Twin Planes Research Articles

Effect of grain size on amorphization mechanism and kinetics of bridgmanite in shocked meteorites

Bridgmanite formation and amorphization in shocked meteorites constrain the pressure and temperature conditions during planetary impact. However, the effect of the bridgmanite grain size on its amorphization kinetics is still unclear. Here, the amorphization mechanism and kinetics of fine-grained polycrystalline bridgmanite were studied at high temperatures up to 1080 K. High-temperature time-resolved synchrotron X-ray diffraction measurements showed that significant volume expansion due to temperature-induced amorphization caused static stress, which then hindered amorphization progress. Further, the temperature required for the amorphization of fine-grained bridgmanite (~ 1 μm) was found to be approximately 100 K higher than that required for the amorphization of coarse-grained samples (> 10 μm). We also noted that amorphization preferentially commenced at the twin planes and subgrain boundaries of bridgmanite grains, resulting in lower amorphization temperatures for the coarse-grained samples. The limited number of such specific locations in fine-grained natural bridgmanite suggested that grain boundary amorphization may be the dominant mechanism for bridgmanite amorphization in shocked meteorites. This unique amorphization kinetics would support the preservation of bridgmanite during the post-shock annealing in the shocked meteorite. Although bridgmanite amorphization starts easily at temperatures above ~ 420 K, a small amount of bridgmanite grains can survive at temperatures above 800 K by the effect of amorphization-induced stress.

Detection of Be dopant pairing in VLS grown GaAs nanowires with twinning superlattices

Control over the distribution of dopants in nanowires is essential for regulating their electronic properties, but perturbations in nanowire microstructure may affect doping. Conversely, dopants may be used to control nanowire microstructure including the generation of twinning superlattices (TSLs)—periodic arrays of twin planes. Here the spatial distribution of Be dopants in a GaAs nanowire with a TSL is investigated using atom probe tomography. Homogeneous dopant distributions in both the radial and axial directions are observed, indicating a decoupling of the dopant distribution from the nanowire microstructure. Although the dopant distribution is microscopically homogenous, radial distribution function analysis discovered that 1% of the Be atoms occur in substitutional-interstitial pairs. The pairing confirms theoretical predictions based on the low defect formation energy. These findings indicate that using dopants to engineer microstructure does not necessarily imply that the dopant distribution is non-uniform.

EBSD patterns simulating multilayer twins based on dynamical theory of electron diffraction.

Electron backscatter diffraction (EBSD) can be employed to determine crystal structures but has not been used alone to identify defects at the atom scale due to the lack of understanding of the EBSD patterns generated by various structure defects. In the present work, the EBSD patterns of FCC-Fe with 9-layer, 6-layer and 3-layer twin structures are simulated, respectively, using the revised real space (RRS) method and compared with the counterpart of perfect crystals. Our results show that when the electron beam is incident along a direction parallel to the twin plane, the pattern appears symmetrical with respect to the corresponding Kikuchi band of the twin plane, and the diffraction details within the Kikuchi band also exhibit symmetry with respect to the middle line of the Kikuchi band. Moreover, the overall clarity of the patterns decreases, and the pattern becomes more blurred with increasing the distance from the Kikuchi band corresponding to the twin plane. By contrast, the incident electron beam along the direction perpendicular to the twin plane results in diffraction superposition of the matrix region and the shear region, which shows twofold rotational symmetry with respect to the Kikuchi pole corresponding to the normal to the twin plane. In addition, some extra Kikuchi bands appear in the EBSD patterns due to the long-period structures of the multilayer twins. As the number of multilayer twins decreases, the number of extra Kikuchi bands decreases and the area of the blurring pattern increases. The correlation between twin structures and EBSD patterns provides theoretical insights for identifying twin structures by the EBSD technique.

