This is something of a practical question more than anything else. Consider a setting in the modern world, where a large country e.g. the U.S. has collapsed and been replaced by a sort of modernized version of a feudal monarchy. Let's call it "federal monarchy". In this new Kingdom of America, the benevolent Philosopher-King has decided he doesn't want to have to deal with fiat money. Maybe he's concerned that the Royal Bank of America will manipulate the money supply to the detrement of the People, or whatever, I'm not really concerned with the rationale.
Now, the King has decided that his biggest denomination will be one ounce (not a troy ounce, he wants to keep things simple) of 20 carat gold. The idea here is that, roughly speaking, the purchasing power of that amount of gold will be more or less the same as 5lbs of sterling silver (which he's going to make the next denomination down). Ignore the practical concerns of lugging this much metal around for the moment.
He wants to make his gold "Crowns" tough, but keep the weight relatively manageable at an ounce and still be worth about 5lbs sterling silver. What should the remaining 4 carats' worth of alloying metal be, if he wants to maximize the durability of the alloy? I've heard that titanium really strengthens the metal significantly, but I don't know if there would be enough in there to strengthen the alloy enough to justify the expense.