Twin Planes Research Articles

Coupling defect-inherent built-in electric fields to promote directional charge migration for rapid photocatalytic degradation of levofloxacin

By enhancing the intrinsic built-in electric field (IEF) of the catalyst through defect engineering, and coupling the IEF of the catalyst via an S-type heterojunction structure, a 3-in-1 IEF is formed, which drives the directional migration of electrons and holes, and can effectively improve photocatalytic performance. In this paper, an oxygen vacancy-type Bi2WO6 (Ov-BWO) composite with twin-type Zn0.5Cd0.5S (ZCS) was synthesized to construct an S-scheme heterojunction. The twin structure results in an IEF in ZCS that points from the twin plane Zn to the twin boundary S. The oxygen vacancies, on the other hand, create an internal electric field from Bi and W to O by altering the local charge density. By coupling the IEF of the catalyst through the S-type heterojunction, an enhanced IEF is formed, pointing from the twin plane Zn to the O in Ov-BWO, enabling rapid directional migration of photogenerated charge carriers. Furthermore, potential Levofloxacin (LVFX) intermediates and degradation pathways were found by combining Liquid chromatography-mass spectrometry (LC-MS) data with Density Functional Theory (DFT) simulations. This study systematically elucidates the photocatalytic degradation system, from catalyst design to the degradation process of pollutants, through the analysis of theoretical calculations and experimental results.

Atomistic investigation on the anisotropic elastic and plastic responses of nanotwinned metals

Introducing nanotwins into materials is one of the important strengthening methods to achieve the synergistic improvement of strength and ductility. The anisotropic mechanical behaviors of nanotwinned materials have been widely studied by experimental and computational methods. The dominant deformation mechanisms about dislocation slippages can be effectively switched among three modes, and controlled by twin spacing and the angle between twin boundaries (TBs) orientation and loading direction. Particularly, most of previous researches mainly focused on the deformation mechanisms during the plastic flow stage and researchers paid little attention on the anisotropic characteristics of TBs at the elastic stage which are also essential to manufacture the high-performance materials. Therefore, this study is aiming to systematically investigate the anisotropic effect of TBs both at the elastic and plastic stages within the single crystalline or polycrystalline systems by molecular dynamics (MD) simulations. It is revealed that, when the loading direction is parallel to twin planes, the introduction of TBs in single crystalline models will significantly affect the characteristics of atomic bond rotation and elongation dominated elastic deformation, which can alter the Poisson’s ratio of materials, generate elastic-softening behavior and inhomogeneous elastic deformation. At the plastic flow stage, the deformation mechanism transforms from trans-twin dislocation slippage into the coexistence of Hard Mode I and threading dislocation slippage (Hard Mode II) when the loading direction changes from parallel to perpendicular direction with respect to TBs. Moreover, the dislocation segments at the conjunction of trans-twin dislocation play a momentous role in enhancing material strength. The results for polycrystalline models are consistent with that of single crystalline ones. These findings are expected to be beneficial for the development of high-performance nanostructured materials for structural and functional applications by strain engineering and defect regulation.

Dislocation motion, interaction and twinning during microindentation of CdWO4 single crystals

In this paper dislocations and twins formed under Vickers indentation of different planes of CdWO4 crystals are studied. It has been found that on (010) and (100) faces fresh dislocations run along [001] from an imprint of Vickers pyramid. On (001) twins with (010) twinning plane induced by indenter propagate along [100]. Twins with {011} twin planes are induced by indentation on (100). Possible dislocation dissociation reactions and role of the resulting partial dislocations in twinning during microindentaion in cadmium tungstate are discussed.

Solute Segregation and Pinning Effect on Lateral Twin Boundary in Magnesium

Deformation twinning creates a three-dimensional twin domain via the migration of forward, normal and lateral twin boundaries (TBs) with respect to twin shear direction, normal to twin plane and twin lateral direction. Solute segregation and pinning effect on the forward and normal TBs have been experimentally observed and demonstrated via atomistic simulations. Here, we conducted a comprehensive study of solute segregation and the pinning effect on the lateral TBs in Mg. First-principles density functional theory calculations were used to obtain the segregation and formation energies of 19 alloying elements in coherent regions of lateral TBs. Alloying elements with greater difference in atomic radius from Mg generally show more negative segregation energy. Moreover, alloying elements with good solubility are selected to demonstrate the pinning effect on a coherent interface. Ge, Ga, Y, Gd, La and Ca show negative segregation energy and solubility energy, indicating that these elements can form stable segregation and have a strong pinning effect at the lateral boundary. Molecular dynamics simulations revealed that solutes in coherent regions are more effective in pinning lateral TBs than those in misfit regions. The results provide insight into the selection of solute atoms for tailoring twinning behavior.

