Tuesday, March 27, 2018

Meet the Undergrad Helping to Make Ultralight, High-Performance Metals a Reality

Adam Shaw is still working toward his degree, but he’s also working toward the creation of next-gen materials that could change the world of modern manufacturing. A senior at Harvey Mudd College in California, Shaw is part of an international team of physicists and materials scientists whose research could hold the key to making an entirely new class of durable, lightweight alloys—mixtures of metals that can crystallize together to be greater than the sum of their parts.

Shaw, seen here presenting his work at the American Physical Society's March meeting in Los Angeles
Image Credit: Harvey Mudd College
Designing and manufacturing is always a balancing act, whether you’re creating a car engine, an aircraft, or a laptop—and it’s an engineer’s job to find the ideal middle ground between performance and price, or between power and efficiency. While that means finding just the right material for the job, many chemists and physicists are in the business of making new materials, expanding our range of possibilities, and pushing machines ever closer to peak performance.

That can mean a lot of different things, though: some scientists work experimentally, testing elaborate arrangements of layered atoms to see if they can find a new superconductor that works at high temperatures. Others focus on theory, trying to understand how the orbital structure of an atom’s electrons changes its bonding characteristics, and how those changes might influence a material’s macro-scale properties.

But Shaw is part of a team that’s coming at the problem from both sides: while a group of scientists in Australia attempts to synthesize new super-alloys, their Californian collaborators crunch the numbers, simulating new compounds and crystal structures, in order to try and predict what’s going to work well. The hope is that if they find a recipe that looks promising, the team on the other side of the world can synthesize it and test it in the lab.

“In recent years, there's been a lot of research into magnesium-rare earth alloys,” says Shaw. They “combine both high strength and ductility, while maintaining that desired low density.”

Magnesium is one of the lightest metals on the periodic table, beating out even aluminum, and alloys of magnesium containing more obscure elements like yttrium and scandium can have an extraordinary strength-to-weight ratio—an important factor in the design of things like aircraft.

The rare earth elements, shown outlined in red on the periodic table. They're not as rare as their name might imply—some are even more abundant than copper—but tend not to occur in nice concentrated veins, the way many other minerals do.
Image Credit: CurioCity
Ductility, the ability to flex and stretch without breaking, is another crucial feature. Typically, the harder a metal is, the more brittle it becomes, as you might know if you’ve ever done research into wedding rings made of unconventional materials like tungsten. While tungsten rings are some of the most scratch-resistant jewelry on earth, a hard impact can cause them to shatter to pieces. They’ll withstand far more abuse than a gold ring ever could without warping, but when they reach their break point, it’s game over—in the form of what engineers call "catastrophic failure"…not something you want happening to your car’s engine.

These magnesium alloys—and one in particular, with about 20% scandium blended in—seem promising: one form of this MgSc mixture “shows very high strength and generally good mechanical properties,” according to Shaw. But there’s a problem; “it’s only stable far above room temperature.”

The properties of a material don’t just depend on which atoms are in it. Graphite and diamond are chemically identical, and the only difference is how the atoms are arranged: the crystal structure. It takes immense heat and pressure to turn graphite into diamond, but once it’s been converted, the end product is stable at a wide range of temperatures. Not all materials are like that, though: watch what happens to tin when it transitions from its “beta” structure, stable above 56°F, to its “alpha” structure.

Yeah…you don’t want that happening to your engine, either.

Shaw explains that, “In general, there are three main crystal lattices competing in most metallic alloys: face centered cubic (FCC), body centered cubic (BCC), and hexagonal close packed (HCP).” It’s the BCC arrangement of MgSc that displays exceptional durability and other exciting properties, but it’s not much good if you have to keep it at hundreds of degrees for it to stay in the proper structure.

