Coherent Twin Boundaries Research Articles

Diffusion of Alkali Metals in Polycrystalline CuInSe2 and Their Role in the Passivation of Grain Boundaries.

The behavior of alkali atom point defects in polycrystalline CuInSe2 is studied. In this work, three grain boundary models, one coherent twin boundary and two twin boundaries with dislocation cores, are considered. Total energy calculations show that all alkali metals tend to segregate at the grain boundaries. In addition, the segregation of alkali atoms is more pronounced at the grain boundaries with the dislocation cores. The diffusion of alkali metals along and near grain boundaries is studied as well. The results show that the diffusion of alkali atoms in the grain boundary models is faster than within the bulk. In addition, the ion exchange between Na and Rb atoms at the grain boundaries leads to the Rb enrichment at the grain boundaries and the increase of the Na concentration in the bulk. While the effects of Na and Rb point defects on the electronic structure of the grain boundary with the anion-core dislocation are similar, Rb atoms passivate the grain boundary with the cation-core dislocation more effectively than Na. This can explain the further improvement of the solar cell performance after the RbF-postdeposition treatment.

Interstitial triggered grain boundary embrittlement of Al–X (X = H, N and O)

With the quick development of high-speed railway and the service of the series of CRH (China Rails High-speed) for almost a decade, one of the greatest challenges is the managements/maintenances of those trains in environmental conditions. It is critical to estimate the intergranular corrosion damage initiations and propagations, thus, to setup the relative database in order to support foundations for an interactive corrosion risk management. In this work, ∑301111¯1 coherent twin boundary (CTB) of FCC Al–X is utilized to reveal the interactions between interstitials (X = H, N and O) and CTB together with their embrittlement mechanism. The HCP-type fault layers within the CTB are same as those fault layers in growth fault, indicating the local FCC-HCP transformation at CTB and revealing the physical foundation for those criteria designing growth twins. The notable accumulation of bonding electrons of interstitial atoms occupying either T-site or O-site yields a dramatically reduction of Al–X and Al–Al bonding charge density and bonding strength, revealing the embrittlement mechanism of Al CTB.

Evolution of solidification defects in deformation of nano-polycrystalline aluminum

Formation of solidification defects and their evolution in uniaxial tensile deformation of solidified polycrystalline aluminum (Al) were investigated by molecular dynamics (MD) simulations. First, solidification process was simulated both isothermally and with different quench rates. At the initial stages of nucleation, coherent twin boundaries and/or fivefold twins formed depending on the quench rate or the undercooling temperature. The solidified polycrystalline Al consisted of randomly distributed grains, twin boundaries, and vacancies. Evolution of nanostructures and defects in uniaxial tensile deformation of solidified Al under different temperatures and strain rates were studied. Void formation at grain boundaries and detwinning of preexisting solidification twins and deformation twins were observed during the uniaxial deformation. It was also found that the temperature of deformation has a stronger effect than the applied strain rate on the strength of solidified samples. For solidified cases with grain sizes lower than 10 nm, the yield strength and Young’s modulus increased with increasing grain size, indicating an inverse Hall-Petch relationship. Similar to experimental data, MD simulations showed a higher yield strength for single crystal Al and a large plastic deformation for polycrystalline Al.

Combined in situ microstructural study of the relationships between local grain boundary structure and passivation on microcrystalline copper

In situ Electro-Chemical Scanning Tunneling Microscopy (ECSTM) and Electron Back-Scatter Diffraction analysis of the same local microstructural region were combined to study the relationships between grain boundary (GB) type and structure and passivation on microcrystalline copper in 0.1 M NaOH aqueous solution. The results show that, for high angle random boundaries, passivation in the Cu(I) oxidation range is characterized by a decrease of the depth of GB edge region due to the formation of a passive film locally thicker than on the adjacent grains and thus to anodic oxidation being locally less efficient since it consumes more copper at the grain boundaries. For Σ3 CSL boundaries, the Cu(I) passivation efficiency was observed to be dependent on the local structure of the boundaries. A transition from more to less efficient passivation was observed for a deviation angle of 0.4–0.5° of the {111} GB plane with respect to the perfect geometry of an ideal coherent twin boundary. This transition would result from the effect on the local anodic oxidation properties of an increased density of misfit dislocations accommodating the increased deviation of the boundary plane from the exact CSL plane.

