Structure of Metals and Alloys | Metallurgy

In this article we will discuss about the structure of metals and alloys.

1. Grain Structure:

The microstructure of solid metallic bodies consists of grains. Grains consist of unit cells in which atoms are arranged in a particular order. The cell structure repeats itself throughout the volume of the grain (Fig. 1.1). That is why the grains are also called crystallites. The structure is called lattice in which atoms are placed at lattice points. In metals, generally there is only one atom at a lattice point.

There are many types of structures of unit cells for different materials, however, metals generally possess one of the following three cell structures:

(i) Body centered cubic structure (BCC).

(ii) Face centered cubic structure (FCC).

(iii) Hexagonal closed packed structure (HCP).

The three cell structures are illustrated in Fig. 1.2. Some metals such as iron (Fe), cobalt (Co), titanium (Ti), etc. change their cell structure at different temperatures.

(i) Body Centered Cubic Structure:

The structure consists of one atom at each of the eight corners of a cubical element and one atom at the center of the cube in Fig. 1.2(a). Metals with this structure are chromium (Cr), hafnium (Hf) at temperatures greater than 1975°C, iron (Fe) except at temperatures 911°C to 1392°C, molybdenum (Mo), niobium (Nb), tantalum (Ta), titanium (Ti) at temperatures greater than 882°C and tungsten (W), etc.

(ii) Face Centered Cubic Structure:

In this structure there is an atom at each of the eight corners of the cubical element and there is an atom in the middle of each face of the cube as shown in Fig. 1.2(b). Metals with this structure are aluminum (Al), cobalt at temperature greater than 1120°C, copper (Cu), gold (Au), iron (Fe) between temperatures 911°C – 1392°C, lead (Pb), nickel (Ni), silver (Ag), platinum (Pt), etc.

(iii) Hexagonal Closed Packed Structure:

In this cell structure there is an atom at each corner of a hexagonal prismatic element, besides, there are three atoms symmetrically placed between the two end faces as shown in Fig. 1.2(c) and one atom each at center of the flat end faces. Metals with this structure are beryllium (Be), cobalt at temperatures less than 1120°C, magnesium (Mg), zinc (Zn), titanium (Ti) at temperatures less than 882°C, etc.

2. Lattice Defects:

Ideally there should not be any defect in lattice structure, however, imperfections and defects may occur due to alloying elements, plastic deformation, grain boundaries, etc.

The lattice structure generally contains following types of defects which are illustrated in Fig. 1.3:

(i) Point defects or imperfections.

(ii) Line defects which are also called dislocations.

(iii) Surface defects—grain boundaries.

Point defects are caused by various reasons such as:

(i) Absence of an atom from a lattice point,

(ii) An atom getting to a site which is not a lattice point,

(iii) An atom of a different element (alloy) substituting an atom of parent metal, etc.

Point defects disturb the natural arrangement of atoms in its vicinity and consequently atoms surrounding the point defect are either stretched apart or are pushed too close. This gives rise to additional pull or push among the atoms.

The type and concentration of these imperfections or defects greatly influence the properties of metals and alloys. The defects may also be induced or controlled by alloying, heat treatment or plastic deformation in order to obtain a change in the mechanical properties.

Line defects or dislocations are important for plastic deformations.

The following two types of dislocations are observed:

(i) Edge dislocations.

(ii) Screw dislocations.

These are illustrated (Fig. 1.4). The figure also shows how these defects travel through the lattice when subjected to shear forces. The dislocations travel from one layer of atoms to another as the shear stresses increase. Slipping of all the atoms simultaneously would take enormous forces which is not explained by much lower yield strength of metals. A dislocation may be obstructed in its movement by atoms of alloying elements or point defects, by stationary dislocations and grain boundaries or by other defects.

More force is then required to overcome the obstruction. Sometimes, the ends of a dislocation may get pinned down, in such a case, increase in applied shear force results in curving of the line defect (Fig. 1.5). With further increase in stress the dislocation may get curved inward, ultimately transforming into a ring dislocation and a new dislocation at the initial points. Thus this becomes a source of production of dislocations.

Plastic deformation creates a large number of dislocations. Higher the dislocation density in the material, higher is the resistance to movement of dislocations and hence higher forces are required for plastic deformation. This explains the increase in strength during plastic deformation, which is called strain hardening or work hardening.

Surface defects are the irregular arrangement of atoms at the grain boundaries. In a grain the atoms are arranged in particular order, however, the directions of the arrangement are different in neighbouring grains. The boundaries are irregularly shaped with the effect that at some places atoms are too far apart while at other places they are too close than the normal distance, thus giving rise to tensile and compressive forces.

These defects also cause restriction to movement of dislocations. In a structure with small grains the dislocations can move only a small distance before encountering an obstacle, i.e. grain boundary. Therefore, a metal is stronger when it has small grains structure than when it has a large grain structure.