Twin Lamellae Research Articles

Characterization ofAIIIBVsuperlattices by means of synchrotron diffraction topography and high-resolution X-ray diffraction

New possibilities are presented for the characterization ofAIIIBVmixed superlattice compounds by the complementary use of synchrotron diffraction topography and rocking curves. In particular, using a synchrotron white beam and the section diffraction pattern of a 5 µm slit taken at a 10 cm film-to-crystal distance, it was possible to reproduce a set of stripes corresponding to interference fringes. These are analogous to the interference maxima revealed in high-resolution rocking curves, but are created by the changes in orientation of the planes inclined to the surface which are induced by unrelaxed strain. The section diffraction topographic method enabled examination of the sample homogeneity along the narrow intersecting beam. This was important in the case of the present sample containing a twin lamella in the InP substrate wafer. Both the section and projection Bragg case topographic methods enabled the crystallographic identification of the twin lamella. Another characteristic feature indicated in the section topography was the bending of the stripes corresponding to the superlattice peaks close to the boundaries of the twin lamella. The most probable interpretation of this phenomenon is an increase in the thickness of the deposited layers close to the lamella, together with possible changes in the chemical composition, leading to a decrease in the mean lattice parameter in the superlattice.

Micromechanical modeling for mechanical properties of gradient-nanotwinned metals with a composite microstructure

Nanotwinned metals with a gradient microstructure have attracted a great deal of attention due to their excellent mechanical performance of combining high strength and high ductility. In this work, a micromechanical model is developed to describe the stress-strain response of gradient-nanotwinned metals with a composite microstructure. The deformation mechanisms originated from bimodal grain size distribution in nanostructured materials and nanoscale twin lamellae in a grain are involved in derivation of flow stress. The contributions from the gradient distribution of microstructural size and the microcracks during plastic deformation are taken into account in simulating the mechanical properties such as the yield strength and ductility. Using the proposed model, we figure out the stress-strain relation of gradient nanostructured metals and analyze the quantitative relation between the mechanical properties and the geometrical/physical parameters related to the gradient-nanotwinned composite copper. Numerical results show that, the strength and ductility of the gradient-nanotwinned bimodal metals are both improved as twins spacing decreases. With the volume fraction of coarse-grained phase decreased, the strength is improved significantly accompanied by slight reduction of the ductility. In addition, the simulated results are in a good agreement with experimental results. The present work could be helpful to describe and predict the elastic-plastic deformation behavior of gradient nanostructured composite -metals.

Tailoring the Microstructure and Mechanical Property of AZ80 Alloys by Multiple Twinning and Aging Precipitation

AZ80 alloy is a relatively inexpensive age‐hardenable Mg alloy. To improve its precipitation strengthening effect, the discontinuous precipitates generally nucleating from grain boundaries should be reduced. In this study, the authors applied strain‐path change compression on a wrought AZ80 alloy to generate multiple {10‐12} twins and then the pre‐deformed alloy is subjected to aging at 180 °C for different period. The effects of the pre‐generated twins on precipitation behavior and hence the mechanical properties are investigated. It is found that multiple {10‐12} twins are formed in the AZ80 alloy after two paths of compressions along the transverse and rolling directions. Consequently, continuous precipitation is largely promoted during the subsequent aging process. Moreover, the continuous precipitates preferred to form inside the {10‐12} twin lamellae. Discontinuous precipitation is almost inhibited during aging at 180 °C for 4–96 h in the sample containing many multiple twins. Compare to the directly peak‐aged sample, the pre‐twinned and subsequently peak‐aged alloys generally exhibit greatly superior mechanical properties. The yield strength, ultimate strength, and elongation for tension along RD are 242 MPa, 476 MPa, and 19.4%, respectively. These excellent properties are attributed to the combined effects of grain refinement, texture weakening, and enhanced precipitate strengthening.

Local stress field induced by twinning in a metastable β titanium alloy

A crystal plasticity based finite element model incorporating deformation twinning is applied in 3D to simulate the stress field around a twin lamella in a metastable β titanium alloy. Compared with conventional 2D simulations, the stress field determined by the 3D simulation shows better agreement with that measured experimentally using high resolution electron backscattered diffraction. Reasons leading to the different results in 2D and 3D simulations are discussed. It is also discussed in which circumstances 2D simulation is sufficient, and in which 3D is necessary.

