA molecules occupy the G- and H-subsites, respectively. The thiol nucleophile of the GSH molecule is positioned for SN2 attack at carbon 3 of the bound HOPDA. The respective sulfur atoms of conserved Cys-10 and the bound GSH are within 3.0A, consistent with product release and the formation of a mixed disulfide intermediate. In the BphK-(GSH)2 complex, a GSH molecule occupies each of the two subsites. The three sulfur atoms of the two GSH molecules and Cys-10 are aligned suitably for a disulfide exchange reaction that would regenerate the resting enzyme and yield disulfide-linked GSH molecules. A second conserved residue, His-106, is adjacent to the thiols of Cys-10 and the GSH bound to the G-subsite and thus may stabilize a transition state in the disulfide exchange reaction. Overall, the structures support and elaborate a proposed dehalogenation mechanism for BphK and provide insight into the plasticity of the H-subsite.

Transformation twins in the weld HAZ of a low-carbon high-strength microalloyed steel

Transformation twins were found in the weld heat-affected zone (HAZ) of a low-carbon microalloyed steel. The effect of weld thermal cycle and homogenisation of austenite with respect to dissolution of microalloy carbides on twinning is examined by applying a range of heat inputs as well as conducting simulated heat treatments with short and long annealing periods. Although twinning is more frequent in the martensitic coarse-grained HAZ when low values of heat input are applied, twinning can occur in the ferritic fine-grained HAZ when the peak temperature is not very high and cooling rates are low. It is also found that the annealing period during high-temperature holding of austenite does not significantly affect the volume fraction of twins, suggesting that inhomogeneities in the austenite cannot be a major cause of twinning. Weld effects, especially thermal stresses, are therefore concluded to be a major reason for twinning in the HAZ.

Influence of energetic-driven “Taylor-Wave” shock-wave prestraining on the structure/property response of depleted uranium

The influence of shock prestraining, via direct energetic Taylor-wave (triangular wave) loading, on the post-shock structure/property behavior of depleted uranium (DU) was studied. Samples were shock prestrained within a soft shock recovery fixture composed of momemtum traps and a spall plate to assure 1-dimensional loading. The DU samples exhibit roughly a 30% increase in yield strength following shock prestraining to ∼45 GPa. The texture evolution in DU was quantified using electron-backscatter diffraction (EBSD). Detailed quantification of the substructure evolution following shock prestraining revealed high volume fractions of {130), '{172}', and {112) deformation twins. The volume fraction of (130) twins was found to be ∼10x the volume fraction of '{172}' and {112} twins. Details of the twin system activation and volume fraction relative to the local Schmidt factor within grains are presented. The influence of HE-driven shock prestraining on the structure/property response of DU is compared and contrasted to that seen in 304SS and 316SS subjected to Taylor-wave shock prestraining.

Detecting and overcoming hemihedral twinning during the MIR structure determination of Rna1p

The structure of Rna1p was originally solved to 2.7 A resolution by MIRAS from crystals with partial hemihedral twinning in space group I4(1) [Hillig et al. (1999), Mol. Cell, 3, 781-791] by finding a low-twinned native crystal (twin fraction alpha=0.06) and after twin correction of all data sets. Rna1p crystals have now been used to examine how far twinning and twin correction affect MIR phasing with a higher resolution but highly twinned native data set. Even high hemihedral twinning [alphanative=0.39, alphaderivative=0.24] would not have hindered heavy-atom site identification of strong derivatives using difference Patterson maps. However, a weaker derivative could have been missed and refinement would have stalled at high R values had twinning not been identified and accounted for. Twin correction improved both site identification, experimental phasing statistics and MIR map quality. Different strategies were tested for refinement against twinned data. Using uncorrected twinned data and TWIN-CNS, Rna1p has now been refined to 2.2 A resolution (final twinned R and Rfree were 0.165 and 0.218, respectively). The increased resolution enabled release of the NCS restraints and allowed new conclusions to be drawn on the flexibility of the two molecules in the asymmetric unit. In the case of Rna1p, twinned crystal growth was possible owing to the presence of a twofold NCS axis almost parallel to the twin operator.

Crystal structure of a major fragment of the salt-tolerant glutaminase from Micrococcus luteus K-3

Glutaminase of Micrococcus luteus K-3 (intact glutaminase; 48 kDa) is digested to a C-terminally truncated fragment (glutaminase fragment; 42 kDa) that shows higher salt tolerance than that of the intact glutaminase. The crystal structure of the glutaminase fragment was determined at 2.4 Å resolution using multiple-wavelength anomalous dispersion (MAD). The glutaminase fragment is composed of N-terminal and C-terminal domains, and a putative catalytic serine–lysine dyad (S64 and K67) is located in a cleft of the N-terminal domain. Mutations of the S64 or K67 residues abolished the enzyme activity. The N-terminal domain has abundant glutamic acid residues on its surface, which may explain its salt-tolerant mechanism. A diffraction analysis of the intact glutaminase crystals (a twinning fraction of 0.43) located the glutaminase fragment in the unit cell but failed to turn up clear densities for the missing C-terminal portion of the molecule.

