Crystal Defects and Noncrystalline Structure PDF

1 Crystal Defects and Noncrystalline Structure—Imperfection In contrast to the “perfect” structures, this high-resolution transmission electron micrograph shows irregularities in the Packing of atoms including the twin boundaries in the lower right-hand corner of this image of a diamond film....

1 Crystal Defects and Noncrystalline Structure—Imperfection In contrast to the “perfect” structures, this high-resolution transmission electron micrograph shows irregularities in the Packing of atoms including the twin boundaries in the lower right-hand corner of this image of a diamond film. (two-dimensional defect) It is not possible to avoid some contamination of practical materials. Even high-purity semiconductor products have some measurable level of impurity atoms. Many engineering materials contain significant amounts of several different components. Commercial metal alloys are examples. As a result, all materials that the engineer deals with on a daily basis are actually solid solutions. At first, the concept of a solid solution may be difficult to grasp. In fact, it is essentially equivalent to the more familiar liquid solution, such as the water–alcohol system. The complete solubility of alcohol in water is the result of complete molecular mixing. Solid solution of copper and nickel atoms sharing the fcc crystal structure. Nickel acts as a solute dissolving in the copper solvent. This particular confguration is referred to as a substitutional solid solution because the nickel atoms are substituting for copper atoms on the fcc atom sites. This configuration will tend to occur when the atoms do not differ greatly in size. The water–alcohol system represents two liquids completely soluble in each other in all proportions. For this complete miscibility to occur in metallic solid solutions, the two metals must be quite similar, as defned by the Hume- Rothery* rules. The Hume-Rothery* rules: 1. Less than about 15% difference in atomic radii 2. The same crystal structure 3. Similar electronegativities (the ability of the atom to attract an electron) 4. The same valence If one or more of the Hume-Rothery rules are violated, only partial solubility is possible Some systems form random solid solution. By contrast, some systems form ordered solid solutions. A good example is the alloy AuCu3. At high temperatures (above 390°C), thermal agitation keeps a random distribution of the Au and Cu atoms among the fcc sites. Below approximately 390°C, the Cu atoms preferentially occupy the face-centered positions, and the Au atoms preferentially occupy corner positions in the unit cell. Ordering may produce a new crystal structure similar to some of the ceramic compound structures. For AuCu3 at low temperatures, the compound-like structure is based on a simple cubic Bravais lattice. When atom sizes differ greatly, substitution of the smaller atom on a crystal structure site may be energetically unstable. In this case, it is more stable for the smaller atom simply to fit into one of the spaces, or interstices, among adjacent atoms in the crystal structure. Such an interstitial solid solution is displayed below which shows carbon dissolved interstitially in α-Fe. This interstitial solution is a dominant phase in steels. Although more stable than a substitutional confguration of C atoms on Fe lattice sites, the interstitial structure shown below produces considerable strain locally to the α -Fe crystal structure, and less than 0.1 at % C is soluble in α -Fe. Structural defects exist in real materials independently of chemical impurities. Imperfections associated with the crystalline point lattice are called point defects: (1) The vacancy is simply an unoccupied atom site in the crystal structure, and (2) the interstitial, or interstitialcy, is an atom occupying an interstitial site not normally occupied by an atom in the perfect crystal structure or an extra atom inserted into the perfect crystal structure such that two atoms occupy positions close to a singly occupied atomic site in the perfect structure. Vacancies can occur independently of chemical factors (e.g., by the thermal vibration of atoms in a solid above a temperature of absolute zero). The Schottky defect is a pair of oppositely charged ion vacancies. This pairing is required in order to maintain local charge neutrality in the compound’s crystal structure. The Frenkel defect is a vacancy–interstitialcy combination.