Orientation and length scale effects on dislocation structure in highly oriented nanotwinned Cu

Highly oriented nanotwinned Cu has been compressed to 6% strain in directions 90°, 0° and 45° with respect to the twin boundaries of the almost parallel twins. In the 90° and 0° compressed samples Mode I and Mode II dislocations and their interactions with twin boundaries dominate the deformation of twin/matrix (T/M) lamellae with thickness less than 500 nm. In 45° compressed samples, Mode III dislocations, especially partial dislocations moving along the twin boundaries, dominate the deformation of fine T/M lamellae with thickness less than 100 nm, while dislocations from slip Modes I, II and III are identified in T/M lamellae more than 100 nm thick, where these dislocations extensively interact in the T/M lamellae with thicknesses more than 200 nm. Dislocation cells are observed in a twin lamella with a thickness of about 500 nm.

Synthesis, synchrotron diffraction study and twinning in Na2Ca4Mg2Si4O15 – a heteropolyhedral framework compound

Abstract The formation of polycrystalline Na2Ca4Mg2Si4O15 from solid state reactions has been studied between 800 and 1050°C. Single crystals of the compound have been grown in a closed platinum capsule by slow cooling in the temperature range between 1300 and 1000°C. Basic crystallographic data are as follows: monoclinic symmetry, space group P12/c1, a=7.1717(3) Å, b=5.3512(2) Å, c=16.4789(7) Å, β=90.911(4)°, V=632.33(4) Å3, Z=2. A conspicuous feature of the crystals is an intensive lamellar non-merohedral twinning clearly observable already under a petrographic microscope. The diffraction pattern can be explained as a superposition of two reciprocal lattices with a two-fold axis parallel to [001] being the twin element. Using synchrotron radiation it was possible to solve the crystal structure of Na2Ca4Mg2Si4O15 from a twinned data set. Least-squares refinements resulted in a residual of R(|F|)=0.031 for 2899 observed reflections with I>2σ(I) and 127 parameters. The crystal structure contains both [Si2O7]-dimers and insular [SiO4]-moieties. Tetrahedra and [MgO6]-octahedra form a three-dimensional framework whose topological characteristics have been studied. The remaining calcium and sodium cations are distributed among four crystallographically independent positions located in voids of the network. On a microscopic scale the twinning observed in the diffraction experiments could be explained by the existence of a 21-screw axis parallel to [001] in (¼, 0, z) mapping both domains onto each other. A comparison with related compounds having an A+ 2B2+ 6Si4O15 stoichiometry is presented. More than 25 years after its first observation in refractories our investigation clarifies the crystal structure of a silicate that is of relevance for both Materials science and high pressure research.

Molecular dynamics simulation of BCC Ta with coherent twin boundaries under nanoindentation

We performed molecular dynamics (MD) simulations of nanoindentation on monocrystalline and nanotwinned Ta (nt-Ta) films, aimed to uncover the effects of twin boundary (TB) and twin thickness on hardness and the corresponding deformation mechanism. We found that the inelastic deformation of monocrystalline Ta under indentation could be attributed mainly to the propagation of dislocation loops and emergence of deformation twin. Inverse Hall-Petch-like relationship was found in the material with various twin thicknesses. It was also found that dislocation absorption, desorption and direct transmission are three kinds of dislocation-TB interaction in nt-Ta, resulting in the migration of TBs, the formation of steps along TBs, and the formation of new sites for dislocations nucleation, which contribute to the softening. On the other hand, TBs can block the propagation and migration of dislocations and confine dislocations in the twin lamellae, which may contribute to hardening. Overall, softening mechanisms dominate the deformation of nt-Ta with small twin thickness during nanoindentation process.

Probing lamellar twins in spark plasma sintered CaTiO3 using Electron Backscattered Diffraction

The twinning characteristics of spark plasma sintered CaTiO3 with orthorhombic perovskite structure is critically analysed using Electron Backscatter Diffraction (EBSD) technique. The twins are characterized by a sub-micron interlamellar spacing with a misorientation of ≈ 90°. The crystallographic relationship between the matrix and the twin has been analysed in terms of misorientation matrix using the Kikuchi patterns obtained from the EBSD scan. From the analysis of angle/axis pairs at the boundary, the twin axis was found to be of type [101]. These twins with high angle boundaries are expected to enhance fracture resistance through crack deflection and crack tip shielding mechanisms.

Twinning-induced dynamic recrystallization and micro-plastic mechanism during hot-rolling process of a magnesium alloy

{101̅1} twinning played an important role during hot-rolling process of AZ31 alloy. In the present work, TEM was employed to investigate such twinning related plastic mechanism. The dislocations were usually lying on the basal plane in matrix, while a⃑+c⃑ slip was usually activated near grain boundary as well as in twinned area even when the orientation was quite soft for basal slip. The combination of a⃑ and a⃑+c⃑ slip created the characteristic misorientation relationship along 〈112̅0〉 in twin region. The activation of non-basal slip met the requirement to accommodate adequate localized stain in twinned area, which made twinning induced DRX kinetically possible. The twin lamella with the highest SF, which was supposed to be the most effective in accommodating imposed strain, showed the highest tendency for localized strain and led to complete DRX inside twinned region.