Between microscopic and mesoscopic descriptions of twin–twin interaction

On a microscopic scale, deformation twinning is carried by the motion of twinning disconnections. A disconnection is an interfacial line defect characterized by a Burgers vector, a line vector, and a step vector. The Burgers vector (dislocation component of the disconnection) carries the deformation while the step vector (ledge component) carries the transformation from one twin variant to the other. On a mesoscopic scale, the deformation produced by twinning is a simple shear. A moving disclination dipole provides a mesoscopic model accounting for the twinning shear. Twin-twin interaction processes including the intersection of twins, the formation of structured twins, and the nucleation of cracks, may feature very complex mechanisms when analyzed on a microscopic scale. It turns out, however, that these mechanisms are controlled by the properties of large disconnection groups containing up to 10000 disconnections and more. These properties are sufficiently well approximated in the disclination dipole model. The disclination model for twin-twin interaction predicts orientation and volume fractions of secondary twins. The model also predicts the nucleation of cracks and crack growth. The disclination model was used to analyze the ductile-to-brittle transition of austenitic steel deformed at low temperature. The mesoscopic disclination model for twinning is successful because it accounts for the properties and mechanisms of disconnection groups.

Structural Basis for Activation of the Thiamin Diphosphate-dependent Enzyme Oxalyl-CoA Decarboxylase by Adenosine Diphosphate

Oxalyl-coenzyme A decarboxylase is a thiamin diphosphate-dependent enzyme that plays an important role in the catabolism of the highly toxic compound oxalate. We have determined the crystal structure of the enzyme from Oxalobacter formigenes from a hemihedrally twinned crystal to 1.73 A resolution and characterized the steady-state kinetic behavior of the decarboxylase. The monomer of the tetrameric enzyme consists of three alpha/beta-type domains, commonly seen in this class of enzymes, and the thiamin diphosphate-binding site is located at the expected subunit-subunit interface between two of the domains with the cofactor bound in the conserved V-conformation. Although oxalyl-CoA decarboxylase is structurally homologous to acetohydroxyacid synthase, a molecule of ADP is bound in a region that is cognate to the FAD-binding site observed in acetohydroxyacid synthase and presumably fulfils a similar role in stabilizing the protein structure. This difference between the two enzymes may have physiological importance since oxalyl-CoA decarboxylation is an essential step in ATP generation in O. formigenes, and the decarboxylase activity is stimulated by exogenous ADP. Despite the significant degree of structural conservation between the two homologous enzymes and the similarity in catalytic mechanism to other thiamin diphosphate-dependent enzymes, the active site residues of oxalyl-CoA decarboxylase are unique. A suggestion for the reaction mechanism of the enzyme is presented.

EBSD Study of Annealing Twinning during Recrystallization of Cold Rolled Copper

In this work, EBSD (Electron Back Scattered Diffraction) measurements have been performed on deformed, partially and fully recrystallized cold rolled copper to 70 and 90 % reduction. The twin fraction as well as its existing relation with the parent crystallographic orientation has been followed in relation with respect to the recrystallized fraction. It has been shown thanks to this quantitative analysis that annealing twinning is more active when recrystallization nuclei slowly develop. The experimental observations are briefly discussed according with the twins selection principles [1].

Structure of a pseudomerohedrally twinned monoclinic crystal form of a pyridoxal phosphate-dependent catalytic antibody

The purification, crystallization and structure determination at 2.3 A resolution of the complex of the pyridoxal-5'-phosphate (PLP) dependent catalytic antibody 15A9 with a phosphopyridoxyl-L-alanine (PPL-L-alanine) substrate analogue are described. The crystal belongs to space group P2(1), with two molecules in the asymmetric unit related by non-crystallographic symmetry. The unit-cell parameters are a = 63.5, b = 81.7, c = 79.3 A and beta is fortuitously 90 degrees . Refinement of the structure converged at unacceptably high R factors. Although the traditional analysis of intensity distribution did not indicate twinning, pseudomerohedral twinning was revealed by a newer test based on local intensity differences [Padilla & Yeates (2003), Acta Cryst. D59, 1124-1130]. When the potential twinning operator was included in SHELX, the structure could be satisfactorily refined with a twinning fraction of 0.46, indicating a nearly perfect hemihedrally twinned crystal. One of the active sites is occupied by the phosphopyridoxyl-L-alanine ligand, while one iodide ion mimics the cofactor phosphate group in the other. Four other iodide ions are present in the structure: two are involved in specific intermolecular contacts and two dictate the conformation of the CDRH3 loop in each molecule.