Aspect Ratio Of Grains Research Articles

Influence of Beam Energy on the Properties of Pulsed Laser Deposited Al-Doped ZnO Thin Films

The influence of beam energy on the structural, optical, and electrical properties of Al-doped ZnO (Al:ZnO) thin films deposited using the pulsed laser deposition technique is observed systematically. X-ray diffraction analysis shows the crystalline nature of the thin films with preferred orientation (0 0 2) plane and hexagonal wurtzite structures. The impact of beam energy on the lattice strain is examined by Williamson–Hall (W–H) plots. The surface morphology examined by field emission scanning electron microscopy (FESEM) and atomic force microscopy analysis show the increased aspect ratio of grains, particle density, and surface coverage with the beam energy. The elemental composition of the films depends on the beam energy. Optical properties characterized by UV-visible spectroscopy indicate high transparency over 75% in the visible region and a sharp UV absorption. The optical band gap, $E_{g}$ obtained from Tauc's plot indicates a decrease in the energy gap with an increase in laser beam energy. Burstein–Moss (B-M) shift is observed in the band gap due to Al-doping. Photoluminescence (PL) properties of the thin films indicate visible emission peaks, corresponding to deep level defects. The electrical properties analyzed by Four probe technique depict that the thin films possess a resistivity ∼10−3 Ω·cm. Al:ZnO films show good optoelectronic properties for their practical applicability in devices such as solar cells and transparent display electrodes.

Homogeneity of microstructure and Vickers hardness in cold closed-die forged spur-bevel gear of 20CrMnTi alloy

Cold closed-die forging is a suitable process to produce spur-bevel gears due to its advantages, such as saving materials and time, reducing costs, increasing die life and improving the quality of the product. The homogeneity of microstructure of cold closed-die forged gears can highly affect their service performance. The homogeneity of microstructure and Vickers hardness in cold closed-die forged gear of 20CrMnTi alloy is comprehensively studied by using optical microscopy and Vickers hardness tester. The results show that the distribution homogeneity of the aspect ratio of grain and Vickers hardness is the same. In the circumferential direction of the gear tooth, the distribution of the aspect ratio of grain and Vickers hardness is inhomogeneous and they gradually decrease from the surface to the center of the tooth. In the radial direction, the distribution of the aspect ratio of grain and Vickers hardness is inhomogeneous on the surface of the gear tooth; while it is relatively homogeneous in the center of the gear tooth. In the axial direction of the gear tooth, the distribution of the aspect ratio of grain and Vickers hardness is relatively homogeneous from the small-end to the large-end of the gear tooth.

Morphological Transitions during Au Electrodeposition: From Porous Films to Compact Films and Nanowires

Gold electrodeposition was studied in a sulfite electrolyte to which micromolar concentrations of Tl2SO4 were added. Hysteresis and a regime of negative differential resistance (NDR) evident in electroanalytical measurements are correlated with deposit morphology and interpreted through measurements of thallium underpotential deposition (upd). Deposit morphologies range from specular surfaces to highly faceted dendrite-like grains of moderate aspect ratio and, for potentials within the NDR region, sub-50 nm diameter, high aspect ratio 110 oriented single crystal nanowires. The nanowires exhibit an epitaxial relationship to the substrate that permits one step fabrication of surfaces densely covered with high aspect ratio nanowires having controlled orientations. The NDR and nanowires are a consequence of the non-monotonic relationship between Tl coverage and growth velocity; at low coverage Tl accelerates Au deposition while at higher coverage it inhibits deposition. Immiscibility of the Tl and Au supports on-going surface segregation during area expansion that accompanies nanowire growth leading to greater dilution of the additive coverage and more rapid growth at the nanowire tips, while the sidewalls remain passivated by a saturated Tl coverage.

