Introduction
Perfect crystals formed by periodic repetition of a unit cell are ideal crystals. Real crystals have structures that differ, locally at least, from this periodic arrangement as they include various types of defects. These defects can be dimensionless (point defects), have one dimension (dislocations), two dimensions (grain boundaries, stacking faults, twin boundaries) or three dimensions (precipitates, inclusions). Certain material properties depend essentially on structure and very little on defects (Young's modulus, linear dilation coefficient, magnetic and ferroelectric properties). Others, on the other hand, are heavily influenced by the nature and concentration of the defects they contain (behaviour under stress, diffusion phenomenon and heat treatment, electric conductibility, etc.). The following table describes these different types of defects, their average concentration or dimension and their influence on the properties of the materials.
Dimension | Type | Characteristic concentration or dimension | Influence |
|---|---|---|---|
0 | vacancies | concentration: \(\approx 10^{-4}\) at melting temperature \(\approx 10^{-15}\) at room temperature | solid-state diffusion |
interstitial defects | concentration: \(\approx 10^{-10}\) at melting temperature \(\approx 10^{-30}\) at room temperature | solid-state diffusion | |
foreign atoms | mechanical properties; electric conductibility of insulators and semi-conductors | ||
Frenkel and Schottky defects | concentration: \(\approx 10^{-15}\) at room temperature | electric conductibility of ionic crystals | |
1 | dislocations | density: \(\approx 10^6\) to \(≈ 10^{12}{ \rm \,cm.cm}^{-3}\) (metals) \(\approx 10^{4}{\rm \,cm.cm}^{-3}\) (ionic crystals) | mechanical properties; ductility; toughness |
2 | twins; grain boundaries stacking defects | grain size: from \({1}{\rm\, \mu m}\) to \({1}{\rm\, cm}\) | ductility; mechanical properties |
3 | precipitates inclusions | dimensions: from \({5}{\rm\, nm}\) to \({100}{\rm\, mm}\) | mechanical properties; heat treatments; magnetic properties |