Mechanism Of Grain Research Articles

Verification of the Mechanism of Grain Refinement by Ultrasonic Treatment of Aluminum-4 Wt Pct Silicon Molten Alloy

Ultrasonic treatment (UST) of molten metals produces a fine grain microstructure. Several mechanisms of grain refining by UST have been suggested; however, experimental verification has not yet established the actual mechanism. In this study, UST was applied to Al-4 wt pct Si molten alloy (1) above the liquidus temperature, (2) during undercooling before recalescence, (3) during recalescence, and (4) after recalescence. After UST treatment, the average grain sizes of the solidified samples were measured, and the effects of UST were evaluated. In the case of (1), the temperatures of the crucible, ultrasonic horn, and the atmosphere of the molten alloy were also confirmed above the liquidus temperature, and a finer grain structure was obtained. This result reveals that UST promotes the nonequilibrium nucleation mechanism. UST during (2) and (3) also caused finer grain microstructures, whereas (4) did not. UST during undercooling hastened nucleation. However, the previously reported mechanism of the breaking dendrite was not activated by UST after recalescence.

The morphological evolution of the axial structure and the curved columnar grain in the weld

The competitive growth of microstructures in the entire weld pool for both the Al–Cu alloy and the pure aluminum was simulated by the cellular automata method to comparatively investigate the micro-mechanisms for the morphological evolution of the axial structure and the curved columnar grain in the weld. The competitive mechanism of grains during the epitaxial growth and the morphological evolution of the grain structure in the weld with various welding speeds were studied. The results indicate that both the thermal conditions and the solidification characteristic of the weld metal exert an important influence on the grain competition and the resulting structure in the weld. For the Al–Cu alloy, the dendritic structure with a large S/L interface curvature appears during the epitaxial growth. The preferential orientation affects the competition result obviously. Owing to the anisotropic growth kinetics, the straight axial structure forms at low welding speeds. With the increase of the welding speed, the width of the axial region decreases and eventually disappears. For the pure aluminum, the S/L interface during the epitaxial growth is planar, and the grain competition is controlled by the thermal conditions completely. The columnar grains curve gradually to follow the highest temperature gradient direction at low welding speeds and become straight at high welding speeds.

The role of Co in the BaTiO3–Na0.5Bi0.5TiO3 based X9R ceramics

Ceramics in BaTiO3–Na0.5Bi0.5TiO3–Nb2O5–Co3O4 (BT–NBT–Nb–Co) system were fabricated to satisfy the EIA-X9R specification (−55~200°C, ΔC/C≤±15%) of multilayer ceramic capacitors. The role of Co in this system was investigated. The core–shell structure was found to exist in the system, as confirmed by transmission electron microscopy (TEM), accounting for the high dielectric temperature stability. The Co can regulate the reaction between Nb and BT–NBT to adjust the core–shell structure, where the optimum dielectric properties were achieved at Nb/Co=3, with dielectric constant being 930 at room temperature and capacitance variation being less than 15% in the temperature range from −55°C to 200°C. The impedance spectroscopy was fitted by 3RC equivalent circuit, corresponding to the grain, grain boundary and ceramic/electrode interface region for the Co modified BT–NBT system. The conduction activation energy of grain was found to decrease with increasing Co concentration, where the conduction mechanism of grain and grain boundary are both sensitive to the Co concentration.

Progress of Research in the Action Mechanism of Grain Growth Inhibitor for Cemented Carbide

Three hypotheses for the grain growth inhibiting mechanism of carbide grain were described in this paper: the dissolution hypothesis, the adsorption hypothesis and the segregation hypothesis. The weakness of these three existing hypotheses has been pointed out. On the basis of analyzing the existing research results of other researchers, this paper proposed a new grain growth inhibiting mechanism: segregation - dissolution hypothesis.

