Large Abnormal Grains Research Articles

Interfacial microstructure and mechanical property of the discontinuous short tungsten fiber-reinforced tungsten matrix composites

For tungsten fiber-reinforced tungsten matrix (Wf/W) composites, the interface between W matrix and W fiber has a dominant effect on the mechanical properties. In this study, the interfacial microstructure in the Wf/W composites with or without Y2O3 coating deposited on W fiber is investigated. The crystal structure of the Y2O3 coating composed of nano-sized columnar grains on the W fibers is cubic. And the crystal structure of Y2O3 coating changes to be face centered cubic after sintering in the Wf/W composites. Two kinds of interfaces, including the interface between W matrix and Y2O3 coating, the interface between W fiber and Y2O3 coating, are present in the Wf/W composites. Different orientation relationships are observed in these two kinds of interface. The stable orientation relationship with 111Y2O3 // 01¯1W have the lowest interfacial energy. The Y2O3 interface is essential for improving the toughening behavior of Wf/W composites and impeding the abnormal grain growth on the surface of W fiber sintering at low temperature. The recrystallization nucleus is developed by the bonding between W powder and W fiber during sintering, and then grow fast into W fiber. The apparent activation energy of abnormal grain growth of W fibers is about 242.7 KJ/mol sintering at temperature between 1500 and 1800 °C. The abnormal large grains usually show intergranular brittle fracture and make the W fiber be brittle.

Simulation of strain induced abnormal grain growth in aluminum alloy by coupling crystal plasticity and phase field methods

A mesoscale modeling methodology is proposed to predict the strain induced abnormal grain growth in the annealing process of deformed aluminum alloys. Firstly, crystal plasticity finite element (CPFE) analysis is performed to calculate dislocation density and stored deformation energy distribution during the plastic deformation. A modified phase field (PF) model is then established by extending the continuum field method to consider both stored energy and local interface curvature as driving forces of grain boundary migration. An interpolation mapping approach is adopted to transfer the stored energy distribution from CPFE to PF efficiently. This modified PF model is implemented to a hypothetical bicrystal firstly for verification and then the coupled CPFE-PF framework is further applied to simulating the 2D synthetic polycrystalline microstructure evolution in annealing process of deformed AA3102 aluminum alloy. Results show that the nuclei with low stored energy embedded within deformed matrix tend to grow up, and abnormal large grains occur when the deformation is close to the critical plastic strain, attributing to the limited number of recrystallized nuclei and inhomogeneity of the stored energy.

Importance of uniformity of grain size to reduce dc degradation and improve reliability of ultra-thin BaTiO3-based MLCCs

Excellent direct current (dc)-bias and reliability have become increasingly important for ultra-thin BaTiO3-based multilayer ceramic capacitors (MLCCs). Herein, X5R-MLCCs with a thickness of ∼1 μm are fabricated using BaTiO3 with varied grain size. It is shown that the uniformity of the grain size plays an important impact on the direct current (dc)-bias and the reliability of ultra-thin MLCCs. Uniform grain size, which is indicative of good distribution of doping element contributes to improved temperature stability. By contrast, abnormal large grains induce reinforced space charge polarization. Chips with non-uniform grains exhibit dramatic dielectric constant change under dc-bias due to the high tetragonality and irreversible domain-wall motion. Weibull distribution and highly accelerated life test (HALT) reveal that non-uniform grains contain more oxygen vacancies supported by impedance spectra analysis at 300–450 °C. This study provides a feasible strategy to improve the dc-bias and the reliability of the ultra-thin MLCCs.

Grain Growth Behavior and Electrical Properties of 0.96(K0.46-xNa0.54-x)Nb0.95Sb0.05O3-0.04Bi0.5(Na0.82K0.18)0.5ZrO3 Ceramics.