Kyanite microstructural and microchemical characteristics reveal differences in growth, deformation and chemical modification: A case study from the Paleoproterozoic suture zone of South Harris, NW Scotland

Based on petrological association, cathodoluminescence (CL), trace element signatures and orientation relationships, two generations of kyanite are distinguished in a high temperature, high pressure garnet-biotite-aluminosilicate bearing migmatite of South Harris, NW Scotland. The migmatite shows a garnet and biotite rich domain (Grt-Bt domain) which is cut at a low angle by a dominantly coarse-grained plagioclase-quartz leucosome. In addition, a fine-grained plagioclase-quartz-kyanite domain (Plag-Qtz-Ky domain) is present intercalated with the Grt-Bt domain and subparallel to the plagioclase-quartz domain. Type 1 kyanite is coarse grained and associated with Bt clusters and garnet within the Grt-Bt domain. It grew relatively early, syn- to post-garnet growth, in a suprasolidus environment resulting in crystallographically determined oscillatory CL and trace element zoning. Grains record evidence of progressive deformation in the crystal plastic regime, where deformation is accommodated by dislocation glide, climb and deformation twinning. The dominant activated slip system is (100)<001> with minor component of <100>, while deformation twins show a ∼ 180° rotation around ∼<001> axis and a twin plane near (001). Grains appear to be impervious to deformation induced diffusion with all zoning remaining sharp despite crystal plastic deformation. Grains in direct contact with the Plag-Qtz-Ky domain show late modification of the CL and trace element signature suggesting melt-mediated interface-coupled dissolution-precipitation reaction. This modification resulted in crosscutting lobate high trace element regions and irregular rims with low Cr and V content. These rims show similar CL and trace element characteristics as Type 2 kyanite which are exclusively seen within the Plag-Qtz-Ky domain suggesting that Type 2 grains are cogenetic with Type 1 rims. Type 2 grains are finer grained than Type 1 grains and show near uniform CL and trace element distributions with rare oscillatory zoned and relatively higher Cr & V bright cores. Type 2 show either no or very localized internal deformation features. However, they exhibit a clear shape preferred orientation which coincides with a crystallographic preferred orientation where the longest shape axis is parallel to <001>. We propose that Type 2 kyanite grains underwent melt-present deformation by rigid body rotation in an externally derived melt with different trace element chemistry than the host rock. This melt thus interacted chemically by melt-mediated interface-coupled dissolution-precipitation reactions with the surrounding rocks forming the Type 1 rims.Our study shows detailed analysis of kyanite is an important tool for giving constraints on the deformation, P–T and melting history of high-grade metamorphic rocks and migmatites.

Unveiling the kinetics of oxygen evolution reaction in defect-engineered B/P-incorporated cobalt-oxide electrocatalysts

Defect-rich transition-metal oxide electrocatalysts hold great promise for alkaline water electrolysis due to their enhanced activity and stability. This study presents a new strategy that significantly improve the OER activity of Co-oxide nanosheets through incorporation of B and P (B/P-CoOx NS), eventually leading to abundant surface defects and oxygen vacancies. The B/P-CoOx NS demonstrates low overpotential of 220 mV to achieve 10 mA/cm2. The electrochemical and kinetic studies coupled with conventional morphological and structural characterizations, reveal that various crystalline defects like vacancies, dislocations, twin planes, and grain boundaries play crucial roles in promoting the OH− ion adsorption, the formation of intermediates, and the desorption of oxygen molecules. The industrial viability of the developed electrocatalyst is substantiated through assessments under harsh industrial conditions of 6 M KOH at 60 °C in a zero-gap single-cell alkaline electrolyzer which achieves 1 A/cm2 at 1.95 V. Chronoamperometry tests (100 h) highlight remarkable robustness of the electrocatalyst. This work establishes a new strategy to fabricate defect-rich OER electrocatalysts, setting a precedent to achieve better OER rates with non-noble materials.

