Grain Boundary Character Research Articles

Effect of aging time on precipitation behavior, mechanical and corrosion properties of a novel Al-Zn-Mg-Sc-Zr alloy

The precipitation behavior, mechanical properties and corrosion resistance of a novel Al—Zn—Mg—Sc—Zr alloy aged at different time were studied by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), tensile tests, potentiodynamic polarization and electrochemical impedance spectroscopy. The results revealed that with increasing aging time at 120 °C, the hardness and tensile strength of the alloy increased rapidly at first and then slightly decreased. The resistance of exfoliation corrosion (EXCO) and intergranular corrosion (IGC) increased gradually with increasing aging time. The same trend of corrosion properties was demonstrated by electrochemical polarization curves and EIS test. The characteristics of grain boundary precipitates and precipitate free zone (PFZ) had a significant influence on the mechanical and corrosion behaviors of the studied alloy. On the basis of TEM observations, the size of grain boundary precipitates and the width of PFZ became larger, and the distributed spacing of grain boundary precipitates was enhanced with increasing aging time.

Capillarity-driven shrinkage of grains with tilt and mixed boundaries studied by molecular dynamics

Molecular dynamics simulations of isolated cylindrical grains in aluminum shrinking under capillary forces were carried out to systematically study the effect of grain boundary character on grain shrinkage behavior. Grains with pure tilt and various mixed tilt-twist 14.25°〈100〉 boundaries were examined. The simulation results confirmed that the shrinking grains with pure tilt boundary simultaneously rotated towards higher misorientations. However, the results also showed that even a minor change of the grain boundary character from pure tilt to mixed tilt-twist is enough to avoid grain rotation. The observed kinetics of the shrinking grains was strongly dependent on the character of grain boundaries. The mobility of the mixed boundaries was estimated to be much higher than that of the pure tilt boundary. Also, the shrinkage kinetics of computed grains was found to vary with a decreasing grain size. The advanced stage of shrinkage was characterized by the stagnation or decrease of the rate of grain area change. This behavior of small grains was attributed to a lack of mechanisms for the elimination of the dislocations (boundary structural units) in the grain boundary. It is hypothesized that such shrinkage stagnation of nanosized grains (≤6 nm) can contribute to the thermal stability of nanocrystalline materials.

Effects of grain size of AISI 304 on the machining performances in micro electrical discharge machining

The material removal process of micro electrical discharge machining is based on the instantaneous ultra-high temperature generated by a series of repetitive discharge pulses. Due to the size effects, the polycrystal cannot be considered as continuous and homogeneous material when machining is in micron scale, and the effects of material microstructure should not be neglected. In this article, the thermoelectric characteristics of grain and grain boundary are discussed, and the influence of grain size on the machining performances in micro electrical discharge machining is researched. Two kinds of austenitic stainless steels (AISI 304) which are different in grain size are chosen as the workpieces in experiments. It is verified by both theory models and experimental results that the smaller the grain size, the higher the material removal rate, under the same discharge conditions. Both thermal conductivity and melting point of the grain boundary are lower than those of the grain because of the grain boundary segregation. The effective thermal conductivity and local effective melting point of polycrystalline materials vary with their grain sizes since the grain boundary volume fractions change. As a consequence, the material removal rate of micro electrical discharge machining has direct relationship with grain size of the workpiece.

Grain-boundary character distribution and correlations with electrical and optoelectronic properties of CuInSe2 thin films

Thin-film solar cells based on polycrystalline Cu(In,Ga)Se2 absorbers exhibit record conversion efficiencies of up to 22.6%. There is still a lack of a quantitative connection between the grain-boundary character distribution (GBCD) and the corresponding electrical and optoelectronic properties. The present work uses microstructural data from a CuInSe2 thin film acquired by electron backscatter diffraction (EBSD) to evaluate the GBCD. The most prominent features of the GBCD of CuInSe2 are Σ3 twin boundaries and the Σ9 and Σ27a symmetric tilt grain boundaries. Moreover, combining EBSD with electron-beam-induced current and cathodoluminescence (measurements on the same identical area) on a CuInSe2/Mo/glass stack provide the means to relate the grain-boundary character with the corresponding electrical and optoelectronic signals across the grain boundary. In part, determining this relationship is accomplished by means of correlation analysis using measurement data from more than 100 grain boundaries. However, the crystallographic, electrical and optoelectronic data showed no strong correlations, which is attributed to atomic reconstruction found in atomic planes adjacent to planar defects in polycrystalline CuInSe2 thin films and corresponding reductions of excess charge densities at these defects.