Enhanced overall strength and ductility of magnesium matrix composites by low content of graphene nanoplatelets

This work focuses on the microstructure, strengthening and plastic deformation of low content graphene nanoplatelets (GNPs) reinforced magnesium (Mg) matrix composites. The GNPs with the homogeneous dispersion and distribution realised through the disintegrated melt deposition technique improve the overall strength and ductility of composites significantly and efficiently. The yield strength improvement is mainly attributed to (i) the effective load transfer owing to the high specific surface area and the interlock effect from the wrinkled two-dimensional GNPs, and (ii) the grain refinement by the GNPs induced twin lamellae. Besides, (i) the promoted {101¯2} extension twins that maintain the grains with favourable orientations for basal slip and (ii) the initiation of the prismatic 〈a〉 slip play important roles in the toughening effect of composites, which are discussed in detail. This study highlights the advantage of GNPs in tuning the metal matrix composites with high strength and high ductility simultaneously.

Calcite twinning strain variations across the Proterozoic Grenville orogen and Keweenaw-Kapuskasing inverted foreland, USA and Canada

We report the calcite twinning strain results of a traverse across the Grenville orogen from Parry Sound, Ontario (NW) to Ft. Ann, New York (SE), including the younger, adjacent Ordovician Taconic allochthon. Fifty four carbonates (marbles, calcite veins, Ordovician limestone) were collected resulting in 68 strain analyses on mechanically twinned calcite (n = 2337 grains) across the Central Gneiss Belt (CGB; 3 samples), the Central Metasedimentary Belt (CMB; 27 samples), the Central Granulite Terrane (CGT; Adirondack's; 13 samples) and the Ottawan Orogenic Lid (OOL; 11 samples). Twinning strains in the greenschist-grade OOL marbles preserve N–S shortening and U-Pb titanite ages (∼1150 Ma; n = 4) document these marbles formed during the Shawinigan (1190–1140 Ma) part of the Grenville orogen. From northwest to southeast, the Ottawan (1095–1020 Ma) twinning strain is dominantly a layer-parallel shortening fabric oriented N–S (Parry Sound), then becomes parallel to the Grenville thrust direction (NW–SE) across the CMB to the Adirondack Highlands where the sub-horizontal shortening strain becomes margin-parallel (SW–NE). Within the regional sample suite there are two areas studied in detail, the Bancroft shear zone (n = 11) and a roadcut on the southeast side of the Adirondack Mountains (Ft. Ann, NY; n = 8). Marbles from the Bancroft shear zone contain calcite grains with 2 sets of twin lamellae (e1 and e2). The better-developed e1 sets (n = 406) record a horizontal fabric oriented NW–SE whereas the younger e2 lamellae (n = 146) preserve a margin-parallel (SW–NE) horizontal fabric. Both the e1 and e2 strains record an overprint vertical shortening strain (NEV), perhaps related to extensional orogenic collapse. We also report an Ottawan orogen-aged granoblastic mylonite (1093 Ma, U-Pb zircon; 1102 Ma Ar-Ar biotite) in the Keweenaw thrust hanging wall 500 km inboard of the Grenville front and interpret the relations of Grenville-Keweenaw far-field dynamics.

Single and double twin nucleation, growth, and interaction in an extruded magnesium alloy

The objective of this study was to identify twinning characteristics and mechanisms in an extruded AZ31 magnesium alloy under favorable conditions of profuse 101¯2 extension twinning using in-situ optical microscopy, electron backscatter diffraction, and X-ray diffraction analysis. The propagation of a single twin variant led to a relatively fast saturation of twin nucleation after which the increase in strain was predominantly accommodated by the growth of existing twin lamellas. For distinct twin variants, the intersecting twins led to the confinement of the spaces constrained by the fine twin lamellas. Embryonic twin structures acknowledged theoretically or through atomistic simulations were experimentally observed, including the vanishing of primary112¯1 embryonic twin via the nucleation and growth of either single or multiple 101¯2 secondary extension twin variants during deformation. Newly identified twin-twin interaction scenarios included the ladder-like and branching-like twinning structures which occurred depending on the applied strain, incoming twin paths, and their impingement on the pre-existing twin boundaries. The formation of the branching-like structures involved three or more non-co-zone 101¯2 extension twin variants where the activation and growth of one variant among them were found to be favored by the external stresses via Schmid factor analysis.

