NIST materials scientist makes new metal alloys for cheaper nickel coins

Carelyn E. Campbell remembers learning about materials science from a National Geographic magazine article she read as a high school student in 1987. “The goal to be able to design materials for specific applications really drew my interest,” she says.

As an undergraduate in materials science and engineering at Northwestern University, Campbell was drawn to metallurgy, which she likens to her hobby of baking: by taking different metals and mixing them in specific proportions, you can process them and create an alloy that has unique properties that are different from its individual elements. At the National Institute of Standards and Technology (NIST), Campbell develops new alloys, such as high-strength steels and high-performance superalloys for turbine blades.

Last year, NIST awarded Campbell and a colleague, materials research engineer Mark Stoudt, a medal for their work on developing three patented coinage alloys for the US Mint, one of which is the lowest-cost alternative the Mint has identified for the 5-cent nickel. Prachi Patel spoke with Campbell about the work that went into these new alloys and about her latest endeavors. This interview was edited for length and clarity.

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.

How did you design and develop the new alloys?

In materials science, we do everything based on the mantra of “processing, structure, property”: processing a material controls the structure, and structure controls the properties. So first we sat down to think about the properties of the desired alloy that we wanted. Then we worked backwards to see what microstructural features we need to get those properties and what processing we need to get that structure.

Knowing the important elements and relationships then lets you develop the correct models to link processing, structure, and properties. For that, we relied heavily on the CALPHAD [calculation of phase diagrams] method, which is a tool for modeling composition and temperature-dependent properties. And then finally we used the models to design specific alloys.

My collaborators Eric Lass (now at the University of Tennessee) and Mark Stoudt at NIST did a lot of the experimental work. We made a few half-kilogram ingots in our melting furnace at NIST, and then we rolled them into sheets, which the Mint took to its facility in Philadelphia to make blank disks.

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.

What are the new alloys you’ve made? Are they being used by the US Mint?

We developed three alloys that would be seamless replacement materials for coinage. Their compositions are based on different ratios of copper, nickel, zinc, and manganese. We also developed a few cladding materials, potentially for new quarters. The Mint is still doing some large-scale testing of these alloys with their producers. The alloys need to meet certain metrics, and then some legislative work needs to occur for the alloys to be implemented.

What is your latest project?

At NIST we are working on designing a variety of metals specifically for the additive-manufacturing process, also known as 3D printing. Most metals that are 3D printed now are materials that were designed to be wrought or cast, which are different processing methods. With 3D-printing machines used today, you have a rapid solidification initially, and then as you put down your next layers, you’re still heating and cooling that first layer. So the material can go through some phase transitions during those secondary heating and cooling processes, and sometimes you get unexpected properties in the final part you print. We’re looking at new materials for additive manufacturing and also trying to optimize the current ones so they can be more efficiently processed.

Plus, with some partners at the Center for Hierarchical Materials Design, a consortium funded by NIST, we are trying to develop a new high-temperature, cobalt-based superalloy. This is geared towards aerospace and land-based turbine engines. The alloy has a similar microstructure to some of the high-temperature Ni-based superalloys we’ve developed before. But cobalt has a higher melting temperature than nickel, so the hope is that we might be able to design these alloys to operate at higher temperatures, which would then increase engine efficiency.

Prachi Patel is a freelance writer in Pittsburgh who covers energy, materials science, and nanotechnology. A version of this story first appeared in ACS Central Science: cenm.ag/campbell.