Phase transformation behavior and mechanical properties of NiTi shape memory alloys fabricated by directed laser deposition

Directed laser deposition shows promise as a method for fabricating complex structure NiTi shape memory alloy parts in engineering, aerospace, automobile, and biomedical fields. The dislocation density has an impact on phase transformation behavior and mechanical properties of NiTi manufactured by directed laser deposition. In this work, NiTi samples were produced by directed laser deposition, and the mechanism of martensitic phase transformation and its interactions with dislocation density were revealed through molecular dynamics. Laser power and scanning speed are the main factors affecting dislocation density, the phase compositions of the samples are B2, B19′, and Ti2Ni phase, and the samples with dislocation density below 2 × 1015 m−2 show single-step B2–B19′ martensitic transformation, stress plateau and improved elongation, the samples with dislocation density above the value exhibit no significant transformation peak and no stress plateau. The movement of (100) twin planes and the growth of advantageous variants are observed in the simulation which can explain stress-induced martensitic reorientation and the stress plateau phenomenon. The bending test suggested that the sample under the optimal parameters has an 85 % recovery ratio within 8 % pre-strain.

Optimizing Tensile Properties and Hardness of Inconel 718 by Cold Rolling

The as-received commercial Inconel 718 material was solid solution heat treated (ST), cold-rolled (CR), and precipitation-hardened (PH) to investigate the effects of deformation on the tensile properties, hardness, and texture. Three sets of specimens (0%, 20%, and 50% CR) were ST at 1100 °C/1 h, CR, and aged (720 °C/8 h + 650 °C/8 h) for the analysis. The ultimate tensile strength (UTS), 0.2% yield strength (YS), and elongation of 50% deformed condition were 1645 MPa, 1512 MPa, and 3.8%, respectively. The 20% deformation resulted in a balanced UTS (1348 MPa), YS (1202 MPa), and elongation (11%). The contribution of precipitation hardening to the strength decreased, while the contribution of CR increased with an increasing percentage of deformation. As the level of deformation increased, the size and quantity of γ″ decreased proportionally. The CR specimens produced a high density of nano/micro twins with twin planes oriented perpendicular to the RD-ND surface. The gradient of crystal orientation and internal features of large austenitic grains were generated by their preferred rotation. The cross-slip of screw dislocations induced a complete β-fiber consisting of {110}&lt;112&gt;, {112}&lt;111&gt;, and {123}&lt;634&gt;, with doubled intensity at the higher deformation. Additionally, the specimens were highly susceptible to the twinning-induced orientation emerged by a predominant &lt;110&gt;//ND. In the as-deformed condition, an incomplete but intense α-fiber, clustered between {110}&lt;001&gt; and {110}&lt;112&gt;, was characterized. Apart from achieving the highest strength, the current work demonstrates the effects of CR on the material strength without the complex influences of δ precipitates.

Effect of Al-Sr-Y intermediate alloy on microstructure and mechanical properties of A356 alloy

In this experiment, an intermediate alloy (Al-3Sr-8Y) that can refine and modify A356 alloy simultaneously was developed, and the synergistic effect of Sr and Y on the microstructure and mechanical properties of A356 alloy was investigated. The results show that when the content of Al-3Sr-8Y intermediate alloy reaches 0.3 wt%, the morphology of α-Al dendrites is significantly refined and the secondary dendrite arm spacing (SDAS) was significantly reduced. Moreover, the morphology of eutectic Si transforms from acicular to small fibrous, and the average area and aspect ratio of eutectic Si decrease to 0.81 μm2, and 2.01. This change is caused by the twin plane re-entrant edge (TPRE) poisoning mechanism, where the addition of Al-3Sr-8Y can poison the intrinsic growth position of the Si phase, reduce the growth rate of the Si phase, promote isotropic growth, and achieve a highly branched morphology of the Si phase to form a fibrous structure. Due to the excellent synergistic effect of Sr and Y, the tensile strength and elongation of the alloy reached the maximum values of 303.5 MPa and 9.5% when 0.3 wt% Al-3Sr-8Y was added after heat treatment, which is an increase of 18.2% and 86.3% compared with the untreated A356 alloy, respectively.

