Bulk Metallic Glasses: Constructing the Future

Robyn Shaffer March 14, 2012 0

Pick up your cell phone and look at its casing. What is it made of? How thick is it? How durable, how flexible, how strong is it? In the mind of Jan Schroers, Associate Professor of Mechanical Engineering & Materials Science, those questions play a vital role in designing and manufacturing new metallic alloys known as bulk metallic glasses.

Bulk metallic glasses, referred to commonly as metallic glasses, are technically defined as alloys with critical cooling rates low enough to allow the formation of layers of over 1mm in thickness. Colloquially, they are a metallic alloy that exploits the favorable properties of both metals and glasses, including flexibility, strength, and durability.

Optimization of Metals and Glasses

Ordinary metals are crystalline solids, with high density, conductivity, and strength. Their molecules are ordered in a lattice structure, and disruptions in that lattice structure are known as dislocations. Common metals have approximately one dislocation in one billion molecules, or 1014 dislocations in a cubic centimeter. It is these dislocations, rather than the natural ordering of metals, that ultimately define the properties of metals.

Glasses, on the other hand, are amorphous, or non-crystalline, solids. As anyone who has broken a window knows, glasses are brittle when solid; however, they become molten and exhibit fluid properties at high temperatures. This characteristic, known as the “glass transition,” allows glasses to be molded into creative and complex shapes, most famously exhibited by artist Dale Chihuly.

These physical characteristics of glass and metals significantly affect their processing potential. During processing, metals display plasticity – the permanent deformation of the metal material. Glasses, however, with no lattice structure or dislocations, instead displays elasticity – a reversible deformation of the material under a force. Both plastic deformation and elastic deformation are necessary when shaping complex objects, yet neither glass nor metal can fully encompass both properties.

This is where Professor Jan Schroers steps in. “We combine two previously mutually exclusive properties,” remarked Schroers, resulting in the metallic alloys known as bulk metallic glasses. Joining advanced structural properties – strength, durability, and deformation – with the flexibility sufficient for molding complex shapes, these bulk metallic glasses are a ‘super-metal’ of sorts, an enhanced alloy with a range of capabilities.

Works by the artist Dale Chihuly, who blows glass using a process similar to Schroers procedure.

Skirting the Temperature Boundary

Vital to the success of bulk metallic glasses is their behavior at varying temperatures. The “ideal service region” describes the functioning properties of metals in use. At low-to-normal temperatures of approximately 200°C or lower, the alloys perform as metals with high durability and strength. At temperatures above 200°C, however, the alloys enter an “ideal processing region” in which they can be shaped and molded while avoiding full liquidity.

Finding the optimal composition of alloys that displays these specific temperature behaviors however, is no easy task. Schroers’ laboratory has labored in “trial and error” procedures, investigating different elements and percent-composition in search of an ideal alloy. Now, however, the laboratory has been given a $13 million grant by the National Science Foundation to “look into new methods to find metallic glasses more effectively,” said Schroers. He hopes that the new methods his laboratory is developing would find appropriate metallic glass alloys up to 100 times faster than the current processes.

In order to convert bulk metallic glasses into complex objects, the Schroers laboratory begins the manufacturing process by super-cooling the alloys. Researchers must be wary of the “time-temperature-transformation diagram” (TTT) – a graph that shows the transformation points of a steel alloy and describes its different states, namely crystalline and non-crystalline or glass. If the metallic alloy were cooled slowly, it would cross a TTT boundary and crystallize, losing all the advantages of an amorphous material.

Schroers’ super-cooling process bypasses the crystallization state and instead forms glass. Simple shapes are formed first, and then the alloy is reheated for further processing, all the while avoiding the pitfalls of an ordered crystalline structure. This process “decouples the required fast cooling from forming,” noted Schroers, giving researchers a long processing window to create complex and unique shapes.

Blow Molding, Complex and Seamless

Within this window, Schroers uses a technique called “blow molding” to shape the metallic alloys into objects. The method is similar to glass blowing, in which molten glass is shaped by air blown through a blow tube. While artists may blow mold in a free-form style, Schroers’ laboratory uses carved molds to obtain a precise shape. These molds are made using common, cheap materials, just one more benefit of this processing method.

To optimize the blow molding process, Schroers’ laboratory developed two novel conditions for shaping the metallic glasses: in vacuum and in liquid. These conditions rely on a basic property of metal: thermo-conductivity, i.e. the tendencies of metals to lose heat to the environment after separation from a heat source, in this case the hot mold. Heat is lost mainly through conduction, a transfer of heat between two physically adjacent materials. In a vacuum, however, there is no atmosphere carry away heat; thus, the metallic glasses retain the heat. In a liquid, researchers can more easily maintain the temperature of the liquid environment, preventing heat transfer to the liquid as well.

Heat retention ensures that the bulk metallic glasses maintain their non-crystalline, fluid state over a longer period of time. Schroers’ laboratory takes advantage of this prolonged fluid state to shape complex molds. This process can shape, join, and finish metal objects in one step without seams, according to Schroers, with the capability to attain “an almost infinite level of complexity.”

Products from Schroers lab show the detail in form achieved by bulk metallic glasses. Courtesy of Jan Schroers.

Materials of the Future

Beyond the laboratory, the complex objects of bulk metallic glasses may find a home in medical, electronic, and commercial applications. With the prerequisite that each layer must be less than 1 mm thick, metallic glasses are optimal for electronic casings, including those on cell phones, laptops, and other precision technology.

Schroers views small electronics as a starting point, requiring minimal material investment, to demonstrate the capabilities and desirability of bulk metallic glasses as a metal replacement. Though steps to commercialization will span many years, Schroers optimistically says, “I envision this as an energy-efficient, cost-efficient process to build cars, anything now built out of metals and plastics.”

Bulk metallic glasses also have unique advantages in medical applications, due to their precise composition and shaping. The main challenge to biomedical materials is biocompatibility – the behavior and interaction between the body and the material. Bulk metallic glasses can be shaped specifically to optimize this interaction, with air bubbles intentionally placed to match metal density to tissue density and metal elasticity to tissue elasticity. Matching these properties may lead to improvements in biocompatibility over current medical technologies, granting these novel materials a wide range of applications, ranging from stents to tissue implants. Given the low-cost nature of disposable molds, Schroers imagines “custom-shaped implants,” which match not only the shape of the tissue but also the strength and durability.

With broader commercial applications ahead, Schroers and his team are deftly navigating the world of metallurgy to produce alloys and materials at the highest level. Through optimizing the composition and processing methods, bulk metallic glasses may soon reign as the technological material of choice. Look at your cell phone again. Is it too stiff, or too flexible? Is it cracked and brittle? In the not too distant future, those defects and weaknesses might be corrected by Schroers and his team, bringing bulk metallic glasses to revolutionize the processing, design, and industry of all things metal.

About the Author
ROBYN SHAFFER is a junior History of Science, History of Medicine major in Berkeley College. She is an Articles Editor for Yale Scientific Magazine.

Acknowledgements
The author would like to thank Professor Schroers for his time and enthusiasm about his research.

Further Reading
Schroers, Jan et al. “Thermoplastic blow molding of metals.” MaterialsToday 14:1-2 (2011): 14-19.
Schroers, Jan. “Processing of bulk metallic glass.” Advanced Materials 22 (2010): 1566-1597.
Kumar, Golden, Amish Desai and Jan Schroers. “Bulk Metallic Glass: The Smaller the Better.” Advanced Materials 23:4 (2011): 461-476.

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