Strategies and practices for suppressing abnormal grain growth during liquid phase sintering

Abstract

Abnormal grain growth (AGG), where a small number of grains grow to sizes much larger than the neighboring matrix grains, is a frequent occurrence in liquid phase sintering of ceramics and cermets. As AGG can be detrimental to the material properties, a considerable amount of research on the nature, causes and suppression of AGG has been carried out. In this review, we outline the mixed control theory of grain growth and the principle of microstructural evolution that have been developed by Kang and coworkers over the last two decades. The theory and the principle, which are based on theories of crystal growth from a liquid, state that grain growth behavior is controlled by the nature of the solid-liquid interfaces, either atomically rough (macroscopically rounded) or smooth (macroscopically faceted). For grains with atomically rough solid-liquid interfaces, growth is controlled by diffusion of solute through the liquid phase and normal grain growth always occurs. For grains with faceted solid-liquid interfaces (or a mixture of rough and faceted interfaces), growth is interface reaction-controlled and diffusion-controlled below and above a critical driving force for growth, respectively. Depending on the relative values of the critical driving force for growth Δg c and the maximum driving force for the largest grain in the system Δg max , pseudo-normal, abnormal, and stagnant grain growth can take place. Based on this theory and principle, we present strategies for suppressing AGG by adjusting Δg c and Δg max to avoid AGG and examples of the successful use of these strategies.

Dynamic Crystallization of a Haplogranitic Melt: Application to Pegmatites

“…One appealing possibility is abnormal grain growth (AGG) where coarsening of angular crystals occurs by surface-nucleation and leads to exclusive growth of a few large crystals relative to the others (e.g. Kang et al, 2002;Jung et al, 2009;Fisher & Kang, 2019).…”

Controlled-Atmosphere Sintering of KNbO3

“…If the grains contain screw dislocations, the grain growth rate increases parabolically with ∆G until ∆G c is reached, at which point grain growth becomes diffusion-limited again. According to the relative values of ∆G c and ∆G max (where ∆G max is the value of ∆G for the largest grain), different grain growth behaviors (pseudo-normal, abnormal, and stagnant) can be observed [54,56,67,68]. This grain growth theory, called the mixed control theory, was originally developed to describe liquid phase sintering, but similar behavior also takes place in single phase systems with faceted grain boundaries in which the formation and lateral spreading of steps governs grain boundary migration [54,66,67,[69][70][71][72][73][74][75][76][77][78].…”

“…As ε decreases, ∆G c also decreases. Grain morphology is a qualitative indication of the value of ε [55,68,80]. The cubic morphology of the KNbO 3 grains (Figure 3) with faceted faces, sharp edges, and corners ( Figure 7) indicates a high value of ε.…”

Current status of solid-state single crystal growth

“…The appearance of AGG has been observed in many different systems both ceramic and metallic. Many authors have tried to explain the occurrence of AGG, suggesting different mechanisms and models, however, the underlying reasons for AGG are still under debate [26]. Generally, the following phenomena have been suggested as the possible causes of AGG: (a) the presence of second phases, pores or impurities (b) high anisotropy of the interfacial energy and grain boundary mobility, and (c) the presence of a thin liquid film at the grain boundary which facilitates grain boundary mobility [23,24].…”

“…Because of their atomically disordered structures, rough interfaces allow for a large number of attachment sites for atoms, which then enables a high rate of interfacial reactions. Since the migration kinetics are governed by the slowest process, in the case of rough grains, the diffusion, as the slowest process, will be the rate-determining process for grain boundary migration [26]. On the other hand, for faceted grains, the experimental results have shown that the grain growth is controlled by either interface reaction (attachment of atoms from one grain to an adjacent grain) or atomic diffusion across the grain boundary, depending on which process is slower.…”