Grain growth

Grain growth occurs in polycrystalline solids. At sufficiently high temperatures and once crystal recovery and recrystallization have fully taken place, a further lowering of the internal energy can only be achieved through a reduction of the grain boundaries. This is done by growing the biggest and most favorable oriented crystallites ( " grains " ) at the expense of smaller representative. This process can be ideally, but in practice rarely, to the extent continue that eventually a single crystal is formed.

The term is commonly used in the metallurgy, but can also refer to ceramic and mineral substances.

Importance

The majority of the materials has at room temperature the Hall -Petch effect on, and therefore achieved at smaller grain sizes a higher yield strength. At high temperatures, the opposite is the case, because the open and disordered structure of the grain boundaries causes voids can move along the boundaries easier and faster, resulting in higher Coble creep ( a kind of creep). Since grain boundaries are regions of high energy, they make excellent nucleation sites for precipitates and other secondary phases such as Mg -Si - Cu phases in some aluminum alloys or martensite plate in steel. Depending on the involved second phase, the positive or negative impact on the macroscopic properties.

Regulate

Grain growth has been studied for a long time, especially through the study of cut, polished and etched samples under the optical microscope. Although this is a broad inventory empirical intuition material originated, particularly with regard to temperature gradients and compositions, limited the lack of knowledge of the crystal structure, the understanding of the fundamental physical processes. Nevertheless, you could define the following basic properties of grain growth:

Normal and abnormal growth

Together with crystal recovery and recrystallization growth phenomena can be divided into continuous and discontinuous processes. In the first case, the structure of state A to state B occurs uniformly, thus the grain sizes are higher. In the second case, the changes are heterogeneous place so that transformed and untransformed specific regions can be identified.

Discontinuous grain growth, also called secondary recrystallization is characterized by a rapidly growing proportion of grains that use up their neighbors quickly; it tends to form a resultant structure in which a few very large grains dominate. So that takes place, a subset of grains must have an edge over the competitors, ie about high interfacial energy, high local mobility of the interface (ie, high temperature), a cheap fabric or lower particle density of a secondary phase, the " pinning " the grain boundaries.

Driving force

The boundary between a grain and its neighbor forms a defect in the crystal structure, and therefore is associated with a certain amount of energy. The result would be a thermodynamic effort to reduce the total area of ​​the grain boundaries. This occurs when grain growth while reducing the total number of individual grains.

Compared to phase transitions in the internal energy of the grain growth can be achieved is low, so there takes place much more slowly and can be easily attenuated by particles or loose atoms.

Ideal growth

Ideal growth is a special case in which only affects the reduction of the general interfacial energy. It further environmental conditions such as elastic stresses or temperature gradients are neglected. If you can maintain that the growth rate is proportional to the acting force, and the urging force proportional to the total surface energy, it can be shown that the time T required to achieve a given particle diameter, may be approximately shown by the following equation:

Where d0 is the initial particle size, d is the finite grain size and k is a temperature-dependent constant that is exponentially formed:

Where k0 is a constant, T is the absolute temperature, and Q is the activation energy for the interface displacements. Theoretically, the activation energy for boundary shifts is as much as that for the self-diffusion, but this is often not accurate.

In general, these equations are obviously for high purity materials but fail already when tiny amounts of dissolved substances are introduced.

Growth factors preventing

If other factors that tend to hinder the border movements, such as the Zener pinning by small particles, the particle size can be limited to a much lower value than would normally be expected. Which is an important mechanism of manufacturing technology in order to prevent the softening of materials at high temperatures. In fine-grained steel the carbonitrides of microalloying elements titanium, niobium and vanadium take on this role.