Occurrence Of Abnormal Grain Growth Research Articles

Conditions for the Occurrence of Abnormal Grain Growth Studied by a 3 D Vertex Dynamics Model

The conditions for the nucleation of abnormal grain growth were investigated using a three dimensional vertex dynamics model. Potentially abnormal growing grains characterized by their size and topological class, respectively and embedded in an isotropic grain ensemble were subjected to annealing varying their grain boundary properties. The simulation results indicate that the classical mean field approaches underestimate the role of the grain boundary energy advantage, while the impact of a mobility advantage is overestimated.

Effect of Annealing on Microstructural Development and Grain Orientation in Electrodeposited Ni

Thick (up to 5 mm) Ni electrodeposits were produced by the pulsed electrodeposition (PED) technique. The PED-Ni was investigated in planar and cross-sections using high resolution scanning electron microscopy. Grain size and local texture were studied by electron backscatter diffraction. Thermal stability and grain growth behaviour were investigated using in-situ annealing in the transmission electron microscope. It is observed that columnar grains are present in the material and that the orientation of grains is not uniform. Textures and in-situ annealing behaviour are compared to previous data on nanocrystalline PED-Ni and Ni-Fe, where a subgrain coalescence model adopted from recrystallization is used to describe the occurrence of abnormal grain growth upon annealing and where twinning was found to be responsible for the texture development.

Grain growth behavior of coarse-grained austenite in a Nb–V–Ti microalloyed steel

Grain growth of coarse-grained austenite during equalization in a simulated thin slab casting and direct rolling process (TSDR) has been studied on a Nb–V–Ti microalloyed steel. Coarse-grained austenite produced by reheating at 1350 °C for 90 min were reheated to four equalization temperatures (1100, 1150, 1200 and 1250 °C) for 30 min, then the austenite grain sizes and precipitates were examined. The experimental results show that coarse-grained austenite keep its wide range of grain sizes at elevated equalization temperatures. Ti-rich carbonitrides retain its strong pinning effect on grain boundary of coarse-grained austenite at temperatures below 1250 °C. The occurrence of abnormal grain growth in coarse-grained austenite may be difficult when particle coarsening occurs as it is hard for the smaller grains which has the larger driving forces to grow into neighboring larger grain matrix.

Effect of Grain Orientation on the Abnormal Grain Growth with Magetoplumbite Crystal Structure

The nucleation and growth behavior of magentoplombite has been observed during the abnormal grain growth in the presence of liquid phase. Abnormal grain growth was explained in terms of grain orientation due to liquid phase. It has been observed that the nuclei formation for abnormal grain growth was driven by the capillary force due to the liquid phase. Because magnetoplumbite crystal structure has an anisotropic crystal nature, abnormal grain growth with faceted grain shape was observed depending on its chemical composition. The formation of nuclei and particle orientation was important for the occurrence of abnormal grain growth.

Abnormal Grain Growth Induced by Boundary Segregation of Solute Atoms

Abnormal grain growth (AGG) proceeds in case that normal grain growth is inhibited. It has long been known that the inhibition involves finely dispersed particles and/or the development of specific textures. There is another strong obstacle against the grain boundary (GB) motion; the solute atoms can reduce their energy by moving from the bulk into a GB. Resultant interaction between the solute atoms and a GB makes the GB motion more difficult. However the role of the GB segregation effect on AGG has not been clarified. In this study we simulate the 2D and 3D grain growth accompanying boundary segregation of solute atoms by using a phase-field model. It is shown that the segregation plays an important role on the occurrence of AGG. The boundary-segregation-induced AGG can take place when the average driving force of grain growth approaches a critical condition for pinning-depinning transition in solute-drag atmosphere.