Fabrication and mechanical properties of ultrafine structured dissimilar laminated metal composite sheets (LMCS)

AbstractThe ultrafine grain structure is very difficult to fabricate by severe plastic deformation (SPD) for metals with poor formability. In this paper, a fabrication technology of ultrafine structured dissimilar laminated metal composite sheets (LMCS) was developed for poor plastic metals which have low elongation by hot accumulative roll bonding (ARB) in conjunction with cold rolling. The hot ARBed 1100/7075 LMCS was cold rolled at room temperature after recrystallization annealing treatment. An ultrafine structured dissimilar LMCS was obtained without undergoing severe cold rolled deformation. The mechanical properties were enhanced and optimized by using heat treatment technology. The accelerated refining mechanism of grain was revealed by microstructure characterization of the composite sheet. The enhanced strength was mainly derived from the fine layers, refined grains, increased dislocation accumulation, and abundant dispersoids. The results of the research are helpful in improving the mechanical properties of dissimilar LMCS and optimizing the preparation technology.

Homogenization of anisotropic elastoplastic behaviors of a porous polycrystal with interface effects

SUMMARYThis paper is devoted to develop a theoretical framework to predict the macroscopic transversely isotropic elastoplastic behavior of clay‐like material, which is viewed as a porous polycrystal. We consider evolutions of two local plastic mechanisms of grains and interface simultaneously, for which a Schmid criterion is used for the strength of sheet‐like grains and a Tresca criterion for the strength of interfaces between particles. By adapting the standard incremental method, we propose firstly a classic self‐consistent model, which does not consider the effect of interface, then a generalized self‐consistent model in which the solid phase is represented by laminated (or isotropic) spherical grains surrounded by interfaces. Comparisons of numerical predictions between these two methods are performed and have demonstrated the validity of the generalized self‐consistent model taking account of interface effects. Numerical simulations of uniaxial compression tests have shown that the macroscopic elastoplastic behavior of polycrystalline (clay‐like) material can be successfully predicted by the way of considering the two local plastic mechanisms at microscopic scale. Copyright © 2013 John Wiley & Sons, Ltd.

Metallographic structure evolution of 6061-T651 aluminum alloy processed by laser shock peening: Effect of tempering at the elevated temperatures

The aim of the study was to investigate the influences of laser shock processing (LSP) on residual stress relaxation and metallographic structure evolution of 6061-T651 aluminum alloy at elevated temperature (up to 500°C). Residual stress was determined by XRD with sin2ψ method, optical microscopy and TEM observations of the grain, and precipitated phase evolutionary process after LSP were carried out. The results showed that the effects of temperature on residual stress relaxation could be described, and the residual stress relaxation was analyzed precisely using the Zener–Wert–Avrami function. The main mechanism of stress relaxation between 200°C to 300°C was the dislocation slip and dislocation climb, and dynamic recovery was the main mechanism during the stress relaxation deformation between 400°C to 500°C. The evolution mechanism of grain and precipitated phase of 6061-T651 aluminum alloy between 25°C and 500°C was proposed. LSP still had a beneficial effect on residual stress at the elevated temperature 300°C. Laser shock processing was the principal element for inducing the “metallographic variation” (MV) effect of 6061-T651 aluminum alloy at the elevated temperature.

Dielectric relaxation with polaronic and variable range hopping mechanisms of grains and grain boundaries in Pr0.8Ca0.2MnO3

Polycrystalline Pr0.8Ca0.2MnO3 has been synthesized through solid state reaction route and phase purity is analyzed using synchrotron XRD. Magnetization M(T) showed Mn spin alignment due to ferromagnetic ordering around 125 K (TC), whereas M(H) showed an onset of non-linear behavior from 200 K and with temperature re-orientation of magnetic moment is discussed. The modulation of relaxation processes revealed different formations of impedance plane plots with temperatures. A change in equivalent circuit models from (R1C1)(R2Q2) to (R1C1)(R2Q2)(R3Q3) at 100 K is being reported. The analysis of the impedance data is carried out by calculating impedance of grains and grain boundaries. Fitted parameters derived from these equivalent circuit parameters showed a change in conduction mechanism from small polaronic hopping model (SPH) to Mott's variable range hopping (MVRH) model, around 125 K (TC). Carriers hop to larger distance with multiple activation energies and are described by MVRH below TC. Above this temperature, different trap centers start facilitating these carriers through SPH. Dielectric relaxation shows dispersion around 125 K and the origin of this polarization lies close to the relaxation derived from the grains and their interfaces.