Numerical Study on Bicrystalline Micropillar Compression Using High-Order Gradient Crystal Plasticity

Dislocation structures at crystalline scale play an important role in the scale effect of materials. The higher-order crystal plasticity, in which a dislocation information is introduced as the gradient of slip and affects the hardening behavior of slip, is a useful model to describe a scale dependency of metallic material. In this study, a large deformation finite element analysis of a bicrystalline micropillar is demonstrated to investigate the grain boundary effect on the dislocation motion. The effect of condition on the grain boundary is numerically discussed. It is suggested that the large angle grain boundary and the coherent twin boundary can be represented by boundary conditions of non-penetration and penetration of dislocation.

Interactions between dislocations and three-dimensional annealing twins in face centered cubic metals

Annealing twins often form in metals with a face centered cubic structure during thermal and mechanical processing. Here, we conducted molecular dynamic (MD) simulations for copper and aluminum to study the interaction processes between {111}1/2<110> dislocations and a three-dimensional annealing twin. Twin boundaries are characterized with Σ3{111} coherent twin boundaries (CTBs) and Σ3{112} incoherent twin boundaries (ITBs). MD results revealed that dislocation-ITB interactions affect slip transmission for a dislocation crossing CTBs, facilitating the nucleation of Lomer dislocation.

The irrational shear of [formula omitted] twinning in Mg

The rational shear associated with {101¯1} twinning in hexagonal metals is accommodated by the glide of four-layer twinning dislocations (TDs). However, it is questionable about the elementary TD of {101¯1} twinning, i.e., two-layer TD or four-layer TD. We characterized interfacial steps along {101¯1} coherent twin boundary (CTB) using transmission electron microscopy and identified two-layer steps. However, the shear resulted from the glide of two-layer TDs is irrational. Molecular dynamics simulations revealed that the activation of two-layer TDs relied on the shear stress applied along <12¯10> direction corresponding to the large component of the Burgers vector, while the migration direction of the CTB depended on the shear stress along <101¯2> direction corresponding to the conventional twinning shear direction. The irrational shear deformation of {101¯1} twinning is further discussed on basis of the character of two-layer TDs.

Atomic-scale structural characterization of grain boundaries in epitaxial Ge/Si microcrystals by HAADF-STEM

The atomic structure of grain boundaries in Ge micro-crystals grown on Si pillars for the fabrication of a monolithically integrated X-ray detector was studied by high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Three different boundaries are found in Ge: Σ3{111} coherent twin boundaries, Σ3{112} incoherent twin boundaries, and Σ9{122} and Σ27{552} grain boundaries. They are described using the structural unit models containing single columns. Remarkably, we find for the first time a Σ3{112} incoherent twin boundary exhibiting two different atomic structures; one symmetric and one asymmetric. Their co-occurrence is explained by the presence of a small step in the boundary plane and the introduction of dislocations. Likewise, the atomic structure of junctions formed by the interaction of twin boundaries which result in Σ9{122} and Σ27{552} grain boundaries is also revealed. Geometrical phase analysis is applied to map the strain fields at two triple junctions and to uncover the position of the dislocations.

Solidification dynamics and microstructure evolution in nanocrystalline cobalt

Solidification and microstructures of fast quenched cobalt (from 1950K to 600K) are studied using molecular dynamics simulations. Two types of microstructures, a nanocrystalline type consisting of multiple grains and a lamellar type are obtained. The lamellar type microstructure shows a dominant layering direction with fcc phases separated by stacking faults, coherent twin boundaries or hcp phases. The nanocrystalline type is dominant at fast quenching rates. Both microstructures are highly twinned. Twins can originate either from ab initio faults of several morphologies in the nuclei or from stacking accidents in 〈111〉 directions during growth. It is supported that fivefold twins form by a successive growth of the fcc subunits, but not by an equilateral layer-by-layer growth from a five-folded symmetric core that exists at the very beginning of nucleation. Annealing of the as-quenched samples above the phase transformation temperature leads to gradual growth of the fcc phase and local reversible fcc-hcp transformation facilitated by existing Shockley dislocations. Under uniaxial tensile loading, the nanocrystalline samples show an early initial yielding, followed by continuous work hardening, reaching an ultimate tensile strength of about 3000 MPa. The lamellar samples, in contrast, show a much more intermittent plastic flow. The strongest sample has a nanotwinned structure resembling TEM images of vapor deposited films of fcc metals. The ultimate tensile strength is over 5000 MPa, assisted by coherent twin boundaries serving as strong obstacles to dislocation motion.

