Papers by Author: Suk Joong L. Kang

Abstract: Recent investigations suggest that general grain boundaries can be categorized into two types: rough (atomically disordered) and faceted (atomically ordered). This paper reports our recent investigations on the migration behaviour of faceted boundaries and its effect on grain growth in polycrystalline ceramics. A model experiment has been performed using bi-layer samples of polycrystals with different average grain sizes and single crystals of BaTiO₃ to study the migration behaviour of faceted boundaries. A non-linear relationship between grain boundary migration and the driving force for migration is revealed. Grain growth behaviour with respect to boundary faceting has also been studied in perovskites. The structural transition of boundaries between rough and faceted can be induced by changing oxygen partial pressure, adding dopants and changing temperature. The fraction of faceted boundaries was changed by changing oxygen partial pressure and donor doping. As the facet fraction decreased, the grain growth behaviour changed from stagnant and abnormal to normal. The different types of growth behaviour observed can be explained by the coupling effect of the maximum driving force for the boundary migration and the critical driving force for appreciable migration of faceted boundaries.

Abstract: When a polycrystal is in chemical equilibrium, the microstructure evolves as a result of grain growth under the capillary driving force arising from the interface curvature. As the growth rate of an individual grain is the product of the interface mobility and the driving force, the growth of the grain can be controlled by changing these two parameters. According to crystal growth theories, the growth of a crystal with a rough interface is governed by diffusion and its interface mobility is constant. In-contrast, the growth of a crystal with faceted interfaces is governed by the interface reaction and diffusion for driving forces below and above a critical value, respectively. As the growth rate is nonlinear for the regime of interface reaction control, the grain growth is nonstationary with annealing time. Calculations reveal that the types of nonstationary growth behavior including pseudo-normal, abnormal, and stationary are governed by the relative value of the maximum driving force, gmax, to the critical driving force for appreciable growth, gc. Recent experimental observations showing the effects of critical processing parameters on microstructural development also support the theoretical prediction. The principles of microstructural design are deduced in terms of the coupling effects of gmax and gc.

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: Twinned WC grains are sometimes observed in WC powder and sintered WC-Co alloys. The present investigation has studied the formation of twinned WC grains during carburization of an Eta phase. Eta grains were carburized at 700-1450°C for 1 min to 9 h. Twinned WC grains formed during the carburization. Crystallographic characterization of the formed twins were made using SEM and TEM. The formation of twins was found to be affected by the carbon activity during carburization. The twins formed under high carbon activities while no twins formed under low carbon activities. Two kinds of twins with different orientations were observed. The present experimental observation suggests that the twins formed via 2-dimensional nucleation and layer-bylayer growth on small WC clusters under high supersaturation and high driving force for the growth of WC grains.

Abstract: Growth behavior and kinetics of grains in a liquid matrix has been studied by computer simulation for various physical and processing conditions. The kinetics of growing and dissolving grains is considered to follow that of single crystals in a matrix. Depending on the crystal shape, i.e. rounded or faceted, different kinetic equations were adopted for growing grains and a single equation was assumed for dissolving grains. Effects of critical parameters such as step free energy, temperature, and liquid volume fraction were evaluated.

Abstract: The design of microstructure in materials, ranging from ultrafine, moderately sized, duplex to single crystalline, has long been a challenging subject to material scientists. A basic means to achieve this goal is related to the control of grain growth. Taking BaTiO3 as a model system, this investigation shows that control of grain boundary structure between rough and faceted and control of initial grain size can allow us to achieve the goal. When the grain boundary is rough, normal grain growth occurs with a moderate rate. On the other hand, for faceted boundaries, either abnormal grain growth or grain growth inhibition occurs resulting in a duplex grain structure or fine-grained structure, respectively. Growth of single crystals is also possible when the boundary is faceted. During crystal growth amorphous films can form and thicken at dry grain boundaries above the eutectic temperature. As the film thickness increases, the growth rate of the crystals is reduced. This observed growth behavior of grains with boundary structure is explained in terms of the difference in mobility between the two types of boundaries. The results demonstrate the basic principles of obtaining various microstructures from the same material.