In-situ strain localization analysis in low density transformation-twinning induced plasticity steel using digital image correlation

The effect of deformation temperature on the strain localization has been evaluated by an adapted digital image correlation (DIC) technique during tensile deformation. The progress of strain localization was traced by the corresponding strain maps. The electron backscatter diffraction analysis and tint etching technique were utilized to determine the impact of martensitic transformation and deformation twinning on the strain localization in both elastic and plastic regimes. In elastic regime the narrow strain bands which are aligned perpendicular to the tension direction were observed in temperature range of 25 to 180 °C due to the stress-assisted epsilon martensite. The strain bands were disappeared by increasing the temperature to 300 °C and reappeared at 400 °C due to the stress-assisted deformation twinning. In plastic regime strain localization continued at 25 °C and 180 °C due to the strain-induced alfa-martensite and deformation twinning, respectively. The intensity of plastic strain localization was increased by increasing the strain due to the enhancement of martensite and twin volume fraction. The plastic strain showed more homogeneity at 300 °C due to the lack of both strain-induced martensite and deformation twinning.     

Coherent X-ray diffraction investigation of twinned microcrystals

Coherent X-ray diffraction has been used to study pseudo-merohedrally twinned manganite microcrystals. The analyzed compositions were Pr(5/8)Ca(3/8)MnO(3) and La(0.275)Pr(0.35)Ca(3/8)MnO(3). The prepared loose powder was thermally attached to glass (and quartz) capillary walls by gentle heating to ensure positional stability during data collection. Many diffraction data sets were recorded and some of them were split as expected from the main observed twin law: 180° rotation around [101]. The peak splitting was measured with very high precision owing to the high-resolution nature of the diffraction data, with a resolution (Δd/d) better than 2.0 × 10(-4). Furthermore, when these microcrystals are illuminated coherently, the different crystallographic phases of the structure factors induce interference in the form of a speckle pattern. The three-dimensional speckled Bragg peak intensity distribution has been measured providing information about the twin domains within the microcrystals. Research is ongoing to invert the measured patterns. Successful phase retrieval will allow mapping out the twin domains and twin boundaries which play a key role in the physical properties.     

Competition between ferroelasticity and ferroelectricity

In the crystals of the KH₂PO₄ family, the properties of ferroelasticity and ferroelectricity strongly interact. It is for example possible to change by application of an electric field the orientation of the phase front between high temperature and low temperature phases, and to observe the rotation of reticular planes during an electric hysteresis cycle drawing. These phenomena, as the existence of a quasi constant domain width illustrate the particular competition between ferroelasticity and ferroelectricity in these compounds.     

Interlayer inclusion of tetracyanonickelate anion and water molecules: The crystal and molecular structure of [-ZN(en)2-μ-(NC)2-NI-μ -(CN)2-ZN(en) -] n 2n+ ·n[NI(CN)4]2− · 3n H2O

The crystal structure of [Zn(en)₂- (NC)₂Ni(CN)₂ Zn(en)][Ni(CN)₄]· 3 H₂O consists of infinite positively charged wave-shaped layers of composition [Zn(en)₂-(NC)₂Ni(CN)₂- Zn(en)] _n ²ⁿ⁺ with the [Ni(CN)₄]^2– anions and water molecules included between them. Both Ni atoms exhibit square-planar coordination. The chelate bonded en (=ethylenediamine) and N-bonded cyano ligands around two independent zinc atoms form a deformed tetrahedron and a deformed octahedron, respectively. Yellow needles of the complex belong to the orthorhombic space groupPbcm witha = 6.977(1),b = 25.407(4),c = 14.876(2)Å,Z = 4,D _m = 1.74(1) g cm^–3 andD _c = 1.739 g cm^–3. The structure was refined toR = 6.31 %.     

High-pressure synthesis and crystal structure of silicon phosphate hydroxide, SiPO4(OH)

A new high-pressure phase, silicon phosphate hydroxide, was prepared at and 1000 °C in >98% purity. From X-ray diffraction on a pseudo-merohedrally twinned crystal, it was found that SiPO₄(OH) crystallizes in a monoclinic cell with space group P2₁/n (No. 14), , and Z=4. The refinement gave a conventional R_obs of 0.0320 and wR_obs of 0.0864 for the overlapped data from both twin components. In the structure, SiO₆ octahedra form chains along [101], with PO₄ tetrahedra alternating along the chain in the b-direction. The parallel chains link up with tetrahedral corners from other chains to form a 3-dimensional network. SiPO₄(OH) belongs to a structural family that includes HgSeO₄·H₂O. It is also related to the SbOPO₄ structure by a small distortion that lowers the symmetry from C2/c in SbOPO₄ to in SiPO₄(OH).     

