ORNL
Breaks into Metallic Glass Field
They are hard, strong, tough yet springy, so these relatively lightweight materials could be used to make golf clubs, fishing rods, car bumpers, aircraft skins, artificial joints, dies, cutting tools, and transformer coils. Bulk metallic glasses have unusual mechanical strength and magnetic properties, as well as resistance to wear and corrosion. Although they have been around for four decades, only recent developments at ORNL and elsewhere have made bulk metallic glasses practical and affordable for various applications.
These materials were first
discovered in the 1960s at the California Institute of Technology. Unlike
conventional metals, whose atoms are arranged in repeating patterns typical
of crystals, metallic glasses have a noncrystalline, or amorphous, structure—that
is, their atoms have a close-to-random arrangement. At Caltech it was
found that the secret to forcing a molten material to retain its liquid,
or amorphous, state was to quench it—cool it rapidly at a rate of
a million degrees Celsius per second. The resulting metallic glass ribbons,
which were only a thousandth of an inch thick, had very limited use.
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C. T. Liu’s reflection shows in this sample of a bulk metallic glass
produced at ORNL. Liu’s group has demonstrated that bulk amorphous
alloys prepared by melting and casting show an excellent surface finish.(Image
enhanced by Judy Neeley.) |
In 1989-90 Japanese researchers
demonstrated that thicker metallic glasses could be produced by conventional
casting, without fast cooling, if three conditions were met: combine three
or more elements; use elements that differed from each other in atomic
size by at least 12%; and select elements that have a strong affinity
for each other. It would then be possible to melt these elements together,
cool the material at a regular rate, and cast it in dies to make shapes
such as rods and disks.
In the past three years, researchers at ORNL and elsewhere have produced bulk metallic glasses that are 500 times thicker than the Caltech ribbons of the 1960s. A group led by Chain T. Liu, a senior corporate fellow in ORNL’s Metals and Ceramics Division, has produced metallic glasses up to 1.5 in. thick, or 1500 times thicker than the original Caltech products.
With funding from DOE’s Office
of Basic Energy Sciences, the ORNL group has made metallic glasses of
several different compositions, using ingredients ranging from aluminum
to zirconium. The thickest material they have produced—a metallic
glass 1.5 inch thick—consists of zirconium, titanium, aluminum, nickel,
copper, and beryllium.
Joe Horton, a researcher in Liu’s group, holds a patent for a metallic-glass biomaterial that could be used as surgery tools or to replace bone or form implants, such as artificial knees and hips. Liu and his associates have applied for a patent on a method for making a zirconium-containing metallic glass more cheaply.
“Pure zirconium costs $500 a pound, but zirconium combined with oxygen costs only $50 a pound,” Liu says. “A bulk metallic glass with pure zirconium has 1760 times the fracture strength of an alloy with impure zirconium, which results in a crystalline material rather than a metallic glass. We developed a microalloying method for purifying impure zirconium while making the metallic glass. We add tiny amounts of boron, silicon, silver, and lead to the impure zirconium, which tie up the oxygen in the impure zirconium. As a result, the metal acts as pure zirconium, enabling the material to end up in the glass phase.”
The ORNL material could be used not only for golf clubs but also to store hydrogen in the “atomic holes” in its loosely packed atoms. The hydrogen-containing material could then be heated up under reduced pressure to release the stored hydrogen from its gaps, which in turn could be used in fuel-cell cars of the future (which might even include golf carts).
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