Yusei does not use "Iron Chain Dragon". If you are wondering if it is one the Dragons used by the Signers, it's not either. It's just a regular monster.
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You pull it from The Duelist Genesis, you'll unlock the booster by defeating every duelist in prison at least three times (that includes the guards and the Iron Chain guy). It's an ultra rare, so you need to be quite lucky to pull it. Also, although I'm not 100% sure it's possible, you should be able to get it as a random card gift from Yusei as well after beating him. The odds he'll give you Stardust Dragon are slim to none though (if it's possible in the first place).
No, Diamond is the strongest, followed by Iron. Chain is next, offering only a tiny amount more protection than gold. In terms of durability, chain is the same as iron, but diamond again is the best.
they're the same
Use Iron Ingots on an Anvil in Minecrafft 1.4+. (It requires XP)
As I've said before there is no exact percentage on how often iron dragons drop draconic visage's but just that it is a very rare drop.