Disequilibrium reaction pathways and the twin-mediated growth of tabular forsterite during contact metamorphism of quartz-bearing dolomite

The forsterite zone of the Ubehebe Peak contact aureole, Death Valley, USA consists of an outer zone of tabular/jack-straw olivine and an inner zone of subequant polyhedral olivine. Subequant polyhedral forsterite crystals close to the intrusion are small and tabular forsterite crystals farther away are larger. To investigate the formation of the two morphologies, forsterite growth experiments were conducted in cold seal pressure vessels in the CaO-MgO-SiO2-CO2-H2O system. Forsterite precipitation follows a disequilibrium reaction pathway made of three reactions: [1] tabular forsterite growth from quartz and dolomite, [2] forsterite growth from tremolite dissolution, and [3] subequant polyhedral forsterite growth from tabular forsterite dissolution. Initially, quartz reacts with dolomite to simultaneously form twinned tabular forsterite and tremolite. As quartz reacts away, forsterite precipitation continues at a slower rate through tremolite dissolution. A second generation of forsterite then precipitates on top of some tabular forsterite but has different habit and tracht. Once all the tremolite reacts away, subequant polyhedral forsterite precipitation continues at an even slower rate through dissolution of tabular forsterite. The tabular morphology of jack-straw olivine is a consequence of twin-mediated unidirectional growth; the abundance of twins being due to rapid nucleation and growth at initially high reaction affinities. Twin junctions are preferential nucleation centers for steps, so faceted growth is enhanced on {100}. This phenomenon is the twin plane re-entrant effect. Subequant polyhedral forsterite in the Ubehebe Peak inner contact aureole recrystallized and ripened from tabular forsterite. In the outer contact aureole, conditions were not conducive to recrystallization and ripening so well-developed tabular forsterite persists.

Role of induced elastic deformations at the Mg/MgH2 transformation

The kinetics of magnesium hydride formation depends on many factors such as thermodynamic conditions, the presence/absence of additives and the mechanical treatments applied to the starting material. As a result, the impact on the reaction of the material is complex. Additionally, during repeated hydrogenation/dehydrogenation cycles, the microstructure of the material becomes significantly different from the original one. In such case, it is not so simple to order the relative importance of the different experimental contributions. In fact limited research efforts have been devoted to the impact of elastic deformation and its direct consequences on the hydride nucleation process. This motivates the present theoretical analysis of the mechanical energy balance between the Mg and MgH2 entities. Here the formation and interaction of a hydride nucleus with structural defects as a pore or a free surface is carried out in the Mg-MgH2 system.The present work evidences that calculation makes it possible to underline and unequivocally analyze the importance of a single parameter among a set of multiple factors that potentially have an impact on the kinetic processes. It is demonstrated here that due solely to the change in volume during hydride nucleation, conditions develops in the magnesium matrix for predominant release of hydride onto free surfaces, including pores.The calculation of the energy balance made it possible to classify the local impact on the nuclei for different typical configurations such as the vicinity of a free surface, a boundary and the vicinity of low rigidity defects (e.g. pores, vacancies, etc.). Hydride formation near a surface (external or internal, a twin plane or a grain boundary, etc.) results in a reduction in mechanical energy terms, which favors the easy nucleation step in magnesium. Nucleation occurs primarily on free surfaces rather than in bulk.

Uneven Strain Distribution Induces Consecutive Dislocation Slipping, Plane Gliding, and Subsequent Detwinning of Penta-Twinned Nanoparticles.

Twin structures possess distinct physical and chemical properties by virtue of their specific twin configuration. However, twinning and detwinning processes are not fully understood on the atomic scale. Integrating in situ high resolution transmission electron microscopy and molecular dynamic simulations, we find tensile strain in the asymmetrical 5-fold twins of Au nanoparticles leads to twin boundary migration through dislocation sliding (slipping of an atomic layer) along twin boundaries and dislocation reactions at the 5-fold axis under an electron beam. Migration of one or two layers of twin planes is governed by energy barriers, but overall, the total energy, including surface, lattice strain, and twin boundary energy, is relaxed after consecutive twin boundary migration, leading to a detwinning process. In addition, surface rearrangement of 5-fold twinned nanoparticles can aid in the detwinning process.