Topological electronic structure of twin boundaries and twinning superlattices in the SnTe material class

The topological electronic structure of a single twin boundary and coherent twinning superlattices (TSLs) based on the SnTe class of materials is calculated and discussed within a supercell implementation. The superlattices consist of two twin planes (TPs) in the supercell arranged in such a way that each of the boundaries forms a mirror plane for the entire structure. Two types of TP boundary, cationic and anionic, can exist, and so three types of supercells can be constructed. We study the topological phases of each twinning configuration using the tight-binding approximation and calculating the topological invariants. We show that they differ by topological properties and find that all-cationic TSLs are topologically distinct from the anionic case due to the opposite sign of the Berry curvature around the $\mathrm{\ensuremath{\Gamma}}$ point of the TSL's Brillouin zone. Our findings are consistent with a complementary analysis of (111)-oriented slabs with a single twin boundary in the presence of a Zeeman field. They are also consistent with the calculated number of spin-polarized Dirac-like edge states of both superlattices and slabs. We conclude that each type of TP forms a two-dimensional mirror-plane-protected topological crystalline insulator.

Engineering twin boundaries for enhancing strength and ductility of thermoelectric semiconductor PbTe

Twin boundary engineering is a potential strategy for achieving robust mechanical properties of materials. Our previous molecular dynamics simulations indicated that the nanotwin could significantly enhance the ductility of thermoelectric (TE) semiconductors PbTe due to coherent twin boundary (CTB) migration accompanied by the ‘catching bond’ at room temperature. To further improve the mechanical strength or ductility of PbTe, we investigated the role of the shear direction, the CTB orientation and the temperature on mechanical properties of nanotwinned PbTe. Under the shear stress along [11̅0] loading direction, the partial dislocations with a/6 [12̅1] and a/6 [21̅1̅] Burgers vectors are preferentially activated on (111) twin plane with higher yield strength and ultimate shear strength than that of the (111)[112̅] slip system. The nanotwinned PbTe with CTB orientation ranging from 125° to 161° has both higher fracture strain and larger ultimate shear strength than 0° CTB orientation. This is attributed to the motion of the twinning partial dislocation significantly enhancing the ductility while the blocking of dislocations by CTBs further improving the shear strength and deformability of PbTe. Moreover, the low temperature (below 100 K) energetically enables the partial dislocation to nucleate and glide on the strong Te-CTB plane, which induces successive CTB migration along Pb- and Te-CTB planes, resulting in enhanced ductility of nanotwinned PbTe.

A new synergy to overcome the strength-ductility trade-off dilemma in Al–Si–Cu alloy by micro-nano-particle complex clusters

The effects of the novel micro-nano-particle clusters (nano-AlN and submicron-TiC particles doped with B, referred as TiCB) on the microstructure and mechanical properties of the Al–9Si–3Cu alloy are investigated herein. Comprehensive analysis revealed that the nano-AlN particles resulted in a significant modification of eutectic Si by causing the poisoning of twin plane re-entrant edge growth of Si and restricting the growth of Si crystals. Correspondingly, the morphology of Si transformed from flaky to fibrous. Furthermore, the synergy of areas with and without particle clusters was emphasized to address their contributions to the simultaneous improvement of the strength and ductility. When inoculating the micro-nano-particle clusters into the Al–9Si–3Cu alloy, the yield strength, ultimate tensile strength, and elongation were 138 MPa, 253 MPa, and 5.8%, respectively, corresponding to increases of 23.2%, 23.4%, and 123.1% compared with those of the uninoculated Al–9Si–3Cu base alloy (112 MPa, 205 MPa, and 2.6%, respectively). After T6 heat treatment, the spheroidal Si and θ′ phases on the Al matrix further improved the strength of the Al–9Si–3Cu alloy. This work may provide a promising strategy to design and fabricate Al–Si–Cu alloy with high strength and ductility.