The Effect of Grain Boundary Structure on the Intergranular Degradation Behavior of Solution Annealed Alloy 690 in Simulated Pressurized Water Reactor Primary Environment

The environmental degradation of four different types of grain boundaries were investigated on alloy 690 following slow strain rate tensile tests in 360 oC hydrogenated water. Random high angle boundaries (RHABs) support fast Cr diffusion that promotes the formation of a compact surface oxide film and grain boundary migration. Surprisingly, coherent twin boundaries (CTBs) are susceptible to intergranular oxidation and do not exhibit Cr diffusion or grain boundary migration. When CTBs are changed to transformed twin boundaries (TTBs) by cold work, they behave like RHABs. Finally, incoherent twin boundaries (ITBs) undergo intergranular oxidation with limited Cr depletion but no boundary migration beyond the oxide. The Cr diffusivity along grain boundary in this alloy is directly related to the density of coincident site in the grain boundary plane and determines the morphology of oxide formed near the grain boundary. CTBs are still highly resistant to stress corrosion cracking (SCC) due to the semi-coherent interface between the intergranular chromia and grain matrix. In contrast, the intergranular oxides formed along RHABs inherit highly-disordered boundary structure from the original grain boundaries and show much higher SCC susceptibility. The grain boundary structure dependence of SCC resistance should be understood from its effects on solute diffusivity, structure of intergranular oxide and the local stress-strain state.

Large Scale 3-Dimensional Atomistic Simulations of Screw Dislocations Interacting with Coherent Twin Boundaries in Al, Cu and Ni Under Uniaxial and Multiaxial Loading Conditions

Large Scale 3-Dimensional Atomistic Simulations of Screw Dislocations Interacting with Coherent Twin Boundaries in Al, Cu and Ni Under Uniaxial and Multiaxial Loading Conditions

Эволюция икосаэдрических частиц меди в процессе их роста при электрокристаллизации

Metal icosahedral microparticles with six fifth-order symmetry axes, a specific faceting and high catalytic activity, open up wide opportunities for their usage as catalysts capable of operating in fluidized bed reactors. In this paper, microparticles of copper of the icosahedral habitus about 15 microns in size in the form of truncated icosahedra have been grown for the first time by electrodeposition from sulfuric acid phate electrolyte. Such icosahedra are faceted by twelve {110} crystallographic planes and twenty {111} planes. Studies of the morphology of the surface of copper truncated icosahedra using metallography and electron microscopy have shown that in the process of growth they transform into perfect icosahedra faceted only by {111} planes. The structure of icosahedral copper particles has been studied in detail. Electron microscopic studies of the cross section of copper icosahedral particles show that they consist of sub-structural volume elements (fragments) that are separated from each other by twin and (or) dislocation boundaries. Often inside the fragments there are broken boundaries, near which extinction contours of dislocation origin are observed. In addition, coherent twin boundaries of the {111} <112> type are observed inside the fragments; they do not create long-range stress fields, twin layers, insertions and dislocations. Pentagonal pits appear on the surface of particles in the process of electrodeposition or etching, which indicate the presence of partial disclinations there. According to the results of experimental data, a scheme for the evolution of truncated icosahedra in the process of copper electrocrystallization into perfect icosahedra is proposed. Using the disclination approach, the density of the total free energy of a truncated and perfect icosahedron has been estimated and it is shown that the transformation of a truncated icosahedron into a perfect one is energetically justified.

Dynamic recrystallization in Monel400 Ni-Cu alloy: Mechanism and role of twinning

The role of twin boundaries (TBs) in the dynamic recrystallization (DRX) of Ni-30Cu (Monel400) was investigated through hot compression tests in the temperature interval of 750–1150 °C and strain rate of 0.01 s−1. Electron Back Scattered Diffraction (EBSD) analysis showed that DRX at 750 and 950 °C was limited to bulging and sparse nucleation on random high angle grain boundaries (HAGBs) while the primary coherent twin boundaries (TBs), with a length density of 55% in the starting microstructure, did not take part in the bulging process before conversion into HAGBs. The sluggish DRX was therefore attributed to the necessary conversion of primary TBs into HAGBs. The investigation also revealed that a new variant of TBs formed behind the protruding grain boundaries separated the bulges from the parent grains, indicating that unlike primary TBs, the newly formed TBs contributed to the occurrence of DRX. The formation of the new TBs was related to the dissociation of well-developed subgrains and the migration of random HAGBs. A mechanism for the occurrence of DRX in hot deformed Monel 400 is proposed that takes into account the contribution from the new TBs.

