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Concept
Abnormal grain growth
Summary
Abnormal or discontinuous grain growth, also referred to as exaggerated or secondary recrystallisation grain growth, is a grain growth phenomenon through which certain energetically favorable grains (crystallites) grow rapidly in a matrix of finer grains resulting in a bimodal grain size distribution.
In ceramic materials this phenomenon can result in the formation of elongated prismatic, acicular (needle-like) grains in a densified matrix with implications for improved fracture toughness through the impedance of crack propagation.
Abnormal grain growth (AGG) is encountered in metallic or ceramic systems exhibiting one or more of several characteristics.
Secondary phase inclusions, precipitates or impurities above a certain threshold concentration.
High anisotropy in interfacial energy (solid-liquid))or grain boundary energy (solid-solid) in bulk materials.
Highly anisotropic surface energy in thin film materials.
High chemical inequilibrium.
Although many gaps remain in our fundamental understanding of AGG phenomena, in all cases abnormal grain growth occurs as a result of very high local rates of interface migration and is enhanced by the localised formation of liquid at grain boundaries.
Abnormal grain growth is often recorded as an undesirable phenomenon occurring during the sintering of ceramic materials as rapidly growing grains may lower the hardness of the bulk material through Hall Petch type effects. However, the controlled introduction of dopants to bring about controlled AGG may be used to impart fibre-toughening in ceramic materials. In piezoelectric ceramics the occurrence of AGG may bring about the degradation of piezoelectric effect and thus in these systems AGG is avoided.
Rutile (TiO2) frequently exhibits a prismatic or acicular growth habit. In the presence of alkali dopants or a solid state ZrSiO4 dopant, rutile has been observed to crystallise from a parent anatase phase material in the form of abnormally large grains existing in a matrix of finer equiaxed anatase or rutile grains.
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