Polysynthetic Twinning Research Articles

Deformation behavior of PST-TiAl bicrystals at 800 °C

PST (polysynthetic twinned) TiAl crystal exhibits excellent mechanical properties, and thus, it has been regarded as a critical candidate for turbine blades in the aero-engine industry. For the first time, the deformation behavior of PST-TiAl bicrystals with two different α2/γ lamellar orientations was studied in this work. Ti-46Al-6Nb bicrystals were cut from directionally solidified ingots, and three bicrystals of B1 (10°–14°), B2 (10°–19°), B3 (6°–75°) together with a single crystal of S1 (3°) were stretched at 800 °C. Then, the deformation behaviors, including fracture morphology, crack propagation, and deformation mechanism, were analyzed. Interestingly, these bicrystals show good high-temperature tensile performance, and their deformation behaviors seem to be very anisotropic. The findings indicate that the misorientation disparity in bicrystals primarily determines their deformation coordination capacity and corresponding mechanical performance. Subsequently, the difference in deformation orientation between the two lamellae significantly controls fracture behavior and deformation compatibility. High slip potential and abundant variant choices in γ phases are supposed to be beneficial for deformation capacity improvement. Meanwhile, the deformation potential of ordinary dislocation and superlattice dislocation slip systems in the α2 phase was also examined. Results show that the disparity in dislocation motion ability between γ variants gradually amplifies the advantages of the easy-mode phase in deformation through strain coordination, while mitigating the disadvantages associated with the difficult-mode phase. During further deformation, the relatively low bicrystal misorientation and small deformation capacity differences between the two lamellae determine high deformation coordination and improved performance. Overall, our work reveals that bicrystal deformation initiates at the relatively softer lamellae and proceeds to the harder lamellae, and its final comprehensive performance is determined by the deformation coordination capacity between the two lamellae.

Advanced TiAl Based Alloys: From Polycrystals to Polysynthetic Twinned Single Crystals

AbstractTiAl alloys stand out for low density, high specific strength, and excellent creep resistance, making them promising for high‐temperature aerospace applications. However, traditional TiAl alloys suffer from poor room‐temperature ductility and low service temperature that limit their critical applications in aerospace structures. To address these issues, research has focused on improving the mechanical properties of TiAl alloys through alloying and microstructural design. After decades of effort, the evolution of TiAl alloys has progressed from polycrystalline TiAl to high‐performance polysynthetic twinned (PST) TiAl single crystals. The well‐aligned PST TiAl single crystals enriched with Nb enable an excellent combination of strength and ductility, significantly outperforming polycrystalline TiAl alloys. This review summarizes recent progress on TiAl alloys, particularly focusing on newly developed PST single crystals. First, the development history of TiAl alloys is overviewed; then their crystal structures, phase diagrams, and typical microstructures are systematically discussed, along with the design strategies based on alloying elements. Additionally, recent advances in TiAl columnar crystals, which are between polycrystals and single crystals, are reviewed. Subsequently, the mechanical anisotropy, preparation methods, and superior mechanical properties of the PST single crystals are analyzed in detail. The final remark highlights the future development and application prospects of TiAl alloys.

Size-dependent deformation mechanisms in two-phase γ-TiAl/α2-Ti3Al alloys

Polysynthetic twinned (PST) TiAl single crystals, consisting of alternating layers of γ-TiAl and α2-Ti3Al, exhibit excellent mechanical properties. However, size-dependent deformation mechanisms have not yet been fully elucidated. In this work, we investigated the deformation process of two-phase γ-TiAl/α2-Ti3Al alloys with different layered sizes via uniaxial tensile loading using molecular dynamics simulations. The results show that with the decreasing phase boundary (PB) spacing λ or domain boundary (DB) spacing d, the excess free volume at the intersection of DB and PB decreases, leading to increased yield stress. We found that a higher interface density (smaller λ or d) can accommodate more dislocations, and the size-dependent ultimate strength follows an approximate Hall-Petch relation. These findings offer an essential understanding for the size-dependent PST TiAl single crystals with high strength.

