10.5 Grain Formation

In the process of solidification, if all the nucleation sites except one are suppressed, the liquid solidifies into a single grain. The orientation of the grain can be engineered by placing a seed crystal of known orientation.

SiC and Si₃N₄ filamentary single crystals (known as whiskers) are used as reinforcing materials in composites because they exhibit superior strength and elastic modulus. Single crystals also exhibit excellent high-temperature properties due to the absence of grain boundaries.

Single crystal turbine blades used in present jet engines exhibit superior high-temperature properties. Silicon single crystals are the backbone of the electronic industry because of their excellent electronic properties.

When a melt is cooled slowly under equilibrium conditions in the presence of abundant nucleation sites, the liquid forms a large number of small grains. Although solidified grains physically look alike, the crystallographic orientations of the grains are different and orientation changes randomly from one grain to another. Such a solid is known as a polycrystalline material.

The number of grains, and hence, material properties can easily be varied by controlling the nucleation process. This can be achieved by adding inoculants or seed crystals. For example, a fine grain steel is produced by adding refractory elements (e.g., W, V, Cr, Mo). These elements combine with carbon and produce carbides that provide effective seeding sites for nucleation of grains.

Materials in high-temperature service usually fail by crack formation and growth along grain boundaries. Thus, high temperature capabilities can be improved by eliminating grain boundaries and using single crystal materials. Single crystal blades of nickel-base superalloys are currently used as gas turbine blades to increase the high temperature capability of the turbines.