Small-angle Grain Boundaries Research Articles

Misorientation increase of small-angle grain boundaries during directional solidification of silicon

In situ observation of the solidification process of Si combined with ex situ characterization revealed that small-angle grain boundaries (SAGBs) increase misorientation through two paths. First, coalescence with other SAGBs leads to a boost in misorientation and SAGBs tend to coalesce during solidification. Second, the absorption of individual lattice dislocations also increases the misorientation of an SAGB. Three possible mechanisms behind dislocation absorption by SAGBs have been proposed and are discussed in this paper. Among these mechanisms, the incorporation of same-sign lattice dislocations is regarded as the prominent mechanism when misorientation increases during solidification.

Phase-Field Crystal Studies on Grain Boundary Migration, Dislocation Behaviors, and Topological Transition under Tension of Square Polycrystals

In this paper, the tensile deformation behaviors of polycrystals after relaxation were studied using the phase-field-crystal (PFC) method. Here, the free energy density map characterized the 2D energy distribution of atomic configuration effectively. The application of the Read–Shockley equation distinguished high-energy grain boundary (HEGB) and low-energy grain boundary (LEGB) in large-angle grain boundary (LAGB), and they demonstrated different migration behaviors at the early and later stages. The behaviors of small-angle grain boundary (SAGB), including its migration and grains’ rotation, were also studied. Two different mechanisms of dislocation emission and absorption were explored, which demonstrates the possibility of dislocation elevating interfacial energy. The simulated results on the topological transition of grain boundaries prompted us to propose the thinking about the applications of the Neumann–Mullins law and Euler formula.

Creep-fatigue voids and sub-grain boundaries assisted crack initiation for titanium alloy in VHCF regime with high mean stress at 400°C

Very high cycle fatigue failure of bimodal titanium alloy at 400 °C and high mean stress has been investigated. Many circular creep-fatigue voids (CFVs) were found underneath the crack initiation zone, which were distributed in the β transformed matrix and near the grain boundaries (GBs) of primary α grains. By the energy dispersive spectroscopy (EDS) element mapping, the oxygen element was found to be enriched around the CFVs. In addition, many sub-GBs of small-angle GBs could be observed inside the primary α grains. Based on these, a new mechanism of CFVs and sub-GBs assisted crack initiation and early propagation in bimodal titanium alloys was proposed in very high cycle fatigue regime.

Three-Dimensional Morphology and Analysis of Widmanstätten Sideplates Ferrite

The three-dimensional (3D) morphology and crystal structure of Widmanstätten sideplate ferrite were simulated using a focused ion beam (FIB) scanning electron microscope equipped with electron backscatter diffraction (EBSD). The primary Widmanstätten sideplates nucleated and grew directly at the austenite grain boundary (GB). A certain included angle between the sideplates and the austenite GB was observed. The sideplates grew approximately parallel to the grain, and were separated by a small-angle GB. The primary Widmanstätten sideplates are best described as “∃” shaped, with a long intermediate ferrite strip. The interface with the austenite GB was smooth and flat, and the sideplate surface contained pits and holes. The secondary Widmanstätten sideplates nucleated and grew on the surface of the proeutectoid GB ferrite, with the sideplates and GB ferrite perpendicular to each other. Sideplates parallel to one another grew into the grain, and were separated by small-angle GB. The 3D morphology was distinguished by its “comb” shape. The sideplates’ tail was clustered and its front end remained sharp. The contact side of the GB ferrite was smooth and flat. The surface contained several uneven pits and defects.

Grain boundary segregation transitions and critical phenomena in binary regular solutions: A systematics of complexion diagrams with universal characters

A systematics of grain boundary (GB) segregation transitions and critical phenomena has been derived to expand the classical GB segregation theory. Using twist GBs as an example, this study uncovers when GB layering vs. prewetting transitions should occur and how they are related to one another. Moreover, a novel descriptor, normalized segregation strength (ϕseg), is introduced. It can represent several factors that control GB segregation, including strain and bond energies, as well as misorientation for small-angle GBs (in a mean-field approximation), which had to be treated separately in prior models. In a strong segregation system with a large ϕseg, first-order layering transitions occur at low temperatures and become continuous above GB roughing temperatures. With reducing ϕseg, the layering transitions gradually merge and finally lump into prewetting transitions without quantized layer numbers, akin to Cahn's critical-point wetting model. Furthermore, GB complexion diagrams with universal characters are constructed as the GB counterpart to the classical exemplar of Pelton-Thompson regular-solution binary bulk phase diagrams.