Regulating phase ratios and mechanical properties of polysynthetic twinned TiAl single crystals via annealing

Polysynthetic twinned (PST) TiAl single crystal specifically refers to a fully lamellar TiAl single crystal with parallel phase interfaces and twin interfaces grown by directional solidification. In this paper, PST single crystals with different phase ratios are obtained by annealing at specific temperatures and holding times. The results show that the diffusion rates of Ti and Al elements at various temperatures directly trigger and propel the surface recrystallization and variation in the internal phase ratio. When the temperature is lower than 1448 K, the diffusion rate of Ti is obviously higher than that of Al, which causes one dense α2 recrystallized layer to form on the surface of TiAl single crystals. Meanwhile, as more Ti elements migrate to the surface, the α2 phase ratio inside the TiAl single crystal thereby decreases. When the temperature exceeds 1448 K, the diffusion rate of Al gradually reverses to exceed that of Ti, which forms the surface sandwiched recrystallization dominated by γ phase and simultaneously increases α2 phase ratio inside the TiAl single crystal. The variation in the two-phase ratio directly induces a significant change in the lamellae thickness, which exhibits different tensile behaviors of PST-TiAl single crystal. When the α2 phase content is less than 20 %, wider γ lamellae make it easier for dislocations to be activated within its lamellae and continuously move across the γ/α2 interfaces, thereby obtaining better tensile plasticity. As the α2 phase content exceeds 30 %, finer γ lamellae inhibit the dislocation initiation, resulting in the fracture occurrence of TiAl single crystal before yielding. No matter how the phase ratio changes, the crack preferentially initiates within α2 lamellae. However, the crack propagation follows different paths based on various γ lamella thicknesses. The fracture mode of PST-TiAl single crystal also changes from shear fracture along slip bands within the γ lamella to brittle fracture along the {11¯00} planes within α2 lamella.

Crystal organisation and material properties of Chama and Glycymeris myostraca and shells

Movement of bivalve hard and soft tissue requires muscular action. Despite diverse bivalve lifestyles and living environments, the myostracum, a specific hard tissue formed where muscles attach to the shell, appears similar in structure for species of many bivalve orders.We investigated myostracal and non-myostracal, valve, microstructure, texture and material properties of Chamidae and Glycymerididae species with electron-backscatter-diffraction, laser-confocal and backscatter electron imaging and nanoindentation testing. Chamidae and Glycymerididae follow different lifestyles and live in distinct environments. Chamidae are cemented to substrate and live in wave-swept, shallow, waters. Glycymerididae dwell in calm water and burrow into sandy/muddy sediment.We found that myostracal aragonite of all investigated species has a crystal assembly pattern that reflects crystal growth through growth competition. Aragonite is extensively twinned in the myostracum and non-myostracal, valve, layers, not in the calcitic ornamentation. For myostracal aragonite, we found cyclic twinning, for non-myostracal aragonite the twinning was polysynthetic or polycyclic. We show how twinning and crystallographic texture are transmitted between myostracal and non-myostracal, valve, layers. Relative to non-biological aragonite, myostracal and non-myostracal, valve, indentation elastic modulus is reduced by 10–15 % and 15–20 %, respectively; myostracal and valve hardness is increased by 15–20 % and 5–10 %, respectively. Comparing modulus and hardness between aragonitic microstructures, we found that, relative to other microstructures, myostracal modulus is increased by 5 % and myostracal hardness by 15 %. Hence, the myostracal material shows a unique and specific microstructure, texture, modulus, and hardness that might be necessary for muscle attachment to enable the lifestyle-controlled requirements posed onto the organism.

Pseudo-twin boundary improves flow stress and cyclic stability of TiAl single crystal