High-strength porous Si3N4 ceramics prepared by freeze casting and silicon powder nitridation process

Porous Si3N4 ceramics with uniform morphology were fabricated via freeze casting and silicon powder nitridation process. Because of the interlocking grains, Si3N4 porous ceramics obtained high porosity of 70.51% and high flexural strength of 16.3MPa despite of its nitridation degree 88.8%. With SiC addition, a much higher nitridation degree 97.9% was achieved. But the relatively low porosity of 66.7% with low flexural strength of 9.6MPa was gained owing to the restrained growth and deflection of grains. After post-sintering, the increased aspect ratio of grains gave rise to the improved flexural strength for Si3N4 porous ceramics, while the contrary phenomenon was observed with SiC addition.

L1 FePt-ZrO2 (001) nanostructured films with high aspect ratio columnar grains

In order to increase the signal-to-noise ratio of heat assisted magnetic recording, it is desirable to fabricate high magnetic anisotropy FePt media with small grain size and high aspect ratio (grain height to size ratio). In the present paper, we report that FePt media with small grain size and high aspect ratio were achieved by doping ZrO2 into FePt film grown on TiON intermediate layer. The grain size was around 5.6 nm, and the aspect ratio was as high as 2.6. It is believed that this originated from the ZrO2 (002) tetragonal crystalline phase epitaxially grown on TiON intermediate layer. With a 5 vol. % carbon doping into FePt-ZrO2 films, the perpendicular anisotropy was improved and the out-of-plane coercivity was around 23.2 kOe. Doping of crystalline phase material with certain required crystal structure may offer a method for fabrication of nanostructured thin films with high aspect ratio grains at high processing temperature.

Processing of ultrafine-grained aluminum by cross accumulative roll-bonding

In this paper, the structural and mechanical characteristics of aluminum strips subjected to two severe plastic deformation processes, namely accumulative roll bonding (ARB) and cross accumulative roll bonding (CARB), are compared. Transmission electron microscopy studies on the strips showed that ultrafine grains are formed by both of the processing routes to eight passes. However, the structure of the CARB-processed specimen was less elongated than that of the ARB-processed specimens; that is, the aspect ratio of grains in the CARB case was less than that in the ARB specimens. Tensile tests indicated that the tensile strength is increased with the number of rolling passes, and the strength of the CARB specimens is higher than that of the ARB specimens.

Domainal fabrics of hematite in schistose, shear zone-hosted high-grade Fe ores: The product of the interplay between deformation and mineralization

Schistose high-grade hematite orebodies (>64 wt % Fe) in the Iron Quadrangle, Minas Gerais, were formed in shear zones by hydrothermal alteration of the Paleoproterozoic Cauê BIF during the Transamazonian orogenesis. The ore is comprised of platy hematite (specularite) grains that define the foliation and overprint a relict banded martite-hematite fabric resembling, at first sight, a mylonite. The EBSD analyses of a m-scale schistose orebody from the Pau Branco mine show that specularite grew as elongated plates with the (00.1) plane parallel to the foliation. The population of the measured grain aspect ratio (GAR) is homogenous in different scales, and the longest axes of the crystals align with the stretching lineation (L//X) building continuous domains, or anastomose around stretched iron oxide aggregates and rootless fold hinges. The pole figure of the (00.1) plane shows usually a maximum centered on the pole of the foliation Z often elongated on a girdle perpendicular to the lineation L. The {10.4} pole figure has the configuration of a symmetric cleft girdle and the corresponding {11.0} and {10.0} pole figures present well developed girdles parallel to the foliation with an elongated maximum centered on X. Microstructures associated with crystal-plastic behavior and dynamic recrystallization are missing and the fabric of the orebody probably results from precipitation of strain-controlled oriented hematite plates and anisotropic syntaxial growth of favorably oriented grains with the intervention of hydrothermal fluids during Fe enrichment. The shear zone provided pathways for the percolation of mineralizing fluids under temperatures that varied from 140 to 350 °C or higher, under ductile or ductile–brittle conditions. The orthorhombic fabric and CPO (crystallographic preferred orientation) of the ore nevertheless contrast with the asymmetry of simple shear as observed in the torsion experiments by Siemes et al. (2010, 2011), probably due to volume loss and possibly a flattening component of deformation in the ore zone.