Central role of TiO2 anatase grain boundaries on resistivity of CaCu3Ti4O12-based materials probed by Raman spectroscopy

This study focuses on characterization and control of grain boundaries to enhance the properties of CaCu3Ti4O12 (CCTO) ceramics capacitors for industrial applications. A novel factor deals with TiO2 anatase revealed by Raman scattering in grain boundaries, found as a dominant parameter of largest sample resistivity, consistent with higher grain boundary resistivity and higher breakdown voltage. Four selected samples of CCTO-based compositions showing very different properties in terms of permittivity ranging from 1000 to 684 000 measured at 1 kHz, capacitance of grain boundaries ranging from 8 10−10 to 4.5 10−7 F cm−1, grain boundary resistivity ranging from 193 to 30,000,000 Ω cm and sample resistivity extending from 450 to 1011 Ω cm. The relationship between permittivity weighted by grain size and capacitance of grain boundaries confirms the internal barrier layer capacitance model over 5 orders of magnitude.

Dielectric and impedance characteristics of Na1/3Ca1/3Y1/3Cu3Ti4O12 ceramic prepared by solid-state reaction method

Dielectric response of Na1/3Ca1/3Y1/3Cu3Ti4O12 ceramic, fabricated via a conventional solid-state method, has been investigated in the frequency range 100–2 MHz at temperatures from −80 to 200 °C. Single phase of Na1/3Ca1/3Y1/3Cu3Ti4O12 with cubic perovskite-related structure was confirmed by X-ray diffraction. High dielectric permittivity (>3 × 103) and low dielectric loss (~0.08) were observed at room temperature. The dielectric relaxation peaks of $$\varepsilon^{\prime \prime }$$ shifts to higher frequencies, indicating a thermally activated relaxation process. The activation energy for the dc conduction process (0.501 eV) is comparable to that for conduction at the grain boundaries (0.514 eV), indicating the dc conduction process is closely associated with the characteristics of grain boundaries in Na1/3Ca1/3Y1/3Cu3Ti4O12 ceramic.

Effect of grain boundary character of multicrystalline Si on external and internal (phosphorus) gettering of impurities

AbstractWe investigated the effect of the grain boundary (GB) character of multicrystalline Si (mc‐Si) on the efficiency of external and internal gettering of impurities during phosphorus diffusion gettering (PDG). We utilized seed crystals with an artificially designed GB configuration to grow mc‐Si ingots with different artificial GB characters. PDG combined with an originally developed multiple‐cycle gettering technique at low temperature was introduced on intentionally Fe‐contaminated mc‐Si samples to enhance external and internal gettering. A significant positive PDG effect was observed after PDG combined with the multiple‐cycle technique, as evidenced by the increase in lifetimes after PDG. A bright cloud‐like photoluminescence signal around contaminated GBs was observed for artificial Σ5‐GBs and tilt‐GBs after PDG, suggesting the enhancement of the internal gettering efficiency by leaving a cleaner area around the GBs. This result suggests the importance of the control of crystal defect character as well as impurities in mc‐Si ingots, which could strongly affect the PDG efficiency. Copyright © 2016 John Wiley & Sons, Ltd.

Implementing Transmission Electron Backscatter Diffraction for Atom Probe Tomography

There are advantages to performing transmission electron backscattering diffraction (tEBSD) in conjunction with focused ion beam-based specimen preparation for atom probe tomography (APT). Although tEBSD allows users to identify the position and character of grain boundaries, which can then be combined with APT to provide full chemical and orientation characterization of grain boundaries, tEBSD can also provide imaging information that improves the APT specimen preparation process by insuring proper placement of the targeted grain boundary within an APT specimen. In this report we discuss sample tilt angles, ion beam milling energies, and other considerations to optimize Kikuchi diffraction pattern quality for the APT specimen geometry. Coordinated specimen preparation and analysis of a grain boundary in a Ni-based Inconel 600 alloy is used to illustrate the approach revealing a 50° misorientation and trace element segregation to the grain boundary.

Twin boundary activated α → ω phase transformation in titanium under shock compression

The role of grain boundary structures on the shock response of hexagonal-close-packed (hcp) metals is little understood. We use molecular dynamics simulations to investigate deformation mechanisms in shock compressed Ti bicrystals with three types of grain boundary (GB) microstructure. Our results show the shock response of phase Ti polycrystals are influenced by the GB characteristics, i.e., elastic shock wave induced inelastic deformation occurs on both sides of the {101¯2} coherent twin boundaries (CTBs) but only on one-side of the symmetric incoherent twin boundaries (ITB) or {12¯10} tilt grain boundaries regions. In particular, we find that the elastic shock wave can readily trigger the α→ω transformation at {101¯2} CTBs but not the other two GBs, and the α→ω transformation at CTBs leads to considerable wave attenuation (i.e., the elastic precursor decay). Combined with first principle calculations, we find that CTBs can facilitate the overcoming of the energy barrier for the α→ω transformation. Our findings have the potential to influence interface engineering and materials design under extreme conditions.

