What Is Bulk Metallic Glass?
Keywords: bulk metallic glass, amorphous metal, amorphous alloy
Bulk metallic glass, a.k.a. amorphous metal, appears to have a very bright future. Being twice as strong as titanium, tougher and more elastic than ceramics, and having excellent wear and corrosion resistance makes them attractive for a variety of applications. It can even be cast in a mold to near net shapes.
Conventional Metals
In an ordinary metal the atoms of the metal arrange themselves into a repeating pattern of crystals or grains with different sizes and shapes upon cooling from the liquid state. Because metals typically do not solidify into single crystals, they have inherent weaknesses.
The boundaries between the grains are weak spots and under high enough stress and temperature the grains will slide past each other resulting in metal deformation. In addition, extra atoms are often present in grains causing planes of distortion called dislocations. Dislocations easily move through metal that is under stress, again causing deformation. Grain boundaries and dislocations greatly lower a metals strength compared to its theoretical maximum.
Casting of conventional metals also requires more manufacturing steps than bulk metallic glass. Conventional metals shrink significantly as they cool in the mold from liquid to solid form and often develop surface roughness. Secondary steps are usually required to get at the final product, such as grinding and polishing.
Bulk Metallic Glass
The structure of metallic glass is very different from that of conventional metals. Rather than arranging themselves into repeating patterns of grains, the atoms of metallic glasses are “frozen” in a random, disordered structure, similar to regular window glass. It even has a smooth surface like glass. So smooth, in fact, that paint does not adhere well to metallic glass. It is this amorphous structure, lacking in grain defects, that gives metallic glasses their strength, toughness, hardness, elasticity and corrosion and wear resistance.
First discovered by Pol Duwez in 1960 at Caltech, the technique to create metallic glasses required undercooling a molten metal uniformly and rapidly. Rapidly as in 1,000,000°C per second! The molten metal reaches its glass transition temperature without enough time or energy to crystallize, and instead solidifies as metallic glass. Because the material did not conduct heat well, only thin ribbons of metallic glass could be created because of the uniformity and speed of cooling that was required.
Around 1990 Akihisa Inoue and his team at Tohoku University in Japan discovered new alloys that could form thicker metallic glasses at cooling rates as low at 1°C to 100°C, as long as three conditions were met:
1) Use three or more elements in the alloy
2) The atomic size of the elements must differ from each other by at least 12 percent
3) Use elements that have a strong affinity for each other
Soon after, William Johnson and Atakan Peker at Caltech did the same. The lower cooling rates allowed for thicker materials to be created, up to four inches. These thicker materials are referred to as bulk metallic glass (BMG).
Currently available bulk metallic glasses are malleable at around 400°C, compared to over 1000°C for steel. This allows the material to be processed similarly to polymers, with high volume production via casting up to a thickness of four inches. The material has low shrinkage during solidification and can therefore be cast in near-net shapes with microscale precision. The smooth shiny surface eliminates secondary finishing processes. Scalpels made from bulk metallic glass come out of the mold sharp and ready to use.
Some Disadvantages
As with any material, BMG cannot be everything to every application. Its plastic like manufacturability also means that it cannot be used in high temperature applications, i.e., above 260°C, because it becomes soft and weakened. Pure bulk metallic glasses also exhibit cyclic fatigue from repeated stress. Because of their high elasticity and low plasticity, catastrophic failure occurs after only a small amount of plastic deformation.
BMG Composites
New developments in BMG composites are helping to reduce the limitations of the material. In a BMG composite the BMG is the matrix and a ductile crystalline-phase is the reinforcement material. The reinforcement can either be an added material, such as metal or ceramic fibers, or internally created by precipitating ductile dendrites within the BMG, yielding partial crystallinity. These composites combine the ductility, fracture toughness and plasticity of conventional metals with the high strength of pure BMG.
Applications
BMGs are being examined for or currently used in a wide variety of applications including:
- Industrial coatings for improved wear and corrosion resistance
- As a replacement for depleted uranium in Kinetic Energy Penetrators for the military.
