Grain Growth Inhibition Research Articles

Transparent Y-α SiAlON:Ce3+ Ceramics Fabricated by Low-Temperature Liquid Phase Sintering Technique

As the output of solid-state lighting and light sources increases, the demand for transparent phosphor inorganic plates with excellent heat resistance is increasing. It is known that α-SiAlON, which has excellent heat resistance as engineering ceramics, shows photoluminescence by stabilized ions, but it is not possible to obtain α-SiAlON bulk single crystals. In this study, we fabricated transparent Y-α SiAlON:Ce3+ ceramics for use as wave conversion materials for high-power solid-state semiconductor light sources. For this purpose, gas pressure sintering, followed by hot isostatic pressing at lower temperatures, were carried out using homogeneous and dense green bodies. In particular, cyclic cold isostatic pressing increased the density and homogeneity of the green body, which promoted the densification at lower temperatures and the efficient disappearance of pores during sintering. As a result of the suppression of grain growth by low-temperature densification, Y-α SiAlON:Ce3+ ceramics with high in-line transmittance were successfully obtained. The transparent Y-α SiAlON:Ce3+ ceramics exhibited photoluminescence due to the 5d-4f transition of Ce3+. The peak wavelength of the emission intensity depended on the concentration of Ce3+, and the luminescence color was in the range of blue to blue-green.

Studying the Effects of Cu Doping on Structure and Photoluminescence Properties of SnO2 Nanoparticle with Its Effectiveness towards the Mineralization of Toxic Industrial Dye

The structural and optical properties of tin oxide (SnO2) nanoparticles doped with Cu2+ ions have been investigated. Suppression of grain growth of tetragonal rutile structured SnO2 was observed with copper doping which generates more and more nucleation sites, resulting weak crystallinity in Cu-SnO2. Morphological analysis by FESEM showed the formation of nanoparticles with uniform size and shape for the pristine sample which altered significantly with copper doping. The optical absorption spectra of the samples show the compression of the band gap (BG) of SnO2 with Cu doping which is due to the creation of defect states linked with oxygen vacancies. The photoluminescence (PL) spectra of the doped samples possess enhanced visible emissions , which is mainly due to the oxygen vacant defects generated by the doping of copper impurity. The degradation efficiency of Cu-SnO2 was found to be higher (91.32%) in degrading Congo red dye as compared to pure SnO2 owing to a better change separation efficiency and prohibition of e − -h + recombination. The effect of photocatalyst loading, initial dye concentration, and pH on degradation efficiency was also studied in detail and the prepared nanocatalysts were found to be stable even after fourth cycle of degradation.

Structural, nonlinear electrical characteristics, and stability against DC-accelerated aging stress behavior of Er-doped 20 nm ZnO-Bi2 O3 -Mn2 O3 -based varistors.

In this research, ZnO-Bi2 O3 -Mn2 O3 varistors manufactured from 20 nm ZnO powder and doped with different amounts of Er2 O3 were fabricated via the conventional ceramic processing method. The effect of various Er2 O3 concentrations (0.5-2.0mol%) on the sintered density, structural improvement, and nonlinear behavior of the 20 nm ZnO-Bi2 O3 -Mn2 O3 varistors was discussed. Different Er2 O3 concentrations exerted considerable influence on the varistors. The addition of large amounts of Er2 O3 led to the inhibition of grain growth by increasing the amount of the Er2 O3 -rich spinel phase. X-ray diffraction analysis showed that the addition of Er2 O3 to the varistor systems led to the development of Er2 O3 -rich phases during sintering. During sintering, the considerable surface area of the nanoparticle powder resulted in numerous interactions in the surfaces of the ceramics manufactured from 20 nm ZnO powder. The electrical behaviors of the varistors were substantially influenced by Er2 O3 . E1 mA evidently increased from 2,144.7 to 5,482.2V/mm with the increment in doping amount. The nonlinear coefficient also dramatically increased with the increase in Er2 O3 content. The ZnO nanoparticle-Bi2 O3 -Mn2 O3 varistors doped with 1.0-2.0mol% Er2 O3 possessed high voltage and nonlinearity and were stable against DC-accelerated aging stress.

Influence of Cr3C2 and VC Content on WC Grain Size, WC Shape and Mechanical Properties of WC-6.0 wt. % Co Cemented Carbides.