High resolution characterization of grain boundaries in Cu2ZnSnSe4 solar cells synthesized by nanoparticle selenization

We report a novel fabrication and multiple high resolution characterizations of Cu2ZnSnSe4 solar cells. The fabrication is based on nanoparticle precursor production by liquid-phase pulsed laser ablation (LP-PLA), electrophoretic deposition of precursor thin film under ambient condition, and selenization. Columnar grain boundaries are studied using spatially mapped Raman spectroscopy and scanning probe microscopy for their compositional and electrical properties. We observe high electrical conductivity near columnar grain boundaries, and propose poor Cu composition as the cause of the enhancement of the collection of minority carriers. We also study horizontal grain boundaries using cross-sectional scanning transmission electron microscopy (STEM). Combining with the device I-V and quantum efficiency, we suggest that the horizontal grain boundaries act as barriers to the transportation of minority carriers. CZTSe cells with efficiencies of 4.77% and 2.20% are compared.

Shear-coupled grain boundary migration assisted by unusual atomic shuffling.

Shear-coupled grain boundary (GB) migration can be an efficacious mechanism to accommodate plastic deformation when the grain size of polycrystalline materials goes small. Nevertheless, how this kind of GB motion comes into play at the atomic level has not been fully revealed. Here, we have investigated the shear-coupled migration (SCM) of typical [100] group symmetrical tilt GBs in bcc W using atomistic simulations. Depending on GB character, the SCM is found to proceed via dislocation slipping in the 〈100〉 or 〈110〉 mode with striking shear strength difference between them. We demonstrate that there exists an unusual atomic shuffling along the tilt axis, which greatly assists SCM to operate in the easier 〈110〉 mode instead of the 〈100〉 one. The present results highlight the significant role of GB character in the atomistic SCM process and contribute to the future design and fabrication of high-performance materials in GB engineering.

Visualization of Grain Structure and Boundaries of Polycrystalline Graphene and Two-Dimensional Materials by Epitaxial Growth of Transition Metal Dichalcogenides.

The presence of grain boundaries in two-dimensional (2D) materials is known to greatly affect their physical, electrical, and chemical properties. Given the difficulty in growing perfect large single-crystals of 2D materials, revealing the presence and characteristics of grain boundaries becomes an important issue for practical applications. Here, we present a method to visualize the grain structure and boundaries of 2D materials by epitaxially growing transition metal dichalcogenides (TMDCs) over them. Triangular single-crystals of molybdenum disulfide (MoS2) epitaxially grown on the surface of graphene allowed us to determine the orientation and size of the graphene grains. Grain boundaries in the polycrystalline graphene were also visualized reflecting their higher chemical reactivity than the basal plane. The method was successfully applied to graphene field-effect transistors, revealing the actual grain structures of the graphene channels. Moreover, we demonstrate that this method can be extended to determine the grain structure of other 2D materials, such as tungsten disulfide (WS2). Our visualization method based on van der Waals epitaxy can offer a facile and large-scale labeling technique to investigate the grain structures of various 2D materials, and it will also contribute to understand the relationship between their grain structure and physical properties.

Influence of Grain Boundary Character and Annealing Time on Segregation in Commercially Pure Nickel

Commercially pure nickel (Ni) was thermomechanically processed to promote an increase in Σ3 special grain boundaries. Engineering the character and chemistry of Σ3 grain boundaries in polycrystalline materials can help in improving physical, chemical, and mechanical properties leading to improved performance. Type-specific grain boundaries (special and random) were characterized using electron backscatter diffraction and the segregation behavior of elements such as Si, Al, C, O, P, Cr, Mg, Mn, B, and Fe, at the atomic level, was studied as a function of grain boundary character using atom probe tomography. These results showed that the random grain boundaries were enriched with impurities to include metal oxides, while Σ3 special grain boundaries showed little to no impurities at the grain boundaries. In addition, the influence of annealing time on the concentration of segregants on random grain boundaries was analyzed and showed clear evidence of increased concentration of segregants as annealing time was increased.

Characterization of grain boundaries in CdTe polycrystalline films

Characterization of grain boundaries in CdTe polycrystalline films

Precipitation phenomena in ultrafine grained Al–Mg–Si alloy with heterogeneous microstructure

In this study we discuss the effect of heterogeneous microstructure obtained by hydrostatic extrusion on precipitation phenomena in commercially available Al–Mg–Si alloy. After deformation double fibrous texture is present with fiber axes parallel to extrusion direction. Grains with dominance of low angle grain boundaries with <001> orientation and high angle grain boundaries with <111> orientation were found. HRTEM investigation revealed that the precipitation sequence is different than in conventional coarse grained materials and depends on local character of grain boundaries. The reason for this occurrence is the series of thermal shocks during deformation, which allows firstly Mg–Si co-clusters to be formed and secondly incorporates Cu atoms into them hindering the growth of copper free β″. Microhardness measurements for different aging time show that up to 4 h the precipitation hardening effect is stronger than weakening by dislocation annihilation or grain growth. After 14 h, precipitation condition in UFG samples remain almost unchanged but mechanical properties decrease.

