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 Matrix most commonly refers to: * ''The Matrix'' (franchise), an American media franchise ** ''The Matrix'', a 1999 science-fiction action film ** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchis ...

of finer grains resulting in a

bimodal In statistics, a multimodal distribution is a probability distribution with more than one mode (statistics), mode. These appear as distinct peaks (local maxima) in the probability density function, as shown in Figures 1 and 2. Categorical, ...

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.

Mechanisms

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.

Significance

Abnormal grain growth is often recorded as an undesirable phenomenon occurring during the

sintering Clinker nodules produced by sintering Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction. Sintering happens as part of a manufacturing ...

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

dopant A dopant, also called a doping agent, is a trace of impurity element that is introduced into a chemical material to alter its original electrical or optical properties. The amount of dopant necessary to cause changes is typically very low. When ...

s to bring about controlled AGG may be used to impart fibre-toughening in ceramic materials. In

piezoelectric ceramics Piezoelectricity (, ) is the electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins—in response to applied mechanical stress. The word ''p ...

the occurrence of AGG may bring about the degradation of piezoelectric effect and thus in these systems AGG is avoided.

Example systems

# 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. # Alumina, Al₂O₃ with

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

and/or

yttria Yttrium oxide, also known as yttria, is Y2 O3. It is an air-stable, white solid substance. The thermal conductivity of yttrium oxide is 27 W/(m·K). Uses Phosphors Yttria is widely used to make Eu:YVO4 and Eu:Y2O3 phosphors that give the red ...

dopants/impurities has been reported to exhibit undesirable AGG. #BaTiO₃ barium titanate with an excess of TiO₂ is known to exhibit abnormal grain growth with profound consequences on this materials piezoelectric performance. #

Tungsten carbide Tungsten carbide (chemical formula: WC) is a chemical compound (specifically, a carbide) containing equal parts of tungsten and carbon atoms. In its most basic form, tungsten carbide is a fine gray powder, but it can be pressed and formed into ...

has been reported to exhibit AGG of faceted grains in the presence of a liquid cobalt-containing phase at grain boundaries # Silicon nitride (Si₃N₄) may exhibit AGG depending on the size distribution of β-phase material in an α-Si₃N₄ precursor. This type of grain growth is of importance in the toughening of silicon nitride materials # Silicon carbide has been shown to exhibit improved fracture toughness as the result of AGG processes yielding elongated crack tip/wake bridging grains, with consequences for applications in ballistic armor. This type of crack-bridging based enhanced fracture toughness of ceramic materials exhibiting AGG is consistent with reported morphological effects on crack propagation in ceramics #Strontium barium niobate, used for electro-optics and dielectric applications is known to exhibit AGG with significant consequences on the material's electronic performance # Calcium titanate (CaTiO₃, perovskite) systems doped with BaO have been observed to exhibit AGG without the formation of liquid as the result of polytype interfaces between solid phases

References

External links

Abnormal Grain Growth by Cyclic Heat Treatment

{{DEFAULTSORT:Abnormal grain growth Materials science Crystallography Mineralogy concepts

Mechanisms

Significance

Example systems

See also

References

External links