(Invited) Nanowires from Dilute Nitride and Dilute Bismide Alloys for Nanophotonics

GaAs-based nanowires (NWs) have a great potential for applications in photonics and optoelectronics, partly owing to flexibility in band structure engineering via alloying, strain and structural polymorphism. Among promising GaAs-based materials for device applications are highly mismatched GaNAs and GaAsBi alloys. The giant bowing in the bandgap energy characteristic for these materials allows easy tuning of emission wavelengths, while other modifications of their electronic structure promise to improve performance of infrared lasers and solar cells. In this talk we will review our recent results related to the electronic properties of novel GaNAs and GaAsBi NWs and provide several examples highlighting their advantages.First of all, we will show that a GaNAs-based mutishell NW can be used as an efficient coherent photon source, where the NW acts both as a miniaturized optical resonator and as a photonic gain medium. Its wavelength can be efficiently tuned by minor changes in N composition, e.g. down to 1 µm with only 2.5 % of nitrogen [1]. The lasing mode is found to arise from the fundamental HE11a mode of the Fabry-Perot cavity from a single NW, exhibiting optical polarization along the NW axis. Moreover, due to superior nonlinear properties of GaNAs, self-frequency conversion of the stimulated emission through second harmonic generation and sum-frequency generation is observed, providing coherent light emission in the cyan-green range.Secondly, we will demonstrate that alloying of GaAs with N and/or Bi leads to the formation of self-assembled quantum dots (QDs) embedded in NWs and, therefore, is an efficient way to fabricate hybrid QD-NW structures with high optical quality. In the case of dilute nitrides, optically active and highly localized QD states are formed due to local fluctuations in N composition [2], whereas in dilute bismides they are caused by Bi segregation at twin planes [3]. Such QDs act as single photon emitters, promising for application in quantum communication technologies.And finally, we will show that favorable band alignment at the interface between a dilute nitride alloy and parental N-free material leads to very efficient energy upconversion due to two-step two-photon absorption in the related NW heterostructures, with the efficiency of this process being among the highest reported in semiconductor nanostructures. Specifically, we show [4] that, in radial GaAs(P)/GaNAs(P) core/shell NW heterostructures, the upconversion efficiency increases by 500 times as compared with that of the constituent materials, even under an excitation power as low as 100 mW/cm2 that is comparable to the one-sun illumination. The upconversion efficiency can be further improved by 8 times through engineering the electric-field distribution of the excitation light inside the NWs so that light absorption is maximized within the desired region of the heterostructure.

Multiple morphologies and twin structure generated by a definite growth behavior of grain boundary M23C6 in tempered Fe–15Mn–3Al-0.7C steel

M23C6 carbide can precipitate simultaneously on both sides of austenite grain boundary in Fe–15Mn–3Al-0.7C steel tempered at 600 °C. In this paper, the morphology and growth behavior of M23C6 are studied by means of OM, SEM and TEM. The results show that the morphology of M23C6 located on both sides of grain boundary is different. On one side of the grain boundary, the carbides appear to be dispersed and disconnected, each with an irregular particle-like morphology, while on the opposite side, they are dispersed along the grain boundary in a short-rod morphology, and the adjacent ones tend to join together to form a network structure. In fact, regardless of which side of the grain boundary the carbides are on, they exhibit a definite growth behavior, growing along the close-packed plane and grain boundary. Also, the morphology of M23C6 depends on the angle between the close-packed plane and the grain boundary. In addition, twin structures with the same orientation can be found in both austenite and M23C6. A notable distinction between them is that there is a flat twin plane in the austenite twin structure but not in the carbide twin structure. Therefore, this paper highlights that the formation of these two twin structures is different. The austenite twin structure is caused by quenching treatment, while the carbide twin structure is formed by a new method called “growing-contacting”.

Microscopic Visualization of Crystal Twinning in Accessory Zircon

Abstract In general, crystal twinning in accessory zircon represents a rare, but in almost all granites and gneisses, encounterable phenomenon. Since only the crystal face {101} occurs as a twin plane, formation of crystal twins turns out to be relatively simple. Besides geniculate twins, which develop by rotation of the crystal structure about the line [101], parallel twins that are arranged parallel to the crystallographic main axis of zircon can also be observed by light and electron microscopy. While geniculate twins usually show a frequency within the region of a permille, the number of parallel twins can rise to several percent depending on the investigated rock. In the present contribution, a brief insight into the microscopic world of zircon twinning is provided.

