Seminar

Date 2015-09-08 
The Fall Semester Seminar


 
■ Topic :  What governs microstructural evolution in polycrystalline materials?
                   
■ Speaker :  Prof. Suk-Joong L. Kang (Dept. of MSE, KAIST) 
 
■ Date & Time :  September 8 (Tue), 16:00
 
■ Venue :  KAIST Applied Engineering B/D(W1), Multimedia Lecture Hall (1st Floor)
 
■ Abstract :  Since the pioneering work of Burke and Turnbull on grain growth in the early 1950s, grain growth and microstructural evolution in polycrystalline materials have become a key subject of materials science and engineering. For an ideal system, where the migration rate is simply proportional to the driving force for the migration, the classical square law of normal grain growth was deduced and is well documented in the literature. A characteristic of normal grain growth is an invariable distribution of grain sizes relative to the average grain size. In real systems, however, grain growth behavior often deviates from the normal behavior, showing non-normal behavior with variable distribution of relative grain sizes with the annealing time. Non-normal grain growth is typified by abnormal grain growth (AGG) with some exceptionally large grains embedded in a fine matrix. Several causes of non-normal, in particular abnormal, grain growth have been suggested and analyzed, including (i) the drag of the grain boundary by second phase particles (or pores), (ii) the drag of the grain boundary by impurities, (iii) the enhancement of boundary motion by a liquid film, which is highlighted as the introduction of several complexions of the boundary, and (iv) anisotropies in boundary energy and mobility. When explaining AGG, a common feature of the mechanisms related to causes (i) – (iv) is that the kinetics of boundary migration and grain growth is governed by the diffusion of atoms across the boundary. All these mechanisms, however, appear to fail to explain the AGG behavior observed in many different systems.
Apart from the classical mechanisms listed above, quite recently we suggested a mixed control mechanism, governed either by diffusion or interface reaction, of boundary migration. The migration of a rough (atomically disordered) boundary is governed by diffusion of atoms across the boundary, as in the classical models and theories. In contrast, the migration of a faceted (atomically ordered) boundary is observed to be governed either by diffusion or interface reaction for a driving force smaller or larger than a critical value, respectively. The migration rate is negligible for a driving force smaller than the critical value, and linear with a driving force larger than the critical value. Grain growth behavior in polycrystalline materials can be predicted in terms of the coupling effect between the maximum driving force for the growth of the largest grain, Δgmax, and the critical driving force for appreciable migration of the boundary, Δgc (the Principle of Microstructural Evolution): normal for Δgc = 0, pseudonormal for 0 < Δgc << Δgmax, abnormal for 0 < Δgc ≅ Δgmax, and stagnant for Δgmax << Δgc. Many experimental results for metals as well as ceramics support the microstructural evolution principle, showing its generality. Application of the principle has also been demonstrated for fabrication of polycrystals with desired microstructures and for solid state conversion of single crystals. Theoretical and simulation studies that are related to the microstructural evolution principle are expected to be explored in the future.