Formation Mechanism of Grain Cutting Edges in Micro Dressing of Polycrystalline cBN Grinding Wheels

This paper presents a mechanism for the formation of grain cutting edges in the micro dressing of coarse-grain polycrystalline cubic boron nitrite (cBN) grinding wheels using a fine-grain diamond dresser. Many grain cutting edges having a flat surface consisting of ductile smooth surfaces as well as many brittle micro dents, are formed on the working surface of the coarse-grain (#80 mesh) cBN wheel by micro dressing using a fine-grain (#1200 mesh) diamond dresser. This result shows that the flat surfaces of cutting edges on the wheel surface are formed on the basis of a ductile removal process as well as a brittle micro-fracturing of cBN grains by the diamond cutting edges. Moreover, cylindrical grinding experiments with these wheel working surfaces were conducted to clarify the feasibilyty of creating a ground mirror-like surface. As a result, it was confirmed that high-quality mirror surfaces with roughness less than 0.03 m Ra can be efficiently formed using the working surface prepared by this micro dressing method.

Electrical conduction of intrinsic grain and grain boundary in Mn-Co-Ni-O thin film thermistors: Grain size influence

Mn1.85Co0.3Ni0.85O4 (MCN) thin films with pure spinel phase and different grain size were prepared on Al2O3 substrates by chemical deposition method. Temperature dependent ac impedance spectroscopy was employed to analyze the grain size influence on the electrical conduction of intrinsic grain and grain boundary (GB) in MCN thin films. The conduction mechanisms of grain and GB both followed the small-polaron hopping model. It was found that the hopping types of GB (nearest-neighbor-hopping (NNH)) and grain (a transition from variable-range-hopping (VRH) to NNH) were not affected by the grain size, while the resistance, characteristic temperature, and activation energy of grain and GB were affected by the grain size in varying degrees. Additionally, the mechanisms concerning the dependence of electrical conduction of grain and GB on the grain size of MCN thin films were discussed in detail. These studies will also provide a comprehensive understanding of the conduction behaviors of a system with mixed NNH and VRH.

Effect of Ultrasonic Cavitation Acting on Solidified Structures of 7050 Aluminum Alloy

Imposing ultrasound on D.C.casting of the 7050 aluminum alloy could refine grains significantly. Among the physical effects accompanying the propagation of high power ultrasonic oscillations in melt, cavitation, or the formation of cavities filled mainly with gases dissolved in the molten melt. In this research, we have analyzed solidified structures which influenced by different power of ultrasound via using cavitation theory and experiment. The results indicated that after treating by ultrasound, the solidified structures of the aluminum alloy could be refined considerably and the degree of the structure refining rose along with the increase of the ultrasound power. Software MATLAB was chosen to simulate the relationship between acoustic pressure and relative radii of cavitation bubbles, ultrasonic frequency and relative radii and initial equilibrium radii and relative radii respectively. After that, the mechanism of grain refining have been discussed under the condition of stable cavitation and transient cavitation, providing some related references for ultrasound applying in the field of foundry industry.

Fabrication of nanocrystalline copper by explosive loading and its dynamic mechanics properties

Nanocrystalline (NC) copper is fabricated by the method of severe plastic deformation of coarse-grained copper under explosive dynamic loading at high strain rates. The dynamic mechanical properties of NC copper are studied by the split Hopkinson pressure bar method. The results show that it is feasible to fabricate nanocrystalline copper by explosive dynamic plastic deformation of coarse-grained copper and the grain size of NC copper can be smaller than 100 nm. Twinning and formation of dislocations are the main mechanisms of grain refining. The dynamic yield strength of NC copper increases with decreasing average grain size and increasing strain rate.