Formation of intermetallic compounds during reaction between Ti and Al–Mg alloys with various Mg contents

The reactions between Ti and Al–Mg alloys were systematically investigated by heating the Ti/Al–Mg alloy powder mixture compacts with Mg contents ranging from 5.09 to 40.36 wt.% at 660 °C. Mg element in the Al–Mg alloys contributed to the breakage of the oxide films on the surfaces of the Ti and Al–Mg alloy powders, thus, promoting the reactions. In the samples with Mg contents varying from 5.09 wt.% to 25.93 wt.%, the first-formed intermetallic compound was Al18Ti2Mg3, and then Al3Ti generated at the Al18Ti2Mg3/Ti interface. Since the Mg content in the Al–Mg melts decreased during the subsequent heating process, the previously formed Al18Ti2Mg3 in the Ti/Al–5.09Mg sample transformed into Al3Ti accompanied by the reaction of Al with Ti to newly form Al3Ti, generating a single Al3Ti phase. In contrast, the new formation of Al3Ti and its transformation to Al18Ti2Mg3 proceeded continuously in the Ti/Al–13.54Mg and Ti/Al–25.94Mg samples, resulting in a mixture of Al18Ti2Mg3 and a little Al3Ti residue. In the Ti/Al–40.54Mg sample, Al3Ti was the unique reaction product throughout the heating process. The final Al3Ti particles in the Ti/Al–5.09Mg sample were in a blocky shape, while they were in a plate-like form in the Ti/Al–40.54Mg sample, which was caused by the enhanced growth anisotropy due to the decreased interfacial energy between Al3Ti particles and Al–Mg melts with the increase of Mg content. A twin plane re-entrant mechanism was suggested for the growth of Al18Ti2Mg3 particles, weakening the anisotropic growth caused by the directional supply of Ti atoms, and contributing to the blocky morphology of Al18Ti2Mg3 particles. In addition, the volume difference between the formed Al18Ti2Mg3 and consumed Ti was greater than that between the Al3Ti and Ti, and the Al18Ti2Mg3 has worse plasticity than the Al3Ti, resulting in the Al18Ti2Mg3 reaction product fracturing more easily during thickening, thus forming a petal-like structure, but the Al3Ti product being in an almost continuous layer.

101 contact twins in gypsum experimentally obtained from calcium carbonate enriched solutions: mineralogical implications for natural gypsum deposits.

Gypsum twins are frequently observed in nature, triggered by a wide array of impurities that are present in their depositional environments and that may exert a critical role in the selection of different twin laws. Identifying the impurities able to promote the selection of specific twin laws has relevance for geological studies aimed at interpreting the gypsum depositional environments in ancient and modern deposits. Here, the effect of calcium carbonate (CaCO3) on gypsum (CaSO4·2H2O) growth morphology has been investigated by performing temperature-controlled laboratory experiments with and without the addition of carbonate ions. The precipitation of twinned gypsum crystals has been achieved experimentally (101 contact twin law) by adding carbonate to the solution, and the involvement of rapidcreekite (Ca2SO4CO3·4H2O) in selecting the 101 gypsum contact twin law was supported, suggesting an epitaxial mechanism. Moreover, the occurrence of 101 gypsum contact twins in nature has been suggested by comparing the natural gypsum twin morphologies observed in evaporitic environments with those obtained in experiments. Finally, both orientations of the primary fluid inclusions (of the negative crystal shape) with respect to the twin plane and the main elongation of sub-crystals that form the twin are proposed as a fast and useful method (especially in geological samples) to distinguish between the 100 and 101 twin laws. The results of this study provide new insights into the mineralogical implications of twinned gypsum crystals and their potential as a tool to better understand natural gypsum deposits.

Regio-selective growth of plates on Au nanorods

Morphological control is extremely difficult during the second-stage growth on existing nanoparticles. Here, we show that it is possible to combine nanorod and nanoplate morphologies by using partially encapsulated polystyrene-block-poly(acrylic acid) shells as the mask. Au nanoplates were grown selectively from the exposed surface of the Au nanorod, with consistent orientation for each nanorod. As such, we achieve three levels of synthetic control simultaneously: 1) the polymer mask focuses the Au deposition onto a few sites, so that the nanoplates could extend out prominently; 2) the longitudinal extension of the nanorod could be shifted to the traverse growth of nanoplates by using iodide ions as the inhibitor; 3) the nanoplates grow from only one of the twin planes because of the dynamic competition via the depletion sphere mechanism.

