Materials for engineering, 3rd Edition - (Malestrom)

Structure of engineering materials 15
of a two-dimensional plot of temperature versus composition, which marks
out the composition limits of the phases as functions of the temperature. We
will now introduce some simple examples of phase diagrams, which we will
correlate with some microstructures.
Solid solubility
In a solid solution, the crystal structure is the same as that of the parent
element – the atoms of the solute element are distributed throughout each
crystal and a range of composition is possible. The solution may be formed
in two ways:
(a) in interstitial solid solutions, the atoms of the solute element are small
enough to fit into the spaces between the parent material atoms, as
illustrated in Fig. 1.10(a). Because of the atom size limitation involved,
interstitial solid solutions are not common, although, in steel, carbon
atoms dissolve to a limited extent in iron crystals in this way.
(b) in substitutional solid solutions, the atoms share a single common array
of atomic sites as illustrated in Fig. 1.10(b). Zinc atoms may dissolve in
this way in a copper crystal (up to approximately 35% zinc) to form brass.
A few pairs of metals are completely miscible in the solid state and are
said to form a ‘continuous solid solution’; copper and nickel behave in this
way and the phase diagram for this system is shown in Fig. 1.11. The horizontal
scale shows the variation in composition in weight per cent nickel and the
vertical scale is the temperature in °C. The diagram is divided into three
‘phase fields’ by two lines – the upper phase boundary line is known as the
liquidus and the lower line as the solidus. At temperatures above the liquidus,
alloys of all compositions from pure copper to pure nickel will be liquid,
while at temperatures below the solidus, all alloys are in the solid state. It
will be apparent that, unlike pure metals, alloys freeze over a range of
temperature whose magnitude depends upon the composition of the alloy
(a)
1.10 Solid solutions formed (a) interstitially and (b) substitutionally.
(b)