Story takes place in the world of the movie 'Moon Child' (the one with Japanese musicians HYDE and Gackt) and set in the near future (2024 or so) where technology is more or less stagnated to early 2000's due to an near economic collapse. It all takes place in the fictional city of 'Mallepa', a melting pot for various ethnic immigrants, most notably poor Japanese. Oh, and Vampires exist.
In fic, Kei (HYDE's character) bargains himself off to the antagonist (another, older vampire with human and younger vampire minions) in order to free a friend and ends up being restrained inside a room with a chain and collar by said antagonist to prevent him from leaving once the friend is freed. Kei by this point is very weakened due to going without drinking blood for an extended amount of time but still stronger than an average human. They never say how old Kei is so age in terms of strength isn't really a factor here.
What I had planned on doing was having the chain be made mostly of silver until I remembered that silver is actually a pretty weak metal, and that even with a weakened and with a presumed aversion to silver, it probably wouldn't be enough to hold a vampire. I'm trying to figure out a good metal that could be mixed with silver and be strong enough to hold something stronger than human.
I've waded through the tags on here and google searched using the terms 'strength of silver', 'strength ratio of metals' and 'types of chains'. Unfortunately, I wasn't sure how exactly to word these in a way that would be even remotely helpful.