In this paper, the influences of Cr3C2/VC content on WC grain size, WC grain shape and mechanical properties of WC–6 wt. % Co cemented carbides were investigated. The results showed that the grain size first rapidly decreased and then slightly decreased with the increasing Cr3C2/VC content, and VC led to finer grain size and narrower size distribution. HRTEM/EDS analysis of the WC/Co interface indicates that the segregation concentration of V is much larger than that of Cr, which may be a strong response to the higher inhibition efficiency of VC. The addition of Cr3C2 induced triangular prism shape WC grains while VC generated stepped triangular prism grains. Despite the grain growth inhibitor (GGI) mechanisms of Cr3C2/VC have been extensively studied in the literature, the doping amount, especially the doping limit, has not been systematically investigated. In this work, the saturated solubilities of Cr and V in cobalt binder phase along with carbon content hare been predicted based on thermodynamic calculations. Based on the theoretical calculations, the doping amount of Cr3C2/VC is designed to be gradually increasing until more or less over their maximum solubilities in the binder phase, thereby investigating the subsequent microstructure and mechanical properties. When the doping of Cr3C2/VC exceeds the maximum solubility in Co phase, Co-rich Cr-carbides and Co-deficient V-carbides would form respectively, which were detrimental to the transverse rupture strength (TRS) and impact toughness. The hardness increased with the increasing Cr3C2/VC content, while the fracture toughness decreased with the increasing Cr3C2/VC content. The TRS initially enhanced and then declined, but the stepped triangular prism shape grains and low fraction of WC/Co interface in WC–6Co–VC cemented carbide led to a more pronounced decline in the TRS. The sample with 0.6 wt. % Cr3C2 addition had good comprehensive mechanical properties, its hardness, fracture toughness and TRS were 1880 kg/mm2, 9.32 MPa·m1/2 and 3450 MPa, respectively.

Very high upper critical fields and enhanced critical current densities in Nb3Sn superconductors based on Nb–Ta–Zr alloys and internal oxidation

The inhibition of Nb3Sn grain growth in the presence of ZrO2 nanoparticles appears to be one of the most promising method for pushing the critical current densities of Nb3Sn superconducting wires to levels that meet the requirements set for the Future Circular Collider. We have investigated the effect of ZrO2 nanoparticles formed by the internal oxidation of Zr on the superconducting properties and microstructure of Nb3Sn formed from Nb-1 wt%Zr, Nb-7.5 wt%Ta, Nb-7.5 wt%Ta-1 wt%Zr and Nb-7.5 wt%Ta-2 wt%Zr alloys. A monofilamentary wire configuration was used, with a 0.22 mm outer diameter Nb-alloy tube containing a core of powdered metal oxide (SnO2, CuO or MoO3) as oxygen source and successive deposits of Cu, Sn and Cu on the outer surface. As determined from inductive measurements, the layer critical current densities of the samples based on Nb alloys with internally oxidized Zr were superior to those based on Nb-7.5 wt%Ta. The samples based on Nb-7.5 wt%Ta-1 wt%Zr and Nb-7.5 wt%Ta-2 wt%Zr showed higher critical current densities at high magnetic fields (above 10–15 T), and upper critical fields exceeding 28.5 T at 4.2 K (99% normal state resistivity criterion). A record value of 29.2 T of the upper critical field at 4.2 K was obtained on samples based on Nb-7.5 wt%Ta-2 wt%Zr. Hypotheses are proposed and discussed for explaining this unexpected increase of the upper critical field, by considering the possible effects of non-oxidized Zr on the superconducting properties of Nb3Sn and of the oxidized Zr on the formation and microchemistry of Nb3Sn. Regardless of sample type the Nb3Sn grains observed in our samples have an aspect ratio of 1.5–1.7. When compared in the short axis direction, the mean distance between grain boundary intercepts (lineal intercept method) is ∼40% smaller in the samples with internally oxidized Zr than in the reference samples based on Nb-7.5 wt%Ta. In the long axis direction the reduction is of 20%–30%.

Microstructure, Electrical and Thermal Conductivity of the As-Extruded Al-xMM (Mischmetal) Based Alloys.