Influences of Super-Gravity Field on Aluminum Grain Refining

The influence of a super-gravity field on metal grain refining has been mainly investigated using industrial pure aluminum as an example. The relationship between the super-gravity field (gravity coefficient of less than 1000) and the grain size in the equiaxed crystal zone was studied, and the corresponding mechanism for grain refining was discussed in detail. The effect of the super-gravity field on grain refining was remarkable, and the relationship could be well fitted by the second-decreasing-exponential function. For aluminum, the grain size decreased rapidly as the gravity coefficient increased from 1 to 250, and then remained nearly unchanged from 250 to 1000. Under the experimental conditions, the main mechanism was that the fragments of dendritic grains—generated by super-gravity—acted as the nucleation sites and resulted in grain refining. Further, the change of nucleation Gibbs free energy, directly caused by super-gravity, was not remarkable enough to refine the grains, which could be suggested by the stable freezing points under different super-gravity fields.

Effect of mould D/H ratio on solidification structure of Al–2·0%Si alloy ingot under ultrasonic vibration

A series of experiments were conducted by Al-2·0Si%Si alloy to investigate the effect of mould D/H ratio and shape on solidification structure under ultrasonic vibration (UV). Finally, the mechanism of grain refining was discussed. The results show that equiaxed grain occupancy is maximized when D/H is equal to 0·7 for ingots with mould volume of 80 and 120 cm3, however, the equiaxed grain occupancy has the maximum when D/H is equal to 2·0 for the ingot with mould volume of 200 cm3. Otherwise, the equiaxed grain occupancy has downtrend with increasing of mould volume. The columnar structure is easily formed in ultrasonic ingots with complex mould shape and more corners.

Preparation of Silver Nanopowder by Freeze-Drying Procedure

A freeze-drying procedure is developed for manufacturing silver nanopowder with regular shape and uniform diameter. The procedure includes four steps: preparing the precursor solution, freezing the solution, drying the congelation, and calcining the drying product. The starting substances are silver nitrate and caustic soda and the precursor is diammine silver solution. Three freezing styles including direct freezing, vacuum evaporation freezing, and spray freezing are adopted to freeze the precursor solution. In the drying stage, two heating styles for the frozen solution are compared. X-ray diffraction (XRD) and energy disperse spectroscopy (EDS) spectrograms show that the products of freeze drying are pure amorphous silver nanopowder without hard agglomerations. Silver cubic nanocrystals are subsequently obtained by calcination. The chemical fundamentals and the drying curves of freeze drying are given. The processes of preparing silver nanopowder are described in detail. The effect of freezing and heating on characteristics of the powder are discussed. The mechanism of grain forming and growing up is analyzed according to the observation results. The characteristics of powder freeze drying and its feasibility for volume production are presented.

Grain growth kinetics of textured 0.92Na0.5Bi0.5TiO3—0.08BaTiO3 ceramics by tape casting with Bi2.5Na3.5Nb5O18 templates

Plate-like Bi2.5Na3.5Nb5O18 particles were used as templates to fabricate grain-oriented Na0.5Bi0.5TiO3—BaTiO3 (NBTBT) ceramics by reactive-templated grain growth. The effects of sintering conditions on the grain orientation and microstructure of the textured NBTBT ceramics were investigated, and the kinetic mechanism of grain growth is discussed. The results show that textured ceramics were successfully obtained with orientation factor more than 0.6. NBTBT specimens are composed of strip-like grains and equiaxed shaped grains. The textured ceramics have a microstructure with strip-like grains aligning in the direction parallel to the casting plane and exhibit an {h00} preferred orientation. The degree of grain orientation increases initially, then decreases with increasing sintering temperature and soaking time. The maximum texture fraction is 0.69 when sintered at 1185 °C for 6 h. The kinetic exponent n and activation energy Q of the two types of grain in textured NBTBT ceramics were calculated. The results show that the grain growth mechanism of oriented grains is controlled not only by grain lattice diffusion, but also by grain boundary diffusion. The grain growth mechanism of matrix grains is mainly controlled by the grain boundary curvature.