Effect of machining on performance enhancement of superficial layer of high-strength alloy steel

The microstructure of machined superficial layer significantly affects the mechanical properties and service life of mechanical components. Herein, to obtain a machined surface with excellent properties, a comprehensive application of theoretical calculations and test analysis research methods was conducted to investigate the influence mechanism of machining on performance and quality of machined superficial layer of high-strength alloy steel during machining, and reveal the intrinsic correlation between microstructure evolution, quality and performance of machined superficial layer. The results indicate that the machining can achieve gradient microstructure on the machined surface, which can improve the performance of machined surface greatly. From the surface to the inside region, the layers are respectively the recrystallized layer where compact nano-sized equiaxed grains are located, the rheological layer with clusters of high-density sub-grain structures, and the distorted layer with residual distorted grains. The maximum strain of superficial layer in the machined state is 2.81 times that of the original state. The grains are evidently refined and the grain size is reduced by 49.03%. The dislocation density is enhanced by 100.69%, while the number of small-angle grain boundaries (SAGB) and other substructures increase sharply, being 5.26 times that in the original state. By performance testing, the machining substantially enhances the hardness of the superficial layer of high-strength alloy steel with certain rise in toughness simultaneously, which is exactly the result of the combined effect of fine-grain strengthening and dislocation strengthening caused by the machining process above.

Influence of interfacial structure on propagating direction of small-angle grain boundaries during directional solidification of multicrystalline silicon

Small-angle grain boundary (SAGB) growth in multicrystalline Si was observed in situ during directional solidification, and the misorientation and boundary structure were analyzed by electron backscatter diffraction and high-resolution transmission electron microscopy. It was found that SAGBs change their orientation in order to reduce the number of grain boundary dislocations and thus the energy. This suggests that grain boundary dislocations have an influence on SAGB behavior during solidification.

Effect of misorientation angle of grain boundary on the interaction with Σ3 boundary at crystal/melt interface of multicrystalline silicon

Interactions between Σ3 grain boundaries (GBs), small-angle GBs (SAGBs), and general GBs at a silicon crystal/melt interface are studied by in situ observation during directional solidification. The interactions exhibit a dependence on the misorientation angle. SAGBs can propagate through Σ3 GBs, but this behavior transitions into coalescence with Σ3s when the misorientation approaches the limit of a SAGB, i.e. 15°. On the other hand, general GBs show the ability to terminate Σ3 GBs continuously. The present findings suggest that the presence of intrinsic dislocations in GBs plays a role in GB interactions.

Microstructure and texture evolution near the adiabatic shear band (ASB) in TC17 Titanium alloy with starting equiaxed microstructure studied by EBSD

The microstructure and texture evolution near the adiabatic shear band (ASB) in TC17 Titanium alloy, during hot compression, were studied by electron back-scattered diffraction (EBSD). The ASB was induced in the bulk cylindrical TC17 Titanium alloy with an initially equiaxed microstructure, by hot compression with a relatively low strain rate. When the equiaxed-microstructure TC17 Titanium alloy was compressed at 600 °C, 700 °C, and 850 °C, two kinds of ASB were observed: D-ASB, which is almost parallel to the diagonal of the longitudinal section, and H-ASB, which is almost parallel to the horizontal of the longitudinal section. Dynamic recrystallization took place in β phase grains at the ASB center, while dynamic recovery dominated at the transition region. The large angle grain boundaries (LAGBs) were mainly concentrated in β phase, while the small angle grain boundaries (SAGBs) were mainly concentrated in α phase distributed along the ASB direction. Meanwhile, both α and β phase grain sizes slightly increased with increasing hot compression temperature and deformation, except for the alloy compressed at 850 °C with 70% deformation due to the phase transformation. At the ASB center, prismatic textures existed in α phase grains except for the basal textures in the alloy deformed at 600 °C. In β phase grains, the {001} planes were always normal to CD, while the {111} planes were normal to CD in the initial microstructure of the TC17 alloy. Furthermore, in β phase grains, the cubic texture {100} 〈001〉 dominated in the TC17 alloy deformed at 600 °C, 700 °C, and 850 °C. With increasing compression temperature, the crystal plane parallel to the ASB direction in α phase grains changed from {0001} to {112¯0} while it was stable in β phase grains, which is not beneficial to the mechanical performance of this alloy.

