Key Engineering Materials Vol. 352

Abstract: Diffusion phenomena in solid particles were analyzed with the new material transport concept. It was assumed that total excess free energy in a system acted as a driving force for material transport so that the system changed to an equilibrium state. The new rate equation was adopted to analyze shape change, sintering and growth of grains. It was found that surface energy or ratio of grain boundary energy to surface energy was key factor for shape changes in these processes.

Abstract: Kinetics of porous layer evolution during high-temperature annealing was investigated by Monte Carlo simulation. Sintering process of spongy one-component films with randomly distributed pores was studied. Layers with porosity from 20% to 50% with simple cubic and diamond-like lattices were under examination. Sintering rate was demonstrated to be non-monotone in time for any film porosity and different lattice coordination number. Metastable states of the system, dependent on time and temperature of annealing process, were revealed. Estimation of annealing time necessary to reach the definite sintering level under changes of annealing temperature was suggested.

Abstract: In this paper, we investigated numerically gravity induced skeletal settling during liquid phase sintering. The microstructural evolution will be simulated by simultaneous computation of displacement of the center of mass and mass transport due to dissolution and precipitation at the interfaces between solid-phase and liquid matrix. Common to this study based on domain methodology for definition of regular multi-domain model will be the need to relate some diffusional phenomena to essential geometric and topological attributes of the W-Ni porous microstructure influenced by skeletal settling combined with extrication of some solid-phase domains during liquid phase sintering.

Abstract: A finite difference method based on control volume methodology and interface-tracking technique for simulation of rapid solidification accompanied by melt undercooling will be described and applied to analyze the solidification of alumina sample on copper substrate.

Abstract: Strength reliability of ceramics depends on accuracy of parameters involved in the probability distribution function for fracture. The parameters are usually estimated by use of strength data. However, one may have additional information in the experiment, such as fracture cause data, fracture location data, flaw-size data and flaw-orientation data. In this paper, we will incorporate these additional information in the parameter estimation to improve the accuracy of the reliability. A new theory on the asymptotic variances is presented.

Abstract: First-principles grain boundary (GB) tensile deformation simulations were performed to investigate the atomic-scale mechanism of GB fracture of the Σ13 pyramidal twin GB in α-Al2O3. It was found that the specific Al-O bond broke at the GB core in the early stage of tensile deformation. From chemical bonding analyses, the first breaking bond was the weakest bond in the GB core. However, when the catastrophic GB fracture started, initially strong Al-O bond broke. This indicates that local atomic bonds should determine the microscopic GB fracture behavior.

Abstract: The effect of step free energy on the grain growth behavior in a liquid matrix is studied in a model system BaTiO3-SiO2. BaTiO3-10SiO2 (mole %) powder compacts were sintered at 1280°C under various oxygen partial pressures (PO2), 0.2, ~ 10-17 and ~ 10-24 atm. As the step free energy decreases with the reduction of PO2, it was possible to observe the change in growth behavior with the reduction of the step free energy. At PO2 = 0.2 atm, essentially no grain growth (stagnant grain growth) occurred during sintering up to 50 h. At PO2 ≈ 10-17 atm, abnormal grain growth followed stagnant grain growth during extended sintering (incubation of abnormal grain growth). At PO2 ≈ 10-24 atm, normal grain growth occurred. These changes in growth behavior with PO2 and the step free energy reduction are explained in terms of the change in the critical driving force for appreciable growth relative to the maximum driving force for grain growth. The present experimental results provide an example of microstructure control in solid-liquid two- phase systems via step free energy change.

Abstract: Damage evaluation for alumina/graphite refractory was conducted under uni-axial compressive loading. Apparent sonic velocity during a loading-unloading cycle was measured by ultrasonic method. Quasi-elastic-plastic behavior was observed in the stress-strain curve for each cycle. However, it is difficult to detect damage from the stress-strain curve during each loading- unloading cycle. On the other hand, using the result of change in apparent sonic velocity during a loading-unloading cycle, it is possible to estimate damage to some extent. The apparent sonic velocity kept approximately constant during the first loading process, but it decreased remarkably during the first unloading one. In the subsequent loading-unloading cycles, it increased in the loading process and decreased in the unloading one. Consequently, it is concluded that damage mechanism during the first loading-unloading cycle is different from that during the subsequent loading- unloading cycles for alumina/graphite refractory.

Abstract: Effect of microstructure of silicon nitride on the fracture toughness, KIc evaluated by the IF method was studied with various indentation loads ranging from 49 N to 490 N, since practical assessment of fracture toughness of small Si3N4 parts is needed in the ceramic ball bearing market. The plot of KIc against the as-indented crack length revealed the rising R-curve behavior for the coarse Si3N4 and slight R-curve for the fine Si3N4. By comparing KIc estimated from the SEPB and IF methods using 4 different equations, it was revealed that the IF equation which gave the nearest value to KIc from SEPB was different depending on the microstructures. These results were discussed in conjunction with their R-curve behavior and the effective crack length in the SEPB specimens.