Polysynthetically twinned (PST) TiAl single crystal with lamellar structures exhibits great mechanical properties at room temperature. Therein twin boundaries (TBs) are important for achieving optimized ductile and fatigue performance of PST TiAl, but their role and the associated mechanism are elusive. Herein, we decipher the role of true TB (TTB) and pseudo TB (PTB) by a combined atomistic simulation and mesoscopic modeling, and find that PTB could remarkably improve room-temperature flow stress and cyclic stability of TiAl single crystal. It is revealed that dislocations pile up at PTB while unobstructedly traverse TTB. The emergency of back stress and the movement of dislocations along PTB contribute to the strengthening mechanism. The flow stress of TiAl single crystal with PTB is 34% higher than that with TTB. It is further found that as the twin thickness decreases, the flow stress of TiAl single crystal with TTB initially increases and then decreases (i.e., inverse Hall–Petch like behavior), whereas that with PTB always increases owing to the extra back stress and interfacial stress (i.e., Hall–Petch like behavior). Atomistic-informed mesoscopic theoretical models are then proposed to describe the flow stress as a function of twin thickness. Under cyclic loading, PTB is found to facilitate strain delocalization of TiAl single crystal during plastic deformation and thus noticeably improve the cyclic stability. These findings should shed light on achieving strong TiAl alloys with enhanced fatigue performance by the introduction and design of PTB.

Mostly solidified hardground at the top of the crystal pile in the Bushveld magma chamber

Mafic layered intrusions show abundant evidence for the formation through the progressive inwards growth of a mushy solidification layer, with a central body of nearly crystal-free resident melt. The thickness and permeability of the mushy layer play a vital role in operation of several petrogenetic processes but remain poorly constrained in layered intrusions. To address this issue, we have re-examined the cumulate sequence in the Upper and Critical Zones of the Bushveld Complex. Although some observations (e.g., soft-sediment deformation) are indicative of cumulate rocks being mushy and having experienced interstitial melt percolation (e.g., metasomatic anorthosites), other field and textural evidence suggest the contrary. Some of the clearest evidence comes from sections through chromitite and magnetitite layers as well as through the Merensky Reef, layers that were deposited on the eroded rocks of the temporary chamber floor. Below the erosional surface, the layered rocks (1) were tilted and truncated without losing their internal coherence, (2) have knife-sharp contacts with the overlying rocks, (3) show minimal evidence of penetration by downward-percolating melt, (4) reveal sharp truncation of individual crystals, particularly beheading of pyroxene oikocrysts and intersecting of polysynthetic twinning in plagioclase, and (5) show resumed growth on the truncated crystals into the overlying rocks. These observations indicate that the cumulates of the inward-growing chamber floor were almost entirely solid at the time of the erosion and deposition of these layers. To reconcile these two sets of contrasting observations, we propose that only a few metres of the uppermost portion of the cumulate pile of the Upper and Critical Zones of Bushveld magma chamber were mushy, with other underlying rocks being almost totally solidified. The mushy layer tends to be thin and locally even non-existent (e.g., in magnetitite layers) due to high efficiency of primary adcumulus growth at the crystal-liquid interface of the temporary chamber floor. The solid chamber floor results from thermochemical erosion of the entire mushy layer by magmas replenishing the evolving chamber. The study implies that such petrogenetic processes as the hydrodynamic sorting of crystals and reactive porous flow may only operate within this very thin mushy portion of the cumulate pile and cannot therefore be responsible for any large-scale petrological features (e.g., platinum reefs, massive chromitites, stratiform anorthosite layers) of the Bushveld Complex.

Micromechanical analyses on bending of polysynthetically twinned single crystal of titanium aluminide

Micromechanical bending analyses via CPFEM of polysynthetically twinned (PST) single crystal of TiAl composing of one α2-phase lamella and six γ–phase lamellae have been performed. The results have demonstrated that a decrease in the domain size aspect ratio λD/λL or an increase in the volume fraction of α2 phase fα2 have an effect of increasing the induced bending moment and consequently bending resistance under both bending modes of hogging and sagging. The similar variations in λD/λL and fα2 results in a decreasing trends of maximum tensile and compressive initial yield stresses (normal bending stresses), obtained through small strain beam theory. Results have also shown the effects of imposed constraints on the bending behavior; traction-free boundary conditions result in activation of prismatic slips instead of pyramidal slips in α2-phase, leading to a lower observed bending moment for the variables λD/λL and fα2 considered. A scaling law is proposed that relates bending moment to domain size aspect ratio and bending angle.