Coupling effect of twin boundary and void on the mechanical properties of bulk nanotwinned copper: an atomistic simulation

Molecular dynamics simulations are performed to analyze the uniaxial tensile deformation behaviors of nanotwinned copper bulk containing pre-existing multiple spherical/ellipsoidal voids. The effects of twin-boundary spacing, initial void size, void fraction, void shape, void orientation angle and loading orientation on the mechanical properties are examined by considering the stress-strain response and dislocation activity. Results illuminate that the coherent twin boundary can significantly enhance the strength of bulk copper. The critical stress for dislocation emission in the copper specimen depends mainly on the size of voids, which can influence the yield stress of nanotwinned copper. Less stress is required to emit dislocations from larger voids than smaller ones. In general, the larger initial void porosity is accompanied by smaller elastic modulus and critical stress. Furthermore, it is found that different dislocation patterns can be triggered under different loading orientations.

The role of slip transfer at grain boundaries in the propagation of microstructurally short fatigue cracks in Ni-based superalloys

Crack initiation and propagation under high-cycle fatigue conditions have been investigated for a polycrystalline Ni-based superalloy by in-situ synchrotron assisted diffraction and phase contrast tomography. The cracks nucleated along the longest coherent twin boundaries pre-existing on the specimen surface, that were well oriented for slip and that presented a large elastic incompatibility across them. Moreover, the propagation of microstructurally short cracks was found to be determined by the easy slip transfer paths across the pre-existing grain boundaries. This information can only be obtained by characterization techniques like the ones presented here that provide the full set of 3D microstructural information.

Steps and [formula omitted] secondary twinning associated with [formula omitted] twin in titanium

{112¯2} twinning commonly takes place in α-titanium (α-Ti). High-resolution transmission electron microscopies (HRTEM) explored various steps along {112¯2} coherent twin boundary. Topological model of {112¯2} twin revealed twinning disconnections (TDs) that are represented by (bi, ih{112¯2}) corresponding to a step height ih{112¯2} and a shear vector bi. Atomistic simulations were conducted to study the energies and kinetics of TDs. Combining microscopies and atomistic simulations, we concluded that (b3, 3h{112¯2}) is the elementary TD and (b1, h{112¯2}) is the reassembly TD. Steps observed in HRTEM thus can be treated as a reassembly of (b3, 3h{112¯2}) TDs and (b1, h{112¯2}) TDs. In addition, Electron Backscatter Diffraction (EBSD) maps revealed {112¯2}→{112¯1} double twins in α-Ti. Using two-dimensional and three-dimensional atomistic simulations, we demonstrated the nucleation of (b1, h{112¯2}) TD and {112¯2}→{112¯1} double twin through the interaction between basal <a> dislocation and {112¯2} twin. Our results enrich the understanding of {112¯2} twinning including TDs, steps, and {112¯1} secondary twins in hexagonal metals.

Serrated and stepped-like twin boundary of nano-sized extension twin in a deformed magnesium alloy

Twin boundary of a nano-sized {1 0 −1 2} extension twin far away from its tip has been experimentally characterized to be about 3.5° from the fully coherent twin boundary (CTB), which is the twinning plane of (1 0 −1 2)t//(1 0 −1 2)m. Subscripts ‘t’ and ‘m’ refer respectively to twin and its matrix. In atomic scale, the twin boundary consists of PtBm-CTB step, PtBm-BtPm serration, and BtPm-CTB-PtBm serration, where PtBm and BtPm refer respectively to (1 0 −1 0)t//(0 0 0 1)m and (0 0 0 1)t//(1 0 −1 0)m. The relationship between the deviation angle of a twin boundary from CTB and the structure of the twin boundary is deduced on the basis of an explicit geometric model. When the deviation angle is 3.5° as measured, the ratio of the average length of CTB terraces to the average height of PtBm-CTB steps is calculated to be 15.28, representing a twin boundary dominated by CTB with relatively small strain.