Influence of cube texture development on magnetic properties of Ni–5 at.%W alloy substrates

The metallic Ni–5 at.%W (Ni5W) substrate with sharp cube texture is found as one of the promising candidates used for fabrication of coated conductors. However, the alternating current (AC) loss, partly contributed by the ferromagnetic Ni5W substrate is unavoidable. In this work, we analyzed the evolution of magnetic properties of Ni5W substrates via the cube texture development. The results revealed that the cube texture has an important impact related to size advantage during recrystallization, leading towards the consumption of the deformation and other orientational grains, and hence giving rise to form a sharp cube texture. Increasing the annealing temperature, the grain size is found to be increased. In the fully developed cube-textured substrate, the grain boundary was replaced by small angle grain boundaries (SAGBs). Also the magnetic domain structure was transformed from 180°-structure to a branched one with the increasing annealing temperature. This transformation is dependent on the increase of grain size, particularly, the SAGBs plays an important role in the formation of perfect domain structure. Increased coercivity and hysteresis loss in the fully textured substrate can be attributed towards the ideal cube texture and perfect complex domain structure. Therefore, a potential solution has been proposed to resolve this limitation of the sharp cube texture and increased hysteresis loss in Ni5W substrates.

Origin of small-angle grain boundaries during directional solidification in multicrystalline silicon

The formation of small-angle grain boundaries (SAGBs) during directional solidification of multicrystalline silicon was directly observed in situ. SAGBs were confirmed by preferential etching due to the aggregation of dislocations at the solid/melt interface. Dislocations self-arrange by polygonization into an energetically favorable SAGB configuration. The misorientation of the SAGBs increases during crystal growth, possibly through the continual incorporation of dislocations. The present results suggest that there is considerable dislocation motion at the solid/melt interface and polygonization can occur during solidification.

Chemical Nanoanalyses at Grain Boundaries By Joint Use of Scanning Transmission Electron Microscopy and Atom Probe Tomography

Polycrystalline materials with grain boundaries (GBs), involving excess free energy because of their structural imperfection, can reduce their energy by the nanoscopic structural changes of the GBs via impurity segregation. Those local changes at GBs can stabilize non-equilibrium nanostructures, resulting in the drastic change in the macroscopic properties of those materials. The mechanism of GB segregation is, however, far from being understood due to difficulties in characterizing both crystallographic and chemical properties of the same GB at atomistic levels. We have therefore developed an analytical method to determine the impurity segregation ability on the same GB at the same nanoscopic location by a joint use of scanning transmission electron microscopy (STEM) and atom probe tomography (APT) combined with ab-initio calculations, and discussed the segregation mechanism in terms of bond distortions at the GB. In the present work, we examined the GBs in Si ingots used for solar cells. They have serious impacts on the solar cell efficiency via the segregation of detrimental impurity atoms, such as oxygen and transition metals introduced inevitably during crystal growth and cell processing, depending on their structural condition at those GBs. Accordingly, precise understanding of the segregation mechanism of impurity atoms is one important issue to produce cost-effective solar cells by engineering the structural condition of impurity atoms segregating at GBs. Three-dimensional distribution of impurity atoms was systematically determined at the typical large-angle GBs (i.e., Σ3{111}, Σ5{013}, Σ9{221}, Σ9{114}, Σ9{111}/{115}, and Σ27{552}) [1-3] and small-angle GBs [3-5] by APT with a low impurity detection limit (0.01 at.% on a GB plane) simultaneously with a high spatial resolution (about 0.4 nm), and it was correlated with the atomic stresses around the GBs estimated by ab-initio calculations based on atomic-resolution scanning TEM data (for large-angle GBs [2]) and by calculations with the elastic theory based on dark-field TEM data (for small-angle GBs [4]). It was hypothesized that oxygen atoms segregate at the bond-centered positions under tensile stresses above about 2 GPa so as to attain more stable bonding network by reducing the local stresses [6]. The number of segregating atoms per unit GB area (N GB ) is in proportion to both the number of the stressed positions per unit GB area (n bc) and the average concentration of oxygen atoms around the GB ([Oi]) with N GB ~ 50n bc[Oi]. This indicates that the probability of oxygen atoms at the segregation positions would be, on average, fifty times larger than in bond-centered positions in defect-free regions. This nanoscopic finding may provide a general guidance to control compositions and band structures at GBs via oxygen segregation.

Preferential Pt Nanocluster Seeding at Grain Boundary Dislocations in Polycrystalline Monolayer MoS2.