- Casings for cell phones
- Scalpels
- Sporting goods such as bats and tennis racquets
- Jewelry
The Defense Advanced Research Projects Agency (DARPA) also funding a three-year program called Structural Amorphous Metals (SAM). The aim of the program is to demonstrate the viability of BMG in structural applications. Specific applications being investigated include “corrosion-resistant, reduced magnetic mass hull materials; moderate temperature, lightweight alloys for aircraft and rocket propulsion; and wear-resistant machinery components for ground, marine, and air vehicles.”
U.S. Patent Situation
Upon examining several patents and class codes on amorphous metals it appears that the main U.S. patent classification codes for these materials are:
148/304 - Amorphous: Stock material which has no regular crystal structure but rather has a series of noncrystalline areas much like a glass.
148/403 - Amorphous, i.e., glassy: Stock material which has no regular crystal structure, but rather has a series of noncrystalline areas much like a glass.
148/561 - Passing through an amorphous state or treating or producing an amorphous metal or alloy: Process wherein a metal or metal alloy having no regular crystalline structure or periodicity (i.e., amorphous) in any amount is produced or treated by a process under the class definition or wherein a metal or metal alloy passes through a physical state having no regular crystalline structure or periodicity during the treatment of the metal or metal alloy.
Guideline examined patents assigned to these codes that were granted during the period from 1987 to 2003. We then compared the top patent holders for the above class codes in terms of number of patents published from 1987 to 2003.
Top BMG Patent Holders from ‘87 to ‘03
55 patents - YKK Corp.
43 patents - Honeywell
33 patents - Tsuyoshi Masumoto & Unitika Ltd.
26 patents - Akihisa Inoue
15 patents - Alps Electric Co.
14 patents - Koji Hashimoto
13 patents - California Institute of Technology
13 patents - Nippon Steel Corp.
11 patents - Hitachi Ltd.
11 patents - Kabushiki Kaisha Toshiba
One method Guideline uses to compare patent holders is by calculating an index referred to as Technology Influence. Technology Influence represents how often an assignee’s patents from the previous five years (in this case, 1998-2002) are referenced by patents published in the year of comparison (in this case 2003). A Technology Influence value of 1 represents the average. This shows how much a patent holder’s past technology developments are influencing current development. From this analysis Guideline determined that Caltech’s work has been most influential as their Technology Influence value is 5.06, whereas the next closest value is only 1.46, held by Alps Electric.
Applied Science is another calculation used to compare patent holders. This refers to the average number of non-patent references cited by a patent holder’s patents, such as scientific papers from journals, conference proceedings, etc. This gives an indication of which companies are working on the leading edge. Again, Caltech stands out as a clear leader with an Applied Science value of 7.3. This makes sense considering that Caltech is known to be one of the leaders in developing this technology. As mentioned earlier, metallic glass was first discovered at Caltech.
An analysis of patent assignees and inventors revealed that Akihisa Inoue has done extensive work and collaboration. He is listed as an inventor or co-inventor on a little over 60 patents with about 120 other Japanese researchers. All of this work was done with the following Japanese organizations, and this is only in regards to U.S. patents.
- Tsuyoshi Masumoto and Unitika, Limited
- Teikoku Piston Ring Company Limited
- Alps Electric Co., Ltd.
- YKK Corporation
- Honda Motor Co., Ltd.
- Yamaha Corporation
- Japan Science and Technology Corporation
- Unitika Ltd.
- Toyota Jidosha Kabushiki Kaisha
- Research Development Corporation of Japan
- Japan Metals & Chemicals Co., Ltd.
- Sumitomo Rubber Industries, Ltd.
- Mitsubishi Materials Corporation
Indeed, Inoue led a five year project sponsored by the Japanese government (Inoue Supercooled Liquid Glass Project), which reported the development of a less expensive copper alloy based BMG with a tensile strength over 2 Gpa. Currently Inoue is leading a five-year project sponsored by the Japanese New Energy and Industrial Technology Development Organization.
Although Inoue has done the most extensive work in terms of U.S. patenting on amorphous and glassy metal technology, the work being done by William Johnson’s group at Caltech appears to be having a larger impact on the overall body of work in U.S. patents over recent years.
Brian Reuter is Director of Product Realization at Guideline, Inc. Guideline provides research, product realization, expert witness and consulting services. Learn more at www.intota.com.
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