A game-changing class

C&I Issue 13, 2009

A breakthrough by US researchers has produced a glassy material that is stronger and more resistant to fatigue than high-strength steel and aluminium alloys. It is not what we normally think of as glass though – it is metallic. And the new materials could challenge titanium in aerospace, biomedical and aeronautics applications, say the scientists.

Though metals and metal alloys seem hard, the arrangement of their atoms in regular crystal lattices is an inherent weakness. The lattices can slide over one another and dislocate under stress. Deny the metal this crystalline architecture, by solidifying its atoms in a more random arrangement, and you produce an amorphous metal, also known as a metallic glass, that is twice as strong as its crystalline counterpart.

Earlier metallic glasses had failed to live up to their initial promise as good structural materials because they suffered poor fatigue resistance and fracture toughness. Materials scientist Douglas Hofmann from the California Institute of Technology (Caltech), US, believes that his group has now solved this problem. Hofmann, who also works for Liquidmetal Technologies (see Box), says that they created a metallic glass with a crystalline microstructure throughout. The softer crystalline material does not detract from the metal’s strength, but it blocks cracks from propagating through the glass. Hofmann says his new processing strategy can control the size and distribution of these softer crystalline particles so as to produce orders of magnitude increases in ductility, toughness and fatigue endurance ( PNAS 2009, 106, 4986).

Metals can be hard but brittle and easy to fracture or can be tougher but a bit softer. You push one property at the expense of the other, explains Lindsay Greer, a metallic glass expert at the University of Cambridge, UK. But the new glass– crystalline composites offer a new regime of property combinations that cannot be achieved with any other known metallic material, he says.

Bulk metallic glasses
Theoretically, any metal cooled rapidly enough can become glass-like and the first such glass was created in 1959 by cooling a gold-silicon alloy extremely rapidly. However, these metallic glasses were measured in microns, as they did not cool rapidly enough to become amorphous at any other scale.

Then, in 1990, a team led by Akihisa Inoue in Japan created metallic glasses at much lower cooling temperatures by including three alloys with varying atomic sizes so that the atoms were unable to settle in a crystal structure. The lower cooling rates paved the way for centimetre-thick materials, so-called bulk metallic glasses (BMG).

The race was now on to find better combinations of metals with good glass forming ability. Palladium and platinum were good but expensive so Caltech and Tohoku University in Japan produced cheaper alloys based on zirconium. Lower cooling rates allowed for thicker materials, eventually producing a world record diameter, for a glassy metal, of 10cm.

At the end of 2008, Hofmann and his colleagues reported new metallic-glass alloys based on titanium, along with beryllium and zirconium, that were lighter and cheaper, but still tough and ductile ( PNAS 2008, 105, 20136). The alloys’ densities were similar to titanium alloys, a useful characteristic for aerospace applications.

For now, Greer believes pure metallic glasses, without crystalline inserts, will find niche markets outside structural applications. ‘It’s still not remotely the case that you would build a bridge out of metallic glasses,’ he says. Crystalline metals like steel harden when under strain, as crystal distortions bunch up around cracks and provide support. Metallic glass weakens as it bends. A huge structure could shatter like a pane of glass, so components must measure in millimetres to be as reliable as structural steel.

That still leaves a lot of small component uses that would be very exciting, says Greer. Microelectrical systems, like pressure sensors or motor sensors, which can set off airbags in cars, are a largely unexplored application, he notes.

Tiny precision gears are possible using metallic glasses because they do not shrink when cast. Inoue has described a micro-motor with metallic glass gear parts that has a lifetime 300 times longer than a conventional motor with steel parts ( MRS Bulletin 2007, 32, 651). Such robust precision gears for micro-motors could extend the lifetime of medical devices such as catheters.

The excellent energy absorption and durability of metallic glasses has also led to their use as valve springs by the Japanese auto industry. Tens of millions have been made, benefiting from metallic glasses’ ability to flex more under pressure. Metallic glasses are also used in industrial coatings due to their durability, resistance to corrosion and low friction. The reason for their relatively limited applications so far, according to Greer, is the price premiums inherent to BMG processing, the small market size and expensive raw materials.

Liquidmetal Technologies in California, the company built on Caltech’s metallic glass patents, has sold thousands of golf clubs with metallic glass heads and has moved into tennis rackets, baseball bats and ski equipment. Indeed, sporting equipment is an ideal market for any new material, as consumers are prepared to pay more for the latest thing, especially if it gives better performance. It is also a perfect test bed for the material, as a golf club breaking is nothing like as serious as an aeroplane engine failure.

Metallic glasses have been available as upmarket mobile phone casings and in jewellery and expensive watch boxes. They can be cast in a mould like plastic and could be used for things like laptop casings, though cost will likely restrict this to high end machines.

Composite metallic glasses
The new composites with crystalline inserts have similar strength, toughness and fatigue endurance to high performance steel, but with a lower density, says Hofmann. ‘We have the missing piece we needed in metallic glasses to push them into structural applications where they couldn’t go before because of their poor failure characteristics.’

Hofmann says they are comparable to high performance titanium alloys, including Ti64, the alloy that accounts for 80% of titanium use. Our goal has always been to make these materials go after the applications of Ti64, he says. ‘We’re talking aerospace, aeronautics, biomedical implants and sports equipment.’ And metallic glass costs could compare favourably to titanium and aluminium alloys, given the right economies of scale.

BMGs have low melting temperatures, compared to titanium, so hundreds of parts/hour could be made using reusable moulds – impossible for molten titanium as it melts any cast. ‘The cost of sacrificial casts and machining greatly outweighs the cost of producing metallic glass parts, even though metallic glass has more expensive starting materials,’ says Hofmann. He hopes to see Liquidmetal’s materials put into applications like cars, satellites and aircraft by the end of the year. ‘Right now we are working with the [US] government and with commercial companies and people are at prototype stage,’ he adds.

Military applications
The US Defense Advanced Research Projects Agency (DARPA) has funded research on metallic glasses for military technologies. BMGs have excellent energy absorption and hardness, so could be used as armour.

Another, less benign, research area is armour piercing bullets. Depleted uranium shells pierce armour because they are heavy, but also because they self-sharpen when they undergo deformation as they hit their target. Metallic glasses have that same property. Work so far has focused on zirconium metallic glass bullets packed with tungsten rods.

Greer says new structural applications could include critical components in military equipment that need extreme properties. And it is conceivable that metallic glass applications could be used in some key aerospace components such as undercarriage parts, he adds.

Inoue believes the use of BMGs will ‘increase steadily in future’ and lists many potential uses, including flow meters, fuel cell parts, information storage materials, casing materials, torque sensors and soft magnetic materials. It is certain that pure BMGs will increase steadily in myriad small applications, but the new super composites may allow for structural materials in aerospace and military technologies, as well as high end consumer electronics, all offering potentially lucrative business opportunities.

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