Papers by Keyword: Grain Growth

Abstract: This work intends to show the effect of the iron ion doping in LaCoO₃ perovskite, both in powders and in sintered samples obtained from combustion reaction route. The phase formation and particle morphology and particle size distribution of the powders were analysed by XRD, SEM and sedimentation techniques, respectively. Relative density, microstructure (secondary phases and grain size) and pore size distribution of LaCo_1-xFe_xO₃ sintered ceramics have been investigated by SEM/EDS and Hg porosimetry analysis. Although LaCo_1-xFe_xO₃ powders obtained from combustion reaction exhibited smaller grain sizes when sintered at high temperatures, they showed higher volume fraction of secondary phases. The presence of these crystalline phases in addition to the desired perovskite affected the microstructure acting as grain growth inhibitors by grain boundary pinning. It is believed by observing three grain junction pores that LaFeO₃ phase has a smaller dihedral angle than LaCoO₃. This fact would explain why LaFeO₃ presented a smaller driving force for sintering with higher tendency of pore and inclusion coarsening at higher temperatures (1400°C).

Abstract: A phase field model has been established to simulate the grain growth of AZ31 magnesium alloy containing spherical particles with different sizes and contents under realistic spatial-temporal scales. The expression term of second phase particles are added into the local free energy density equation, and the simulated results show that the pinning effect of particles on the grain growth is increased when the contents of particles is increasing, which is consistent with the law of Zener pinning. There is a critical particle size to affect the grain growth in the microstructure. If the size of particles is higher than the critical value, the pinning effect of particles for grain growth will be increased with further decreasing the particle size; however the effect goes opposite if the particle size is lower than the critical value.

Abstract: Nanocrystalline Al₂O₃-ZrO₂ powders were prepared through controlled crystallization of amorphous phase, and kinetic studies on crystallization and grain growth were carried out. Based on Kissinger equation, the activation energies for crystallization were determined to be 981.8 kJ·mol^-1 for (Zr_0.94Y_0.06)O_1.88 and 1364.1 kJ·mol^-1 for α-Al₂O₃. Average size of crystallite was obtained by Scherrer equation according to FWHMs in XRD peaks; the grain growth was slow below 1100°C, and turned severe at higher temperatures. The appropriate calcination temperatures was determined to be 1100°C, at which stable phases were formed and further severe grain growth can be restrained. SEM micrograph of Al₂O₃-ZrO₂ powder after calcination at 1100°C revealed uniform distribution of spherical particles with size of about 20 nm, close to that determined by XRD results.

Abstract: The paper studies deformation mechanisms of nanocrystalline (NC) pure Al and its binary alloys with various distributions of an alloying element which can be Co or Mg via molecular dynamics simulations. It is revealed that a shear deformation of the pure Al is associated with the grain boundary sliding (GBS) and their simultaneous migration. Mg atoms in grain boundaries (GBs) of an Al-Mg alloy lead to GBS which does not accompany with a grain growth, while the deformation process of the corresponding alloy with a random distribution of Mg is close to that for the pure Al. Unlike Mg, GB segregations of Co atoms detain both GBS and GB migration and result in high strength of an Al-Co alloy. On the contrary, the strength of the alloy with the Co atoms distributed randomly is very low due to the structure amorphisation leading to the ease of plastic flow.

Abstract: Sintering involves several interactions as particles bond and enable microstructure evolution toward a minimized energy condition, resulting in a complex interplay of measurement parameters. Overriding the evolution is energy minimization, and from that perspective some simple relations emerge. The natural progression is determined by energy reduction, measured by surface area, density, and grain boundary area (grain size). Contrary to the usual sintering analysis that starts with atomic level mass transport mechanisms, presented here is an approach that links to global energy reduction during sintering to simple monitors. Initially sintering converts surface area into lower energy grain boundary area. Subsequently grain growth annihilates grain boundary area. Thus, grain boundary area peaks at intermediate sintered densities, while surface area continuously declines. The trajectory follows a straightforward dependence on density as illustrated using data for a wide variety of materials and consolidation conditions.

Abstract: The grain evolution of multicrystalline Si was studied using the ingot grown from microcrystalline template. The grain shape evolution and width increase are not monotonic but may have 3 stages. On the other hand, the grain boundary (GB) analysis suggests that there exit 2 reactions, namely random GB annihilation at the initial stage and Σ3 generation and annihilation at the steady state.

Abstract: The aim of this work is to show the effect of the iron ion doping in LaCoO₃ perovskite, both in powders and in sintered samples obtained from combustion reaction and solid state route. The phase formation and particle morphology and particle size distribution of the powders were analysed by XRD, SEM and sedimentation techniques, respectively. Relative density, microstructure (secondary phases and grain size) and pore size distribution of LaCo_1-xFe_xO₃ sintered ceramics were investigated by SEM/EDS and Hg porosimetry analysis. Although LaCo_1-xFe_xO₃ powders obtained from the combustion reaction exhibited smaller grain sizes when sintered at high temperatures, they showed a higher volume fraction of secondary phases. The presence of these crystalline phases in addition to the desired perovskite affected the microstructure acting as grain growth inhibitors by grain boundary pinning. It is believed that by observing three grain junction pores that the LaFeO₃ phase has a smaller dihedral angle than LaCoO₃. This fact would explain why LaFeO₃ presented a smaller driving force for sintering with a higher tendency of pore and inclusion coarsening at higher temperatures (1400°C).

Abstract: Grain growth in open systems is analyzed for the cases of flux-driven ripening during soldering, flux-driven lateral growth during deposition of thin films, flux-driven lateral growth during reactive growth of intermediate phase, flux-driven lateral growth of antiphase domains in FCC-phase A3B and BCC-phase AB during the diffusion growth of ordered phase layer.

Abstract: The mechanical and thermal properties of metallic materials are strongly related to theirmicrostructure. An accurate and quantitative prediction of microstructural evolutions is then crucialwhen it comes to optimize the forming process. Recently a new full field approach, based on a Level-Set (LS) description of interfaces in a finite element (FE) context has been introduced to model 2D and3D primary recrystallization (ReX), including the nucleation stage [1, 2], and has been extended to takeinto account the grain growth (GG) stage [3, 4]. The ability of this approach to model also the Zenerpinning (ZP) phenomenon without any assumption concerning the shape of second phase particleswas also demonstrated [5]. Moreover, recent developments have also illustrated the capability of thisapproach to take into account the characteristics of twin interfaces during grain boundary motion [6,7]. Current work concerns also the improvement of the numerical cost of this new approach [8]. Allthese developments are necessary to account for the microstructural complexity of ReX phenomenon.

Abstract: Alumina, Silicon Carbide, Boron Carbide and Magnesium Aluminate Spinel were directly joined by a Spark Plasma Sintering (SPS) apparatus. The optimal parameters for joining (temperature, holding time and applied pressure) were experimentally found. The joined regions were investigated using scanning electron microscopy (SEM) ultrasonic and micro hardness testing. Alumina, Boron Carbide and Silicon Carbide parts were successfully joined at 1300, 1800 and 1900°C, respectively, for 30 min under argon atmosphere (10^{- 2} torr) and 40 MPa uniaxial pressure. Spinel parts were successfully joined at 1300°C for 60 min. Micro hardness of the interfacial regions were similar to those of the bonded specimens. The effect of a prolonged heat treatment on the microstructural evolution of the joined regions was investigated and the grain growth mechanism was discussed.