Cellular-dendritic interface

The presence of dislocations can have marked effects. A number of factors can lead to the production of dislocations during growth from the melt. In the presence of impurities when cellular or dendritic interfaces are formed (see Section 4.5), a well-developed dislocation substructure is produced. On a large scale the trapping of foreign particles can produce dislocations both to accommodate mismatch at the particle-solid interface and as a result of misfit stresses.The presence of impurity particles also leads to the development of the dislocation array of sub-boundaries produced during solidification, known as the Uneage structme. "... 

I u. 4.14). At the same time changes in the interface structure can I nr, such as a coarsening of the cellular-dendritic structure. 

When the liquid/solid interface is unstable according to the criteria discussed in Section 20.3.3, a cellular or dendritic structure is developed. When the degree of instability is relatively low, an array of protuberances develops on the interface as shown in Fig. 20.8a. These protuberances, called cells, advance perpendicular to the interface. Their shapes vary depending upon the type of material, the orientation of the interface, and other factors. For (100) liquid/solid interfaces in cubic metals, equiaxed cells form like those in Fig. 20.86. However, for a (110) interface, the cells take on a corrugated configuration of long hills and furrows. When the degree of... 

Connection between Transport Processes and Solid Microstructure. The formation of cellular and dendritic patterns in the microstructure of binary crystals grown by directional solidification results from interactions of the temperature and concentration fields with the shape of the melt-crystal interface. Tiller et al. (21) first described the mechanism for constitutional supercooling or the microscale instability of a planar melt-crystal interface toward the formation of cells and dendrites. They described a simple system with a constant-temperature gradient G (in Kelvins per centimeter) and a melt that moves only to account for the solidification rate Vg. If the bulk composition of solute is c0 and the solidification is at steady state, then the exponential diffusion layer forms in front of the interface. The elevated concentration (assuming k < 1) in this layer corresponds to the melt that solidifies at a lower temperature, which is given by the phase diagram (Figure 5) as... 

Favier used the USMP-1 opportunity to explore the interfacial breakdown in Bi-doped Sn, which, like most metals, solidifies as a plane front with little kinetic undercooling. His U.S. co-investigator, Abbaschian, investigated interfacial stability on the other side of the phase diagram i.e., Sn-doped Bi, which solidifies with a faceted interface. The purpose was to test the extension of the Mullins-Sekerka stability criterion to include the effects of anisotropy, which acts to stabilize the interface against breakdown into cellular and dendritic growth. " ... 

The bone matrix that comprises lamellar and woven bone contains another level of porosity on the order of 5 to 10 frm that is associated with the bone cells (see Fig. 8.2a, b, c). Osteocytes, the most common type of bone cell, are surrounded by a thin layer of extracellular fluid within small ellipsoidal holes (5 /zm minor diameter, 7 to 8 /xm major diameter) called lacunae, of which there are about 25,000 per /iim in bone tissue. The lacunae are generally arranged along the interfaces between lamellae. However, the lacunae also have a lower-scale porosity associated with them. Each osteocyte has dendritic processes that extend from the cell through tiny channels ( 0.5 /on diameter, 3 to 7 /zm long) called canaliculi to meet at cellular gap junctions with the processes of surrounding cells. There are about 50 to 100 canaliculi per single lacuna and about 1 million per mm of bone. 

Growth front instability during transformation can lead to cellular or dendritic microstructures, depending on the severity of the instability. Minor instability leads to the formation of primary protuberances, called cells, which advance perpendicular to the interface. If the instability increases, these primary protuberances can themselves spawn secondary protuberances perpendicular to the primary protuberances, and a dendritic microstmcture develops. Cellular and dendritic microstructures are most commonly observed in vapor-solid or liquid-solid phase transformations, although they can also be formed in solid-solid phase transformations. 

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