Atomic-Crystal Structure of Metals

 

1.3.1. General Features of Metals

 

Among 106 elements known today, 76 are metals. Such elements as Si (silicon), Ge (germanium), As (arsenic), Se (selenium) and Te (tellurium) are considered to be intermediate between metals and nonmetals. In the solid state metals possess a number of typical properties:

- high thermal and electrical conductivity;

- positive temperature coefficient of electrical resistivity (the electrical resistance of pure metals increases with the temperature; about 30 metals display superconductivity, their electrical resistance drops abruptly to practically zero at temperature near absolute zero);

- thermionic emission, i. e. capacity to emit electrons when heated;

- good reflection of light; metals are opaque and have a specific metallic luster;

- they lend themselves well to plastic deformation.

All metals and alloys are crystalline bodies. Their atoms are arranged in a regular order repeated periodically in three directions. This distinguishes them from amorphous bodies, whose atoms are arranged in random disorder.

All typical properties of metals can be explained by the fact that they contain highly mobile collective conduction electrons. The bonds between the outer electrons and atoms' nuclei are weak, so metals have so-called “collective” electrons, which move freely between the positively charged and periodically located ions. Therefore metals have high electric and heat conductivity as well as electron emission capacity. By regular atoms arrangement metals possess high light reflection and high plastic properties.

 

1.3.2. Concept of Crystal Lattice

 

The atomic-crystal structure is the mutual positions of the atoms existing in a crystal. A crystal consists of the atoms arranged in a definite order, periodically repeated in three coordinate directions.

The concept of a space lattice, or crystal lattice describes the atomic-crystal structure of substances. Fig. 1.16 illustrates an example of such a crystal lattice. Heavy lines indicate the smallest parallelepiped (crystal lattice), which could be repeated consecutively along its three axes to build up the whole crystal.

Fig. 1.16. Schematic representation of a crystal lattice

 

The metals and alloys have various types of crystal lattices. But the great majority of commercially important metals have one of the following highly symmetrical compound lattices with close-packed atoms: body-centered cubic (bcc) (Fig. 1.17a); face-centered cubic (fcc)
(Fig. 1.17b); hexagonal close-packed (hcp) (Fig. 1.17c).

Fig. 1.17. Types of crystal lattices of metals: a – body-centered cybic,

b – face-centered cybic, c – hegagonal-close-packed

 

1.3.3. Allotropic (Polymorphic) Transformations

 

At different temperatures many metals exist in different crystalline forms. These forms are sometimes called allotropic (polymorphic) modifications. As a result of polymorphic transformation, atoms of a crystalline body with a lattice of one type are rearranged to form a crystal lattice of another type.

The curve of cooling the pure iron is represented in Fig. 1.18. Iron exists in the α-Fe and
g-Fe allotropic forms. α-Fe exists at temperatures below 911 and above 1392°C (sometimes it is named d-Fe (see Fig. 1.18)). In the temperature range from 911 to 1392°C iron is in gamma form. The crystal lattice of α-Fe is the body-centered cubic type. Up to 768°C iron is magnetic, above this temperature it is nonmagnetic. The crystal lattice of g-Fe is face-centered cubic type.

Polymorphic transformations are known for metals: Fe, Co, Ti, Mn, Sn, Li, Ca, Sr, Zr, rare-earth metals, etc.

 

1.3.4. Anisotropy of Metals Properties

 

Different densities of their atoms in various planes and directions cause that many properties of metal crystals (chemical, physical, mechanical) depend on the direction, they are measured in. This inequality in the properties of a monocrystal in various crystallographic directions is called anisotropy.

All crystals are anisotropic bodies. They differ from amorphous solids (glass, plastics, etc.), which display the same properties in any direction.

Engineering metals are polycrystals. Their crystallites have a statistically disordered mutual orientation. Consequently, the general properties of a polycrystalline body are more or less the same in all directions, i.e. engineering metals are isotropic materials.

 

Fig. 1.18. Cooling curve of pure iron

 

1.3.5. Defects in Crystal Lattices of Metals

 

A real crystal always has defects in its structure. These defects are classified by their geometric features into:

- point (zero-dimensional)

- linear (one-dimensional)   defects

- plane (two dimensional)

Point defects are the small ones along all three dimensions. Their size does not exceed several atomic diameters. Point defects include (Fig. 1.19)

- vacancies (Schottky defects), i.e. lattice points where atoms are absent;

- interstitial atoms (Frenkel defects), i.e. additional atoms in crystal lattice;

- substitutional atoms, i.e. foreign atoms in lattice nodes of the basic metal.

Fig. 1.19. Point imperfections in a crystal lattice

 

Vacancies are most frequently formed by atoms leaving their regular position at the lattice points, jumping to the surface of the crystal and evaporating from the surface of a crystal. Less frequently, vacancies may be formed by atoms jumping into an interstitial position.

The vacancy concentration increases with the temperature growth. The number of vacancies at temperatures near the melting point may reach one per cent of the total number of atoms in the crystal. At a given temperature not only single vacancies are formed in crystal, but double, triple and even larger ones as well.

Vacancies are formed not only by heating but also in the process of plastic deformation and in bombardment of a metal with high-energy atoms or particles (irradiation in a nuclear reactor).

Interstitial atoms are formed by jumping of an atom to interstitial position, which leaves a vacant site. A vacancy positions itself at the lattice point that was previously occupied by the atom. In the closely packed lattice, typical for the most metals, the energy required to form interstitial atoms is several times greater than that required to form thermal vacancies. For this reason, interstitial atoms are rare in metals, and thermal vacancies are the main point defects in this kind of crystals.

Point defects cause local distortion of the crystal lattice and influence certain physical properties of metals (electrical conductivity, magnetic properties, as well as the phase transformations in metals and alloys).

Linear defects are very small in two dimensions and of great extent along the third dimension. They are called dislocations. A crystal lattice with edge dislocation is shown schematically in Fig. 1.20. The edge dislocation is a localized distortion of the crystal lattice due to the presence of an “extra” atomic halfplane or extraplane B.

In addition to edge dislocations there are also screw dislocations. The screw dislocations are formed by incomplete shear of crystal in vertical plane (Fig. 1.21.)

Dislocation density r is defined as the total length of the dislocations l in cm per unit volume V of the crystal in cm³. The technical metals have dislocation density 10⁴…10¹³ cm^-2, i.e. up to one hundred million km in cm³.

Plane defects or surface imperfections are small only in one dimension and constitute the interfaces between the separate crystallines (grains) or their blocks in polycrystalline. By different orientation of the neighboring grains, the atoms are arranged much less regularly at the boundary between grains than within the volume of the grains (Fig. 1.22.)