Grain Growth Mechanism Research Articles

Effect of CaCO3 and V2O5 Composite Additives on the Microstructure and Magnetic Property of MnZn Ferrites

Manganese-zinc (MnZn) ferrites have important application in convertors and switching mode power supplies. The core losses ( $\text{P}_{L}$ ) of MnZn ferrites have serious influence on the performance of power conversion devices. It is worth emphasizing that reducing the core losses at high frequency is of great significance for the miniaturization and integration of power conversion devices. Here in this paper, MnZn ferrites have been fabricated by the method of solid-state reaction, and the effect of CaCO3 and V2O5 co-additives on the microstructure and magnetic property has been investigated. This paper illustrated the competition mechanism of grain growth, which originated from the hindrance effect of CaCO3 and acceleration effect of V2O5, as well as the effect of CaCO3 and V2O5 on core losses by means of loss separation method. In addition, as for the MnZn ferrite sample with 0.1 wt% CaCO3 and 0.01 wt% V2O5 co-additives, core losses were suppressed to 46 and 664 kW/m3 corresponding to 3 MHz 10 mT and 3 MHz 30 mT at 25 °C, respectively. The state-of-the-art MnZn ferrites with ultralow core losses own potential application for high-frequency power conversion devices.

Effect of Added Mullite Whisker on Properties of Lithium Aluminosilicate (LAS) Glass-Ceramics Prepared for Dental Restoration.

In this work, lithium aluminosilicate (LAS) glass-ceramics were prepared from lithium disilicate glass powder and mullite whiskers. Lithium disilicate glass powder (28.6Li-68.6Si-2K-0.8La, mol.%) with contents of 5, 10 and 15 wt.% mullite whiskers were hot-pressed at 860 °C for 1 h. The effects of added mullite on phase composition, microstructure, mechanical properties, coefficient of thermal expansion (CTE) and translucency were systematically studied. Also, the mechanism for phase transformation and grain growth was discussed. The LAS glass-ceramic samples exhibited relative densities above 98% with main crystalline phases of lithium disilicate (Li2Si2O5, LD), lithium silicate (Li2SiO3, LM), and β-spodumene (LiAlSi2O6). The β-spodumene whiskers were in-situ synthesized through the reaction between mullite whiskers and glass matrix. As β-spodumene fraction rose, lithium disilicate crystals content decreased and that of lithium silicate increased, resulting in declined bending strength and improved fracture toughness. The composite containing 10 wt.% mullite whisker showed optimal performance with bending strength above 300 MPa and fracture toughness of 2.7 MPa·m1/2. The formation of β-spodumene in the obtained composite declined CTE, with values ranging from 10.5×106 °C-1 for LD glass-ceramic to 6.03×10-6 °C-1 for LAS glass-ceramic doped with 15 wt.% mullite whiskers. Finally, though the real in-line transmission of obtained glass-ceramic composites declined as β-spodumene content rose, the specimens containing less than 10 wt.% mullite whiskers showed favorable translucency, which would suit applications in dental restoration.

An overview on severe plastic deformation: research status, techniques classification, microstructure evolution, and applications

The present overview gives a new approach toward developments and recent achievements in severe plastic deformation. The review focuses on several subjects. First, an outline of SPD research status in the world is presented by literature analysis based on the total number of publications, citations, and the contribution of the top-ranked countries. Second, the mechanisms of grain refinement and grain growth during SPD processing are discussed by means of the latest concepts. Third, all SPD methods invented so far are classified based on a new approach. Up to now, the growing tendency of researchers to introduce new SPD techniques results in a large number of SPD methods which can be considered as new or modified techniques or a combination of previous ones. Such a reference can help to prevent the future duplication to introduce the SPD processes, which are technically similar. At the end, the practical applications of ultrafine/nanostructured materials and industrial commercialization of SPD methods are summarized.

In Situ Investigation of Grain Evolution of 300M Steel in Isothermal Holding Process.