“Elastic” and “inelastic” twins and pinning of magnetic flux lines (vortices) in single crystals YBa2Cu3O7–x

In this work, we studied the fixation of magnetic vortices (pinning effect) on “inelastic” and “elastic” twins, whose wedge-shaped peaks in the direction of twinning do not reach the crystal surface. The effectiveness of the “elastic” twins effect as pinning centers was studied by measuring of the critical transport current Jc in single crystals YBa2Cu3O7–x. The presented experimental data indicate that the “elastic” twins boundaries are effective pinning centers of the magnetic flux lines in single crystals of the compound under study. The main pinning centers of the vortices are dislocation clusters forming at the vertices of the “elastic” twins. The dislocation lines are located in the twin plane and, if the orientation of the magnetic vortices and dislocation lines coincide, the energy of the vortices decreases, the pinning force appears, fixing their location, which ultimately leads to an increase in the critical density of the transport electric current.

32‐Symmetric Chiral Gold Nanoplates with Near‐Infrared Circular Dichroism

AbstractPlasmonic chiral nanostructures provide a novel route for extraordinary optical properties, such as light polarization control and enhancing chiroptical signals of molecules. The symmetry of chiral nanostructures, such as helices, gyroids, and irregular tetrahedrons, is strongly correlated to chiroptical responses; therefore, it is an important design parameter. In a previous study, a synthetic strategy is demonstrated for 432‐symmetric chiral gold nanoparticles by breaking the mirror symmetry of a 4/m 2/m‐symmetric crystal structure on a high‐index surface. Herein, the aim is to extend the chiral symmetry of gold nanoparticles and their chiroptical responses. The symmetry of gold nanoparticles can be modulated by inducing single or multiple twin planes in seed nanoparticles. The 32‐symmetric chiral gold nanoplates, referred to as 32 helicoids, are successfully synthesized using 3 2/m‐symmetric triangular and hexagonal plates. The growth mechanism of the 32 helicoids, which results in two different characteristic morphologies, is analyzed. The 32 helicoids are significantly important owing to their optical activity over a wide range from the visible to the near‐infrared region. This investigation provides an insight that symmetry control of the nanomorphology can represent an important research direction in chiral plasmonic applications.

Atomically Sharp Internal Interface in a Chiral Weyl Semimetal Nanowire

Internal interfaces in Weyl semimetals (WSMs) are predicted to host distinct topological features that are different from the commonly studied external interfaces (crystal-to-vacuum boundaries). However, the lack of atomically sharp and crystallographically oriented internal interfaces in WSMs makes it difficult to experimentally investigate topological states buried inside the material. Here, we study a unique internal interface known as merohedral twin boundary in chemically synthesized single-crystal nanowires (NWs) of CoSi, a chiral WSM of space group P213 (No. 198). Scanning transmission electron microscopy reveals that this internal interface is a (001) twin plane which connects two enantiomeric counterparts at an atomically sharp interface with inversion twinning. Ab initio calculations show localized internal Fermi arcs at the (001) twin plane that can be clearly distinguished from both external Fermi arcs and bulk states. These merohedrally twinned CoSi NWs provide an ideal platform to explore topological properties associated with internal interfaces in WSMs.

