Materials for engineering, 3rd Edition - (Malestrom)

Metals and alloys 85
At% Al
0 10 20 30 40 50 60 70 80 90 100
1500
1412
1400
1300
1200
Liquid
1100
Temperature (°C)
1000
900
800
700
600
500
Si
Liquid + Si
577
Liquid + Al
Al
660
400
300
Si + Al
200
0 10 20 30 40 50 60 70 80 90 100
Wt% Al
3.9 The silicon–aluminium phase diagram.
ductility. The binary Al–Si alloys are generally used where strength is not a
primary consideration, e.g. pump casings and automobile engine manifolds.
Copper and magnesium additions are made to enhance the strength of such
alloys and more complex compositions (e.g. including nickel additions) lead
to improved elevated temperature properties for such applications as piston
alloys for internal combustion engines.
Small additions of magnesium allow significant age-hardening of the
castings through precipitation of Mg 2 Si in the aluminium matrix. Doubling
of the yield strength may be achieved in this way, and such alloys find use
in aircraft and automotive applications.
Aluminium–copper alloys, although less easily cast than Al–Si alloys,
respond well to age-hardening heat treatments. Several compositions have
been developed with enhanced elevated temperature properties, for example
for use as diesel engine pistons.