Dependence of dislocation structure on orientation and slip systems in highly oriented nanotwinned Cu

To explore the correlation between orientation, active slip systems and dislocation structure, highly oriented nanotwinned Cu has been deformed in compression to 2% and 6% strain. The compression directions are 90°, 0° and 45° with respect to the twin boundaries (TBs) of the almost parallel twins. The dislocation structures are analyzed by the two-beam diffraction imaging in a transmission electron microscope and by a Schmid factor analysis. In structures deformed at 90° a high density of long straight dislocation lines with both slip plane and Burgers vectors inclined to the twin plane (slip Mode I) are observed; they transmit across multiple TBs at a strain of 2% and form a high density of dislocations on TBs at a strain of 6%. In structures deformed at 0° dislocations with Burgers vectors parallel to the twin plane (slip Mode II) are confined within Twin/Matrix lamellae and the analysis shows that both slip Mode I and II are active with dominance of Mode II. In structures deformed at 45° dislocations from slip Modes I, II and III are identified, where Mode III dislocations consist of partial dislocations moving along the TBs and full dislocations inside the twin lamellae gliding on the slip planes parallel to the twin plane. The analysis of the dislocation structures illustrate the strong correlation between active slip systems and the dislocation structure and the strong effect of slip mode anisotropy on both the flow stress and strain hardening rate of nanotwinned Cu.

Distribution effects of secondary twin lamellae on the global and local behavior of hierarchically nanotwinned metals

Twin lamellae in hierarchical nanotwinned metals (HNMs) typically exhibit a random distribution of thickness. Here, a self-consistent model using a micro–macro transition method has been proposed to understand the distribution effects of secondary twin lamellae on the global and local mechanical behaviors of HNMs. The representative volume element (RVE) of this model is composed of secondary twin lamellae with a lognormal distribution. Each RVE in the HNMs follows an elastic–viscoplastic behavior. A mechanism-based strain gradient theory is adopted based on different dislocation densities in twin boundary dislocation pileup zones and grain interiors. Numerical simulation results reveal that not only the mean size of secondary twin lamellae thickness contributes to the global mechanical behaviors, but the variance of the distribution will also affect the material strength, especially for a relatively small mean twin thickness. Additionally, local stress/strain fields are explored under given macroscopic fields. The discrepancy between micro- and macro-fields for two different mean twin lamellae thicknesses is thoroughly discussed. Furthermore, the influence of dispersion, which increases the local stress field while decreasing the local strain field, is also highlighted. This proposed model can help us design functional HNMs by considering the size distribution and local mechanical behaviors caused by size dispersion.

Cryo-rolling enhanced inhomogeneous deformation and recrystallization grain growth of a zirconium alloy

This work focuses on the influence of cryo-rolling on subsequent recrystallization of hexagonal Zr-4 alloy. With initial recrystallized microstructure, the as-received Zr-4 sheets were rolled at liquid nitrogen temperature (LNT) and room temperature (RT). For each rolling temperature, two levels of strain, 20% and 40%, were applied. Then, samples, including as-received and rolled sheets, were annealed at 700 °C for 3 min–5 min. Microstructures of the deformed and annealed Zr-4 samples were characterized in detail by electron backscatter diffraction (EBSD) method. After rolling, no twin lamella is observed in Zr-4 sample. It is found that the intensity of the heterogeneous deformation in cryo-rolling is more severe than that in RT-rolling. After annealing, the recrystallized average grain size of Zr-4 decreases with the increases of the applied strain at both deformation temperatures. For a given rolling reduction, the recrystallized average grain size increases with decreasing deformation temperature. The texture orientation of Zr-4 was not remarkably changed during the rolling deformation and annealing process.

Evolution of planar defects during homoepitaxial growth of β-Ga2O3 layers on (100) substrates—A quantitative model

We study the homoepitaxial growth of β-Ga2O3 (100) grown by metal-organic vapour phase as dependent on miscut-angle vs. the c direction. Atomic force microscopy of layers grown on substrates with miscut-angles smaller than 2° reveals the growth proceeding through nucleation and growth of two-dimensional islands. With increasing miscut-angle, step meandering and finally step flow growth take place. While step-flow growth results in layers with high crystalline perfection, independent nucleation of two-dimensional islands causes double positioning on the (100) plane, resulting in twin lamellae and stacking mismatch boundaries. Applying nucleation theory in the mean field approach for vicinal surfaces, we can fit experimentally found values for the density of twin lamellae in epitaxial layers as dependent on the miscut-angle. The model yields a diffusion coefficient for Ga adatoms of D = 7 × 10−9 cm2 s−1 at a growth temperature of 850 °C, two orders of magnitude lower than the values published for GaAs.