Grain Boundary Junctions in Microstructure Generated by Multiple Twinning

The microstructure of a Cu-Ni alloy after static recrystallization was investigated using electron backscatter diffraction in a scanning electron microscope and the existence of orientationally related clusters of crystallites formed by multiple twinning has been established. Grain boundary and triple junction character within the clusters are analyzed. While the outer boundaries of the cluster are crystallographically random, all the inner boundaries have 3 ⁿ misorientations. A newly developed crystallographic theory of triple junctions and multicrystallite ensembles consisting of CSL boundaries is used to describe the structure of the cluster. The presence of an 1 triple junction is confirmed. Apparently, the microstructure of recrystallized materials susceptible to annealing twinning consists of multiple-twinned clusters. The cluster size cannot be reduced to the grain size excluding twins.     

Effect of the density of twinning dislocations on the configuration of stress fields near a wedge twin with different shapes of the boundaries

The effect of changes in density of twinning dislocations on one boundary of the wedge twin on the configuration of the stress fields generated by the latter is considered on the basis of a macroscopic dislocation model. Specific features of violation of symmetry of the stress-field distribution near the wedge twin with different shapes of the boundaries are demonstrated for the case of different densities of twinning dislocations on the twin boundaries. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 5, pp. 199–204, September–October, 2008.     

Phase‐transformation‐induced twinning of an iron(III) calix[4]pyrrolidine complex

The title compound, tetrachlorido‐1κCl;2κ³Cl‐(2,2,7,7,12,12,17,17‐octamethyl‐21,22,23,24‐tetraazapentacyclo[16.2.1.1^3,6.1^8,11.1^13,16]tetracosane‐1κ⁴N,N′,N′′,N′′′)‐μ₂‐oxido‐diiron(III), [Fe₂Cl₄O(C₂₈H₅₂N₄)], undergoes a slow phase transformation at ca 173 K from monoclinic space group P2₁/n, denoted form (I), to the maximal non‐isomorphic subgroup, triclinic space group P, denoted form (II), which is accompanied by nonmerohedral twinning [twin fractions of 0.693 (4) and 0.307 (4)]. The transformation was found to be reversible, as on raising the temperature the crystal reverted to monoclinic form (I). In the asymmetric unit of form (I), Z′ = 1, while in form (II), Z′ = 2, with a very small reduction (ca 1.8%) in the unit‐cell volume. The two independent molecules (A and B) in form (II) are related by a pseudo‐twofold screw axis along the b axis. The molecular overlay of molecule A on molecule B has an r.m.s. deviation of 0.353 Å, with the largest distance between two equivalent atoms being 1.202 Å. The reaction of calix[4]pyrrolidine, the fully reduced form of meso‐octamethylporphyrinogen, with FeCl₃ gave a red–brown solid that was recrystallized from ethanol in air, resulting in the formation of the title compound. In both forms, (I) and (II), the Fe^III atoms are coordinated to the macrocyclic ligand and have distorted octahedral FeN₄OCl coordination spheres. These Fe^III atoms lie out of the mean plane of the four N atoms, displaced towards the O atom of the [OFeCl₃] unit by 0.2265 (5) Å in form (I), and by 0.2210 (14) and 0.2089 (14) Å, respectively, in the two independent molecules (A and B) of form (II). The geometry of the [OFeCl₃] units are similar, with each Fe^III atom having a tetrahedral coordination sphere. The NH H atoms are directed below the planes of the macrocycles and are hydrogen bonded to the coordinated Cl⁻ ions. There are also intramolecular C—H...Cl hydrogen bonds present in both (I) and (II). In form (I), there are no significant intermolecular interactions present. In form (II), the individual molecules are arranged in alternate layers parallel to the ac plane. The B molecules are linked by a C—H...Cl hydrogen bond, forming chains along [100].     

Coordination preferences of the alkali cations sodium and caesium in the mixed‐cationic Zintl ammoniate Cs3.2Na0.8Ge9·5.3NH3

The involvement of two different alkali cations in the nonagermanide ammoniate Cs_3.2Na_0.8Ge₉·5.3NH₃ [tricaesium sodium nonagermanide–ammonia (1/5.3)] provides insights into the coordination behaviour of ammonia towards sodium and caesium cations within one compound and represents the first mixed‐cationic solvate structure of nonagermanide tetraanions. The compound crystallizes in the monoclinic space group P2₁/m and, with the presence of pseudomerohedral twinning, mixed‐cation sites and disordering of the nonagermanide cage anions, features a combination of crystallographic challenges which could all be resolved during the refinement.     

Use of thin polyparaxylene films to study the plastic deformation bismuth single crystals

The effect of thin polyparaxylene films on the mechanical twinning of bismuth single crystals with the (111) surface subjected to local deformation. It is found that the number of twins formed near the stress concentrator increases in the presence of the film. Possible mechanisms are proposed to explain an increase in the mobility of twin dislocations in a deformable crystal whose surface is coated with a polyparaxylene film. Spalling of bismuth is found in the regions deformed by the indenter. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 4, pp. 162–166, July–August, 2006.