This study investigated the causes of microstructural changes and the resultant electrical properties according to the sintering temperature of 0.96(K0.46−xNa0.54−x)Nb0.95Sb0.05O3-0.04Bi0.5(Na0.82K0.18)0.5ZrO3 lead-free ceramics by analyzing the correlation between vacancy concentrations and 2D nucleation. When sintered for 4 h, no grain growth occurred for the x = 0.000 composition over a wide temperature range, demonstrating that the existence of initial vacancies is essential for grain growth. As x increased, that is, as the vacancy concentration increased, the critical driving force (ΔGC) for 2D nucleation decreased, and abnormal grain growth was promoted. The number and size of these abnormal grains increased as the sintering temperature increased, but at sintering temperatures above 1100 °C, they decreased again owing to a large drop in ΔGC. The x = 0.005 specimen sintered at 1085 °C exhibited excellent piezoelectric properties of d33 = 498 pC/N and kp = 0.45 due to the large number of large abnormal grains with an 83% tetragonal phase fraction. The x = 0.000 specimen sintered at 1130 °C with suppressed grain growth exhibited good energy storage properties because of its very high relative density and small grain size of 300 to 400 nm.

Enhancement of Nanostructured Ferritic Alloy 14YWT Properties via Heat Treatment for Post-Consolidation Processing

The nanostructured ferritic alloy 14YWT is a promising candidate for in-core use in generation IV nuclear reactors, due to a dense dispersion of insoluble, ultrafine-scale Y-Ti-O nano-oxides, which provide a high degree of irradiation tolerance and thermal stability. This study investigates the effects of heat treatment on the workability of 14YWT, along with the effect of processing history on abnormal grain structures and radial microstructural uniformity. In this study, a 14YWT rod consolidated at 850 °C was heat treated in argon at 1100 °C, 1150 °C, 1200 °C, or 1250 °C for 1 or 8 hours and changes in mechanical properties and microstructure were examined using microhardness and electron backscattered diffraction (EBSD). Two distinct types of large abnormal grains were observed, each with unique processing origins, including one with high and the other with low strain energy. Consolidation via direct extrusion resulted in radial microstructural gradients, where the center of the rod was softer with larger grain sizes and lower strain energies. These gradients persisted and intensified throughout heat treatment. Based upon this work, the recommended heat treatment for increased workability with minimal microstructural change is 1150 °C for 1 hours.

Dielectric properties and electrical response of yttrium-doped Bi2/3Cu3Ti4O12 ceramics

([Formula: see text]Bi[Formula: see text]Cu3Ti4[Formula: see text] ([Formula: see text] 0.00–0.30) ceramics were successfully prepared via the conventional solid-state method. X-ray powder diffraction confirmed the lattice constant gradually decreases with increasing Y[Formula: see text] content. SEM images displayed Y[Formula: see text] substitution for Bi[Formula: see text] gave rise to the large abnormal grains, and the size of abnormal grains became larger with the increase of Y[Formula: see text] substitution. ([Formula: see text]Bi[Formula: see text]Cu3Ti4[Formula: see text] ceramics presented the relatively high dielectric constant of 7400 with the dielectric loss of 0.055 when [Formula: see text] 0.20. The analysis of complex impedance suggested the grains are semiconductive and the grain boundaries are insulating. For pure [Formula: see text]Cu3Ti4[Formula: see text] ceramics, the appearance of additional low-frequency peaks in electrical modulus indicated the grain boundaries are heterogeneous. The investigation of modulus peaks fitting with Arrhenius formula implied that the low-frequency permittivity for all ([Formula: see text]Bi[Formula: see text]Cu3Ti4[Formula: see text] ceramics was ascribed to the Maxwell–Wagner relaxation at grain boundaries. In addition, a set of clear dielectric peaks above [Formula: see text]C associated with Maxwell–Wagner relaxation can be found for all ([Formula: see text]Bi[Formula: see text]Cu3Ti4[Formula: see text] ceramics in the temperature dependence of dielectric constant. This set of clear dielectric peaks showed a tendency to shift to higher temperatures with the increase of Y[Formula: see text] substitution. Meanwhile, a tiny dielectric anomaly at room temperature was found in Y-doped [Formula: see text]Cu3Ti4[Formula: see text] ceramics.

Recrystallization mechanism of abnormal large grains during long growth of CVD-ZnS

Abnormal large grains influencing highly the optical and electrical properties are commonly observed in the bottom part of CVD-ZnS layers. We have analyzed the films using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and infrared transmittance tests. Compared to normal grains, more homogeneous and simpler structures have been observed in abnormal large grains in CVD-ZnS. In addition to the obvious difference in sizes, the crystalline outline and interior structure of normal and abnormal large grains are found to be markedly different. Distorted structures and stacking faults, such as wurtzite phase and polytypes of ZnS, are not detected in the large grain areas. Preferred orientation of abnormal large grains is found to be sphalerite (1 1 1). These results could be explained as due to the recrystallization mechanism of abnormal large grains. Samples with large grains show lower infrared absorption band (at 6.2 µm wavelength) compared to samples with normal size grains. However, the infrared transmittance is found to be lower for the samples with large grains. Microstructure patterns show the presence of plenty of microdefects in boundaries between large grains, which indicates that the recrystallization in the volume with large grains is not as perfect as the recrystallization in hot isostatic pressure process (HIP).