Structural evaluation of defects in β-Ga2O3 single crystals grown by edge-defined film-fed growth process

We have structurally evaluated β-Ga2O3 crystals grown by edge-defined film-fed growth process using etch pitting, focused ion beam scanning ion microscopy, transmission electron microscopy, and related techniques. We found three types of defects: arrays of edge dislocations corresponding to etch pit arrays on -oriented wafers, platelike nanopipes corresponding to etch pits revealed on the (010)-oriented wafers, and twins including twin lamellae.

Grain size constraints on twin expansion in hexagonal close packed crystals

Deformation twins are stress-induced transformed domains of lamellar shape that form when polycrystalline hexagonal close packed metals, like Mg, are strained. Several studies have reported that the propensity of deformation twinning reduces as grain size decreases. Here, we use a 3D crystal plasticity based micromechanics model to calculate the effect of grain size on the driving forces responsible for expanding twin lamellae. The calculations reveal that constraints from the neighboring grain where the grain boundary and twin lamella meet induce a stress reversal in the twin lamella. A pronounced grain size effect arises as reductions in grain size cause these stress-reversal fields from twin/grain boundary junctions to affect twin growth. We further show that the severity of this neighboring grain constraint depends on the crystallographic orientation and plastic response of the neighboring grain. We show that these stress-reversal fields from twin/grain boundary junctions will affect twin growth, below a critical parent grain size. These results reveal an unconventional yet influential role that grain size and grain neighbors can play on deformation twinning.

Nanoscale deformation twinning in xenotime, a new shocked mineral, from the Santa Fe impact structure (New Mexico, USA)

Abstract Shock microstructures in refractory accessory minerals such as zircon and monazite provide crucial evidence for deciphering impact-related deformation in a wide variety of planetary materials. Here we describe the first occurrence of shock deformation in xenotime, YPO4, from a shocked quartz–bearing shatter cone in granite at the Santa Fe impact structure (New Mexico, USA). Backscattered electron imaging shows that shocked xenotime grains near the surface of a shatter cone contain multiple orientations of closely spaced planar fractures. High-resolution electron backscatter diffraction mapping reveals that some of the planar microstructures in {112} contain deformation twin lamellae that range from 50 nm to 200 nm in width on the polished surface and occur in up to three crystallographic orientations. Other features attributed to impact, such as planar low-angle boundaries and planar deformation bands, record crystal-plastic deformation. Shatter cone formation and co-existing shocked quartz constrain minimum shock pressure experienced by the xenotime grains to 5–10 GPa. An upper limit of 20 GPa is tentatively assigned based on the absence of YPO4 polymorphs and shock twins in co-existing zircon. We propose that {112} deformation twins in xenotime constitute a diagnostic record of shock metamorphism, similar to {112} twins in zircon; they have not previously been reported in nature and occur in a rock with conspicuous evidence of shock deformation. Documentation of deformation twins in xenotime, a widely applied U-Pb geochronometer, can be used to identify hypervelocity deformation in shocked rocks, detrital grains, and other materials, and may be particularly ideal for recording low-pressure (<20 GPa) impact conditions that do not produce diagnostic shock microstructures in zircon.

Tuning of nanostructure by the control of twin density, dislocation density, crystallite size, and stacking fault energy in Cu100−xZnx (0≤x≤30 wt%)

The present study demonstrate the effect of stacking fault energy (SFE) on the evolution of crystallite size, dislocation density, twin density, stacking faults, and their mutual interactions during cryorolling in pure copper (SFE 78mJ/m2) and copper–zinc alloys with varying Zn content of 10wt% (SFE 35mJ/m2), 20wt% (SFE 19mJ/m2), and 30wt% (SFE 14mJ/m2). The dislocation density, twin density and extrinsic fault probability increase whereas crystallite size decreases upon cryorolling and with the decrease of SFE. Accumulation of dislocations at the twin boundaries causes dynamic recrystallization, promotes grain refinement by forming subgrains inside the twin lamellae. Detail investigation on the activity of structural defects during cryorolling of Cu/Cu-Zn alloys has been done through resistivity measurements. Nanoindentation studies have performed to understand the deformation kinetics of the cryorolled Cu/Cu-Zn alloys with high concentration of structural defects in term of strain rate sensitivity and activation volume. A micromechanical model has been used to demonstrate the effect of SFE and the role of structural defects during nanostructuring at cryogenic temperature.