The Heavy Metal Collection

The Heavy Metal Collection consists of five dragons;
Metal Dragon
Gold Dragon
Mercury Dragon
Platinum Dragon
Armadillo Dragon

Metal dragon is Generation I dragon you get on level 18. (Or "get" - you can buy it when you reach level 18, with gold, but it's not too expensive.)
It's breeding and hatching time is 10 hours each.

Gold Dragon = Metal + Electric
Breeding time 12 hours
Hatching time 12 hours

Mercury Dragon = Metal + Sea
Breeding time 12 hours
Hatching time 12 hours

Platinum Dragon = Metal + Ice
Breeding time 12 hours
Hatching time 12 hours

Armadillo Dragon is a bit tricky, because it's Metal plus Terra, and these are opposite elements, which means that you need a hybrid to breed Armadillo. You need a metal hybrid with Terra or terra hybrid with Metal.
The easiest way to breed Armadillo is to breed Double Metal with Double Terra, but these dragons are exclusive and not breedable, so try with Metal and Flaming Rock (F+T) or Terra and Gold (M+E)
Like with the others, Armadillo Dragon's breeding and hatching time are both 12 hours.

The first 3 Dragons (when babies) have the same animation.
Metal, Gold and Platinum dragon look very much the same when grown up.