The effect of addition of Mischmetal (MM) on the microstructure, electrical and thermal conductivity, and mechanical properties of the as-extruded Al-MM based alloys were investigated. The studied AlxMM alloys (where x = 0.2, 0.5, 1.0, 1.5, 2.0 and 5.0 wt.%) were cast and homogenized at 550 °C for 4 h. The cast billets were extruded into 12 mm bars with an extrusion ratio of 39 at 550 °C. The addition of MM resulted in the formation of Al11(Ce, La)₃ intermetallic compounds and the area fraction of these intermetallic compounds increased with an increase in the MM content. The Al11(Ce, La)₃ phase, which was distributed in the as-cast alloys, was crushed into fine particles and arrayed along the extruded direction during the extrusion process. In particular, these intermetallic compounds in the extruded Al-5.0MM alloy were distributed with a wide-band structure due to the fragmentation of the eutectic phase with a lamellar structure. As the MM content increased from 1.0 wt.% to 5.0 wt.%, the average grain size decreased remarkably from 740 to 73 μm. This was due to formation of Al11(Ce, La)₃ particles during the hot extrusion process, which promoted dynamic recrystallization and suppression of grain growth. The electrical and thermal conductivity of the extruded alloys containing up to 2.0 wt.% MM were around 60.5% IACS and 230 W/m · K, respectively. However, the electrical and thermal conductivity of the extruded alloy with 5.0 wt.% MM decreased to 55.4% IACS and 206 W/m · K, respectively. As the MM content increased from 1.0 wt.% to 5.0 wt.%, the ultimate tensile strength (UTS) was improved remarkably from 74 to 119 MPa which was attributed to the grain refinement and formation of Al11(Ce, La)₃ intermetallic compounds by the addition of MM.

Effects of Rapid Cooling on Properties of Aluminum-Steel Friction Stir Welded Joint.

In this study, dissimilar sheets including AA3003 aluminum and A441 AISI steel were welded via cooling-assisted friction stir welding (FSW). Three different cooling mediums including forced CO2, forced water, and forced air were employed, and a non-cooled sample was processed to compare the cooling-assisted condition with the traditional FSW condition. The highest cooling rate belongs to CO2 and the lowest cooling rate belongs to the non-cooled sample as FSW. The best macrograph without any segregation at interface belongs to the water-cooled sample and the poorest joint with notable segregation belongs to the CO2 cooling FSW sample. The CO2 cooling FSW sample exhibits the smallest grain size due to the suppression of grain growth during dynamic recrystallization (DRX). The intermetallic compound (IMC) thickening was suppressed by a higher cooling rate in CO2 cooling sample and just Al-rich phase was formed in this joint. The lowest cooling rate in the FSW sample exhibits formation of the Fe rich phase. The IMC layers were thicker at the top of the weld due to closeness with the heat generation source. The water cooling sample exhibits the highest tensile strength due to proper mechanical bonding simultaneously with optimum IMC thickness to provide appropriate metallurgical bonding. Fractography observation indicates that there is a semi-ductile fracture in the water cooling sample and CO2 cooling sample exhibits more brittle fracture. Hardness evaluation reveals that the higher the cooling rate formed, the higher the hardness in stir zone, and hardness changes in the aluminum side were higher than the steel side.

Suppression of abnormal grain growth in friction-stir welded Al–Cu–Mg alloy by lowering of welding temperature

The effect of welding temperature on thermal stability of friction-stir welded (FSWed) 2519 aluminum alloy was investigated. A lowering of the welding temperature below the particle dissolution threshold was shown to be a very effective way for suppression of abnormal grain growth during post-weld solution treatment. This effect was attributed to a prevention of the welded material from precipitation of fine dispersoids during subsequent annealing which resulted in relatively low Zener pressure and thus provided an activation of the apparently-normal grain-growth mechanism. This phenomenon was demonstrated for the first time and could be used as a processing strategy to improve thermal stability of FSWed heat-treatable alloys.

Development of New 14 Cr ODS Steels by Using New Oxides Formers and B as an Inhibitor of the Grain Growth

In this work, new oxide dispersion strengthened (ODS) ferritic steels have been produced by powder metallurgy using an alternative processing route and characterized afterwards by comparing them with a base ODS steel with Y2O3 and Ti additions. Different alloying elements like boron (B), which is known as an inhibitor of grain growth obtained by pinning grain boundaries, and complex oxide compounds (Y-Ti-Zr-O) have been introduced to the 14Cr prealloyed powder by using mechanical alloying (MA) and were further consolidated by spark employing plasma sintering (SPS). Techniques such as x-ray diffraction (XRD), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) were used to study the obtained microstructures. Micro-tensile tests and microhardness measurements were carried out at room temperature to analyze the mechanical properties of the differently developed microstructures, which was considered to result in a better strength in the ODS steels containing the complex oxide Y-Ti-Zr-O. In addition, small punch (SP) tests were performed to evaluate the response of the material under high temperatures conditions, under which promising mechanical properties were attained by the materials containing Y-Ti-Zr-O (14Al-X-ODS and 14Al-X-ODS-B) in comparison with the other commercial steel, GETMAT. The differences in mechanical strength can be attributed to the precipitate’s density, nature, size, and to the density of dislocations in each ODS steel.