A probabilistic crystal plasticity model for modeling grain shape effects based on slip geometry

A new statistical theory is introduced that takes into account the coupling between grain size, shape and crystallographic texture during deformation of polycrystalline microstructures. A “grain size orientation distribution function” (GSODF) is used to encode the probability density of finding a grain size D along a direction (given by unit vector n) in grains with orientation g. The GSODF is sampled from the input microstructure and is represented in a finite element mesh. During elastoplastic deformation, the evolution of grain size D (in direction θ) and the orientation g is tracked by directly updating the GSODF probabilities using a Lagrangian probability update scheme. The effect of grain shape (e.g. in high aspect ratio grains) is modeled by including the apparent grain size as seen by various different active slip systems in the grain within the constitutive law for the slip system resistance. The prediction of texture and strains achieved by the statistical approach is compared to Taylor aggregate and finite element deformation analysis of a planar polycrystalline microstructure. The role of grain shape and size in determining plastic response is investigated and a new adaptive GSODF model for modeling microstructures with multimodal grain shapes is proposed.

Model of sand formations for joint simulation of elastic moduli and electrical conductivity

This paper presents an approach for the joint simulation of elastic-wave velocities (compressional and shear waves) and electrical conductivity for clastic rocks using a unified microstructural model. We treat clastic formations as porous composite materials containing two components: solid grains and pores completely saturated with fluids (gas–water–oil mixture). The effective shapes of both components correspond to triaxial ellipsoids. For calculating the effective properties of such a composite, we apply the self-consistent effective media approximation (EMA) method. This method treats all components equally without introducing any host and requires having knowledge of their shapes. To determine the pore and grain effective aspect ratios, we have used a set of experimental data that included the empirical regression equations published for the elastic-wave velocities in clean sandstone formations and Archie’s law for the electrical conductivity. By applying the inversion procedure that consists of minimizing the difference between the experimental and EMA-predicted data, the aspect ratios of grains and pores as a porosity function have been obtained. Based on this relation and knowing individual property components (elastic and electrical conductivity) for grains and conductivity for pore saturating fluids, we can calculate effective properties of the clean sandstone formations. We demonstrate that to simplify the simulation of effective properties, grains and pores can be approximated by spheroids. However, in this case, the relative errors that the inversion process yields are higher in comparison with those of the three-axis ellipsoids. Additionally, we have determined the aspect ratios of components for several velocity equations (including different confinement pressures) and various cementation factors ‘m’ in Archie’s law. We have modelled electrical conductivity for clastic formations with different water mineralization and oil saturation. The effective conductivity obtained by using our approach reflects a good fit with the published experimental data.

The dielectric permittivity of carbonate formations from the unified microstructure model

Abstract In this paper we study the influence of pore microstructure and saturation of pore systems on the dielectric permittivity of double-porosity formations. The dielectric permittivity calculations were fulfilled using the unified model approach. This approach is based on the calculation of different physical properties from the single microstructure model applying the symmetric variant of the effective medium approximation (EMA). Porous rock is considered as a composed material that consists of solid grains and fluid filled pores of a small scale (primary porosity) that form a homogeneous isotropic matrix. Secondary pores of a large scale are represented by inclusions of different shapes placed in this matrix. Each component of rock (grains, primary and secondary pores) is approximated by a three-axial ellipsoid. The aspect ratios of grain and primary-pore ellipsoids are introduced as a function of porosity. The shapes of secondary pores such as a crack, vug, and channel are modeled by varying ellipsoidal aspect ratios. The distribution of saturating immiscible fluids in pore space is described by two models: (1) water (the wetting component) is concentrated in a film attached to the pore walls while oil or gas (the non-wetting component) occupies the central part of a pore; in this model a pore is considered as a layered ellipsoid with the wetting component in the outer layer; (2) the saturating fluid contains spherical drops of oil placed in the water. The dielectric permittivity obtained by using the unified microstructure model with variable aspect ratios of matrix grains and pores demonstrates a good agreement with the experimental data. The influence of secondary pores on the dielectric permittivity depends on pore shape and decreases with oil saturation.

