Neat interview with NIST metallurgist Carelyn E. Campbell (article by Prachi Patel)
Why do we need new alloys for coins?
The project discussion started in 2013. The US Mint asked if we could design new coinage alloys to reduce the cost of making coins. The nickel at the time cost 9 cents to make. It’s made of 75% copper and 25% nickel by mass, and they wanted to reduce the amount of nickel in the alloy. The new alloy still has nickel, but less of it, and would cost only 5.9 cents to make a 5-cent coin. The primary goal was to reduce cost but to keep all the properties of the current coin.
What properties are important for coinage alloys?
One of the most important properties is the electrical resistivity. That’s the signature vending machines use to identify the type of coin, so a dime needs a different signature than a quarter or a nickel. Others are corrosion resistance and wear resistance. We also had processing constraints. The Mint doesn’t want to have to change the processing mechanisms for the new alloy, such as the annealing temperature range.
Was there anything unexpected that rerouted the alloy development?
Yes, color. Initially we weren’t thinking that the color would be a major part of our design, but it became one of the primary constraints of the alloy composition. The Mint wanted to keep color identical to the currently used coin alloy. We were trying to substitute manganese for the nickel. And the more manganese you add, the more yellow the coin becomes.
An interesting problem! Great story, read the whole thing.