We show that Pt nanoclusters preferentially nucleate along the grain boundaries (GBs) in polycrystalline MoS2 monolayer films, with dislocations acting as the seed site. Atomic resolution studies by aberration-corrected annular dark-field scanning transmission electron microscopy reveal periodic spacing of Pt nanoclusters with dependence on GB tilt angles and random spacings for the antiphase boundaries ( i.e., 60°). Individual Pt atoms are imaged within the dislocation core sections of the GB region, with various positions observed, including both the substitutional sites of Mo and the hollow center of the octahedral ring. The evolution from single atoms or small few atom clusters to nanosized particles of Pt is examined at the atomic level to gain a deep understanding of the pathways of Pt seed nucleation and growth at the GB. Density functional theory calculations confirm the energetic advantage of trapping Pt at dislocations on both the antiphase boundary and the small-angle GB rather than on the pristine lattice. The selective decoration of GBs by Pt nanoparticles also has a beneficial use to easily identify GB areas during microscopic-scale observations and track long-range nanoscale variances of GBs with spatial detail not easy to achieve using other methods. We show that GBs have nanoscale meandering across micron-scale distances with no strong preference for specific lattice directions across macroscopic ranges.

Strain Characterization and Microstructure Evolution Under Deformation in 2060 Alloy

A new method of DIC combined with EBSD is developed for the characterization of strain and microstructure evolution during bending. The traditional microhardness point and DIC methods are used to study the microstructure evolution in 2060 alloy during bending; the interested area suffers under tensile stress, the microstructure evolution is collected by SEM, EBSD, digital image correlation (DIC) method during bending. The results shows that the DIC method can both realize the strain tensor characterization of the interested area, and can also express the local strain tensor in the micro-area even more. The degree of grain division in the process of deformation is related to the strain in this region; the grains have larger strain of small angle grain boundary (SLGBs), which results in a new micro-organizational structure. The misorientation is smaller with larger strain degree while the misorientation is larger with smaller strain.

Effect of annealing on microstructure and thermoelectric properties of hot-extruded Bi–Sb–Te bulk materials

The effect of annealing on the microstructure, thermoelectric properties and hardness of the hot-extruded Bi–Sb–Te materials has been investigated systematically to optimize their thermoelectric and mechanical properties. The mechanically alloyed powder was consolidated by hot extrusion at either 340 or 400 °C, followed by annealing in a temperature range of 260–400 °C. The microstructure of the annealed samples contained submicron grains with preferred (001) texture. As annealing temperature increased, the small-angle grain boundaries (SAGBs) increased because the increased amount of Te-rich and Sb-rich phases inhibits the movements of dislocations and SAGBs. The submicron microstructure led to a low thermal conductivity, for example, ~ 0.9 W/mK after annealing at TA ≥ 380 °C. The Seebeck coefficient highly depended on carrier mobility in addition to carrier concentration. For the extruded samples prepared at a lower extrusion temperature of 340 °C, the mobility increased significantly after annealing, resulting in great enhancements in the Seebeck coefficient and electrical conductivity. A peak ZT value of 0.94 and high hardness were simultaneously obtained under the conditions of hot extrusion at 340 °C and annealing at 380 °C. It seems that the combination of low-temperature extrusion and high-temperature annealing is an effective route to prepare high-performance Bi2Te3-based materials.

Injection intensity-dependent recombination at various grain boundary types in multicrystalline silicon solar cells

If the ratio of two open circuit photoluminescence (Voc-PL) images taken at two different light intensities is displayed, some grain boundaries (GBs) may show up as bright lines. This indicates that these special GBs show distinct injection intensity-dependent recombination properties. It will be shown here that this results in an apparent ideality factor of the light emission smaller than unity. The effect is reproduced with numerical device simulations using a usual distribution of defects in the band gap along grain boundaries. Quantitative imaging of this apparent luminescence ideality factor by PL imaging is complicated by the lateral horizontal balancing currents flowing at open circuit. The local voltage response of an inhomogeneous solar cell at different injection levels under open circuit is modelled by Griddler simulations, based on PL investigations of this cell. The evaluation of Voc-PL images at different illumination intensities allows us to conclude that the apparent luminescence ideality factor at the special GBs is about 0.89, whereas in the other regions it is between 0.94 and 0.95. Reverse bias electroluminescence showed no pre-breakdown sites, and hyperspectral PL imaging showed only in one of the investigated GBs particular defect luminescence. TEM investigations of two GBs, one showing distinct injection intensity-dependent recombination and the other one showing none, revealed that the investigated special GB is a large-angle GB whereas the GB not showing this effect is a small-angle GB.