Silicon nitride: the engineering material of the future

The purpose of this review is to present the recent developments in silicon nitride (Si3N4) ceramics and to examine the achievements regarding our understanding of the relationship between processing conditions, chemical composition, microstructure and mechanical properties of Si3N4. Si3N4 is one of the most important structural ceramics because it possesses a combination of advanced properties such as good wear and corrosive resistance, high flexural strength, good fracture resistance, good creep resistance and relatively high hardness. These properties are obtained through the processing method involving liquid phase sintering in which a tailored microstructure, with high aspect ratio grains and chemistry of intergranular phase, triggers the toughening and strengthening mechanisms leading to the development of high fracture toughness and fracture strength. However, despite high fracture toughness and strength, Si3N4 ceramic materials still break catastrophically, and the fracture behaviour of this ceramic is considered to be the major obstacle for its wider use as a structural material. In addition to the macrostructure–mechanical properties relationship, this paper also reviews new designs involving laminates possessing no plane of weakness and some theoretical developments involving crack opening displacement. Proposals of how to improve the fracture resistance were also discussed.

Effect of finite strain on clast-based vorticity gauges

Clast-based vorticity gauges utilize orientations of grains assumed to have behaved as isolated rigid particles suspended in a flowing viscous matrix. A fundamental assumption behind use of the method is that sufficient strain has accumulated for high aspect ratio grains to rotate into positions approaching their stable sink orientation, and that clasts below a critical aspect ratio may be observed in any orientation relative to the flow plane. We constructed a numerical model to explore the effect of variable finite strain on development of the orientation distribution of a large population of rigid clasts embedded in a viscous medium for end-member pure and simple shear and for several distinct general shear flows. Our model predicts the technique will tend to produce vorticity overestimates for lower vorticity flows for a wide range of finite strain. The model also indicates that clast populations in moderate to high vortical flows tend to develop shape preferred orientations that closely resemble those expected for flows of lower vorticity. We conclude that clast-based methods are not effective for extracting detailed kinematic information from a mylonite deformed in a flow with arbitrary boundary conditions. In fact, it appears that most general shear flows continued long enough to develop moderate–high finite strains will tend to produce a clast orientation distribution that will yield a visual estimate of the critical aspect ratio that suggests approximately equal contributions of pure and simple shear components.

Microstructure and mechanical properties of halite/coarse muscovite synthetic aggregates deformed in torsion

We investigated the behavior of a synthetic two-phase aggregate composed of 80% halite + 20% coarse muscovite under torsion deformation, at 100, 200 and 300 °C, a confining pressure of 250 MPa, and a constant strain rate of 3E−4 s −1. At all temperatures, the two-phase aggregate deformed homogeneously at the sample scale. The strength of the aggregate and muscovite deformation depended on temperature, strain rate and initial orientation of muscovite greatest dimension relative to the shear plane. With muscovite initially parallel to the shear plane, halite flowed plastically and long muscovite grains were not strong enough to behave as a rigid inclusion rotating in a ductile matrix. Instead, high aspect ratio grains deformed by folding, which was visibly accomplished by slip along mica {001} cleavage and by great flattening of halite in the core of the folds. When initially normal to the shear plane, high aspect ratio muscovite grains passively rotated towards the shear plane. In both cases, small and low aspect ratio muscovite grains behaved mostly as mica-fish, with mica grains tilted opposite to shear sense. Similarly to natural mylonites, σ-porphyroclast systems and rolling structures were also common in the microstructure. A strong foliation made of halite ribbons and aligned muscovite flakes rapidly developed but did not make the composite aggregate weaker when compared with single-phase halite. Comparison with synthetic aggregates of single-phase halite shows that the two-phase aggregate was much stronger than single-phase halite in all cases. Comparison with a synthetic aggregate with calcite porphyroclasts shows that the strength of the aggregate with muscovite at 100 °C was lower below a strain of ca. 2.6 and higher beyond this value, and that the aggregate with calcite was stronger at 200 °C. Strain rate stepping tests (1E−5 s −1–2E−3 s −1) indicate that the two-phase aggregate behaved as power-law viscous, with stress exponents of ca. 12 and 10 at 100 and 200 °C, respectively. The mechanical data obtained in this study represent the actual rheological behavior of the aggregates because we used soft polymer jackets.