Effect of CoSn3 nanocrystals on Sn3Ag plating for electronic packaging

Plating Sn3Ag on copper substrates represents a crucial electronic packaging technique. In this study, we propose a novel composite plating approach, wherein CoSn3 nanocrystals are deposited within the Sn3Ag coating. The resulting reflowed Sn3Ag joints exhibit a range of distinctive properties. Notably, CoSn3 nanocrystals dissolve in Sn during the reflow process, thereby lowering the supercooling required for Sn nucleation. Consequently, Sn crystals grow in six-fold cyclic twins. Additionally, the dissolution of Co atoms in Sn leads to a reduced solubility of Cu atoms in Sn, consequently lowering the supercooling required for the nucleation of Cu6Sn5. Simultaneously, this phenomenon promotes the nucleation of Cu6Sn5, resulting in a considerable precipitation of Cu6Sn5 nanoparticles within the joints. Therefore, the mechanical properties of the joints are significantly enhanced, leading to a notable 20% increase in shear strength. Furthermore, the presence and distribution of Co elements within Sn induce changes in the growth pattern of interfacial Cu6Sn5. The growth process of Cu6Sn5 is dominated by the interfacial reaction, leading to its growth in a faceted shape. During the aging process, the dissolution of Co elements in Sn impedes the continuous growth of Cu6Sn5 at the interface, causing Cu6Sn5 to be distributed in the form of islands inside the joint. Remarkably, elemental Co acts as an inhibitor for the development of Cu3Sn and reduces the occurrence of Kirkendall voids.

Chiral spiral cyclic twins. II. A two-parameter family of cyclic twins composed of discrete circle involute spirals.

A mathematical toy model of chiral spiral cyclic twins is presented, describing a family of deterministically generated aperiodic point sets. Its individual members depend solely on a chosen pair of integer parameters, a modulus m and a multiplier μ. By means of their specific parameterization they comprise local features of both periodic and aperiodic crystals. In particular, chiral spiral cyclic twins are composed of discrete variants of continuous curves known as circle involutes, each discrete spiral being generated from an integer inclination sequence. The geometry of circle involutes does not only provide for a constant orthogonal separation distance between adjacent spiral branches but also yields an approximate delineation of the intrinsically periodic twin domains as well as a single aperiodic core domain interconnecting them. Apart from its mathematical description and analysis, e.g. concerning its circle packing densities, the toy model is studied in association with the crystallography and crystal chemistry of α-uranium and CrB-type crystal structures.

A re-evaluation of stannopalladinite using modern analytical techniques

AbstractAn investigation of sample 41647 from the Platinum Placer of Ugol'nyi Ruchei, Norilsk Cu–Ni deposit, Russia, stored in the systematic collection of the Fersman Mineralogical Museum, revealed that it contained a mineral that can be identified as the original stannopalladinite described in 1947. As the literature information on the latter is controversial, the mineral was re-investigated using modern analytical techniques. Stannopalladinite occurs as spherical and ovoid-shaped grains up to 0.08 mm, closely intergrown with tetraferroplatinum as part of flattened, prismatic crystals up to 6 mm long. The micro-indentation hardness of stannopalladinite (VHN, 30 g load) is 244 kg/mm2 (range 233–266, n = 4), corresponding to a Mohs hardness of 4. The calculated density is 9.781 g/cm3. In reflected light, stannopalladinite is pale pink. The bireflectance is noticeable only in oil immersion. In crossed nicols the mineral exhibits strong anisotropy with colour effects changing from reddish purple to greyish blue and polysynthetic twinning of grains. The reflectance curves show distinct anomalous dispersion. The chemical composition (wt.%, electron microprobe data, mean of 11 analyses) is: Cu 8.48, Pd 61.21, Pt 0.89, Sn 25.87, Pb 3.70, total 100.15. The empirical formula calculated on the basis of 4 atoms per formula unit is (Pd2.42Cu0.56Pt0.02)Σ3.00(Sn0.92Pb0.08)Σ1.00. The ideal chemical formula of stannopalladinite is therefore proposed as (Pd,Cu)3Sn instead of Pd3Sn2 used in the official IMA List of Minerals. The strongest powder X-ray diffraction lines are [dobs, Å (I, %) (hkl) ]: 2.292 (42) (231), 2.166 (100) (331), 2.034 (10) (710), 1.916 (15) (141) and 1.851 (15) (630). The powder X-ray data are indexed in the orthorhombic unit cell with a = 14.634(2), b = 8.5253(6), c = 4.5946(3) Å and V = 573.24(7) Å3 (Z = 8). Stannopalladinite can be reliably identified among all other minerals belonging to the binary Pd–Sn and ternary Pd–Cu–Sn systems by a combination of chemical, optical and X-ray data.