Structural defects and sp2 localization in CVD diamond

Transmission electron microscopy and energy loss near edge spectroscopy were used to characterize microstructural defects in polycrystalline CVD diamond films. Of particular interest here is the change in sp2 content catalyzed by structural defects such as twin boundaries, random grain boundaries, and dislocations. Changes in bonding configuration from sp3 to sp2 are known to have significant impact on many properties of interest in CVD diamond. I have shown here that coherent Σ = 3 twin boundaries do not lead to sp2 formation, while dislocations and random grain boundaries do. Another feature of interest here is the fivefold twining structure, which is typically found in CVD diamond. It comprises four Σ = 3 coherent twin boundaries, but topological constraint for filling the entire space induces the formation of low-angle grain boundaries and multiple stacking faults in one of the twin domains. I have shown that such defects are indeed present and primarily localized in one domain. The strain associated with these defects increases the localized sp2 content in the diamond.

The interaction and migration of deformation twin in an eutectic high-entropy alloy AlCoCrFeNi2.1

An eutectic high-entropy alloy consisting Al, Co, Cr, Fe and Ni elements was prepared by vacuum directional solidification technology. The alloy exhibits excellent comprehensive mechanical performance during tension at temperature range of 600–700 °C. The microstructure reveals the intersection of twin-twin is the prevailing deformation mechanism and the twins play a dual role in strengthening and toughening the alloy in the thermomechanical process. The deformation twin variants I and Π were formed by the edge dislocation 112¯ and the mixed dislocation 211¯ on the {111} crystal planes, respectively. Besides, the dislocation jogs and kinks caused by twin intersection on the slip planes can strengthen the alloy, which may contribute to the high strength (the tensile strengths at the 600° and 700° tensile tests are respectively780 MPa and 630 MPa.). Moreover, the coherent twin boundary migration has the function of coordinating deformation and contributes to the high ductility of the alloy.

Grain boundary and microstructure engineering of Inconel 690 cladding on stainless-steel 316L using electron-beam powder bed fusion additive manufacturing

This research explores the prospect of fabricating a face-centered cubic (fcc) Ni-base alloy cladding (Inconel 690) on an fcc Fe-base alloy (316 L stainless-steel) having improved mechanical properties and reduced sensitivity to corrosion through grain boundary and microstructure engineering concepts enabled by additive manufacturing (AM) utilizing electron-beam powder bed fusion (EPBF). The unique solidification and associated constitutional supercooling phenomena characteristic of EPBF promotes [100] textured and extended columnar grains having lower energy grain boundaries as opposed to random, high-angle grain boundaries, but no coherent {111} twin boundaries characteristic of conventional thermo-mechanically processed fcc metals and alloys, including Inconel 690 and 316 L stainless-steel. In addition to [100] textured grains, columnar grains were produced by EPBF fabrication of Inconel 690 claddings on 316 L stainless-steel substrates. Also, irregular 2–3 μm diameter, low energy subgrains were formed along with dislocation densities varying from 108 to 109 cm−2, and a homogeneous distribution of Cr23C6 precipitates. Precipitates were formed within the grains (with ∼3 μm interparticle spacing), but not in the subgrain or columnar grain boundaries. These inclusive, hierarchical microstructures produced a tensile yield strength of 0.527 GPa, elongation of 21%, and Vickers microindentation hardness of 2.33 GPa for the Inconel 690 cladding in contrast to a tensile yield strength of 0.327 GPa, elongation of 53%, and Vickers microindentation hardness of 1.78 GPa, respectively for the wrought 316 L stainless-steel substrate. Aging of both the Inconel 690 cladding and the 316 L stainless-steel substrate at 685 °C for 50 h precipitated Cr23C6 carbides in the Inconel 690 columnar grain boundaries, but not in the low-angle (and low energy) subgrain boundaries. In contrast, Cr23C6 carbides precipitated in the 316 L stainless-steel grain boundaries, but not in the low energy coherent {111} twin boundaries. Consequently, the Inconel 690 subgrain boundaries essentially serve as surrogates for coherent twin boundaries with regard to avoiding carbide precipitation and corrosion sensitization.