Macro‑meso scale modeling and simulation of surface roughening: Aluminum alloy tube bending

Surface roughening (SR) is one of the most common defects in deformation-based processing worsening surface quality and service performance of forming components, and in-depth understanding of SR is still a non-trivial and challenging issue for defect free processes since this phenomenon is interactively influenced by so many intrinsic and extrinsic factors. This paper aims to provide a panoramic discussion on the origination and evolution mechanisms of SR considering both intrinsic and extrinsic factors. A macro‑meso scale modeling framework, including Voronoi tessellation based geometrical model and crystal plasticity finite element (CPFE) based constitutive model, is established to explore non-uniform plastic deformation at grain-scale, and then the above framework is numerically implemented and applied into the 5052-O Al-alloy tube bending. The coupling effects of intrinsic factors (i.e., grain size, grain orientation and abnormal large grain) and extrinsic factors (i.e., deformation conditions, strain accumulation and strain rate) on SR are investigated. The results show that SR in plastic deformation stems from the accumulated intergranular non-uniform deformation through thickness direction, indicating that SR is an inherent characteristic and cannot be eradicated. SR is controlled by several factors: grain size, intergranular misorientation, the abnormal large grain, strain accumulation and deformation conditions. In a practical view, these are two effective ways to improve the surface quality by reducing grain size and removing abnormal large grains. From the extrinsic factor, it also makes sense to design the processing parameters reasonably and reduce friction in order to avoid local large deformation.

Elucidating the effect of intermetallic compounds on the behavior of Mg–Gd–Al–Zn magnesium alloys at elevated temperatures

Abstract

Low-temperature sintering and piezoelectric properties of CuO-doped (K,Na)NbO3 ceramics

The lead-free piezoelectric [(K0.485Na0.515)0.935Li0.065](Nb0.99Ta0.01)O3 (KNLNT) ceramics with a high mechanical quality factor (Qm) were prepared at low sintering temperature below 950°C for multilayered piezoelectric devices. Co-doping effect of 1mol% Na2CO3 and 0–1mol% CuO on the densification and piezoelectric properties were investigated. Co-doping effectively decreased the sintering temperature of KNLNT ceramics to 920°C. The co-doped KNLNT ceramics had a bimodal size distribution of large abnormal grains and small matrix grains above 900°C. When the sintering temperature increased above 900°C, a dielectric constant (εr) and loss (tan δ) decreased considerably and concurrently mechanical quality factor (Qm) greatly enhanced. The co-doped KNLNT ceramic with 1mol% Na2CO3 and 0.5mol% CuO had a high Qm and excellent piezoelectric properties when sintered at 940°C: a dielectric constant of 245 and loss (tan δ) of 0.0034, an electromechanical coupling factor (kp) of 0.449, and Qm of 904.

Dynamic abnormal grain growth in tantalum

Dynamic abnormal grain growth (DAGG) is a phenomenon that produces one or more very large, abnormal grains during plastic deformation of polycrystalline material at high temperatures. DAGG was previously observed in commercial-purity molybdenum (Mo) and was used to produce large Mo single crystals of centimeters in length. The present investigation is the first to demonstrate DAGG in commercial-purity tantalum (Ta) sheet, another body-centered-cubic refractory metal. DAGG occurs in Ta at temperatures from 1450–1850°C across strain rates from 3×10−5 to 5×10−4s−1. Grain boundary migration rates during DAGG in Ta are on the order of 10mm/min. DAGG produces large abnormal grains preferentially oriented with the 〈101〉 direction approximately parallel to the tensile axis. A unique observation of this investigation is a preponderance of Σ3 special boundary character along the boundaries of large abnormal grains produced in Ta through DAGG. The propensity toward this special boundary character is a result of a relatively large grain size and strong texture in the polycrystalline material prior to DAGG and the typically low energy of Σ3 boundaries, which suppress boundary migration.

