Liquidmetal

Liquid metal is also a name for Galinstan, a fusible alloy liquid at room temperature, used as a replacement of mercury.

Liquidmetal and Vitreloy are commercial names of a series of amorphous metal alloys developed by a California Institute of Technology research team, now marketed by a firm that the team organized called 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. Although only introduced for commercial applications in 2003, Liquidmetal is already finding a number of uses as varied as golf clubs to the covers of cell phones.

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Background

To understand the qualities of Liquidmetal it is best to start with a more common structural metal, iron. Iron has a relatively large atom, which forms into an open crystal structure given the proper conditions. The atoms can "slide" along the planes of the structure, meaning that pure iron is fairly ductile, often being able to be bent by hand, while still being very strong in tension when one tries to pull the structure apart. In order to improve the strength of iron, impurities can be added to "lock in" the structure to prevent it from sliding. The most common alloying agent is carbon, which results in steel.

When steel solidifies from a liquid after being smelted, it starts to form small crystals of various forms. These crystals grow until they come into contact with other crystals seeded at other points, which have different orientations, and sometimes different mechanical arrangements. When the process is complete, these crystals form a large lattice structure of individual "grains", which are sometimes visible to the naked eye.

Although the alloying process prevents the sort of sliding motions of pure iron, the inter-grain strength is fairly low compared to the strength of the bonds inside the grains. This leads to another form of ductility where the grains themselves slide along their boundaries, or the grains are broken apart from each other. Mechanical cracks formed during the cooling process are another source of potential weakness. Under repeated loading the grains can be forced apart and the cracks forced open; this process, known as crack propagation, leads to metal fatigue.

Numerous processes can be used to reduce this problem. Wrought iron is repeatedly worked to mechanically force these cracks shut during the forming of an item such as a horse shoe, and the famed Japanese katana uses a similar process to produce high quality steels. More modern techniques like cold rolling and forging are able to remove these imperfections on industrial scales. Alternately it is possible to grow single very large crystals that are free from such inter-grain boundaries by definition, but these processes are slow, energy intensive, and fairly expensive. Such materials are typically limited to aerospace roles, for instance the blades of turbines in jet engines which are subject to repeated heat cycling which is a perfect environment for causing metal fatigue.

Molten metals generally have fairly low viscosity and "flow well". This limits the sorts of molding methods that can be used. For instance, casting processes flow molten metal into formed shapes, but these shapes generally have limitations on their complexity. Metals generally shrink as they cool as well, which means that they have to be "finished" after casting to get a quality surface because they do not remain in contact with the form at all times. Additionally, cast metals retain the mechanical imperfections that the forging and rolling processes remove, making them considerably less strong. Metals are simply not ideal for forming complex shapes except for machining and other post-forming processes, which are more expensive and time consuming.

Most of the "problems" with metals are a side effect of their crystalline structure, so producing a non-crystalline amorphous metal would solve many of them. However, crystal growth in a cooling mass of metal is strongly favored, so using any sort of "normal" process will lead to crystal formation. A variety of methods can be used to quickly chill the metal before this can take place, but these are suitable only for small batches.

For all of these reasons, thermoplastics remain a major industrial material. Although they are far less strong than steel, about fifty times less, they can be easily formed into complex shapes and retain a good finish. They can be created from raw materials and formed into a product in a continuous process, something that metals cannot generally match. A mixture of metals for "simple" shapes and plastics for more complex ones forms the basis of almost every product made today, from automobiles to televisions.

Vitreloy

Vitreloy was the end result of a long 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 cooling 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.

Liquidmetal alloys 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 need of finishing. The material properties immediately after casting are much better than of conventional metals; usually, cast metals have worse properties than forged or wrought ones. The alloys are also malleable at low temperatures (400 °C 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^[1], 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 used in similar applications. The zirconium and titanium based Liquidmetal alloys achieved yield strength of over 1723 MPa, nearly twice the strength of conventional crystalline titanium alloys (Ti6A1-4V 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, and led to Vitreloy being fragile, like glass. Although strong, these early batches could easily be shattered if struck. Newer casting methods, adjustment to the alloy mixtures and other changes have improved this.

The lack of grain boundaries may contribute to the high coefficient of restitution (close to 1) these alloys exhibit. In a demonstration, ball bearings dropped on plates of metal will bounce three times as long on Liquidmetal.^[2]

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 temperature before 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.^[3]

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 the shaft and for portions of the face of the club. These were highly rated by users, but the product was later dropped. Since then Liquidmetal has appeared in other sports equipment, including the cores of golf balls, skis, baseball bats and softball bats, and tennis racquets.^[4]

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 a late-model SanDisk "Cruzer Titanium" USB flash drives as well as their Sansa line of flash based MP3 player, and casings of some cellphones (like the luxury Vertu products) and other toughened consumer electronics. They retain a scratch-free surface longer than competing materials, while still being made in complex shapes. The same qualities lend it to be used as protective coatings for industrial machinery, including oil drill pipes and power plant boiler tubes.

It is also considered as a replacement of titanium in applications ranging from medical instruments and cars to military and aerospace industry. In military applications, rods of amorphous metals are considered as a replacement of depleted uranium in kinetic energy penetrators. Plates of Liquidmetal were used in the solar wind ion collector array in the Genesis space probe.

Although Liquidmetal has very high strength and an excellent strength to weight ratio, its commercial success as a structural material may be limited. Work continues on amorphous iron-based alloys that would combine at least some of the advantages of Liquidmetal with even greater strength, estimated to be two to three times the strength of the best steels made today. This would give such an alloy a strength to weight ratio that would easily beat the best lightweight materials such as aluminum or titanium, and be much less expensive than composites.

Commercial alloys

A range of zirconium-based alloys have been marketed under this trade name. Some example compositions are listed below, in atomic percent:

• An early alloy, Vitreloy 1:

Zr: 41.2 Be: 22.5 Ti: 13.8 Cu: 12.5 Ni: 10

• A variant, Vitreloy 4, or Vit4:

Zr: 46.75 Be: 27.5 Ti: 8.25 Cu: 7.5 Ni: 10

• Vitreloey 105, or Vit105:

Zr: 52.5 Ti: 5 Cu: 17.9 Ni: 14.6 Al:10

• A more recent development (Vitreloy 106a), which forms glass under less rapid cooling:

Zr: 58.5 Cu: 15.6 Ni: 12.8 Al: 10.3 Nb: 2.8

References

1. ^ Liquid metal behaves like plastic, Manufacturing Engineering, Mar 2003
2. ^ Ball Bouncer Demonstration - QuickTime movie
3. ^ The case for bulk metallic glass, Materials Today, March 2004
4. ^ Drivers -- Liquid Metal driver - discussion of Liquidmetal golf clubs

See also

• Amorphous metals
• Bulk metallic alloys