The relationships between initial microstructures, process parameters, and grain evolutions in isothermal holdings have drawn wide attention in recent years, but the grain growth behaviors of 300M steel were not well understood, resulting in a failure in precise microstructure controlling in heat treatment. In this work, in situ observations were carried out to characterize the grain evolutions of 300M steel with varying holding time, holding temperatures, and initial microstructures. The intriguing finding was that the grain refinement by austenization of 300M steel was followed by a dramatic grain growth in the initial stage of holding (≤~600 s), and with increasing time (~600–7200 s), the average grain size appeared to have a limit value at specific temperatures. The austenization process accelerated the grain growth by generating large quantity of grain boundaries at the initial stage of holdings, and the growth rate gradually slowed down after holding for ~600 s because the driven force was weakened due to the reduction of grain boundary energy. The initial structure and the initial grain size of 300M steel had no obvious influences on the grain size evolutions. The mechanisms of grain growth were analyzed based on in situ observations and transmission electron microscope (TEM) characterizations. A grain evolution model considering the grain boundary migration of 300M steel was established for the isothermal holding process. Good agreement was obtained between the in situ observation results and the model calculation results. This investigation aimed to understand fundamentally the grain evolutions of 300M steel in the isothermal holding process.

Mechanical behaviour of rapidly solidified aluminium with multiple twinned nanograins: A molecular dynamics simulation study

The mechanical behaviour of rapidly solidified aluminium with multiple twinned nanograins is investigated by a large-scale molecular dynamics simulation. The nanocrystalline structures with mean grain sizes ranging from 3.1 nm to 24.4 nm are prepared by quenching liquid at appropriate cooling rates. It is found that the mechanical properties of rapidly solidified aluminium with multiple twinned nanograins are dependent on its grain size. The flow stress displays inverse Hall-Petch relationship in the present grain-size range. The multiple twinned nanocrystalline aluminium with large and small grain sizes respectively displays different deformation mechanisms during the uniaxial tensile processes. For that with large grain size, the dominated deformation mechanism is twin boundary migration, though co-existing with dislocation activities. And the morphologies of multiple twinned nanograins remain nearly unchanged during the deformation process. For that with small grain size, the deformation is governed by the softening mechanisms of grain growth and detwinning. The grain boundary migration and grain rotation are found in the grain growth process. And the morphologies of multiple twinned nanograins gradually transform into parallel twins or monocrystal.

Mechanism of grain growth and excellent polarization, dielectric relaxtion of La3+, Nd3+ modified PZT nano-films prepared by sol–gel technique

Ferroelectric PbZr0.52Ti0.48O3 film and its partial substitutions by rare earth ions La3+ and Nd3+ Pb0.9(La/Nd)0.1Zr0.52Ti0.48O3, grown on Pt(111)/Ti/SiO2/Si(100) substrates, were prepared via sol–gel and rapid thermal processes. Structural characterization by X-ray diffraction and scanning electron microscopy showed that Pb(Zr0.52Ti0.48)O3 and Pb0.9La0.1(Zr0.52Ti0.48)O3 films are of (111) preferred orientation but Pb0.9Nd0.1(Zr0.52Ti0.48)O3 is more inclined to (100) reflection though both are of tetragonal perovskite structure. The results indicate that the piezoelectric properties of PZT thin films can be improved by doping La3+ and Nd3+ substituted A-site. The d33 can be dramatically improved by doping La3+. Moreover, Pr of Pb(Zr0.52Ti0.48)O3 films reaches up to 120.53 µC/cm2, while the doping samples present relatively inferior ferroelectric hysteresis loops (PrLa = 64.32, PrNd = 53.17 µC/cm2), greater dielectric constants, higher dielectric loss and lower leakage current than the undoped Pb(Zr0.52Ti0.48)O3 sample. And meanwhile, the samples showed a typical non-Debye dielectric spectroscopy of multiple quantum relaxation time distribution observing from the Cole–Cole plot at room temperature.

Grain growth kinetics and electrical properties of CuO doped SnO2-based varistors

Up to now, attempts for developing coarse-grained SnO2-based varistors which exhibit high nonlinearity property at lower voltage have become a challenge without any prominent result because of its unknown grain growth mechanism. In this study, the effect of CuO addition to SnO2-based varistors as a grain growth enhancer additive on microstructural development, grain growth kinetics, and electrical properties was investigated. The characterization of grain growth kinetics showed that CuO addition encouraged grain growth and enhanced the grains size as it could be seen in the activation energy which decreased from 594 kJ/mol to 364 kJ/mol. In the samples with a low amount of CuO, the solute drag force is the controlling mechanism of grain growth. By further addition, the mechanism changed to the Sn4+ solution-precipitation in CuO-rich liquid phase. Also, the electrical properties of CuO doped samples showed that they are so promising for low voltage applications.