Grain Growth Behavior in Sintered Nd-Fe-B Magnets

The Nd 2Fe 14B grain growth behavior in sintered Nd-Fe-B magnets was quantitatively described. The effects of sintering temperature and time, and alloy powder size and its distribution on grain growth process were analyzed. Hence, possible grain growth mechanisms in these magnets were qualitatively discussed. The Nd 2Fe 14B grain growth proceeded at quite a high rate in the initial 0 ∼ 1 h of sintering and from then onwards the grain growth rate decreased. A large average particle size or a wide particle size distribution of initial alloy powders was found to remarkably accelerate the grain growth process and even result in the occurrence of abnormal grain growth. On the basis of experimental results, two grain growth mechanisms were considered to operate during sintering of Nd-Fe-B magnets, that is, dissolution and re-precipitation of Nd 2Fe 14B particles, and Nd 2Fe 14B particle growth by coalescence. It was believed that Nd 2Fe 14B particle growth by coalescence not only produced a large average grain size and a wide grain size distribution, but also was the fundamental reason for the formation of abnormally large grains in the microstructure of sintered Nd-Fe-B magnets.

Study of the Development of Heterogeneous Grain Size in the Through-Thickness Direction of Hot Rolled ELC Steels

The present study aims to investigate the mechanism of the development of abnormal grain sizes in the through-thickness direction of hot rolled steel strips. For this purpose, industrially prepared steel strips were further hot rolled in a laboratory hot rolling mill, setting a variety of rolling parameters. As found, the deformation rate in the hot rolling practice exerts an important role in explaining the mechanism of abnormal grain growth, especially in the close vicinity of the strip surface. Furthermore, the influence of the cooling penetration depth, induced by the roll contact was examined closely, as this phenomenon might support abnormal grain growth mechanisms. Additional information was found in performing a texture analysis in the throughthickness direction of the steel strips, in accordance with the optical metallurgical survey of the microstructures. It will be shown that, the combination of particular hot rolling parameters provokes the occurrence of abnormal grain growth in the through-thickness direction of the ELC steel strips. These particular conditions were considered to be related to the finish hot rolling temperature and thus the roll cooling penetration depth imposed on the steel strip, the finishing reduction degree and especially the strain rate conditions. Moreover, the observed abnormal grain growth is sensitive to the coiling temperature applied. From the experiments, it can be concluded that the mechanism of the formation of a large grained ferrite band below the strip surface is strongly influenced by the development of a fine-grain ferrite layer at some distance below the strip surface. The existence of this layer of very small ferrite grains can be explained on the basis of texture analysis and calculations based on literature data. In this way, it was considered that dynamic recrystallisation of austenite at some depth below the steel strip surface is of most significance in supporting the development of abnormally large ferrite grains. In this paper, further considerations on the mechanism of the abnormal grain growth phenomenon will be dealt with.

Statistical model of magnetization reversal in Nd-Fe-B sintered magnets

Statistical model of magnetization reversal was used to simulate the magnetization reversal behavior in the sintered Nd-Fe-B magnets with double grain-size distributions due to the abnormal grain growth (AGG). The magnetic properties and mechanical properties due to the formation of AGG grains in Nd-Fe-B sintered magnets were tested. The results show that the magnetic properties, especially the rectangularity were severely deteriorated after the formation of the AGG grains and a step was shown on the demagnetization curve, and the occurrence of AGG may account for the poor rectangularity and existence of the step on demagnetization curve according to the statistical model of magnetization reversal. The fracture toughness and bending strength are lowered because of the stress concentration in the AGG grains. The SEM images show that the formation of AGG grains is caused by the solid sintering due to the absence of RE-rich phase. Statistical model of magnetization reversal can qualitative by explain the dependence of the magnetization reversal behavior on the grain size in the Nd-Fe-B sintered magnets.

Individual grain orientations and texture development of nanocrystalline electrodeposits showing abnormal grain growth

The electron backscatter diffraction (EBSD) technique has been used to determine grain orientations of abnormally grown grains in nanocrystalline Ni electrodeposits upon annealing. The results show that the first grown grains have a 〈3 1 1〉//ND orientation. Upon annealing further grain growth occurs and the preferred alignment of the abnormally growing grains changes from 〈3 1 1〉//ND to 〈1 1 1〉//ND. The subgrain coalescence model adopted from recrystallization is used to describe the occurrence of abnormal grain growth, and energy considerations are put forward for explaining the dominance of the 〈1 1 1〉//ND texture component after longer annealing treatments.

