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[Image above] A cylinder of an amorphous iron alloy, or metallic glass, made using an additive manufacturing technique. Credit: Zaynab Mahbooba; NC State University

Metallic glasses are a strange chimera of materials—they have the composition of metals, but the structure of glasses.

Metals generally have their molecules arranged in a crystalline structure, affixed in regular, repeating patterns. I imagine the structure akin to a well-regimented army standing in formation.

Glasses prefer entropy and anarchy—their amorphous structure rejects uniformity and regular spacing, instead adopting a more random arrangement of molecules. In my head, that’s more like a group of kids hanging out on a playground.

And all this is really important because a material’s structure means everything to its overall properties—just as important as which molecules actually make up the material itself.

Because metallic glasses have an amorphous structure, they lack the grain boundaries, or borders between crystals, of crystalline materials. Grain boundaries are weak spots within a material, places where fracture and corrosion can often seed and then spread.

Of course that doesn’t mean that amorphous materials don’t have weak spots—instead, that different structural configurations have different strengths and weaknesses.

Nonetheless, this unique combination of metal and glass materials properties give metallic glasses some really useful characteristics—these materials can be twice as strong as steel, have the wear and corrosion resistance of ceramics, and yet be tougher and more elastic than ceramics, according to a Materials Today article.

Those are incredibly useful properties, making metallic glasses promising constituents to offer entirely new possibilities and enhanced performance to a wide variety of applications—everything from commercial applications such as sports equipment (including golf club heads, tennis rackets, baseball bats, skis, and snowboards) and consumer electronics to aerospace and medical applications. Metallic glasses have even been used to collect solar winds on NASA spacecraft.

Despite their potential, however, metallic glasses have been rather limited so far by their fabrication methods. The materials must cool quickly after heating to prevent crystallization and instead achieve an amorphous structure, which has largely limited fabrication to relatively thin films of metallic glasses to date.

The advent of additive manufacturing, however, has opened new possibilities for producing larger specimens of bulk metallic glasses. And advances in additive manufacturing techniques have continued to extend the possibilities for bulk metallic glasses—so far, in fact, that we’re starting to see things shatter.

Researchers at North Carolina State University (Raleigh, N.C.) have now broken the critical casting thickness—the previous maximum possible size—for fabricating bulk metallic glass.

“We were able to produce an amorphous iron alloy on a scale 15 times larger than its critical casting thickness,” Zaynab Mahbooba, first author of the work and Ph.D. student in NC State’s Department of Materials Science and Engineering, says in a NC State news release.

The scientists achieved this feat of epic proportions using laser-based powder bed additive manufacturing, a 3-D printing technique that is exactly what it sounds like—it uses a laser to heat a powdered material, additively building a final component layer by layer.

The NC State team worked with FeCrMoCB, a high-quality metal alloy powder with excellent strength and hardness. The printing technique laser-heats individual layers of the material, 20 µm at a time, from a powder bed, building up one layer on top of the previous.

Because each individual layer is so thin, it can cool quickly, allowing the metal to solidify with an amorphous structure. Additively building up the material allows each layer to meld together, creating a large-format bulk metallic glass that would be difficult to achieve with other fabrication techniques.

Ultimately, the team printed a cylinder of FeCrMoCB that was 30 mm tall and 45 mm in diameter.

Despite the team’s extensive experience with laser-based powder bed additive manufacturing, the scientists say that working out the conditions to print with FeCrMoCB powder was no simple task.

“What surprised us about the bulk metallic glass alloy was how much effort was needed to actually find a processing parameter space that worked,” Mahbooba explains via email. “We went through hundreds of iterations of different process parameters before finding a space that could produce the dense, amorphous part shown in our paper.”

Part of the challenge, Mahbooba says, is that the FeCrMoCB alloy the team worked with is actually rather brittle. This created challenges not only in fabrication, but in analysis of the printed metallic glass, which cracked so badly that the scientists couldn’t prepare samples to view under the microscope.

Instead, they analyzed crystal structure of the printed material using two methods, X-ray diffraction and another, more sensitive technique called electron-backscattered diffraction.

While X-ray diffraction indicated an amorphous structure, electron-backscattered diffraction identified a low concentration of grains—although less than 1%—in the printed alloy. So can the team say the material had an amorphous structure?

“Other researchers have shown that the change in bulk metallic glass material properties is insignificant if the grain concentration is so low that it goes undetected by X-ray diffraction analysis,” Mahbooba explains by email. “For that reason, we are not really concerned with eliminating the nanograins. In the future, when we get to a stage where we are optimizing a part for an intended application, we will determine whether or not we need to tweak the processing parameters to optimize the materials properties.”

Even though the printed bulk metallic glass isn’t perfect, however, it represents a big step in the right direction for this promising class of materials—and it could enable entirely new applications for bulk metallic glasses.

“We are not trying to replace traditionally manufactured bulk metallic glasses with additively manufactured bulk metallic glasses, we are trying to replace crystalline metal with an additively manufactured BMG in applications that would greatly benefit from the unique properties of bulk metallic glasses,” Mahbooba writes in the email. “To do this we need to design a bulk metallic glass alloy for an intended application, and also for additive manufacturing; this is our focus moving forward.”

The scientists say they have already processed three other Fe-based bulk metallic glasses, which they selected based on published work, previous printing attempts, and educated guesses about which materials would be good glass-formers and have greater plasticity than FeCrMoCB.

“And there is no reason this technique could not be used to produce any amorphous alloy,” Ola Harrysson, corresponding author and Edward P. Fitts Distinguished Professor of Industrial Systems and Engineering at NC State, says in the release. “One of the limiting factors at this point is going to be producing or obtaining metal powders of whatever alloy composition you are looking for.”

So what about traditional, non-metallic glasses? We’ve seen recent advances in 3-D printing glasses using several extrusion based techniques—but could powder bed-based methods offer new opportunities for fabricating glasses?

Unfortunately, the scientists say, that’s not likely.

“The internal stresses resulting from rapid solidification would likely cause the glass to crack and crumble to pieces during powder bed fusion processing,” Mahbooba writes via email. “Depending on the ability of the glass to absorb laser energy, it might be challenging to melt the glass and/or to melt a single layer of glass powder. There is also the added challenge of producing additive manufacturing-quality glass powder (especially considering the viscosity of molten glass!).”

With continued advances in the team’s technique and choice of powdered metals, however, we could at least begin to see additively manufactured bulk metallic glasses in the near future. With such promising materials, let’s hope so—the possibilities could get interesting.

The paper, published in Applied Materials Today, is “Additive manufacturing of an iron-based bulk metallic glass larger than the critical casting thickness” (DOI: 10.1016/j.apmt.2018.02.011).

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