Effects of fluorine on dynamic reaction interfaces in hydrothermal feldspar alteration

Fluid-induced mineral reactions exert an important control on the physico-chemical properties of the crust and hydrothermal plumbing systems. Feldspars are the most abundant minerals in the Earth's crust and a major host of the main cations found in geofluids (Na, K, Ca). In previous work, we discovered that K-feldspar-albite-sanidine zonation textures developed dynamically when sanidine ((K,Na)AlSi3O8) was reacted with NaCl or NaF in H216O- and H218O-enriched solutions at 600 °C and 2 kbar in a closed system. Based on a detailed SEM and TEM characterisation, the present study aims to constrain the reaction mechanism and the effect of fluorine on reaction kinetics and overall reaction textures in alkali feldspar replacement reaction interfaces. Replacement of high-sanidine (Sa) by albite (Ab) and/or K-feldspar (Kfs) proceeded via an interface-coupled dissolution-reprecipitation reaction, with the products preserving the crystallographic orientation of the parent Sa in both NaCl- and NaF-bearing solutions. Mineral interfaces with different micro-textures were observed depending on the nature of the interface (Ab-Sa, Kfs-Ab, Kfs-Sa) and whether the reaction occurred in NaCl-only or NaF-bearing solutions. Abundant amorphous domains (25–50 nm thick) occur both within Ab and at Ab-Sa and Kfs-Ab interfaces formed in NaF-bearing solutions. At the Kfs-Sa interface formed in NaCl-only solutions, a complex interfacial zone characterised by a high density of defects may represent sealed fluid pathways. These results reveal that the nature of the reaction interface depends on both mineral properties, in particular microtextures (i.e. polysynthetic twinning in Ab); and solution chemistry; this directly affects porosity development, the chemical environment at the interface, and chemical exchanges between the reaction front and the bulk solution. Fluorine aids the formation of amorphous layers that modify nucleation mechanisms; and facilitates element transport by forming Na-Si-Al-fluoride complexes, resulting in accelerated reaction rates relative to chloride-solutions. These nm- to μm-level processes contribute to efficient coupling between fluid-flow and mass transfer, explaining why sodic and potassic alteration associated with metal and fluid transport in some of the world's largest ore deposits can develop extensively, and subsequently access metals locked within silicate mineral assemblages.

Petrographic features of Late Cretaceous biotite granites in the Kurtoğlu region (Elazığ/Turkey)

The study area is located in a local area around Kurtoğlu village of Elazığ province, located within the Southeast Anatolian Orogenic Belt. The studied biotite granites belong to the Elazig Magmatic Complex. The Elazığ Magmatic Complex is grouped into volcanic, sub-volcanic and plutonic rocks and mafic and felsic. Felsic rocks belonging to the Elazığ Magmatic Complex are represented by granite, granodiorite, tonalite, quartz monzonite, monzodiorite, and mafic rocks are represented by diorite, quartz diorite and gabbros. The biotite granites are porphyric biotite minerals with an average size of one cm. Petrographically, they are composed of K-feldspar, plagioclase, quartz, biotite and opaque minerals. K-feldspars have low birefringence colours, grey tones, and earthy colours in single nicol. Plagioclases; anhedral, euhedral and mostly subhedral crystals. They typically indicate albite, albite+karlsbad and polysynthetic twinning. In some samples, sericitization and carbonation are observed mostly in the middle parts of the plagioclase resulting from alteration. Biotites; It is generally in the form of subhedral platy-prismatic, rod-like crystals. Pleochroism is seen in brown tones in single nicol. It has high birefringence colours and indicates vivid interference colours in yellowish, blue and green, especially brown tones in double nicol. Porphyritic texture in which phenocryst and smaller crystals are generally observed in granites.