Preparation of slaking resistant CaO aggregate from lightweight CaCO 3 with oxide addition

CaO aggregate was sintered from reagent-grade lightweight CaCO 3 powder by the addition of 0–20% (molar ratio) MgO and ZrO 2, respectively. The results showed that the CaO derived from lightweight CaCO 3 was highly sinterable and compact CaO aggregate with relative density above 96% was obtained after sintering at 1400 °C for 2 h, but further increase of compactness was restrained due to the occurrence of abnormal grain growth. The densification of the aggregate was promoted due to the behavior of oxide addition on restraining the grain growth of CaO. With increasing the amount of oxide addition, the microstructure of CaO aggregate underwent a restructuration process. Homogeneous microstructure, with well growing MgO grains occupying most of the boundary triple points of CaO grain, formed by the addition of 20% MgO. Especially when 20% ZrO 2 was added, CaZrO 3 layer formed around CaO grains. The slaking resistance of the aggregate was appreciably improved due to the promotion of densification, the formation of CaO solid solution (while MgO added) and the modification of microstructure.

摩擦撹拌処理を施した５０８３アルミニウム合金の組織とその高温安定性

5083 aluminum alloy plate of 3 mm thickness was friction stir processed under processing conditions of various rotation speeds and travelling speeds from 300 to 2000 rpm and 100 to 500 mm/min, respectively. Microstructure on cross section of the friction stir processed plate was observed by means of an optical microscopy and SEM-EBSP method. High temperature stability of microstructure in stir zone was evaluated by annealing the specimens at temperatures ranging from 523 to 823 K in a salt bath. Grain refinement was effectively achieved in the stir zone by friction stir processing and grain size was decreased as the rotation speed and/or the travelling speed decreased. Minimum grain size of 1.76 μm was obtained when processed with a rotation speed of 300 rpm and a travelling speed of 100 mm/min. Abnormal grain growth appeared in every specimen when annealed at 823 K regardless of the processing conditions. Critical temperatures of the occurrence of abnormal grain growth tended to decrease as the rotation speed was decreased. No significant change in microstructure was observed at these high temperatures in the specimen when the region that was contacted to shoulder of the tool was removed by mechanically. Possibility of a residual strain in stir zone given by the tool was discussed regarding to the high temperature stability of microstructure fabricated by friction stir processing.

A comparison of thermal stability in nanocrystalline Ni- and Co-based materials

In this paper the microstructural development upon annealing of nanocrystalline Ni- and Co-based electrodeposits is described. New investigations on Ni, Ni–Fe, and Co–P are compared with previous results on Ni, Co, and Ni–P in terms of microstructural changes and stabilizing mechanisms. The conclusions are: pure nanocrystalline Ni and Co are stabilized by impurities in the grain boundaries. In the case of Co, also an allotropic phase transformation influences the occurrence of abnormal grain growth. Alloying and/or adding solutes is found to increase thermal stability. While in Ni-20 at.% Fe the ordering transformation is expected to be the reason for stabilization, in strongly segregating systems (Ni–P and Co–P) the stabilizing effect is the decrease in grain boundary energy due to solute segregation. After precipitation, Zener pinning still hinders grain boundary migration, but not sufficient to stabilize the nanocrystalline structure.

Effect of tungsten carbide additions on the microstructure and properties of hot-pressed alumina

Studies published in the literature have shown that alumina reinforced with refractory carbides is a good alternative to cemented carbides, for cutting tools and wear-resistant applications. In this work, alumina reinforced with tungsten carbide was homogenised and mixed in a planetary ball mill and subsequently hot-pressed at 1450 °C under 20 MPa in flowing argon. Hot-pressing resulted in a material with a hardness and fracture toughness of 19 GPa and 7.1 MPa m 1/2, respectively. X-ray diffraction analysis showed the presence of alumina and WC. No other new crystalline phase was identified. The sintered material showed a very homogeneous distribution of small dark grains of WC in an alumina matrix, which appears to have very large grains with very clean grain boundaries. The WC grains tend to lie on grain boundaries but can also be seen at triple points. The material also shows large alumina grains containing WC inclusions, which points to the possible occurrence of abnormal grain growth.

Modeling of abnormal grain growth in textured materials

The effect of initial texture on the occurrence of abnormal grain growth (AGG) was modeled via a 3D Monte-Carlo approach. A diagram of texture characteristics which give rise to AGG was derived. AGG was associated with periods of linear growth behaviour of the largest grain within the microstructure.