B11-O-01Atomic-Scale STEM Characterization of Grain Boundaries in Oxides

B11-O-01Atomic-Scale STEM Characterization of Grain Boundaries in Oxides

Three-dimensional characterization of grain boundaries in pure nickel by serial sectioningviamechanical polishing

Five macroscopic boundary parameters can be extracted from three-dimensional orientation maps. Serial sectioning, which includes consecutive steps of material removal, and electron backscatter diffraction (EBSD) measurement were employed to extract a stack of two-dimensional sections of a pure nickel sample. The EBSD patterns were collected from large millimetre scale areas and mechanical polishing was applied to prepare the sections. The three-dimensional microstructure was then reconstructed from these sections. A new alignment algorithm based on the minimization of misorientation between two adjacent sections has been developed to accurately align the sections. Differently from the conventional alignment methods, the new algorithm corrects not only the translational misalignment but also rotational and plane parallelity misalignments. The aligned three-dimensional microstructure exhibits smooth grain boundary planes and continuous orientation gradients inside the grains as experimental scatter induced by misalignment was largely removed. Grain boundaries were reconstructed from the aligned three-dimensional map, and the distribution of boundaries in the domain of five macroscopic boundary parameters was computed using kernel density estimation. Methods for estimating the reliability of the distributions are demonstrated. This distribution is compared with the distributions obtained previously for other face-centered cubic materials, including a different pure nickel sample.

Electrically Active and Inactive Single Grain Boundaries in Semiconducting BaTiO3

Bi‐crystal specimens were prepared from Nb and Mn‐doped BaTiO3 poly‐crystals with giant grains of millimeter order in size, and the resistance (R) versus temperature (T) characteristics of these individual grain boundaries was investigated. The electrically active grain boundaries that show normal positive temperature coefficient resistor (PTCR) behavior had no second phases or they were partially distributed along boundaries. On the other hand, electrically inactive grain boundaries that show flat R‐T characteristics were also observed, where continuous Ti‐rich second phases of Ba4Ti12O27 could be detected. Different interface state density and its resultant R–T characteristics were observed for each individual active boundary, which indicates the degree of oxidation and the formation of potential barrier can be different depending on the character of the grain‐boundary plane. The resistance of inactive boundaries was determined by that of insulating second phase showing negative temperature coefficient resistor (NTCR) behavior. These results demonstrate that continuous second phase surrounding a grain deactivates the electrical properties of grain boundary, and thus should be distinguished from insulating depletion layer near grain boundary.

Improved Energy Storage Properties Accompanied by Enhanced Interface Polarization in Annealed Microwave‐Sintered BST

Microwave‐sintering (MWS) technique was employed to fabricate dense (Ba0.4Sr0.6)TiO3 (BST) ceramics. With respect to the high dielectric loss at room temperature, induced by the formation of oxygen defects during the MWS under vacuum atmosphere (−60 kPa), the as‐sintered samples were thermally annealed in air to reduce tanδ and recover the insulating performance. Accompanied by the decreased tanδ, the energy storage properties for annealed MWS BST were optimized, with increasing energy density (γ) from 0.77 to 1.15 J/cm3 and energy efficiency (η) from 60% to 82%. The lower oxygen vacancy concentrations were believed to account for the enhanced insulating characteristics of grain boundaries and contribute to the improved properties after annealing. Electrical characterization of grain and grain boundary by impedance spectroscopy demonstrated that the annealing preferentially modified the grain boundary. In addition, resistances extracted from the high temperature impedance analysis were found to be inadequate for evaluating the electrical characteristics of materials affected by extrinsic mechanisms, such as the interfacial polarization. For comparison, annealing effect on energy storage properties were also discussed for conventionally sintered BST.

Electrical characterization of grain boundaries of CZTS thin films using conductive atomic force microscopy techniques

Electrical characterization of grain boundaries (GB) of Cu-deficient CZTS (Copper Zinc Tin Sulfide) thin films was done using atomic force microscopic (AFM) techniques like Conductive atomic force microscopy (CAFM), Kelvin probe force microscopy (KPFM) and scanning capacitance microscopy (SCM). Absorbance spectroscopy was done for optical band gap calculations and Raman, XRD and EDS for structural and compositional characterization. Hall measurements were done for estimation of carrier mobility. CAFM and KPFM measurements showed that the currents flow mainly through grain boundaries (GB) rather than grain interiors. SCM results showed that charge separation mainly occurs at the interface of grain and grain boundaries and not all along the grain boundaries.