A New Model for Simulating AC and Surge Properties of ZnO Varistors Based on Microstructural Characteristics

The numerical simulation is an effective method for researching the relationship between macroscopic properties of ZnO varistors and their microstructural characteristics. However, the computing efficiency is a big problem for further researches. In this paper, a quick piecewise linear method for solving the complicated nonlinear resistor networks of ZnO varistors is proposed, which can significantly improve the computing efficiency by a hundred times. The simulating models for the microstructural characteristics of ZnO varistors are improved based on actual conduction mechanism of grain boundaries. The simulations of more bulk characteristics of ZnO varistors become possible, such as alternating current (AC) and surge properties.

Argon-plasma-induced growth of crystalline grains in microcrystalline silicon: Formation mechanism of grains

A thick (∼300 nm) microcrystalline silicon (μc-Si:H) film with a low crystalline volume fraction (∼24%) and a columnar grain size of about 100 nm was exposed to an argon plasma at a substrate temperature of 220 °C after deposition. It is shown that argon plasma treatment significantly enhances film-crystallinity throughout the μc-Si:H layer: over a factor of 2 in crystalline fraction and by a factor of 3 in columnar grain size after a 90-min argon treatment. Based on these experimental results, it is proposed that crystallization of μc-Si:H is likely mediated by the energy transferred from energetic argon atoms.

Squeezing the Peasants: Grain Extraction, Food Consumption and Rural Living Standards in Mao's China

At the end of Mao's life farmers still accounted for some 80 per cent of China's population. Its declining share in GDP notwithstanding, agriculture continued to carry a heavy developmental burden throughout the Mao era. The production and distribution of grain – the wage good par excellence – held the key to fulfilling this role. But despite a pragmatic response to the exigencies of famine conditions in 1959–61, state investment priorities never adequately accommodated the economic, let alone the welfare needs of the farm sector. Thanks to the mechanism of grain re-sales to the countryside, the Chinese government's extractive policies were less brutal in their impact than those pursued by Stalin in the Soviet Union. Even so, a detailed national, regional and provincial analysis of grain output and procurement trends highlights the process of rural impoverishment which characterized China's social and economic development under Maoist planning.

Expression Patterns of Genes Encoding Carbohydrate-metabolizing Enzymes and their Relationship to Grain Filling in Rice (Oryza sativa L.): Comparison of Caryopses Located at Different Positions in a Panicle

In rice, caryopses located at the base of the panicle have a lower growth rate than those at the tip of the panicle. The former and latter types of caryopses are called inferior and superior caryopses, respectively. Taking the different growth rate into consideration, sugar status and the expression of genes encoding carbohydrate-metabolizing enzymes in inferior caryopses were compared with those in superior caryopses. During the first 5 d after flowering, superior caryopses elongated rapidly, but inferior caryopses did not. At this phase, inferior caryopses had a low ratio of hexose to sucrose, high activity of acid invertase and the absence of the expression of the genes encoding the above enzymes except for two isoforms of cell wall invertase, OsCIN4 and INV1, in comparison with superior caryopses. At the start of caryopsis elongation in both superior and inferior caryopses, the hexose/sucrose ratio increased accompanied by gene expression of vacuolar invertase (INV3), sucrose synthase (RSus1) and ADP-glucose pyrophosphorylase (AGP-L2: D50317). Furthermore, the genes related to endospermal starch accumulation were expressed highly with the decrease in the hexose/sucrose ratio after its peak. Based on the comparison of superior and inferior caryopses, the possible mechanism of grain filling in rice is discussed.