New materials have remarkable properties and can be customised

Paul Boughton

Imagine a class of material with twice the strength of stainless steelan elastic limit more than double that of most metallic alloysyet it can be processed in ways similar to thermoplastic polymers.

Furthermorethe same class of material can be used to hard-coat metallic components to give exceptional wear resistance and corrosion resistance. If requiredthe properties can be tailored to enhance fatigue resistanceyield strengthdensityelastic modulusimpact resistancethermal or electrical conductivitycoefficient of thermal expansion or acoustic and damping characteristics.

This versatile class of materials is not based on an exotic element or complex alloy; rather it is what is known as glassy metals. These have been discussed as a theoretical possibility for decadesbut only more recently has it been possible to produce them in commercial quantities.

Patents

Numerous patents have also been grantedso the sources of supply are also limited for the time being.

Metals have an amorphous structure while they are in the liquid statebut they normally form crystals as they cool and solidifyoften undergoing phase changes as well. Within the crystals there are atomic vacancies that enable dislocations to occur if the material is subjected to a load – which results in the strength of the bulk material always being much lower than the theoretical maximum that the inter-atomic bonds should allow. In additiongrain boundaries between the crystals promote corrosion and other chemical reactions such as oxidation and sulphidation.

Amorphous metalson the other handdo not form a crystalline structure; instead they maintain a randomamorphous structure (akin to that in glass) without the vacancies that would cause weaknesses in crystalline metalsand without the grain boundaries that promote chemical reactions.

Glassy metals therefore offer a high yield strength (approaching the theoretical limit)high hardness and wear resistancea superior strength-to-weight ratioan increased elastic limitresistance to corrosionand exceptional damping and acoustic properties.

If you are sceptical about how much of an impact the molecular structure can really have on the bulk properties of a materialthink about the alternative forms that carbon can adopt – graphitediamond and carbon nanotubes – and the corresponding material properties.

Yield strength doubled

Liquidmetal Technologies of the USAwhich is at the forefront in this fieldsays that it has produced zirconium-base and titanium-base Liquidmetal alloys (the VIT-001 series) with a yield strength of over 250ksi (1723MPa)which is more than twice the strength of conventional titanium alloys (Fig.1). The same alloys are also claimed to have an exceptionally high elastic limit of 2percentwhich is more than double that of aluminium alloystitanium alloys and cast stainless steel (Fig.2). Note that the material properties are achieved with the material ‘as cast’with no need for work hardeningheat treatment or other processing.

Compared with conventional metalsthe amorphous alloys have a relatively low melting point that is advantageous for two reasons: firstit means that intricate parts can be formed without costly post-finishing processes; and secondvarious other materials can be added to create composites with tailored material properties.

Because of the astonishing properties of glassy metalsit is not surprising that the applications are extremely diverse. One of the first was in golf clubsand another is in tennis racquets. Head has used Liquidmetal within its racquets to deliver a claimed 29percent increase in power and a substantial expansion of the ‘sweet spot’. Further applications are either in production or being investigated in the fields of baseball and softballskiing and snowboardingcyclingknivesgunsscuba equipmentfishing tackle and boating.

While the sport and leisure industries are in pursuit of performance improvementsdesigners of consumer electronics continually seek to create products that are smaller and lighter. Glassy metals are therefore attractive because of their high strength (typically 2.5 times greater than titanium alloy) and hardness (1.5 times harder than stainless steel). Consequently lightweight casings can be manufactured with thin walls and high strength to protect the internal components. Resistance to scratching and corrosion are also benefits in this market.

Vertuwhich supplies hand-crafted mobile telephones for the luxury marketlast year launched the Ascent Motorsport Limited Edition with a Liquidmetal casing (Fig.3). While the price tag on this product (E4595) inevitably restricts its market appeallower-priced electronic products are also taking advantage of Liquidmetal alloys. For examplethe Sandisk 2GB Cruzer Titanium USB Flash drive (around E90) benefits from a ‘crush-resistant’ Liquidmetal casing (Fig.4)and the same company's flagship MP3 playersthe Sansa e200 seriesfeature a scratch-resistant Liquidmetal back (Fig.5).

Many mobile telephones today incorporate hingesbut conventional metalssuch as zincmagnesiumstainless steel and titaniumhave inherent design and performance limitationsespecially with respect to longevity and shock resistance. Liquidmetal Technologieshoweversays that its alloys’ superior yield strength and elasticity resist deformation and provide exceptional durability. In additionstructural parts made from Liquidmetal alloys can be net-shape formed to thinner profiles while maintaining superior strengthresulting in more space to accommodate consumer demands for new technology.


Medical applications

Away from electronicsmanufacturers of medical devices are finding applications for biocompatible glassy metals. Particular advantages here include superior wear resistanceexceptional component strengthimproved manufacturability and control of surface texture during the casting process. Some of the products already utilising glassy metals include reconstructive devicesfracture fixations and spinal implants.

In additionglassy metals can be used for surgical instrumentsas they can be ground to a sharper edge than steelthey are less expensive than diamondand sharp edges do not degrade with use in the same way as steel.

Not surprisinglythe defence industry is also taking a keen interest in the possibilities presented by glassy metals. Liquidmetal Technologies has been awarded a series of multi-yearmulti-million-dollar contracts by the USA Department of Defenseand the company’s technologies are currently being developed for use within a kinetic energy penetrator (KEP) rod. The KEP is a key component for an armour-piercing ammunition system that currently utilises depleted uranium (DU) because of its density and self-sharpening characteristic.

Ballistic tests have shown that the Liquidmetal tungsten composite KEP exhibits self-sharpening similar to the DU KEP. As a resultthe Department of Defense is working closely with Liquidmetal Technologies to develop a new class of effective and environmentally benign KEP rods.

So far we have discussed the use of glassy metals for the production of net-shape componentsbut Liquidmetal Technologies has also developed processes for applying glassy metals as coatings.

Typically the coatings bond extremely well to the substratewithstand repeated thermal cyclingmaintain high hardness at elevated temperaturesexhibit excellent thermal conductivity and are highly resistant to corrosion.

Liquidmetal-Armacor Coatings are alloy steels that are applied by twin-wire arc spraying or a thermal spray technique known as high-velocity-oxygen-fuel (HVOF). Depending on the grade specifiedthe coatings are suitable for use within the oilpowerpulp and paperand glass industries.

In additionapplications have been found in diesel enginesfansnon-slip flooringprocess equipmentmetalworking toolingmining machineryagricultural equipment and steel mill rolls. Compared with electroplated chromiumthe coatings offer improved resistance to corrosionwear and impact damage.

Because of the nature of glassy metals and the ways in which they are processeddeciding whether or not to utilise them in new designs or product upgrades is far more than a question of comparing the cost-per-kilogram with that of conventional materials. Designing with glassy metals opens up a wide range of possibilities for enhancing product performancebut the advice of the material supplier should be sought in order to maximise the benefits. Neverthelessthe opportunities are truly exciting. 

"

Recent Issues