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Metals are aiming the most useful building materials. They exist in nature as compounds like
oxides, carbonates, sulphides and phosphates and are known as ores. Metals are derived from
ores by removing the impurities. Those used for engineering purposes are classified as ferrous
metals, with iron as the main constituent, e.g. cast iron, wrought iron and steel and others like
aluminium, copper, zinc, lead and tin in which the main constituent is not iron as non ferrous
metals.
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Iron is a pure element occurring in four different allotropic structures as alpha, beta, delta, and
gamma iron. -iron is weak and ductile, possesses magnetic properties and is unable to
dissolve carbon. -iron is hard, brittle, non-magnetic and dissolves carbon. v-iron has properties
similar to -iron. -iron absorbs negligible carbon and is non-magnetic. Of these the common
commercial forms are gamma iron with its fcc (face-centered cubic) structure formed at
temperatures from 1394°C to 912°C, and alpha iron which has bcc (body-centered cubic)
structure formed at temperatures from 912°C to 273°C. The delta form is commercially
unimportant. Gamma iron containing carbon, is called austenitic and alpha iron containing
carbon, is called ferritic. The other steel alloys having same gamma structures are also called
austenitic. Similarly alloys having alpha structure are called ferritic.
Even the closed-packed metallic structures contain empty spaces (holes), assume atoms to
be spherical. About 26 per cent of the volume is empty in the fcc metallic structures and about
29 per cent of the bcc volume is empty. The holes in the gamma iron are nearly half the diameter