Evaluation of surface integrity in micro drilling process for nickel-based superalloy

This study focuses on the mechanical drilling of micro-holes in Inconel 718 alloy under wet cutting conditions. Qualitative and quantitative mechanical and metallurgical characterization of the surface and subsurface region was undertaken using nanoindentation, backscatter electron microscopy, electron backscatter diffraction microscopy and transmission electron microscopy. The analysis revealed three different zones, namely, a highly deformed nanostructured surface layer containing ultra-fine and high aspect ratio grains drawn out by large scale deformation, a deformed subsurface layer and finally the unaffected parent metal. The nano-hardness, plastic deformation, microstructure and crystal misorientation were assessed. The correlation between the modified surface and subsurface layers and the cutting conditions was established. The phenomena behind the formation of the different zones were investigated. The results suggest that subsurface alterations are driven by thermo-mechanical loading, causing plasticity and grain refinement by excessive shearing local to the cut surface.

Al-B-C 조제 β-SiC의 스파크 플라즈마 소결에 미치는 α-SiC seed 첨가 영향: 미세 구조 변화

The unique features of spark plasma sintering process are the possibilities of a very fast heating rate and a short holding time to obtain fully dense materials. <TEX>$\beta$</TEX>-SiC powder with 0, 2, 6, 10 wt% of <TEX>$\alpha$</TEX>-SiC particles (seeds) and 4 wt% of Al-B-C (sintering aids) were spark plasma sintered at <TEX>$1700-1850^{\circ}C$</TEX> for 10 min. The heating rate, applied pressure and sintering atmosphere were kept at <TEX>$100^{\circ}C/min$</TEX>, 40 MPa and a flowing Ar gas (500 CC/min). Microstructural development of SiC as function of seed content and temperature during spark plasma sintering was investigated quantitatively and statistically using image analysis. Quantitative image analyses on the sintered SiC ceramics were conducted on the grain size, aspect ratio and grain size distribution of SiC. The microstructure of SiC sintered up to <TEX>$1700^{\circ}C$</TEX> consisted of equiaxed grains. In contrast, the growth of large elongated SiC grains in small matrix grains was shown in sintered bodies at <TEX>$1750^{\circ}C$</TEX> and the plate-like grains interlocking microstructure had been developed by increasing sintering temperature. The introduction of <TEX>$\alpha$</TEX>-SiC seeds into <TEX>$\beta$</TEX>-SiC accelerated the grain growth of elongated grains during sintering, resulting in the plate-like grains interlocking microstructure. In the <TEX>$\alpha$</TEX>-SiC seeds added in <TEX>$\beta$</TEX>-SiC, the rate of grain growth decreased with <TEX>$\alpha$</TEX>-SiC seed content, however, bulk density and aspect ratio of grains in sintered body increased.

Silicon Nitride Ceramics

Silicon nitride is one of the major structural ceramics that has been developed following many years of intensive research. It possesses high flexural strength, high fracture resistance, good creep resistance, high hardness and excellent wear resistance. These properties arise from the processing of the ceramic by liquid phase sintering and the development of microstructures in which high aspect ratio grains and intergranular glass phase lead to excellent fracture toughness and high strength. The glass phase softens at high temperature and controls the creep rate of the ceramic. The purpose of this review is to examine the development of silicon nitride and the related sialons and their processing into a range of high-grade structural ceramic materials. The development of knowledge of microstructure–property relationships in silicon nitride materials is outlined, particularly recent advances in understanding the effects of grain boundary chemistry and structure on mechanical properties. This review should be of interest to scientists and engineers concerned with the processing and use of ceramics for structural engineering applications.

Carbide matrix composites by fast MW reaction-sintering in air of B 4C–SiC–Al mixtures

The behavior of B 4C/SiC/Al mixtures during MW heating in air was studied. It was determined that B 4C/SiC/Al mixtures generate well-densified specimens. The fired specimens are made from a B 4C matrix, the porosity of which is filled with products of the reactions of Al with B 4C and the gases of air. Phases detected included Al 2OC, Al 27O 39N, Al 10N 8O 3, Al 3B 48C and AlN. The SiC high aspect ratio grains are protected in this environment from oxidation, being able to act as a toughening filler.