TEM observation of general growth behavior for silver electroplating on copper rod

The electroplating process is essential to process Cu-Ag complex wires. Therefore, general growth behavior of silver film plated on copper substrate has an instructive effect on the subsequent processes of pinching-out and annealing. In this paper, the substrate, the interface and the coated silver film are mainly characterized in terms of transmission electron microscope (TEM), especially the techniques of selected area electron diffraction (SAED) and displaced-aperture dark field (DADF). The silver grains are random distributed in the intermediate layer between the copper substrate and the coated silver film. Columnar grains with the orientation of {110}//axis direction, are observed prolongating along the normal direction of copper rod in the outer coated film. Two kinds of competing mechanism for grain growth are put forward to reason for the different growth behaviors during electroplating process, on the basis of the results deduced by TEM. Specifically, the formation of the intermediate layer is governed by the chemical reactions, while the electrochemical reactions are responsible for the generation of silver film.

Fabrication and microstructure of liquid sintered porous SiC ceramics through spark plasma sintering

6H-SiC porous SiC ceramics accompanied by the formation of large clubbed and tabular grains were successfully fabricated at 1800 °C under an applied pressure of 50 MPa for 5 min by spark plasma sintering technique without introducing any pore-creating agent. The phase composition, microstructure and properties of porous SiC ceramics were investigated. The measured porosity of the specimens was 35.7% and the mean pore size varied in the range of 0.3–3 μm. The sintered specimen had a strength of 103 MPa, a fracture toughness of 2.89 MPam1/2, and a density of 1.85 g/cm3. The mechanisms of porous structure and grain growth were elucidated.

Dynamic anisotropic grain growth during superplasticity in Al–Mg–Mn alloy

The microstructural mechanisms of dynamic anisotropic grain growth during superplasticity in a quasi-single phase Al–Mg–Mn alloy were characterized. The tensile superplasticity with 320% elongation was mediated by grain boundary sliding accompanied by rigid grain rotation with a limited crystallographic slip. The deformed sample exhibited a bimodal microstructure. Some grains maintained their original size and equiaxed morphology during superplasticity, whereas the others became elongated more than twice in aspect ratio and were composed of equiaxed subgrains that were aligned in the tensile axis. These microstructural features were possibly attributed to a rotation-coupled grain coalescence accompanied by grain boundary sliding.

Effect of Strain Restored Energy on Abnormal Grain Growth in Mg Alloy Simulated by Phase Field Methods

Abnormal grain growth of single phase AZ31 Mg alloy in the spatio-temporal process has been simulated by phase field models, and the influencing factors of abnormal grain growth are studied in order to find the ways to control secondary recrystallization in the microstructure. The study aims to find out the mechanisms for abnormal grain growth in real alloys. It is shown from the simulated results that the abnormal grain growth can be controlled by the strain restored energy. Secondary recrystallization after an annealing treatment can be induced if there are grains of a certain orientation in the microstructure with local high restored energy. However, if the value of the local restored energy at a certain grain orientation is not greater than 1.1E0, there may be no abnormal grain growth in the microstructure.

Secondary recrystallisation behaviours of grain-orientated silicon steel produced by TSCR process

ABSTRACTGrain-oriented electrical steel is mainly used as the core material in electrical transformers, and its magnetic properties are closely related to the sharpness of Goss texture formed by secondary recrystallisation during high-temperature annealing. However, the mechanisms of abnormal Goss grain growth are still disputed in the literature. In this paper, grain-oriented silicon steel strips with AlN as main inhibitor were produced by thin slab casting and rolling process, and the evolution of microstructure and texture during high-temperature annealing was investigated. Moreover, the mechanisms of secondary recrystallisation were further discussed. The results show that secondary recrystallisation is actually a multi-stage process, and different mechanisms are at work at different stages. Coincident site lattice grain boundaries play an important role at the early stage of secondary recrystallisation, however, the role of high-energy grain boundaries and solid-state wetting should be more important at the later stage of secondary recrystallisation.