Interaction Between Second-Phase Particle Dissolution and Abnormal Grain Growth in an Austenitic Stainless Steel

The continuing development of stainless steels has resulted in complex steel compositions with substantial amounts of alloying elements. The benefits of such additions invariably come attached to unavoidable disadvantages. One of the most critical item is the potential microstructural instability of the material. Alloying elements may be in a supersaturated solid solution, in which the precipitation of carbides, nitrides, borides and intermetallic phases occurs in a wide range of temperatures. In order to dissolve the mentioned precipitates, solution annealing is commonly performed. However, at the temperature range in which this treatment is carried out, the onset of abnormal grain growth can occur. The interaction between the dissolution of these second-phase particles and the occurrence of abnormal grain growth is investigated in this work. This study also shows that the thermodynamics and the kinetics of dissolution of precipitates may be used to predict whether abnormal grain growth takes place.

The effects of deformation and pre-heat-treatment on abnormal grain growth in RENÉ 88 superalloy

When an extruded strain-free RENE 88 Ni-base superalloy about 1 to 2 µm in grain diameter is heattreated at 1150°C, abnormal grain growth (AGG) begins after 50 hours. When heat-treated at 1200 °C, AGG occurs at 15 minutes. Some of the grain boundaries are faceted with hill-and-valley structures when observed in transmission electron microscopy (TEM), and the occurrence of AGG is consistent with the boundary step and dislocation mechanism for the migration of singular boundaries with faceted shapes, as observed and proposed in other pure metals and alloys. The dissolution of abundant γ′ precipitates (with a solvus temperature of 1107 °C), which are coherent with the matrix and hence strongly pin the grain boundaries, does not cause AGG during early stages of heat-treatment at 1150 °C. Small deformations drastically alter the AGG behavior. When deformed to 4 pct, AGG begins after heat-treating for 10 minutes at 1150 °C, compared to the apparent incubation time of 50 hours for an undeformed specimen, and very large abnormal grains are produced. With increasing deformation to 6 and 9.2 pct, the abnormal grain size decreases. These results are qualitatively similar to those observed in Cu. This deformation effect on AGG is attributed to the absorption of lattice dislocations in the grain boundaries, which produces nonequilibrium structures that, in turn, can apparently cause rapid boundary migration. When heat-treated at 1200 °C, the largest abnormal grains are found in the specimens deformed to 2 pct. When the initial grain size is increased to about 14 µm by heattreating the extruded alloy at 1150 °C for 30 minutes, similar low deformation effects on AGG are observed. When these specimens are deformed to 10, 13, and 15.2 pct, primary recrystallization occurs during the heat-treatment at 1150 °C, and large abnormal grains are again produced because of the small recrystallized grain size. Therefore, there are two peaks in the grain size vs deformation curve after heat-treating at 1150 °C for 1 hour. A pre-heat-treatment of this alloy at 1050 °C below the solvus temperature of the γ′ phase greatly reduces the size of the abnormal grains obtained in the specimen deformed to 4 pct after the heat-treatment at 1150 °C, probably because some recovery of the dislocations takes place at grain boundaries during the pre-heat-treatment. The deformation effect on AGG observed in this alloy is qualitatively similar to that previously observed in Cu and appears to be consistent with the boundary step and dislocation mechanism for AGG.

Piezoelectric ceramics based on Bi3TiNbO9 from mechanochemically activated precursors

Ceramics based on the Aurivillius type structure compound Bi 3 TiNbO 9 were processed by natural sintering and hot pressing of amorphous precursors obtained by mechanochemical activation of oxides and carbonates mixtures. Synthesis, grain growth and sintering take place in a single thermal treatment at moderate temperatures in comparison with ceramics processed from crystalline precursors. The influence of the processing parameters on the ceramic texture and microstructure at mesoscopic scale were studied by XRD and quantitative optical microscopy. It was possible to obtain both isotropic and textured ceramics. The occurrence of abnormal grain growth was observed under some conditions. An ample electrical characterisation of the ceramics was carried out comprising dielectric, ferroelectric and piezoelectric properties. The influence of the microstructure on the properties and the interest of the materials as high temperature piezoelectrics are discussed.

The Temperature Dependence of Abnormal Grain Growth and Grain Boundary Faceting in 316L Stainless Steel.