Petrogenesis and possible fingerprints of the Najd shear system on the evolution of deformed granitic rocks in the west Wadi Nugrus area, Egypt

In this paper, we present petrological constraints on the geodynamic evolution of deformed granitic rocks in the west Wadi Nugrus area, south Eastern Desert of Egypt. The deformation belt trends NW-SE forming the so-called “Nugrus Shear Zone (NSZ)” a major tectonic feature in the Arabian-Nubian Shield (ANS) at the junction between Wadi El Gemal and Wadi Hafafit areas, namely “Site I″ and “Site II”. The deformed granitic rocks comprise highly sheared arc granitoids and post-collisional to within-plate younger granites. Although the investigated granitic rocks are highly deformed, they show no signs of metamorphism and retain their primary igneous minerals. The melts from which the investigated granitic types originated experienced considerable degrees of partial melting in the transitional lower crust/upper lithospheric mantle as indicated by the contents of incompatible and rare-earth elements (REEs). Typical A-type granites developed before deformation and during emplacement in a continental environment. Zircon has ThO2/UO2 > 0.1, which is typical of igneous zircon, which sometimes experiences little recrystallization during the late magmatic deuteric stage, either pre-or syn-deformational due to circulation of fluids along the shear zone. It is believed that the deformation of all granitic rocks in the west Wadi Nugrus area is possibly synchronous and related to the Najd Fault System (NFS). The (NFS) extends from the western Arabian Peninsula to develop the NW-trending Nugrus Shear Zone (NSZ) in the south Eastern Desert. Sub-magmatic, solid-state, and sub-solidus deformational microstructures are evidence for the deformation related to the (NFS). Chessboard patterns in quartz and sub-magmatic fractures in plagioclase indicate deformation at high temperatures (T > 650 °C) in the presence of melt, which demonstrate the shearing during granitic cooling. On the other hand, quartz grain boundary migration and sub-grain recrystallization are two examples of solid state-related high-temperature deformation-related microstructures (T > 450 °C). Mica kinks along feldspar twinning and bending planes refer to low-temperature sub-solidus microstructures (T < 450 °C).

Evidence of twin mediated growth in the CVD of polycrystalline silicon carbide

The origin of the 〈110〉 texture in polycrystalline 3C-SiC grown by chemical vapor deposition at different temperatures is investigated by thorough EBSD, SEM and XRD characterizations. Regarding the 〈110〉 orientation, although XRD suggests a fiber texture, closer inspection of isolated clusters of grains by EBSD reveals a high symmetrical order in-plane. The pole figures associated with these clusters are in good agreement with cross-twinned pentagonal structures in which each grain shares a common 〈110〉 axis. These structures form via twin-mediated growth involving the formation of twin plane reentrant edges that act as a preferential site for the incorporation of adatoms. A deviation from a perfect cross-twinned structure is observed and is attributed to the presence of additional parallel twins. In this study, the transition to the 〈110〉 orientation occurs when the temperature increases and the partial pressure of reactive species decreases. This can be explained by the enhancement of surface mobility that promotes surface diffusion to the twin plane reentrant edges, thus activating the twin mediated growth mechanism.

Direct observation of the ledge-typed twin boundary in a martensitic steel

The structure at the {112} twin boundaries in martensitic steels has been extensively debated recently. On the one hand, it was claimed to be the omega phase, similar to Ti alloys. On the other hand, it was considered artifacts induced by the double diffraction effect from the overlaps of twin-matrix crystals at their boundaries. In this study, we clarify the twin structure in a martensitic steel providing a detailed investigation of the α′ martensite twin boundaries using experiment and simulation. The fundamental for rejecting the existence of the omega phase in steels is based on the atomic resolution imaging from the <111> direction of twin structure, which was so far challenged to be acquired for such nano-sized twinned systems. The obtained result confirms the ledge-typed twin boundary that causes the visually omega-like forms when observed from other zone axes. The ledges occurred on {110} planes, making them secondary twin planes with coherent characteristics. Furthermore, these ledges were found to transit in a gradual manner from the primary (112) twin plane to (011) and then to (1¯10) secondary twin planes; a perpendicular transition from (112) to (1¯10) was not observed. Such a transitional behavior was explained in view of lowering total twin boundary energy, evaluated by the theoretical prediction of system energy and the geometric phase analysis of the twin planes. Double diffraction and rotational Moiré effects clarified the extra spots in the diffraction pattern obtained at the twin-matrix overlapped regions. This study gives light of understanding twinning in the bcc crystals.