Micron-sized columnar grains of CH3NH3PbI3 grown by solvent-vapor assisted low-temperature (75 °C) solid-state reaction: The role of non-coordinating solvent-vapor

The preparation of hybrid perovskite films with large columnar grains via low-temperature solid-state reaction remains a big challenge. Conventional solvent annealing using DMF, DMSO and ethanol, etc. fails to work effectively at low temperature (<100 °C). Here, we comprehensively investigated the effects of non-coordinating solvent vapor on the properties of perovskite film, and obtained micron-sized columnar grains (with an average grain size of 1.4 μm) of CH3NH3PbI3 even at a low temperature of 75 °C when annealed with benzyl alcohol vapor. The perovskite solar cells based on benzyl-alcohol-vapor annealing (75 °C), delivered much higher photovoltaic performance, better stability and smaller hysteresis than those based on conventional thermal annealing. Additionally, a champion power conversion efficiency (PCE) of 15.1% was obtained and the average PCE reached 12.2% with a tiny deviation. Finally, the mechanism of solvent annealing with non-coordinating solvent was discussed. Moreover, we revealed that high polarity and high boiling point of the solvent used for generating vapor, was critical to grow micron-sized columnar grains at such a low temperature (75 °C). This work will contribute to understanding the mechanism of grain growth in solvent annealing and improving its facility and effectiveness.

Novel mechanisms for solid-state processing and grain growth with microstructure alignment in alnico-8 based permanent magnets

An estimated 750,000 new hybrid electric and plug-in battery vehicles, most with permanent magnet synchronous alternating current (PMAC) drive motors, took to the road in 2016 alone. Accompanied by 40% year over year growth in the EV market significant challenges exist in producing large quantities of permanent magnets (on the order of tens of millions) for reliable, low-cost traction motors [IE Agency, Energy Technology Perspectives (2017)]. Since the rare earth permanent magnet (REPM) market is essentially 100% net import reliant in the United States and has proven to have an unstable cost and supply structure in recent years, a replacement RE-free PM material must be designed or selected, fully developed, and implemented. Alnico, with its high saturation magnetization and excellent thermal stability, appears to be uniquely suited for this task. Further, while alnico typically has been considered a relatively low coercivity hard magnet, strides have been made to increase the coercivity to levels suitable for traction drive motors [W Tang, IEEE Trans. Magn., 51 (2015)]. If a simple non-cast approach for achieving near [001] easy axis grain aligned permanent magnets can be found, this would allow mass-produced final-shape anisotropic high energy product magnets suitable for usage in compact high RPM rotor designs. Therefore, a powder metallurgical approach is being explored that uses classic compression molding with “de-bind and sinter” methods, where a novel applied uniaxial loading, and an applied magnetic field may create final-shape magnets with highly textured resulting microstructures by two different mechanisms. Results indicate a positive correlation between applied uniaxial load and resulting texture (Fig. 1), along with benefits from using an applied magnetic field for improved texture, as well. The apparent mechanisms and resulting properties will be described using closed loop hysteresisgraph measurements, EBSD orientation mapping, and high-resolution SEM.

Understanding the grain-growth mechanism of high-performance organic semiconducting diphenyl-dibenzothiopheno[6,5-b:6′,5′-f]thieno[3,2-b]thiophene molecules

We report here our investigation on the grain-growth mechanism of diphenyl-dibenzothiopheno[6,5-b:6′,5′-f]thieno[3,2-b]thiophene (DPh-DBTTT), which was recently published as a high-performance organic semiconductor. Atomic force microscopy revealed that unconventional needle-like structures grew on the surface of DPh-DBTTT thin films, and these structures became more dominant as the film thickness increased and the deposited film's surface temperature decreased. In combination with the crystal structure simulations, the grazing-incidence wide-angle x-ray scattering data indicated that the DPh-DBTTT molecules preferred an edge-on orientation near the interface between the substrate and DPh-DBTTT thin film, while the needle-like structures consisted of a face-on arrangement of the molecules. We suggest that this structural change originates from the large step-edge energy barrier of the DPh-DBTTT molecules. Our findings would be highly valuable to the design of new high-performance organic semiconducting materials and optimization of the conditions of thin-film deposition.