Dynamic force microscopy study of the microstructural evolution of pulsed laser deposited ultrathin Ni and Ag films

Ultrathin films (6–10 nm) of silver and nickel were deposited by pulsed laser deposition (PLD) in high vacuum (1 × 10−6 mbar). Microstructural evolution of these films as function of incident laser energy, substrate temperature, substrate material [borosilicate glass, fused silica, MgO(100) and Si (311)] and target–substrate distance was studied in detail using dynamic force microscopy. It is shown that with increase in laser energy incident on the target, there is a substantial increase in particle size in the film. The effect of increased laser energy on microstructure is much more drastic than that for the increase of substrate temperature. In general, denser packing of nanoparticles and increased clustering have been observed at elevated substrate temperature. Increase in laser energy gives rise to higher average grain size, packing density, and clustering in comparison to substrate temperature. It is observed that the aspect ratio of grains is dependent on incident laser fluence and substrate temperature, but more drastically on the substrate material. Substrate coverage and aspect ratio of the grains are particularly dependent on the nature of crystallinity of the substrates. It is demonstrated that PLD provides a greater degree of microstructural manipulation than other physical vapor deposition techniques.

Effects of Rare‐Earth (RE) Intergranular Adsorption on the Phase Transformation, Microstructure Evolution, and Mechanical Properties in Silicon Nitride with RE2O3+MgO Additives: RE=La, Gd, and Lu

Silicon nitride powders most often consist primarily of the α phase, which transforms into the β phase during the densification and microstructural evolution of Si3N4 ceramics. The temperature at which the transformation initiates in the presence of a combination of MgO and RE2O3 densification additives is found to decrease with increasing atomic number of the rare earth (RE). This trend coincides with the predicted and observed decrease in the affinity of the RE to segregate to and absorb on the prism planes of hexagonal prism‐shaped β grains with an increase in the atomic number of the RE. When RE adsorption is diminished, Si (and N) attachment on the smooth prism planes is enhanced, which increases diametrical growth rates, normally reaction‐rate limited by an attachment mechanism. Combined with the typically fast [0001] growth, it is this augmented grain growth that contributes toward the initiation of the α–β transformation at lower temperatures. With the enhanced transformation, observations reveal an increase in the number of β grains growing in the early stages of densification. On the other hand, increased RE adsorption leads to greater growth anisotropy, resulting in the formation of higher aspect ratio grains. Thus, Lu2O3 generates larger diameter, yet elongated, reinforcing grains, while La2O3 results in reinforcing grains of a higher aspect ratio. The Gd2O3 additive transformation and microstructural characteristics lie intermediate to those of the lanthanide end‐member elements. Despite these differences, a substantial fraction of large reinforcing grains were found for each additive composition. As a result, the mechanical properties of the resultant ceramics are similar with flexure strengths in excess of 1 GPa, fracture toughness values &gt;10 MPa·m1/2 at room temperature, and excellent strength retention (&gt;800 MPa) at 1200°C.

Microstructure characterization of 7050 aluminum alloy during dynamic recrystallization and dynamic recovery

Dynamic recrystallization and dynamic recovery in 7050 aluminum alloy during high temperature deformation have been investigated by tensile tests conducted at 460 °C and strain rates of 1.0 × 10 − 2 and 0.1 s − 1 . It is shown that continuous dynamic recrystallization and dynamic recovery are the softening mechanisms when strain rates are 1.0 × 10 − 2 and 0.1 s − 1 , respectively. The increase in the average misorientation angle is proportional to the increase in the true strain during continuous recrystallization and dynamic recovery. The grain size of the 7050 aluminum alloy increases with increasing true strain during continuous dynamic recrystallization and dynamic recovery, while the aspect ratio of grains remains unchanged after the continuous dynamic recrystallization; and it increases with the true strain during dynamic recovery. The continuous increase of the average misorientation angle during continuous dynamic recrystallization is attributed to the migration of subgrain boundaries and the absorption of dislocations into grain boundaries.