From an atomic perspective, plastic deformation corresponds to the breaking of bonds with original atom neighbors and then reforming bonds with new neighbors as large numbers of atoms or molecules move relative to one another; upon removal of the stress they do not return to their original positions.
From an atomic perspective, diffusion is just the stepwise migration of atoms from lattice site to lattice site. In fact, the atoms in solid materials are in constant motion, rapidly changing positions. For an atom to make such a move, two conditions must be met:
(1) there must be an empty adjacent site, and
(2) the atom must have sufficient energy to break bonds with its neighbor atoms and then cause some lattice distortion during the displacement. This energy is vibrational in nature.
At a specific temperature some small fraction of the total number of atoms is capable of diffusive motion, by virtue of the magnitudes of their vibrational energies. This fraction increases with rising temperature.
Factors that influence diffusion:
1. Diffusing Species
2. Temperature
There are several features of the solute and solvent atoms that determine the degree to which the former dissolves in the latter, as follows:
1. Atomic size factor. Appreciable quantities of a solute may be accommodated in this type of solid solution only when the difference in atomic radii between the two atom types is less than about . Otherwise the solute atoms will create substantial lattice distortions and a new phase will form.
2. Crystal structure. For appreciable solid solubility the crystal structures for metals of both atom types must be the same.
3. Electronegativity. The more electropositive one element and the more electronegative the other, the greater is the likelihood that they will form an intermetallic compound instead of a substitutional solid solution.
4. Valences. Other factors being equal, a metal will have more of a tendency to dissolve another metal of higher valency than one of a lower valency.
The physical properties of single crystals of some substances depend on the crystallographic direction in which measurements are taken. For example, the elastic modulus, the electrical conductivity, and the index of refraction may have different values in the [100] and [111] directions. This directionality of properties is termed anisotropy, and it is associated with the variance of atomic or ionic spacing with crystallographic direction. Substances in which measured properties are independent of the direction of measurement are isotropic.
The degree of anisotropy increases with decreasing structural symmetry—triclinic structures normally are highly anisotropic.
A crystalline material is one in which the atoms are situated in a repeating or periodic array over large atomic distances; that is, long-range order exists, such that upon solidification, the atoms will position themselves in a repetitive three-dimensional pattern, in which each atom is bonded to its nearest-neighbor atoms. All metals, many ceramic materials, and certain polymers form crystalline structures under normal solidification conditions. For those that do not crystallize, this long-range atomic order is absent; these noncrystalline or amorphous materials are discussed briefly at the end of this chapter.
The crystal structure found for many metals has a unit cell of cubic geometry, with atoms located at each of the corners and the centers of all the cube faces. It is aptly called the face-centered cubic (FCC) crystal structure.
Some metals, as well as nonmetals, may have more than one crystal structure, a phenomenon known as polymorphism. When found in elemental solids, the condition is often termed allotropy.
Isomorphous: This complete solubility is explained by the fact that both Cu and Ni have the
– same crystal structure (FCC),
– nearly identical atomic radii and
– electronegativities, and
– similar valences.
The copper–nickel system is termed isomorphous because of this complete liquid and solid solubility of the two components.
Electroneutrality: is the state that exists when there are equal numbers of positive and negative charges from the ions.
Chemical formula: The chemical formula of a compound indicates the ratio of cations to anions, or the composition that achieves charge balance. For example, in calcium fluoride, each calcium ion has a +2 charge (Ca2+)and associated with each fluorine ion is a single negative charge(F–). Thus, there must be twice F– as many as Ca2+ ions, which is reflected in the chemical formula (CaF2).
Cations (metallic ions), are positively charged, because they have given up their valence electrons to the anions (nonmetallic ions) which are negatively charged.
Ionic Bond: Is always found in compounds that are composed of both metallic and nonmetallic elements. Atoms of a metallic element easily give up their valence electrons to the nonmetallic atoms. In the process all the atoms acquire stable or inert gas configurations and, in addition, an
electrical charge; that is, they become ions. Sodium chloride (NaCl) is the classic ionic material.
Impact of Bond Type:
– Ionic materials are characteristically hard and brittle and, furthermore, electrically and thermally insulative.
Cations-Anions relation: Cations are ordinarily smaller than anions, because the metallic elements give up electrons when ionized, and, consequently, the ratio is less than unity. Each cation prefers to have as many nearest-neighbor anions as possible. The anions also desire a maximum number of cation nearest neighbors.
Cations (metallic ions), are positively charged, because they have given up their valence electrons to the anions (nonmetallic ions) which are negatively charged.
Stoichiometry: may be defined as a state for ionic compounds wherein there is the exact ratio of cations to anions as predicted by the chemical formula. For example, NaCl is stoichiometric if the ratio of ions to ions is exactly 1:1.
Electronegativity: Is a measure of the tendency of an atom to attract bonding pair of electrons.
Impact: The type of strong bond depends on the difference in electronegativity and the distribution of the electron orbital paths available to the atoms that are bonded. The larger the difference in electronegativity, the more an electron is attracted to a particular atom involved in the bond, and the more "ionic" properties the bond is said to have ("ionic" means the bond electron(s) are unequally shared). The smaller the difference in electronegativity, the more covalent properties (full sharing) the bond has.