Formation of columnar/tabular and vermicular kaolinite in the crystal tonsteins within the Late Carboniferous Coal in the Datong Coalfield, Shanxi Province, North China

The Datong Coalfield, Shanxi Province, North China, is rich in high-grade coal-measure kaolin, primarily occurring as partings within the Late Carboniferous–Early Permian coal seams. There is a type of kaolinite-dominant coarse-grained parting similar to sandstone in appearance, whereas the phanerocrystalline minerals are columnar/tabular and vermicular kaolinite rather than terrigenous detrital minerals. The origins of the coarse-grained partings and the formation mechanism of the columnar/tabular and vermicular kaolinite remain ambiguous. Herein, petrographical, mineralogical, whole-rock geochemical, and zircon U–Pb–Hf isotopic studies of the coarse-grained partings were conducted to explore these issues. The zircons in the coarse-grained partings show typical features of magmatic origin and yield a single age group of very latest Carboniferous at ca. 300 Ma, indicating they were transformed from air-fall volcanic ash. Therefore, the coarse-grained partings are essentially crystal tonsteins. The wide range of negative εHf(t) values and arc-related tectonic affinity of the zircons genetically link the primary volcanic ash to the Late Paleozoic continental arc volcanoes that existed in the Inner Mongolia Paleo-Uplift, related to the southward subduction of the Paleo-Asian oceanic plate beneath the North China Block. Two orthogonal cleavages are generally observed on the columnar/tabular and vermicular kaolinite in the crystal tonsteins, contradicting the crystalline structure of kaolinite that belongs to a 1:1 type phyllosilicate with only one cleavage along its (001) lattice plane. These columnar/tabular and vermicular kaolinite are considered to be pseudomorphic kaolinite after plagioclase in the crystal-rich volcanic ash. When the crystal-rich volcanic ash fell into open and acidic peat-forming environments (mires), the low-pH water containing abundant organic acids and good water circulation conditions facilitate dissolution of plagioclase along the chemically unstable (001) lattice planes and leaching out of Na+, Ca2+, and much of silica. Simultaneously, the Al3+ or hydroxides of aluminum and silica recrystallized along the (001) lattice planes of plagioclase as the substrate, constraining layer-by-layer growth of large kaolinite platelets to eventually form pseudomorphic kaolinite. The cleavages perpendicular to the exclusive (001) cleavage of pseudomorphic kaolinite should be inherited from the twin planes and (010) cleavage cracks (if present) of plagioclase.

Plastic deformation of B19’ monoclinic martensite in NiTi shape memory alloys: HRTEM analysis of interfaces in martensite variant microstructures

Martensite variant microstructures in grains of nanocrystalline NiTi shape memory wire deformed in tensile test at room temperature up to 15% strain were analyzed in transmission electron microscope (TEM) with the aim to reveal the mechanism of plastic deformation of B19’ monoclinic martensite which created these microstructures. The analysed microstructures consisted of parallel and/or wedge arranged deformation bands within the martensite matrix oriented in a common <010> low index zone. High resolution TEM (HRTEM) analysis of 35 interfaces within a single grain was performed. The observed interfaces were reasonably planar on mesoscale of individual grains but highly irregular on atomic scale. Majority of analysed interfaces displayed interface planes and lattice misorientations similar to (100), (20-1) and (10-1) martensite twins, but not exactly, many interfaces deviated from the exact twin planes and misorientations. Some interfaces displayed crystal lattices only rotated within the <010> low index zone, i.e. lacking the mirror symmetry of adjoining lattices characteristic for twins. On the other hand, traces of (001) crystal planes were mirror symmetric across all observed interfaces. These experimental observations were rationalized by assuming that the analysed martensite variant microstructures were created by the plastic deformation of B19’ martensite by kwinking deformation proceeding via coordinated [100](001) dislocation slip based kinking combined with (100) twinning.