Since metals in pure forms have limited mechanical properties (e.g strength and hardness) they are mostly used in alloy forms. An alloy consists of at least two metals, namely, the parent metal and the alloying element. However, many metallic alloys contain several or many alloying elements in order to satisfy a combination of properties. Metallic materials can be further subdivided into two groups:
1) Ferrous metals and
2) Non-ferrous metals.
The term ferrous metals is used for all those alloys having iron as the major constituent. Pure iron is a relatively soft material and is hardly of any commercial use in pure state. Alloys of iron with carbon are classified according to their carbon content: Steels with 0.02 - 2 % carbon and Cast Irons with 2-4 % carbon.
Ferrous metals account for about three quarters of the metal tonnage used throughout the world. Steels can be further sub-divided into a number of categories. The term carbon steel is used for those steels in which essentially just carbon and iron are present. The term alloy steel is used where other elements are included. Typical alloying elements include manganese, aluminium, copper, nickel, chromium, cobalt and so on. Stainless steels are one form of alloy steels, which have high percentages of chromium and nickel in its chemical composition for their high resistance to corrosion. Due to their good strength, and relatively low cost, steels are widely used in many different applications including construction, transport, general engineering and consumer products.
Cast Irons as the name implies have been developed for casting process and the terms refer to a family of alloys including gray cast iron, ductile iron, white and malleable iron.
These are all the known metals other than irons. These include alloys of aluminium, copper, magnesium, zinc, titanium and nickel. They provide a wide range of favorable properties including good strength, light weight, resistance to corrosion, and ease of fabrication. Application of these alloys range from basic consumer goods to mechanically demanding aerospace components and structures.
Aluminium is the most important of the non-ferrous alloys and ranks second to steel in worldwide quantity used. It is increasingly used in both automotive and aerospace industry for its low density, good strength and excellent corrosion resistance as well as its ease of manufacture. Both nickel and titanium alloys are extensively used in jet engines and gas turbines because of their advantageous high mechanical properties and excellent corrosion resistance characteristics particularly at high operating temperatures.