Titanium-gold alloy four times harder than most steels

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Titanium-gold alloy is four times harder than most steels
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Andy Pye looks into a discovery by Rice University that the use of added gold can result in improvements to biocompatible titanium for medical implants.

Because it is biocompatible, titanium is the predominant material of choice for artificial knee and hip joints because it is strong, wear-resistant and non-toxic, but an unexpected discovery by Rice University physicists shows that the gold standard for artificial joints can be improved with the addition of some actual gold.

Additional properties that make titanium (Ti) suitable for biomedical applications include its high strength-to-weight ratio and low ion formation levels in aqueous environments. Ti is one of few materials capable of osseointegration — the mechanical retention of the implant by the host bone tissue — which stabilises the implant without any soft tissue layers between the two.

These properties enable the wide use of Ti for devices, such as artificial knee and hip joints, screws and shunts for fracture fixation, bone plates, pacemakers and cardiac valve prostheses. Not surprisingly, the dental applications of Ti are just as common, including implants and their components, such as inlays, crowns, overdentures and bridges.

However, pure Ti is not strong enough for a number of medical devices, necessitating the development of superior alloys. Although hardness can be improved by alloying Ti with another element, care must be taken to preserve biocompatibility. Previously, a two-fold increase in hardness has been achieved by alloying Ti with copper (Cu) or silver (Ag).

The mechanical properties of the intermetallic compound beta titanium-3 gold (beta-Ti3Au) suggest that this material is well suited for medical applications where Ti is already used, with some examples including replacement parts and components (both permanent and temporary), dental prosthetics and implants. The fourfold increase in hardness, as compared with pure Ti, renders beta-Ti3Au as the hardest known biocompatible intermetallic compound. The wear properties of beta-Ti3Au indicate longer component lifetimes and less debris accumulation. Moreover, the ability to adhere to a ceramic surface will result in reducing both the cost and the weight of these components.

Beta titanium-3 gold is three to four times harder than most steels, according to Emilia Morosan, the lead scientist on a new study that describes the properties of a 3:1 mixture of titanium and gold and a specific atomic structure that imparts hardness. “It’s four times harder than pure titanium, which is what’s currently being used in most dental implants and replacement joints,” she says. Conventional knee and hip implants have to be replaced after about 10 years due to wear and tear. Like many other alloys, titanium gold alloys have a higher yield strength, tensile strength and hardness than either of its constituent metals. It may also have applications in the drilling industry, the sporting goods industry and many other potential fields, she claims.

Morosan, a physicist who specialises in the design and synthesis of compounds with exotic electronic and magnetic properties, said the compound is not difficult to make, and it’s not a new material. It’s not even clear that Morosan and former graduate student Eteri Svanidze were the first to make a pure sample of the ultra-hard “beta” form of the compound. But they are the first to document the material’s remarkable properties.

“We published a study not long ago on titanium-gold, a 1-to-1 ratio compound that was a magnetic material made from non-magnetic elements,” Morosan says. “One of the things that we do when we make a new compound is try to grind it into powder for X-ray purposes. This helps with identifying the composition, the purity, the crystal structure and other structural properties.

“But when we tried to grind up titanium-gold, we couldn’t,” she recalled. “I even bought a diamond-coated mortar and pestle, and we still couldn’t grind it up.”

Morosan and Svanidze decided to do follow-up tests to determine exactly how hard the compound was, and while they were at it, they also decided to measure the hardness of the other compositions of titanium and gold that they had used as comparisons in the original study. One of the extra compounds was a mixture of three parts titanium and one part gold that had been prepared at high temperature.

What the team didn’t know at the time was that making titanium-3-gold at relatively high temperature produces an almost pure crystalline form of the beta version of the alloy — the crystal structure that is four times harder than titanium. At lower temperatures, the atoms tend to arrange in another cubic structure — the alpha form of titanium-3-gold. The alpha structure is about as hard as regular titanium. It appears that labs that had previously measured the hardness of titanium-3-gold had measured samples that largely consisted of the alpha arrangement of atoms.

The team measured the hardness of the beta form of the crystal in conjunction with colleagues at Texas A&M University’s Turbomachinery Laboratory and at the National High Magnetic Field Laboratory at Florida State University, Morosan and Svanidze also performed other comparisons with titanium. For biomedical implants, for example, two key measures are biocompatibility and wear resistance. Titanium is one of the few metals that human bone is able to grow around firmly, allowing it to be used widely in medicine and dentistry.

Because titanium and gold by themselves are among the most biocompatible metals and are often used in medical implants, the team believed titanium-3-gold would be comparable. In fact, tests by colleagues at the University of Texas MD Anderson Cancer Center in Houston determined that the new alloy was even more biocompatible than pure titanium. The story proved much the same for wear resistance: titanium-3-gold also outperformed pure titanium.

Morosan says she has no plans to become a materials scientist or dramatically alter her lab’s focus, but she said her group is planning to conduct follow-up tests to further investigate the crystal structure of beta titanium-3-gold and to see if chemical dopants might improve its hardness even further.

Porous material absorbs impacts in car accidents

Superplastic metal foam is being used in Mexico as an impact absorber for the Infierno Exotic Car, a niche model which can reach 395km/hr in 3s. Manufactured by LTM Hot Spot, the foam material is made through a special process involving a phase transfomration from liquid to solid of an alloy formed by combining copper, silver and aluminium.

According to Dr Said Robles Casolco, professor of the Universidad Autonoma del Estado de Morelos (UAEM), the product has memory and can be stretched 100 times more than its normal size, and then go back to its original state.

One of the objectives of this development is to make metal alloys with a porosity similar to bone, sea coral and some rocks, so the metal foam could absorb a strong impact and then revert to its original state. Furthermore, components made from it are light.

Robles Casolco says that the metal foam can also be applied in medical applications by combining it with hydroxyapatite ceramic, which can be used as an implant. The metal foam becomes biocompatible and can be used for hips, for example, due to its low density and low cost. It is possible that it can replace titanium and other heavy and corrosive materials.

About Andy Pye

Andy Pye is a graduate of Cambridge University and has had a high profile career in the technical press as well as being a pioneer in web publishing.

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