In situ observation of interaction between grain boundaries during directional solidification of Si

The interaction between grain boundaries (GBs) at a silicon crystal/melt interface was studied using in situ observations during directional solidification. Two small-angle GBs (SAGBs) interact with each other to produce a new SAGB with increased misorientaion. However, the interaction between Σ3 GBs and SAGBs reveals a different phenomenon. The extending directions and misorientations of the SAGBs and Σ3 GBs show no change before and after the convergence of these two planar defects at the crystal/melt interface.

Preparation of well-distributed titania nanopillar arrays on Ti6Al4V surface by induction heating for enhancing osteogenic differentiation of stem cells

Great effort has recently been devoted to the preparation of nanoscale surfaces on titanium-based implants to achieve clinically fast osteoinduction and osseointegration, which relies on the unique characteristics of the nanostructure. In this work, we used induction heating treatment (IHT) as a rapid oxidation method to fabricate a porous nanoscale oxide layer on the Ti6Al4V surface for better medical application. Well-distributed vertical nanopillars were yielded by IHT for 20–35 s on the alloy surface. The composition of the oxides contained rutile/anatase TiO2 and a small amount of Al2O3 between the TiO2 grain boundaries (GBs). This technology resulted in a reduction and subsequent increase of surface roughness of 26–32 nm when upregulating the heating time, followed by the successive enhancement of the thickness, wettability and adhesion strength of the oxidation layer to the matrix. The surface hardness also distinctly rose to 554 HV in the IHT-35 s group compared with the 350 HV of bare Ti6Al4V. The massive small-angle GBs in the bare alloy promoted the formation of nanosized oxide crystallites. The grain refinement and deformation texture reduction further improved the mechanical properties of the matrix after IHT. Moreover, in vitro experiments on a mesenchymal stem cell (BMSC) culture derived from human bone marrow for 1–7 days indicated that the nanoscale layers did not cause cytotoxicity, and facilitated cell differentiation in osteoblasts by enhancing the gene and osteogenesis-related protein expressions after 1–3 weeks of culturing. The increase of the IHT time slightly advanced the BMSC proliferation and differentiation, especially during long-term culture. Our findings provide strong evidence that IHT oxidation technology is a novel nanosurface modification technology, which is potentially promising for further clinical development.

Jointed magnetic skyrmion lattices at a small-angle grain boundary directly visualized by advanced electron microscopy.

The interactions between magnetic skyrmions and structural defects, such as edges, dislocations, and grain boundaries (GBs), which are all considered as topological defects, will be important issues when magnetic skyrmions are utilized for future memory device applications. To investigate such interactions, simultaneous visualization of magnetic skyrmions and structural defects at high spatial resolution, which is not feasible by conventional techniques, is essential. Here, taking advantages of aberration-corrected differential phase-contrast scanning transmission electron microscopy, we investigate the interaction of magnetic skyrmions with a small-angle GB in a thin film of FeGe1−xSix. We found that the magnetic skyrmions and the small-angle GB can coexist each other, but a domain boundary (DB) was formed in the skyrmion lattice along the small-angle GB. At the core of the DB, unexpectedly deformed magnetic skrymions, which appear to be created by joining two portions of magnetic skyrmions in the adjacent lattices, were formed to effectively compensate misorientations between the two adjacent magnetic skyrmion lattices. These observations strongly suggest the flexible nature of individual magnetic skyrmions, and also the significance of defect engineering for future device applications.

The formation of H bubbles at small-angle tilt grain boundaries in W films.

The accumulation of H at the small-angle tilt grain boundary (GB) in the W(001) surface is investigated, on the basis of the first-principles calculations. By exploring the solution and diffusion behaviors of H at the GB, we find that the small-angle GB can capture the H atoms nearby, serving as a nucleation site of H bubbles. With the increasing number of trapped H atoms, the GB expands gradually, and the GBs can be unripped with an areal density of H up to 5.0 × 1015 H atoms per cm2, leading to the formation of H bubbles. Moreover, H2 molecules are observed, when the areal density of H atoms in GB is over 6.6 × 1015 atoms per cm2. According to our calculations, we propose a possible formation mechanism of H bubbles observed in the experiment, which is valuable for improving the service performance of W as a plasma-facing material in nuclear fusion reactors.