Effects of Cr3C2, VC, and TaC on Microstructure, WC Morphology and Mechanical Properties of Ultrafine WC–10 wt. % Co Cemented Carbides

In this study, the effects of Cr3C2, VC, and TaC on microstructure, WC grain morphology and mechanical properties of WC–10 wt. % Co ultrafine cemented carbides were investigated. The experimental results showed that WC grains size decreased and size distribution became narrow by adding Cr3C2, VC, and TaC. The inhibition efficiency was in the order of VC > Cr3C2 > TaC. Cr3C2 addition would induce triangular prism grains and Co phase was strengthened by Cr3C2, resulting in the enhancement of transverse rupture strength (TRS) and impact toughness. WC morphologies in cemented carbides with VC addition were triangular prisms with multi-steps in basal and prismatic planes due to anisotropic growth. The multi-steps in basal and prismatic planes led to low TRS and fracture toughness. The inhibition mechanism of TaC is to reduce the surface energy of WC and slow down the solution/re-precipitation rate at the WC/Co interfaces by adsorbing on the surface of WC grains. The sample with 0.8 wt. % Cr3C2 had excellent comprehensive mechanical properties. Its Vickers hardness, fracture toughness, TRS and impact toughness were 1620 kg/mm2, 9.94 MPa·m1/2, 3960 MPa and 50.4 J/m2, respectively. In summary, Cr3C2 is the first choice as the grain growth inhibitors (GGI) for the preparation of ultrafine cemented carbides.

A study on the Microstructures and the Suppression of Grain Growth in Rapidly Solidified Fe-Si-B(-Cu-Zr-Ca) Base Nanostructured Alloys

Magnetic induction and resonance wireless power charging generally operates in a high frequency range, and its efficiency is degraded by the generation of heat at high frequency. This makes it necessary to develop new soft magnetic materials capable of maintaining their characteristics even at high frequency. The purpose of this study is to minimize the heat generated by the eddy currents when charging, and to improve the charging efficiency under an alternating magnetic field. In this study, Fe-Si-B alloys containing Cu, Zr and Ca elements were melt-spun to prepare amorphous ribbons. The amorphous ribbons were heat treated to crystallize nanograins, The structure was analyzed by TEM and EELS, As a result, it was found that Zr was distributed mainly at the grain boundaries after heat treatment, whereas Ca was uniformly distributed only along the grain boundaries. It could be concluded that the Ca and Zr elements effectively suppressed the grain growth, and thus maintained the very fine nanograin structure.

Comparison of MnO2 and ZnO Additives on Zircon Decomposition and Formation of Solid Solution

In this paper, the effects of MnO2 and ZnO additives on zircon decomposition were investigated. The substitution of Mn and Zn into zircon lattices and the formation of solid solutions are discussed. The additives were added at 0–4 wt.% to zircon. The phase composition, physical properties, and microstructural changes for the sintered samples were characterized. Results of Rietveld refinement analysis showed that 1 wt.% of both additives retarded the decomposition of zircon. Further addition of additives slightly accelerated the zircon decomposition and improved sample densification. X-ray diffraction peaks of zircon were shifted to higher angles by the addition of MnO2, whereas ZnO did not alter zircon peaks. Energy dispersive spectroscopy analysis detected the presence of Mn and Zn in zircon grains. ZnO has a lower solubility than MnO2 in zircon, leading to the formation of a higher fraction of secondary phases, in turn leading to the suppression of grain growth.

Effect of Nb content on the phase composition, densification, microstructure, and mechanical properties of high-entropy boride ceramics

Dense high-entropy (Hf,Zr,Ti,Ta,Nb)B2 ceramics with Nb contents ranging from 0 to 20 at% were produced by a two-step spark plasma sintering process. X-ray diffraction indicated that a single-phase with hexagonal structure was detected in the composition without Nb. In contrast, two phases with the same hexagonal structure, but slightly different lattice parameters were present in compositions containing Nb. The addition of Nb resulted in the presence of a Nb-rich second phase and the amount of the second phase increased as the Nb content increased. The relative densities were all >99.5 %, but decreased from ∼100 % to ∼99.5 % as the Nb content increased from 0 to 20 at%. The average grain size decreased from 13.9 ± 5.5 μm for the composition without Nb additions to 5.2 ± 2.0 μm for the composition containing 20 at% Nb. The reduction of grain size with increasing Nb content was due to the suppression of grain growth by the Nb-rich second phase. The addition of Nb increased Young’s modulus and Vickers hardness, but decreased shear modulus. While some Nb dissolved into the main phase, a Nb-rich second phase was formed in all Nb-containing compositions.

