There are no naturally occurring ideal crystals. Every solid compound has some deviation from an ideal periodical lattice. These deviations are called defects or imperfections of the solid structure. They tentatively can be structured as dynamic and static. Dynamic defects can happen during mechanical, thermal or magnetic impact. The most frequent example of dynamic defect is phonons – temporary distortion of a crystalline lattice caused by thermal atom motion.
Among the static defects, we can point out point or atomic defects and linear defects. Atomic defects are vacancies and interstitals, impurities, foreign atom integration to the crystalline lattice. Linear defects are dislocations, pores, cracks and grain boundaries.
Imperfection concentration in the solid structure can be small, but their effect on the physical properties of the compound can be significant. For example, 1,000th of a percent of defects in the semiconductor crystal changes its conductivity 103 – 104 times. Linear defects make significant impact on mechanical properties of solid structures.
Before introducing the concepts of defects, we must introduce some basic concepts for materials. As has been said before, pure material with only one type of atom doesn’t exist. There are always some impurities which form defects in a solid. The most frequent metal is alloy. Alloy is a metal where impurities were intentionally added to modify some properties of the solid. A solid solution is a result of adding impurities into a metal. Solvent is an element which is present in a compound in the greatest amount. A Solute denotes an element in a minor concentration in a compound. A Solid solution is a compound which is formed by adding a solute atom to the solvent without changing a lattice. A Solid solution is homogenous, and impurity atoms are randomly dispersed across the compound. There are two types of point defect in solid solutions: substitutional and interstitial.
This equation means that the number of vacancies increases exponentially with temperature. A self-interstitial is an atom in crystal placed into an interstitial site. For metals this kind of defect is quite rare, because the size of the atom of the metal is much bigger then interstitials, and it causes bigger deformations of the crystalline lattice. Types of vacancies and self-interstitials are presented in Figures 2 and 3.
A dislocation is a linear type of defect around which some of the atoms are misaligned. Dislocations can also be edge type, when the extra portion of the atoms are on the edge of the crystal. As far as this is a linear defect, it is also called dislocation line. There is always lattice distortion around the dislocation line. The magnitude of the distortion decreases as it moves away from the dislocation line.
Another type of dislocation is screw dislocation or shear dislocations. This type of dislocation is formed because of the shift of a part of the lattice relative to another part. You can see both types of dislocations in the Figure 4.
Most dislocations in the crystalline compound are neither line, nor screw. These dislocations are called mixed dislocations. Every solid compound has dislocations, caused by mechanical stresses and heat treatment.
Figure 4 shows dislocations in the alloy.