When commercial 316L stainless steel specimens are heat-treated in a single phase state at 1 100°C, abnormal grain growth (AGG) occurs and some grain boundaries are observed to be faceted with hill-and-valley structures in transmission electron microscopy. Some segments of these faceted grain boundaries are expected to be singular. When heat-treated at 1 300°C normal grain growth occurs with all grain boundaries smoothly curved. These grain boundaries are expected to be atomically rough. At 1 200°C AGG still occurs but there is no excessively large grain as in the specimen heat-treated at 1 100°C. This correlation between the grain boundary structure and grain growth is consistent with those observed previously in pure metals, oxides, and a single phase model alloy. The occurrence of AGG with faceted grain boundaries is attributed to grain boundary movement with boundary steps either produced by two-dimensional nucleation or existing at the junctions with dislocations. As the grain boundaries become rough at 1 300°C normal growth occurs because the grain boundaries migrate continuously with their rate expected to increase linearly with the driving force arising from the size difference. If a specimen heat-treated at 1 100°C is further heattreated at 1 300°C, the AGG mode appears to switch to normal growth.

Effects of SiO2, CaO2, and MgO Additions on the Grain Growth of Alumina

When pure alumina is sintered at 1620°C, normal grain growth occurs with equiaxial grains and curved grain boundaries. When 100 ppm of SiO2 together with 50 ppm of CaO is added, abnormal grain growth (AGG) occurs with large grains elongated with straight grain‐boundary segments in the direction of the basal planes. Some of the fine matrix grains also have straight grain boundaries, and ∼10% of the grain boundaries of the matrix grains are faceted when observed by transmission electron microscopy (TEM). Some of these grain boundaries are expected to be singular with low‐energy structures corresponding to the cusps in the polar plot of the grain‐boundary energy against the inclination angle. No frozen liquid is found at the grain triple junctions and grain boundaries by TEM. When 600 ppm of MgO is added together with 100 ppm of SiO2 and 50 ppm of CaO normal growth occurs. The grain boundaries are curved when observed via optical microscopy and TEM and show that all the grain boundaries are defaceted, indicating that they become atomically rough. When sintered at 1900°C after adding 150, 300, or 500 ppm of SiO2, AGG occurs with straight and faceted grain boundaries, similar to the specimens sintered at 1620°C after CaO and SiO2 are added. When MgO is added together with SiO2 and sintered at 1900°C, normal grain growth occurs with rough grain boundaries. High‐resolution TEM observation shows no frozen liquid layer at a grain boundary. The results indicate that the occurrence of AGG in alumina with SiO2 or together with CaO is correlated with the formation of faceted and straight (on large and atomic scales) grain boundaries. It is proposed that these grain boundaries have singular ordered structures with low boundary energies and their growth by lateral step movement can cause the AGG. The addition of MgO causes grain‐boundary roughening and, thus, normal grain growth. The grain boundaries in pure alumina also appear to be rough, and, hence, normal grain growth occurs.

Electrical Properties of Microwave-Sintered ZnO Varistors

Bi2O3-based ZnO varistor materials were prepared by 2.45 GHz microwave, 24 GHz millimeter-wave or conventional sintering processes. Both millimeter-wave and microwave sintering processes are superior to the conventional sintering process for densifying Bi2O3-based ZnO materials. Moreover, the millimeter-wave sintering process not only densified ZnO materials at a higher rate, but also resulted in significantly better varistor characteristics, as compared with the microwave sintering process. A large nonlinear coefficient and low leakage current density were attained by millimeter-wave sintering the samples under 1100°C for 5 min. A density as high as 95% theoretical density (T.D.) has been achieved, accompanied by a nonlinear coefficient (α) larger than 36, and a leakage current density (JL) smaller than 4.1×10-6 A/cm2. The samples had a donor density (Nd) around 2.5× 1024 m-3, a surface density (Ns) around 7.23×1011 m-2 and a potential barrier height (φb) around 2.22 eV. However, the varistor characteristics, including the nonlinear coefficient (α) and leakage current density (JL), degraded markedly for samples sintered at too high a temperature or soaked for too long. This was ascribed to the occurrence of abnormal grain growth, accompanied by the loss of Bi2O3 and ZnO species.