Solid-solubilities of grain-growth inhibitors in WC-Ni-based cemented carbides: experimental investigations and thermodynamic calculations

Cr3C2, TaC and VC are the most widely used grain growth inhibitors (GGIs) in cemented carbides. The inhibition effectiveness and the formation of brittle carbides by over-doping these GGIs are closely associated with their solubilities in the binder phase. A series of WC-Ni-based “model alloys” with high content of binder phase and different dopants of GGIs have been designed based on thermodynamic calculations and then prepared through conventional powder metallurgy method. The subsequent microstructures and the solubilities of GGIs in Ni binder phase were characterized by means of scanning electron microscopy (SEM) and wavelength-dispersive electron-probe microanalysis (WDS-EPMA), respectively. The thermodynamic description of the fcc phase (Ni binder phase) has been remodeled according to the presently obtained experimental data. A good agreement between experimental data and thermodynamic calculations can be obtained, which can provide a theoretical basis to the development of WC-Ni-based cemented carbides.

Directional conductivity in layered alumina

Here we report a novel strategy to consolidate layered alumina demonstrating the directional electrical and thermal conductivity. The material was produced via incorporation of alumina nanofibers (20 ± 2 nm in diameter) decorated by several layers of graphene wrapped around longitudinal axes of the fibers. The graphenated fibers, obtained with the help of one-step catalyst-free CVD process, offered inhibition of grain growth combined with electrical conductivity to the sandwiched composites, which were consolidated by spark plasma sintering. Impact of the concentration of the fillers together with thickness of the conductive graphene-containing layer on thermal properties was studied. A graphene-containing interlayer with 50 μm thickness, sandwiched between two monolithic 10 mm layers of alumina shows ~30% enhancement in isotropic thermal conductivity of monolithic alumina.

Effect of stirring rate on grain morphology of Mg-Al alloy semi-solid structure by phase field lattice Boltzmann simulation

In order to investigate the effects of stirring rate on semi-solid structure of Mg-Al alloy, a PF-LBM model (Phase Field Lattice Boltzmann model) coupled with a rigid body rotation model were adopted in this work. Grain morphology, grain growth rate and grain rotation at different stirring rates were systematically studied. The results indicated that by driving force of melt flow velocity difference around the grain, the grain rotated during the stirring process, and then solute concentration would present to the low flow areas around the grain, which was caused by the forced convection. The solutes are uniformly distributed around the grain as the grain rotates. As a result, the fine and spherical grain can be obtained owing to the inhibition of grain growth.

Parameter Optimization of WC-TaC-6Co Green Part in Injection Moulding using Taguchi Method

This study represents the injection moulding parameters optimization via Taguchi method on density and porosity of the green part of cemented carbide WC-TaC-6Co. The experiment commences with the preparation of WC-TaC-6Co feedstock mixed with 60 % palm stearin (PS), and 40 % of low-density polyethylene (LDPE). The important parameters for this study are the percentage of grain growth inhibitor (GGI), temperature of injection, injection pressure and injection speed. Utilizing orthogonal array L9 (34), signal to noise ratio (S/N) was used to determine the significant levels and its contribution to the responses of density and porosity. The study signified that for density and porosity response, injection pressure and GGI is the most influential parameters. Based on that, it was found that the best parameter combinations for density and porosity is, GGI at 1.2 wt. %, temperature of injection at 155°C, injection pressure at 55% and 50%, and injection speed at 40% and 30%. Thus, it is determined that by controlling the setting of the best parameter, the optimal quality of the desired product can be accomplished and sustained without much complications during the process.

