Materials for engineering, 3rd Edition - (Malestrom)

Structure of engineering materials 25
Phase transformations in the solid state
Changing solid solubility with temperature
Figure 1.20 illustrates this feature in part of a eutectic diagram. Considering
an alloy of composition c, this will solidify to a single-phase solid solution,
α, which is stable only down to temperature T 1 . The line ab is called a ‘solid
solubility’ or ‘solvus’ line and, in the present example, it shows that the
solubility of X in the α phase falls from a value of a% at the eutectic
temperature to b% at the lowest temperature on the ordinate axis.
As the temperature falls below T 1 , the α phase crystals contain more of X
than they would do at equilibrium, that is, they become supersaturated. If
the cooling is slow, crystals of the β phase then form. Initial precipitation
would take place along the grain boundaries of the original α phase – firstly,
because the atoms are more loosely held in the grain boundaries and so
might be expected to ‘break away’ more readily to form the new phase and,
secondly, because the atomic disarray at the α-phase grain boundaries could
help to accommodate any local volume changes associated with the growth
of the new β crystals.
In many systems, the change of solubility with temperature is so great that
the second phase cannot all be accommodated in the grain boundaries of the
primary phase, and precipitation within the primary grains then occurs. This
‘intragranular’ precipitation is usually found to take the form of plates or
needles in parallel array (see Fig. 1.21). This striking geometrical feature
arises from the tendency of the new crystals to grow with their interfaces
aligned parallel with certain definite crystal planes of the primary phase.
These planes will be such that there is a better atomic fit across the α/β
interfaces than if the β phase was randomly distributed inside the α phase.
c
L
α + L
Temperature
T 1
α
a
α + β
b c
%X
1.20 Phase diagram showing decreasing solid solubility in the α-
phase with decreasing temperature.