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  • The Hard Facts About Metallic Glass


    For thousands of years, civilizations have crafted metal and glass, creating not only objects of beauty, but also elements for industry. These distinct materials each possess strengths and weaknesses that determine their applications.

    Common glass is strong and resists being deformed, but is brittle and cracks easily. This is due to its non-crystalline, disordered atomic structure, called an amorphous structure. Metals, on the other hand, resist cracking, but are subject to bending, stretching, and flattening. Their crystalline structure provides micro-structural obstacles that keep metal from cracking and allow it to bend.

    In 1960, at Caltech, a new class of material was discovered called "metallic glass." When we think of glass, we immediately think of window panes. But metal can exist as a glass as well. Why? Because, by definition, "a glass is any material that goes from a liquid to a solid without crystallizing." Metallic glass holds the promise of combining many of the desirable attributes of glass and metals.

    It wasnt until the 1990s that metallic glass was produced in bulk. Although it offered the tantalizing promise of being stronger and tougher than any known material, two major hurdles have kept metallic glass from being widely adopted for useful applications:

    1. The size of metallic glass parts that can be produced

    2. The inherent brittleness that metallic glass exhibits

    The size limitation for parts is related to the way metallic glass is made. First, the material is heated to its glass-transition phase of between 500 and 600 degrees Centigrade. At this point, the material softens into a liquid state that can be molded and shaped. The challenge is that, in this state, the atoms tend to automatically arrange themselves into crystals ? this needs to be avoided to create an amorphous structure, which makes it strong. Common glasses can take hours to crystallize, offering ample time to form and shape the glass. However, metallic glass usually crystallizes almost instantly upon reaching the thick-liquid state.

    To avoid this crystallization, the material needs to be heated quickly and uniformly throughout and then injected into a mold where it freezes. The larger the amount of material, the greater the challenge of achieving this quick heating. Consequently, the size of parts that could be produced has been limited.

    But now, researchers at Caltech have developed a new technique that heats and processes metallic glass extremely quickly, allowing time for injection and freezing before crystallization.1 The technique, called ohmic heating, uses an intense pulse of electrical current that delivers energy of over 1,000 joules in about 1 millisecond. As a result, the material is heated 1,000 times faster than before, enabling parts to be made in milliseconds.

    According to William Johnson, a Professor of Engineering at Caltech, "Weve redefined how you process metals. Weve taken the economics of plastic manufacturing and applied it to a metal with superior engineering properties. We end up with inexpensive, high-performance, precision parts made in the same way plastic parts are made ? but made of a metal thats 20 times stronger and stiffer than plastic."

    This new technique has been patented for commercial use.

    The second important hurdle that had to be overcome is the inherent brittleness that metallic glass exhibits due to its amorphous structure that provides no barriers to the spread of cracks.

    In 2009, researchers at the University of California at Berkeley, working with colleagues at Caltech, addressed this fundamental problem of poor fatigue resistance in bulk metallic glasses by creating a metallic glass alloy named DH3.2 This material is made from five elements: zirconium, titanium, niobium, copper, and beryllium. This alloy is the result of inducing a second phase of the metal, which creates narrow pathways of crystalline metal. These pathways stop any cracks that begin to trickle through the glass.

    Biomedical researchers have produced a metallic glass based on a magnesium-zinc-calcium alloy that can be used in surgery. Its bio-compatible, and it does not produce hydrogen when it dissolves following healing. Because of these characteristics, this metallic glass offers the potential of a new generation of biodegradable bone implants.

    This alloy has proven to be stronger than many structural metal alloys, with a fatigue limit that is more than 30 percent higher than ultra-high-strength steel and aluminum-lithium alloys.

    In light of this trend, we offer the following three forecasts:

    First, applications for metallic glass will be plentiful.

    For instance, a new use is being developed in the medical field by researchers at Switzerlands ETH Zurich.3 Traditionally, surgeons have used screws and metal plates made of stainless steel or titanium to fix certain broken bones. Upon healing, a second surgery is required to remove these parts. Recently, parts made of bio-absorbable metals have been researched, where the metal dissolves in the body over time. Although parts made of magnesium-based alloys are promising, there has been a catch. When they dissolve, they create hydrogen as a by-product, which is harmful to the body, as well as an inhibitor to healing. The ETH Zurich researchers have produced metallic glass based on a magnesium-zinc-calcium alloy, which is not only bio-compatible, but it does not produce hydrogen when it dissolves. Because of these characteristics, this alloy offers the potential of a new generation of biodegradable bone implants. .

    Second, metallic glasses will become a common material in products that need to be strong, yet flexible.

    This means that they will be found in a wide range of industries rather than any specific one. Products such as airplane wings, golf clubs, and engine parts will benefit from these tougher, lighter metals. A critical concern with metal is fatigue that occurs along the boundaries between crystals. Metallic glasses eliminate this concern, leading to parts that are more reliable and therefore safer.

    Third, the strength, electrical properties, and moldability of metallic glass will make this class of material highly attractive to countless additional industries.

    To have the option of a material that is as strong as steel, yet as moldable as putty, will be a dream for many manufacturers. Whole structures will be created that are completely seamless, which means they are free of any weak points. Already, companies such as Apple are considering using this material in products such as future generations of iPhones and iPads.

    References List :
    1. Science, May 13, 2011, Vol. 332, Iss. 6031, "Beating Crystallization in Glass-Forming Metals by Millisecond Heating and Processing," by W.L. Johnson, G. Kaltenboeck, M.D. Demetriou, J.P. Schramm, X. Liu, K. Samwer, C.P. Kim, and D.C. Hofmann. ⓒ Copyright 2011 by the American Association for the Advancement of Science. All rights reserved. http://www.sciencemag.org 2. Nature Materials, February 2011, "A Damage-Tolerant Glass," by M.D. Demetriou, M.E. Launey, G. Garrett, J.P. Schramm, D.C. Hofmann, W.L. Johnson, and R.O. Ritchie. ⓒ Copyright 2011 by Nature Publishing Group, a division of Macmillan Publishers Limited. All rights reserved. http://www.nature.com 3. Nature Materials, November 2009, Vol. 8, No. 11, "MgZnCa Glasses Without Clinically Observable Hydrogen Evolution for Biodegradable Implants," by Bruno Zberg, Peter J. Uggowitzer, and Jorg F. Loffler. ⓒ Copyright 2009 by Nature Publishing Group, a division of Macmillan Publishers Limited. All rights reserved. http://www.nature.com