Polysynthetic Twinning of Diopsides in the Niewang and Tatliksu Nephrite Deposits, Xinjiang, China

Diopside, an important component of the crustal and upper mantle, plays an important role in the formation of nephrite. Polysynthetic twinning in natural diopside, especially from skarns, has rarely been systematically researched. Here, the polysynthetic twinning of natural diopside was investigated in two skarn-type nephrite deposits (Niewang and Tatliksu) in Qiemo, Xinjiang, China. Petrographic observations revealed periodic alternations of dark–light lamellae under cross-polarized light and parallel striations under plane-polarized light, whereas backscattered electron images indicated high homogeneity, which suggested a type of mechanical polysynthetic twinning. According to the optical indicatrix, twins were predominantly oriented as a (100)[100] system. Raman spectra and chemical data show that pyroxenes in nephrite are close to the Di end-member with 0.90–1.04 a.p.f.u. Mg and 0.98–1.05 a.p.f.u. Ca. Both diopside and tremolite from Tatliksu contain slightly higher Fe than those from Niewang. Given the seismic origin of mechanical clinopyroxenes twins, twins of studied diopsides are likely to be attributed to Paleozoic-Mesozoic paleoearthquakes in the Altyn Mountain. A shear stress ≥ 140 MPa and a differential stress greater than 280 MPa are suggested as conditions linked to generation of the twinning of diopsides in the nephrite deposits. The replacement of diopside by tremolite along its twin planes highlights the potential gemological implications, as such replacement by tremolite probably facilitated the formation of dense, fine fibre textures in the nephrites.

CoSn3 Intermetallic Nanoparticles for Electronic Packaging.

At present, composite solder pastes are getting a lot of attention, especially composite Sn based solders reinforced by nanoparticles. Indeed, CoSn3 is a strong nucleating agent of Sn crystal, which has potential application value in the field of electronic packaging. However, there is no reliable synthetic path for CoSn3 nanoparticles at present. In this article, a chemical synthesis method for CoSn3 nanoparticles is developed. Here, CoCl2 and SnCl2 are reduced by NaHB4 in triethylene glycol (TEG), dispersed by ultrasonics, and heated to 350 °C in a tube furnace for growth. The CoSn3 nanoparticles with a diameter of about 150 nm are obtained by heating at 350 °C for 10 min. The CoSn3 nanoparticles undergo a step reaction in the process of synthesis and go through different stages of merging and annexation during their growth. The crystal growth behavior and the process of orientation change during the nucleation and growth of CoSn3 nanoparticles are studied, especially the two growth mechanisms, namely OU (orientation unified) and OA (orientation attached). By mixing CoSn3 nanoparticles with SAC305, we obtain a kind of strengthened composite soldering paste. There are obvious six-fold cyclic twins in the joints made by this soldering paste.

A high-withdrawing-rate method to control the orientation of (γ+α2) lamellar structure in a β-solidifying γ-TiAl-based alloy

β-solidifying γ-TiAl-based alloys with well-controlled lamellar orientation possess excellent mechanical properties, and low withdrawing rates are usually used to control the lamellar orientation by directional solidification. Herein, a high-withdrawing-rate method to control the lamellar orientation in a β-solidifying γ-TiAl-based alloy, TNM alloy (Ti-43.5Al–4Nb–1Mo-0.1B at.%), is proposed by Bridgeman directional solidification. Using this method, polysynthetic twinned (PST) single crystal of TNM alloy with a good lamellar controlling result is obtained. The mechanism of lamellar orientation controlling lies in the process of solidification and β/α transformation, which are governed by thermal stabilization treatment and withdrawing rate. Thermal stabilization treatment for 60 min can make the sample maintain stable interfaces of liquid/solid and β/α phase with a high temperature gradient before the withdrawing process starts, and it also leads to the incline of β dendrites; a high withdrawing rate of 100 μm/s can accomplish the grain selection of α phase during β/α transformation and make complete peritectic reaction occur. Compared with as-cast one, PST single crystal specimen has its room-temperature tensile property enhanced greatly, and presents an ultimate tensile strength/strain of 476 MPa/1.75% with a trans-lamellar fracture morphology.