Effects of BN Dopant on Electrical Characteristics of Barium-Excessive BaTiO<sub>3</sub> Ceramics for Laminated Positive Temperature Coefficient Thermistors Fabricated by Tape Casting Technique

The barium-excessive BaTiO3 ceramics with BN addition for laminated positive temperature coefficient (PTC)thermistors are fabricated. The tape casting technique and reduction/reoxidation process are employed into the fabricating procedure. The influences of BN additive on ceramic characteristics are discussed. The addition over 5% leads to rectangular and abnormal large grains. The 5% BN-doped ceramics shows a resistivity of 233 Ω•cm and PTC jump of 3.4 orders of magnitude. The charge carrier density in grain bulk and acceptor state density at grain boundaries are evaluated. The correlation of BN doping amount with room temperature resistivity, PTC jump, reoxidation effect as well as charge carrier and acceptor state density are investigated.

Influence of Sintering Atmosphere on Abnormal Grain Growth Behaviour in Potassium Sodium Niobate Ceramics Sintered at Low Temperature

The present study aims to identify the effect of sintering atmosphere [O 2 , 75N 2 -25 H 2 (mol%) and H 2 ] on microstructural evolution at the relatively low sintering temperature of 1040℃. Samples sintered in O 2 showed a bimodal microstructure consisting of fine matrix grains and large abnormal grains. Sintering in 75 N 2 - 25 H 2 (mol %) and H 2 caused the extent of abnormal grain growth to increase. These changes in grain growth behaviour are explained by the effect of the change in step free energy with sintering atmosphere on the critical driving force necessary for rapid grain growth. The results show the possibility of fabricating (K 0.5 Na 0.5 )NbO 3 at low temperature with various microstructures via proper control of sintering atmosphere.

Microstructure and Tensile Properties of Ultrafine Grained Pure Al Sheets Produced by Accumulative Roll Bonding

Usually the heat treatment in the cyclic ARB passes is indispensable to reduce work-hardening effects and improve interface bonding quality. The possibility of accelerating grain refinement of aluminum sheets with a dimension of 300 mm×50 mm×1 mm is investigated during the ARB process at room temperature, in which the samples are rotated by 180 degree around normal plane axis perpendicular to the rolling plane between the adjacent cycles. By means of optical microscopy and transmission electron microscopy, it shows that the bonding interfaces can not obviously observed after five cycles, and grains are refined to be ~0.5 μm. Tensile tests show the ARB samples exhibit strain hardening behavior after yielding without a sudden fracture even up to seven cycles of ARB. The softening behavior and enhanced ductility was explained by dynamic recovery, the recrystallization process and even abnormal large grains.

Plastic anisotropy and fracture behavior of AZ31 magnesium alloy

The effects of texture and abnormal large grains on the plastic anisotropy and fracture behavior of hot-rolled AZ31 magnesium alloy were investigated. Uniaxial tensile deformation behaviors of samples with tensile axis tilting 0°, 15°, 30°, 45°, 60°, 75° and 90° to normal direction (ND) respectively were addressed. Tensile deformation anisotropy was observed for samples with different angles to ND. The results show that the specimens with the angle from 0° to 30° exhibit relatively lower yielding strength due to the extension twinning. However, basal slip and prismatic slip are the dominant deformation modes for the specimens with angles larger than 45°. Macro-fractures are parallel to the length direction of abnormal large grains in the specimens with angles less than 60°, while those are serrated fracture edge for specimens with angles 75° and 90°.

The Phase Formation, Microstructure, and Electric Properties of Tungsten Bronze Ferroelectric Sr2NaNb5O15 Ceramics

The high‐density tungsten bronze Sr2NaNb5O15 ceramics without abnormal grain growth were successfully synthesized by a two‐step solid‐state reaction method in this work. The phase evolution and reaction mechanism in SrNb2O6 and NaNbO3 solid solutions were analyzed by X‐ray diffraction and differential scanning calorimetry. The results showed that the Sr2NaNb5O15 phase could exist stably in the temperature range of 1100°–1360°C, and would be decomposed to SrNb2O6 and NaNbO3 phases at temperatures above 1360°C. It was also found that the sintering temperature could significantly affect the density, microstructure, and electric properties of Sr2NaNb5O15 ceramics. When the sintering temperature was 1340°C, the ceramic with a near‐theoretical density and a uniform and fine‐grained microstructure was obtained, exhibiting outstanding electrical properties: ɛm=2210, tan δ=0.023, Tc=280°C, Pr=11.71 μC/cm2, and Ec=16.15 kV/cm. Further increasing the temperature to 1360°C led to the existence of some molten areas and cracks on the surface of ceramics obtained, which could significantly deteriorate the electric properties. In addition, no abnormal large grains (>50 μm) could be found in the system and the reason was also discussed in this paper.