Three-dimensional shape and stress field of a deformation twin in magnesium

While the three-dimensional (3D) shape and stress field of a twin in hexagonal close-packed (HCP) metals have attracted considerable interest in recent years due to their substantial impact on internal stress and mechanical properties, a detailed understanding of their variation with twin size is still lacking. An analytical model that is not restricted by spatial scale is developed in this work by considering the effects of anisotropic twin boundary energy, elastic strain energy and plastic relaxation to predict the 3D shape with the minimum energy, and the stress field, of an isolated ellipsoidal twin of different sizes. The model is applied to Mg with a focus on the {101¯2} twin type. The analytical calculation results show that the nucleation of the nano-sized twin embryos is facilitated by the stress field near structural defects such as dislocations. During the expansion of this nano-sized twin embryo, the interplay between the elastic strain energy and interfacial energy changes the length of the twin along the twin shear (forward) direction from being shorter to longer than that along the lateral direction. In contrast to the current understandings, the maximum shear stress on the twin plane along the twin shear direction occurs at the lateral, rather than the forward, side of the twin. At the forward side, the maximum shear stress occurs at a distance ahead of the twin tip and this distance increases with increasing twin thickness.

Evolution of dislocations and deformation twins in Ti6321 titanium alloy under contact explosion

TEM is used to examine the microstructural evolution of the Ti6321 titanium alloy during the contact explosion. A certain number of dislocations and deformation twins are observed after the explosion. When the slips of dislocations are blocked, {10–11} twins are generated to meet the deformation. The twin boundary deviates from the twin plane and is parallel to the (0001) plane of the matrix. The slide of dislocations and twinning both play a role in the deformation resulting from the explosion.

Dislocation-twin interaction in medium entropy alloy containing a high density of oxygen interstitials

Dislocation-twin interactions play important roles in tuning the plastic deformation of materials. Here, we report the great influence of interstitial oxygen atoms on dislocation-twin interactions in NiCoCrO medium entropy alloy through in-situ transmission electron microscopy mechanical testing, atomic resolution energy-dispersive X-ray spectroscopy characterizations, and integrated differential phase-contrast scanning transmission electron microscopy imaging. The spreading lattice distortion in medium-entropy alloy was found to enable large oxygen interstitial solubility. Oxygen atoms occupied both the tetrahedral and octahedral interstitial sites, and tended to be enriched at defects such as twin boundaries and stacking faults. Oxygen-enriched regions near the twin boundary hindered dislocations from direct transmitting or cross-slip into another twin plane during plastic deformation, resulting in significant dislocation entanglement, partial dissociation of dislocations, and the deviation of slip directions. Our results give insight into the strengthening mechanisms associated with the synergetic effects of twin boundary and interstitial atoms.

Effect of annealing process on microstructure and properties of 980MPa grade hot-dip galvanized dual-phase steels

A hot-dip galvanizing simulator studied the effects of soaking temperature and rapid cooling end temperature on the microstructure and mechanical properties of 980MPa grade hot galvanizing dual-phase steel. The results show that at the end of a rapid cooling temperature of 290°C, with the increase of soaking temperature, the tensile strength of the test steel increases, the yield strength decreases at first and then increases, and the elongation decreases as a whole. The microstructure is composed of ferrite and martensite. At the soaking temperature of 800°C, the strength decreases, and the elongation increases with the increase of the end temperature of rapid cooling. When the end temperature of rapid cooling is lower than 320°C, the martensite tempering is not apparent, and there are many high-density dislocations in the ferrite near the neighbor. When the cooling end temperature is 350°C, some martensites are tempered. The twins in martensite are widened, carbides are unevenly precipitated along the twin planes, and the dislocation density decreases. When the soaking temperature is between 800°C and 820°C, the end temperature of rapid cooling is between 260°C and 290°C. Annealing and zinc plating are beneficial to obtain dual-phase steel with excellent mechanical properties.

