Liquidmetal and Vitreloy are commercial names of a series of amorphous metal alloys developed by a California Institute of Technology (Caltech) research team and marketed by Liquidmetal Technologies. Liquidmetal alloys combine a number of desirable material features, including high tensile strength, excellent corrosion resistance, very high coefficient of restitution and excellent anti-wearing characteristics, while also being able to be heat-formed in processes similar to thermoplastics. Despite the name, they are not liquid at room temperature.
Liquidmetal was introduced for
commercial applications in 2003. It is
used for, among other things, golf clubs, watches and covers of cell phones.
The alloy was the end result of a
research program into amorphous metals carried out at Caltech. It was the first
of a series of experimental alloys that could achieve an amorphous structure at
relatively slow cooling rates. Amorphous
metals had been made before, but only in small batches because cooling rates
needed to be in the millions of degrees per second. For example, amorphous
wires could be fabricated by splat quenching a stream of molten metal on a
spinning disk. Because Vitreloy allowed such slow cooling rates, production of
larger batch sizes was possible. More recently, a number of additional alloys
have been added to the Liquidmetal portfolio. These alloys also retain their
amorphous structure after repeated re-heating, allowing them to be used in a
wide variety of traditional machining processes.
Characteristics
Liquidmetal, created by Dr. Atakan
Peker, contain atoms of significantly different sizes. They form a dense mix
with low free volume. Unlike crystalline metals, there is no obvious melting
point at which viscosity drops suddenly. Vitreloy behaves more like other glasses,
in that its viscosity drops gradually with increased temperature. At high
temperature, it behaves in a plastic manner, allowing the mechanical properties
to be controlled relatively easily during casting. The viscosity prevents the
atoms moving enough to form an ordered lattice, so the material retains its
amorphous properties even after being heat-formed.
The alloys have relatively low softening
temperatures, allowing casting of complicated shapes without needing finishing.
The material properties immediately after casting are much better than those of
conventional metals; usually, cast metals have worse properties than forged or
wrought ones. The alloys are also malleable at low temperatures (400 °C or
752 °F for the earliest formulation), and can be molded. The low free volume also results in low
shrinkage during cooling. For all of these reasons, Liquidmetal can be formed
into complex shapes using processes similar to thermoplastics, which makes
Liquidmetal a potential replacement for many applications where plastics would
normally be used.
Due to their non-crystalline (amorphous)
structures, Liquidmetals are harder than alloys of titanium or aluminum of
similar composition. The zirconium and titanium based Liquidmetal alloys achieved
yield strength of over 1723 MPa, nearly twice the strength of conventional
crystalline titanium alloys (Ti6Al4V is ~830 MPa), and about the strength of
high-strength steels and some highly engineered bulk composite materials (see tensile
strength for a list of common materials). However, the early casting methods
introduced microscopic flaws that were excellent sites for crack propagation
which led to Vitreloy being fragile like glass. Although strong, these early
batches shattered easily when struck. Newer casting methods, adjustments of the
alloy mixtures and other changes have improved this.
The lack of grain boundaries contributes
to the high yield strength (and thereby resilience) exhibited. In a
demonstration, a metal sphere dropped on amorphous steel bounced significantly
longer than the same metal sphere dropped on crystalline steel.
The lack of grain boundaries in a
metallic glass eliminates grain-boundary corrosion—a common problem in
high-strength alloys produced by precipitation hardening and sensitized stainless
steels. Liquidmetal alloys are therefore generally more corrosion resistant,
both due to the mechanical structure as well as the elements used in its alloy.
The combination of mechanical hardness, high elasticity and corrosion
resistance makes Liquidmetal wear resistant.
Although at high temperatures, plastic
deformation occurs easily, almost none occurs at room temperature before the
onset of catastrophic failure. This
limits the material's applicability in reliability-critical applications, as
the impending failure is not evident. The material is also susceptible to metal
fatigue with crack growth. A two-phase composite structure with amorphous
matrix and a ductile dendritic crystalline-phase reinforcement, or a metal
matrix composite reinforced with fibers of other material can reduce or
eliminate this disadvantage.
Uses
Liquidmetal combines a number of
features that are normally not found in any one material. This makes them
useful in a wide variety of applications.
One of the first commercial uses of
Liquidmetal was in golf clubs made by the company, where the highly elastic
metal was used in portions of the club face.
These were highly rated by users, but the product was later dropped, in
part because the prototypes shattered after fewer than 40 hits. Since then, Liquidmetal has appeared in other
sports equipment, including the cores of golf balls, skis,baseball and softball
bats, and tennis racquets.
The ability to be cast and molded,
combined with high wear resistance, has also led to Liquidmetal being used as a
replacement for plastics in some applications.
It has been used on the casing of late-model SanDisk "Cruzer
Titanium" USB flash drives as well as their Sansa line of flash-based MP3
player, and casings of some mobile phones, like the luxury Vertu products, and
other toughened consumer electronics. Liquidmetal
was used in the Biolase dental laser Ilase and the Socketmobile ring bar code
scanner. Liquidmetal has also notably been used for making the SIM ejector tool
of some iPhone 3Gs made by Apple Inc., shipped in the US. This was done by
Apple as an exercise to test the viability of usage of the metal. They retain a scratch-free surface longer
than competing materials, while still being made in complex shapes. The same
qualities lend it to use as protective coatings for industrial machinery,
including petroleum drill pipes and power
plant boiler tubes.
It also replaces titanium in
applications ranging from medical instruments and cars to the military and
aerospace industry. In military applications, rods of amorphous metals replace depleted
uranium in kinetic energy penetrators. Plates
of Liquidmetal were used in the solar wind ion collector array in the Genesis
space probe.
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