Role of sintering time, crystalline phases and symmetry in the piezoelectric properties of lead-free KNN-modified ceramics

Lead-free KNN-modified piezoceramics of the system (Li,Na,K)(Nb,Ta,Sb)O 3 were prepared by conventional solid-state sintering. The X-ray diffraction patterns revealed a perovskite phase, together with some minor secondary phase, which was assigned to K 3LiNb 6O 17, tetragonal tungsten–bronze (TTB). A structural evolution toward a pure tetragonal structure with the increasing sintering time was observed, associated with the decrease of TTB phase. A correlation between higher tetragonality and higher piezoelectric response was clearly evidenced. Contrary to the case of the LiTaO 3 modified KNN, very large abnormal grains with TTB structure were not detected. As a consequence, the simultaneous modification by tantalum and antimony seems to induce during sintering a different behaviour from the one of LiTaO 3 modified KNN.

Normal and abnormal grain growth in nanostructured gold film

Thin nanostructured gold films were deposited on SiO2 by the sputtering technique at room temperature. Films of different thicknesses were deposited ranging from 2 to 16 nm. The film morphology as a function of the thickness was analyzed by microscopic techniques such as atomic force microscopy and transmission electron microscopy. These analyses allowed us to clarify the growth mechanism of the gold nanograins forming the film: in a first stage of growth (2–6 nm) normal grain growth proceeds; then (8–16 nm) the grain surface energy anisotropy drives the growth of abnormal large gold grains by annihilation of the normal ones. During the abnormal growth other normal grain continue to growth. The normal grain size distribution is showed to be a monomodal log-normal distribution that evolves toward larger mean grain radius continuously following a scaling law. By determination of the grain growth exponent, the kinetic mechanism responsible of the grain growth is demonstrated to be the gold atomic diffusion on grain boundaries. By fitting the experimental data using established theoretical models, the room-temperature gold grain boundary coefficient diffusion and mobility were derived. The abnormal grain grows, manifest itself as a bimodal grain size distribution: with the log-normal distribution of the normal grain size, a second Gaussian grain size distribution rises, shifting toward lower size increasing the film thickness. The abnormal grain growth continues until all the abnormal grain boundaries meet each other.

Effect of post weld heat treatment on mechanical properties and microstructures of friction stir welded AZ31B magnesium alloy

The effects of post weld heat treatment (PWHT) on microstructures and mechanical properties of friction stir welded AZ31B magnesium alloy were investigated. Average grain sizes at the upper region and at the bottom region in the stir zone (SZ) of as welded joint were 9.5 μm and 14.4 μm, respectively. When heat treatment temperature at 673 K, a few relatively large grains were formed at the retreating side in upper region, subsequently the region extended in the stir zone as heat treatment time increased. It is believed that the abnormal grain growth is caused by inhomogeneous distribution of dislocation. The elongation to failure of joint was improved by the heat treatment at 573 K for 7.2 ks. However, the formation of abnormal large grains caused degrading elongation. The yield stress at upper side in SZ of as welded joint was larger than that at bottom side and was decreased gradually as the joint was annealed. Because the additional strength at the upper side was due to the work hardning that was added by the shoulder of tool.

Effect of SiO2 and TiO2 Addition on the Morphology of Abnormally Grown Large Pb(Mg1/3Nb2/3)O3–35 mol% PbTiO3 Grains

Abnormal grain growth (AGG) during the sintering of Pb(Mg1/3Nb2/3)O3–35 mol% PbTiO3 was investigated. When a small amount of SiO2 was added, large abnormal grains with the penetration twin characteristic were observed to appear. AGG, in this case, is because of the twin plane re‐entrant edges that provide coarsening advantage. When TiO2 was added, on the other hand, the abnormal grains were simple cubic in morphology with well‐developed {100} planes. In this case, the coarsening advantage is suggested to be because of grain boundary re‐entrant edges.