Al alloys are usually used in Aeroplane construction. They are very strong and light.
Cu alloys are used as electrical conductors of low resistance. Bronze, Menal Metal, Gun Metal are Cu alloys.
Mg alloys aren't very common. Steel has a little Mg ( about 0.5%).
Ni alloys with cromel (Cr) are used as electroplates to insulate metal from air and moisture. Ni alloys are also used in many thermocouples.
Hope this was helpful.