PHYSICAL PROPERTIES OF METALS AND ALLOYS

PHYSICAL PROPERTIES OF METALS AND ALLOYS
The word constitution used with reference to metallic sub - stances does not have the same meaning as composition. Con - stitution denotes the manner of arrangement of the metal atoms as to geometric form in solid crystals, and the regular or or - dered arrangement of different kinds of metal atoms and their relation to each other in such a crystal.
The pattern formed by this orderly arrangement of the atoms is known as the space lattice.
Most metals crystallize with one of the three following lattice structures:
Close-packed cubic: copper, nickel, lead, aluminium, cobalt, silver, gold, platinum.
Body-centred cubic: iron, molybdenum, tungsten, chromium.
Hexagonal close-packed: zink, cadmium, magnesium, beryllium, titanium.
This union of atoms into a geometric array is the physical difference between liquid and solid metal.
The formation of metal crystals within a melt begins at each cooling surface of the liquid mass and extends from the exterior to the interior as heat is lost from the mass. Every change in the conditions of cooling, such as increasing or decreasing the rate at which heat is conducted away from the freezing mass, will have an influence on the size and shape of the crystals and, therefore, on the constitution and properties of the solidified mass.
Melting and Boiling Points. - The temperature at which a metal melts, is called the melting point, the metals of lower melting points are generally the soft metals and those of high melting the hard metals.
The boiling point of a substance depends on the surrounding-pressure. The term "boiling point" refers to the temperature at which the metal boils under normal atmospheric pressure.
Electrical C