Grain Thermal Instability and Heat Treatment Technology for Aluminum Alloy Welded by FSW

The grain thermal instability and heat treatment technology was researched to meet the special requirements of 2219 aluminum alloy plate was obtained by friction stir welding. The microstructures and mechanical properties were investigated correspondingly in order to study the mechanism of abnormal grain growth and put forward the corresponding control measures. The results show that there are no enough precipitation phases existed in the grain boundary of nugget after the friction stir welding, grain is of thermal instability and the grain grows easily, the grain size is up to a millimeter level in the current heat treatment. Through the special recrystallization technology with high temperature oscillating heat treatment to hold the fine grain at short time, the ductility of welded joints can be restored and softening problem of joints can be eliminated, which will improve the performance in mechanical intensity of aerospace aluminum alloy.

Kinetics and mechanisms of the transformation of precipitated amorphous calcium phosphate with a Ca/P ratio of 1:1 to calcium pyrophosphates

The kinetics and mechanisms of crystallization, phase transformation and grain growth in amorphous calcium pyrophosphate (ACPP) derived from the fast nitrate synthesis were studied at 600–800°C. The high-temperature calcium pyrophosphate α-CPP crystallized as the major and metastable phase (α/-CPP), and some stable β-CPP polymorph also formed. The activation energies were determined with 80kJ/mol for α/-CPP grain growth, as well as with 525, 190 and 180kJ/mol for β-CPP nucleation, formation and grain growth during α/-CPP to β-CPP transformation under isothermal annealing, respectively. The transformation processes were mainly proceeded by surface diffusion and additional volume diffusion as the temperature was increased. The metastable α/-CPP crystallization was associated with the Ostwald’s step rule. The formation of two polymorphs from the same ACPP was assumed to occur from two different short-range order units formed in ACPP before the onset of crystallization.

Orientation Relationships in Al0.7CoCrFeNi High-Entropy Alloy.

A detailed microstructural evaluation was executed on the crystallographic texture as well as the mechanisms for nucleation, phase transformation, and grain growth in a Al0.7CoCrFeNi high-entropy alloy. The microstructure and crystallographic orientations were characterized by electron backscatter diffraction, and the chemical composition variations by energy-dispersive X-ray spectroscopy. The cast Al0.7CoCrFeNi alloy started in the BCC phase and partially transformed into the FCC phase. It was found that the Pitsch orientation relationship (OR) dominates the nucleation mechanism of the FCC phase; however, deviations with respect to the Pitsch OR are observed and are attributed to the differently sized atoms forming an ordered B2 phase in the alloy causing lattice distortions. The dual phase BCC-FCC microstructure contains FCC Widmanstätten plates oriented parallel to the {110}BCC planes of the parent grain. It was found that the crystal orientation distribution after the BCC-FCC phase transformation is confined and is explained as a product of the governing mechanisms.

Sintering of GGAG:Ce3+, xY3+ transparent ceramics in oxygen atmosphere

Abstract Cerium doped gadolinium gallium aluminum garnet ceramics (GGAG:Ce) have promising applications such as medical imaging, high power WLEDs and other high energy radiation detection due to their excellent and unique combination of chemical and physical properties. Partial substitution of gadolinium with yttrium can improve some of the optical and scintillation properties even further. The goal of this work is to research the effect of Y 3+ to the sintering of GGAG ceramics. The optical transmittance, micro-structure and grain growth kinetics have been investigated. This research indicates that the doping of Y 3+ ion would transfer the main sintering mechanism for grain growth from solid state mass transport to liquid phase mass transport, thereby reducing the activation energy of the sample.

Novel coalescence-driven grain-growth mechanism during annealing/spark plasma sintering of NiO nanocrystals

A novel oriented attachment growth mechanism of nanograins has been suggested for spark plasma sintering and annealing of NiO nanopowder. A hierarchical microstructure built by cube-shaped nanograins was observed during pressure-less spark plasma sintering at temperatures above 1000°C and when the powders were annealed in ambient atmosphere at and above 700°C. Irrespective of the processing method used, a rotation assisted grain attachment of irregularly shaped nanocrystals results in such a microstructure with parallel epitaxy, twinning or line defects at the grain interface. The unique microstructural features and the governing factors for the attachment process have been discussed.