The BCC, FCC, and HCP crystal structures. As the temperature drops, MgSc can spontaneously transition from the BCC phase to one of the others.
Image Credit: Alex Scrimshire
But the team’s Australian collaborators at University of New South Wales have made progress, with the discovery that a pinch of yttrium—another rare earth metal—helps stabilize the alloy in its BCC form. Although the usefulness of yttrium here was discovered experimentally, it’s also where Shaw and his computational work come into the spotlight.

“Following up on that with computation, we've been able to show that indeed at around 20% scandium…just a small amount of yttrium makes BCC the energetically dominant phase,” allowing it to remain in its useful form at a wider range of temperatures. Bringing the computational approach in line with the experimental results is an important step.

“We can now extend these results, and instead of just explaining what we've already seen in the lab we can start trying to make suggestions to our experimental collaborators.”

Yttrium, the "special sauce" that stabilizes the MgSc alloy, shown in its pure state.
Image Credit: Heinrich Pniok, via Wikimedia Commons (Free Art License 1.3)
Systems in nature tend to move toward the lowest-energy state available to them—this is one way to explain why a ball rolls down a hill: it’s seeking the lowest gravitational potential energy. Similarly, two electrons in a box will generally get as far from one another as possible: being close together puts them at a high electric potential, or voltage.

Believe it or not, this same logic can be applied to dozens, even hundreds of particles at a time, to find the lowest-energy state for the entire system. It takes a lot of computer power, though: the traditional method is to take each individual particle in the system, calculate its electric potential relative to every other particle in the system, then repeat the process for the next particle, and the next one, on down the line until you’ve done every particle in your model. Add them all up, and you’ve got the system’s total energy.

But it turns out that even when using the most powerful supercomputers available, this method of treating every electron separately can become impossibly slow. Luckily, a method known as Density Functional Theory (DFT) gets around this problem by treating all the electrons together at the same time, sort of like a charged gas. Through some mathematical manipulations, this lets scientists like Shaw analyze the energy states and behavior of the system as a whole, while producing results of similar quality at a much faster rate.

That’s important—if running these simulations already sounds like a lot, imagine doing it thousands of times, starting with your atoms in various initial configurations, to find which one has the lowest energy: that’s how you find a crystal structure computationally.

“We use a nonlinear regression model, similar to machine-learning”, to expedite the process, explains Shaw; “once the model has seen enough sample structures, it is able to robustly predict the energy of arbitrarily large crystal structures, which we use to ensure our results are self-consistent." This approach lets the team analyze and predict the stability of multiple different combinations of metals relatively rapidly, something that would be impossible without these advanced techniques.

“We're currently finishing up similar phase stability tests for the MgScLa, MgScGd, [and] MgScCe ternaries,” he says, “and hope to see how their phase properties differ from both the MgSc and MgScY, in case we can suggest a better candidate ternary.”

Soon, some of the Mudd team’s first predictions will be sent to their collaborators at UNSW, who do the nitty-gritty work of trying to turn theory into experimental reality—a task rife with challenges of its own.

Being so low in atomic weight, magnesium has a relatively low boiling point; it’ll turn to a gas around 2000°F. This is the main stumbling block on the way to synthesizing these complex super-alloys, because scandium (the other main ingredient) doesn’t melt until it gets to 2800°F. As a result, blending them together is a little like trying to mix water and liquid nitrogen. Still, it can be done—and although the going is sometimes frustratingly slow, Shaw and his supervisors are hopeful that their work will help make new and groundbreaking materials a reality. In the future, he says, “we hope to extend the computation to alloys which contain four or more different elements, which are even harder to fully probe experimentally.”

As our ability to communicate and collaborate over large distances has evolved, so too has the potential for world-changing science. Though Shaw is just one node in a network of scientists, it’s incredible to see hands and brains—both organic and electronic—working together as one from locations around the world, in an effort to make life better for everyone.

—Stephen Skolnick

1 comment:

  1. Saw this guy at a poster session at TMS a couple of weeks ago - amazing how much he's accomplished as an undergrad!

    ReplyDelete