Structural features of heavily boron-doped graphite and diamond microcrystals synthesized at high pressures

Heavily boron-doped graphite was synthesized from a mixture of adamantane and boron using a high-pressure technique. Ab initio calculations of a carbon cluster of 64 atoms located in two layers of graphene with the inclusion of one boron atom were made. Substitution of boron for carbon in the graphene layer correlates with the experimental data if we consider the change in the cell parameters. The easy transformation of the boron-doped graphite lattice into a diamond one at pressure of about 7.5 GPa may be due to the presence of a small number of boron atoms between graphene layers. Heavily boron-doped diamond crystals in the shape of a five-pointed star were found. The mechanism of their growth includes cyclic twinning and is obviously associated with the presence of a fluid phase in the synthesis medium and facilitated twinning of a diamond lattice containing boron.

Mineralogical and Petrographic Characteristics of the Jurassic Kaban Dacite in the Eastern Sakarya Zone (Olur/Erzurum)

The volcanic rocks of the Kaban dacite, which is the subject of the study, are located in the Olur (Erzurum) region of the eastern Sakarya zone. This zone is divided into four tectonic slices as Hopa-Borçka zone, Artvin Yusufeli zone, Olur Tortum zone and Erzurum Kars Ophiolite zone from north to south, east of the Eastern pontides, and the investigated rocks are located within the Olur-Tortum zone. These rocks are represented by dacites and rhyolites in the study area. Petrographically, it is observed in fine grained grayish, greenish, earthy colors. The main mineral contents are plagioclase, quartz and K-feldspars. Plagioclase, quartz and K-feldspars occur as phenocrysts, but are also observed in microlithic pastes. Plagioclases are characteristic with albite and polysynthetic twinning. Quartz is in the form of anhedral crystals and is distinguished from K-feldspars by its transparent colors in a single nicol.

Formation of contact and multiple cyclic cassiterite twins in SnO2-based ceramics co-doped with cobalt and niobium oxides.

Contact and multiple cyclic twins of cassiterite commonly form in SnO2-based ceramics when SnO2 is sintered with small additions of cobalt and niobium oxides (dual doping). In this work, it is shown that the formation of twins is a two-stage process that starts with epitaxial growth of SnO2 on CoNb2O6 and Co4Nb2O9 seeds (twin nucleation stage) and continues with the fast growth of (101) twin contacts (twin growth stage). Both secondary phases form below the temperature of enhanced densification and SnO2 grain growth; CoNb2O6 forms at ∼700°C and Co4Nb2O9 at ∼900°C. They are structurally related to the rutile-type cassiterite and can thus trigger oriented (epitaxial) growth (local recrystallization) of SnO2 domains in different orientations on a single seed particle. While oriented growth of cassiterite on columbite-type CoNb2O6 grains can only result in the formation of contact twins, the Co4Nb2O9 grains with a structure comparable with that of corundum represent suitable sites for the nucleation of contact and multiple cyclic twins with coplanar or alternating morphology. The twin nucleation stage is followed by fast densification accompanied by significant SnO2 grain growth above 1300°C. The twin nuclei coarsen to large twinned grains as a result of the preferential and fast growth of the low-energy (101) twin contacts. The solid-state diffusion processes during densification and SnO2 grain growth are controlled by the formation of point defects and result in the dissolution of the twin nuclei and the incorporation of Nb5+ and Co2+ ions into the SnO2 matrix in the form of a solid solution. In this process, the twin nuclei are erased and their role in the formation of twins is shown only by irregular segregation of Co and Nb to the twin boundaries and inside the cassiterite grains, and Co,Nb-enrichment in the cyclic twin cores.

Chiral spiral cyclic twins.

A formula is presented for the generation of chiral m-fold multiply twinned two-dimensional point sets of even twin modulus m > 6 from an integer inclination sequence; in particular, it is discussed for the first three non-degenerate cases m = 8, 10, 12, which share a connection to the aperiodic crystallography of axial quasicrystals exhibiting octagonal, decagonal and dodecagonal long-range orientational order and symmetry.