Materials for engineering, 3rd Edition - (Malestrom)

Structure of engineering materials 19
C
g
e
d
h
f
C
b
f ′
b′
a
T 1
T 2
T 3
T 4
Temperature
C
Ni (%)
1.12 Depression of the solidus by rapid solidification.
100% Ni
cored. Each crystal will consist of layers of changing composition – the
‘arms’ of the original dendrite being richer in the higher-melting constituent
(in this case, nickel) than the average and the interdendritic regions being
richer in the other constituent (i.e. copper) than the average. In a microsection
of this structure, therefore, each grain will show a chemical heterogeneity,
which will be reflected in its rate of attack by the etchant, and Fig. 1.13
illustrates this effect in a sample of chill-cast (i.e. rapidly solidified) brass,
which is a solid solution of 30 weight per cent zinc in copper. Depression of
the solidus and ‘coring’ are common features of many cast alloys.
If the cored structure is undesirable, it may be removed by long heat
treatments at high temperatures (known as ‘homogenization treatments’),
which allow the solute atoms to be redistributed by solid state diffusion.
No mutual solid solubility (simple eutectic)
The cadmium–bismuth system is a simple eutectic system (see Fig. 1.14),
which exhibits no solubility of cadmium in bismuth or of bismuth in cadmium.
The phase diagram, therefore, consists of a liquidus line showing a minimum