Effect of δ-ferrite in welded ER308 and ER316 microstructure on hydrogen embrittlement

In this study, the hydrogen embrittlement (HE) behavior confined to focusing the weld microstructure of austenitic stainless steels (ER308 and ER316) was investigated. To investigate the microstructural influence on HE, microstructural changes were induced by different heat-treatment times at 1050 °C. These changes were prepared from microstructures containing δ-ferrite and microstructures where δ-ferrite was removed at welded parts. The results of the slow strain rate test (SSRT) indicated that the HE index, calculated from relative reduction width (RRW), was greatly reduced due to microstructure changes. To demonstrate the correlation between the deformation mechanism and HE index, deformation behavior was analyzed and verified through chemical composition, microstructure, fracture morphology, and modified Curssard–Jaoul (C–J) analysis. In all heat-treatment conditions, more mechanical twinning and a more drastic change in the HE index were observed for ER 308 compared to that of ER 316. The longest heat-treatment time conditions in a greatest decrease in HE index for ER308 and ER316. Hydrogen decohesion effects due to accumulation with dislocation were verified observing a parallel twin plane or microvoid along the twin plane in a transgranular fracture. In additional, as the heat-treatment time increased, the removal of δ-ferrite decreased the HE index for both stainless steels. Because, δ-ferrite was assisted in hindering mechanical twinning by misostrain concentrated at δ-ferrite sites in the early state of SSRT, it was enhanced to the resistance of HE.

Recovery and facets for deformation twins in minerals and metals

Type II and IV twins with irrational twin boundaries are studied by high-resolution transmission electron microscopy in two plagioclase crystals. The twin boundaries in these and in NiTi are found to relax to form rational facets separated by disconnections. The topological model (TM), amending the classical model, is required for a precise theoretical prediction of the orientation of the Type II/IV twin plane. Theoretical predictions also are presented for types I, III, V, and VI twins. The relaxation process that forms a faceted structure entails a separate prediction from the TM. Hence, faceting provides a difficult test for the TM. Analysis of the faceting by the TM is in excellent agreement with the observations.

Pyrite contact twins.

Two examples of contact twins in pyrite from Peru are described. The first one, from Pasto Bueno ore deposit, shows the pyritohedron {120} as principal form, accompanied by the {111} octahedron and {100} cube as secondary forms, giving a lenticular aspect. (111) is the composition plane, and the twin operation is any one of the three binary axes ⟨110⟩ within this plane. The second one (unknown ore deposit) presents two forms, the octahedron {111} and the pyritohedron {120}; the two crystals in the twin are elongated along [101] and [011], respectively, producing a V profile. It is a reflection twin where the twin plane (110) coincides with the composition plane. These twins are by merohedry. Another contact twin is known in the literature, reported by Gaubert [Bull. Soc. Fr. Minéral. Cristallogr. (1928), 51, 211-212] who described it as a spinel twin, i.e. a reflection twin with twin and composition plane (111); here it is shown that it is actually a rotation twin in which the twin operation is a 180° rotation about any of the three equivalent directions ⟨211⟩, contained in the (111) composition plane. The occurrence of these twins as well as the doubtfulness of the spinel twin in pyrite shows a direct relationship with the structural interpretation based on the pseudo-symmetry of the crystallographic orbits.

Twinning in hydrous wadsleyite: Symmetry relations, origin, and consequences

AbstractTwins in hydrous wadsleyite were detected by polarized-light microscopy and characterized with transmission electron microscopy techniques, including precession selected area electron diffraction and large-angle convergent beam diffraction. By inspecting diffracted intensities for high-order Laue zones, we found the symmetry of our hydrous wadsleyite samples to be reduced to monoclinic with respect to the orthorhombic symmetry of most anhydrous wadsleyite samples. Twinned domains in hydrous wadsleyite share the (122) plane as a composition plane and are related to each other by a twofold rotation around a twin axis parallel to [212] or by reflection on (122). The twin axis and twin plane in wadsleyite correspond to the <101> directions and the {101} planes of ringwoodite, respectively. The twin operations exchange the c* and the [120]* directions of wadsleyite, both of which correspond to the directions of the cubic a axes in ringwoodite. Based on our analysis of symmetry relations and pseudo-symmetry in wadsleyite, we conclude that the twins formed during crystal growth under quasi-hydrostatic conditions in the presence of a hydrous fluid. Twinning in wadsleyite may affect the physical properties and deformation behavior of wadsleyite as well as the phase transition to ringwoodite in the Earth’s mantle transition zone.