Materials for engineering, 3rd Edition - (Malestrom)

16
Materials for engineering
1600
C
1500
Liquid (L)
Temperature (°C)
1400
1300
1200
L + S
e
d
b
x
a
T 1
T 2
T 3
1100
Solid solution (S)
1000
0 10 20 30 40 50 60 C 70 80 90 100
Weight
(%Ni)
1.11 Copper–nickel equilibrium diagram.
and is equal to the vertical separation of the liquidus and solidus at a given
composition.
In working from a phase diagram, the beginner should always first consider
some specific composition of alloy and study its behaviour with respect to
change in temperature. There is an important nickel–copper alloy known as
monel, whose retention of strength at high temperatures enables it to be used
for turbine blading: its composition is approximately 65 weight per cent
nickel – 35 weight per cent copper, and the vertical line CC in Fig. 1.11 has
been constructed to correspond to this. Let us consider the solidification of
a casting of this alloy, with molten metal being contained within a mould.
Considering a slow progressive decrease in temperature, at temperatures
above T 1 the liquid phase is stable, but at T 1 solidification commences and
the two-phase field (marked L + S in Fig. 1.11) of the diagram is entered. In
any two-phase field of a phase diagram, the compositions of the two phases
co-existing at a given temperature are obtained by drawing a horizontal (or
isothermal) line. The required compositions are given by the intersections of
the isotherm with the phase boundary lines. In the present case, the isotherms
are shown as dotted lines in Fig. 1.11, and, at temperature T 1 , liquid of
composition c starts to freeze by depositing crystal nuclei of solid solution
composition a, obtained by drawing the isothermal line at temperature T 1 in
the two-phase field. As the temperature continues to fall, the loss of this
nickel-rich solid causes the composition of the liquid to become richer in
copper, as denoted by the line of the liquidus, so that when temperature T 2
is reached, the composition of the liquid (given by the new isotherm) is now
seen to be d. The growing crystals, normally in the form of dendrites, Fig.
1.5(a), remain homogeneous, providing the temperature is not falling too