The single most used material today in the Industry is Metal. The reason for the increased use of the metals is due to its wide range of properties:
|(a) Lustre||(f) Relatively high melting point||(l) Stiffness||(p) Castability|
|(b) Hardness||(g) Strength||(l) Rigidity||(q) Dimensional stability|
|(c) Low specific heat||(h) Ductility||(m) Formability|
|(d) Plastic deformability||(i) Malleability||(n) Machinability|
|(e) Good thermal conductivity||(j) Opaquity||(o) Weldability|
Metals are found in nature in the form of ores. The science of metallurgy includes deriving the metals from its ores, its purification, alloying with other metals and finally manufacture into shape and forms usable in industries.
Metallurgy is a science and technology of:
- Extracting metals from their ores.
- Refining of crude metals.
- Production of alloys to alter constituents and improve properties.
- Studying the relationship of physical, thermal and mechanical treatment of metals and alloys.
Classification of Metallurgy
The study of metallurgy is classified in two major groups,
- Chemical Metallurgy
- Physical Metallurgy
Chemical metallurgy deals with,
- The extraction of metals from the ores in which they are found.
- The process of mining, extraction and refining of metals and alloys that can be used in industries.
Physical metallurgy deals with
- The nature, structure and physical properties of metals and alloys, as well as their applications and behaviour.
- The physical behaviour of the metals during shaping and treating operations.
Structure of Materials
It should be clear that all matter is made of atoms. From the periodic table, it can be seen that there are only about 100 different kinds of atoms in the entire Universe. These same 100 atoms form thousands of different substances ranging from the air we breathe to the metal used to support tall buildings. Metals behave differently than ceramics, and ceramics behave differently than polymers. The properties of matter depend on which atoms are used and how they are bonded together.
The structure of materials can be classified by the general magnitude of various features being considered. The three most common major classification of structure in increasing size, are:
- Atomic Structure. It includes features that cannot be seen, such as the types of bonding between the atoms, and the way the atoms are arranged.
- It includes features that can be seen using a microscope, but seldom with the naked eye.
- It includes features that can be seen with the naked eye.
The atomic structure primarily affects the chemical, physical, thermal, electrical, magnetic, and optical properties. The microstructure and macrostructure can also affect these properties but they generally have a larger effect on mechanical properties and on the rate of chemical reaction. The properties of a material offer clues as to the structure of the material. The strength of metals suggests that these atoms are held together by strong bonds. However, these bonds must also allow atoms to move since metals are also usually formable. To understand the structure of a material, the type of atoms present, and how the atoms are arranged and bonded must be known.
Table 1 Periodical Table of Elements
Structure of Atom
An atom consists of Protons, Neutrons and Electrons. The Protons (+ve charged particles) and Neutrons form the nucleus of the atom around which the Electrons (- ve charged particles) rotate. The number of protons in the nucleus is referred as its Atomic number. The Mass number is the number of protons and neutrons of an atom. The atom is the smallest particle of an element that can enter into a chemical combination with another element. A molecule (group of atoms) is the smallest particle of an element or compound that can exist independently.
Atoms always maintain a balanced electrical charge. Hence, the number of negatively charged electrons surrounding the nucleus is equal to the number of protons. It is also known that electrons are present with different energies and it is convenient to consider these electrons surrounding the nucleus in energy “shells.” The maximum number of electrons that are in any shell is determined by the number of shells. The first shell which is closest to the nucleus marked ‘K’, the second ‘L’, third ‘M’, and so on. The maximum number of electrons that can be in a shell is 2 in K, 8 in L, 18 in M, 32 in N, 18 in O and 8 in P.
All chemical bonds involve electrons. Atoms will stay close together if they have a shared interest in one or more electrons. Atoms are at their most stable when they have no partially-filled electron shells. If an atom has only a few electrons in a shell, it will tend to lose them to empty the shell. These elements are metals. When metal atoms bond, a metallic bond occurs. When an atom has a nearly full electron shell, it will try to find electrons from another atom so that it can fill its outer shell. These elements are usually described as nonmetals. For example, potassium, with an atomic number of 19, has two electrons in the inner shell, eight in the second and third shell, one in the outer shell. Potassium (2,8,8,1) is an unstable atom due to the extra electron in its outer shell and will try to lose this electron. Therefore, it commonly bonds with Chlorine (2,8,7) which has 7 electrons in its outer shell to form KCl.
The bond between two non metal atoms is usually a covalent bond. Where metal and non metal atom comes together, an ionic bond occurs. There are also other, less common, types of bond but the details are beyond the scope of this material. On the next few pages, different types of bonding in the solids will be discussed.