Microstructural evolution and relevant mechanical properties of Al-rich transparent magnesium aluminate ceramics

A novel phenomenon is found that the grain growth of aluminum-rich magnesium aluminate is suppressed to 5 μm, despite of high-temperature hot-isostatic-press at 1750 °C. For the transparent MgO-nAl2O3 (n = 1.2, 1.5, 2.0), the transmittance and strength tended to be enhanced with increasing Al2O3 content, having the highest values 80 % at 550 nm and 214.3 MPa for n = 2.0. This is due to the different microstructural evolution depending on the composition. When the excessive Al2O3 was small, the inhomogeneous microstructure with bimodal grain size and lots of rod-shaped particles were observed. However, as the Al2O3 amount increased, the microstructure became homogeneous with reducing the grain size and the rod-shaped particles. The microstructural and compositional analysis revealed that the rod-shaped particle were generated due to the trace potassium impurity as well as the different cationic diffusion rate, and the suppression of grain growth was induced by the severe Al segregation at the grain-boundaries.

Successful transition from low-temperature superplasticity to high-strain-rate superplasticity with increasing temperature in an ultrafine-grained Mg–Y–Zn–Zr alloy

Simultaneous achievement of low-temperature superplasticity (LTSP) below 0.5Tm (Tm: melting temperature) and high-strain-rate superplasticity (HSRS) above 0.5Tm from the identically processed material is advantageous for industrial applications. Severe plastic deformation by high-ratio differential speed rolling (HRDSR) with different roll speed ratios (2 and 3) was applied to cast Mg–Y–Zn–Zr alloys with the eutectic icosahedral phase (I-phase) structure. Two types of microstructures were obtained, depending on the roll speed ratio. The alloy processed at a speed ratio of 3 (HRDSR3) had an ultrafine-grained microstructure (∼1 μm) and finely broken eutectic I-phase particles. The alloy processed at a speed ratio of 2 (HRDSR2) also had an ultrafine grained microstructure, but with a higher fraction of non-fully fragmented original grains and a lower density of fine I-phase particles (< 1 μm in size). In both alloys, many I-phase particles were agglomerated. Below 523 K, both materials had good LTSP but HRDSR3 showed a better LTSP than HRDSR2. This was because HRDSR3 had a higher fraction of ultrafine grained area participating in grain boundary sliding and a higher density of fine I-phase particles contributing to the pinning effect on grain growth. Above 573 K, however, HRDSR2 showed HSRS but HRDSR3 failed to show HSRS. This resulted because thermal stability of ultrafine grained structure of HRDSR3 became poorer than that of HRDSR2 as the temperature increased above 0.5Tm. When solute solubility increased with temperature, the density of the small I-phase particles (<1 μm) exerting the effective pinning effect on grain boundary migration decreased at a more rapid rate in HRDSR3 than in HRDSR2 during the sample heating and holding duration. This was because a more significant reduction in particle size by higher shear resulted in a higher rate of dissolution of particles into matrix upon temperature increase. Retension of fine grains during the sample heating and holding duration was important in achieving HSRS because it allowed for active occurrence of grain boundardy sliding during deformation. Grain boundary sliding promoted progressive dispersion of agglomerated I-phase particles into the matrix, which was equally important in achieving HSRS because as the I-phase particles were more uniformly dispersed during deformation, the pinning effect of the particles increased, leading to a more effective suppression of dynamic grain growth of the fine grains. The obtained results suggest the importance of control on the size of I-phase particles and grains in achieving LTSP and HSRS from the identically processed Mg alloys in the low and high temperature ranges, respectively. The deformation behaviors of HRDSR2 and HRDSR3 as well as the condition for the successful transition from LSPT to HSRS with increasing temperature could be explained by using the model that assumes that the two deformation mechanisms of grain-size sensitive grain boundary sliding and grain-size insensitive dislocation climb creep compete each other during plastic deformation.

Suppression of abnormal grain growth in K0.5Na0.5NbO3: phase transitions and compatibility

This work presents the suppression of abnormal grain growth in bulk ceramic K0.5Na0.5NbO3 (KNN). The suppression is enabled by precise control of the starting powder morphology through match of milling and calcination duration. A comparative temperature-dependent analysis of the resulting sample morphology, phase transitions and related electronic material properties reveals that abnormal grain growth is indeed a major influence in material property deterioration, as has theoretically been suggested in other works. However, it is shown that this abnormal grain growth originates from the calcined powder and not from sintering and that all subsequent steps mirror the initial powder morphology. In specific, the results are discussed with respect to the predictions of the compatibility theory and microstructure. Despite the material’s multi-scale heterogeneity, the suppression of abnormal grain growth allows for the achievement of significantly improved functional properties and it is reported that this development is correctly predicted by the compatibility theory within the borders of microstructural integrity. It could be demonstrated that functional fatigue is strongly minimised, while thermal and electronic properties are improved when abnormal grain growth is suppressed by powder morphology control.