Lab discovers titanium-gold alloy that is four times harder than most steels

July 20, 2016, Rice University
The crystal structure of beta titanium-3 gold. Credit: E. Morosan/Rice University

Titanium is the leading material for artificial knee and hip joints because it's strong, wear-resistant and nontoxic, 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.

"It is about 3-4 times harder than most steels," said Emilia Morosan, the lead scientist on a new study in Science Advances that describes the properties of a 3-to-1 mixture of and gold with 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."

Morosan, a physicist who specializes in the design and synthesis of compounds with exotic electronic and magnetic properties, said the new study is "a first for me in a number of ways. This compound is not difficult to make, and it's not a new material."

In fact, the atomic structure of the material—its atoms are tightly packed in a "cubic" crystalline structure that's often associated with hardness—was previously known. It's not even clear that Morosan and former graduate student Eteri Svanidze, the study's lead co-author, were the first to make a pure sample of the ultrahard "beta" form of the compound. But due to a couple of lucky breaks, they and their co-authors are the first to document the material's remarkable properties.

"This began from my core research," said Morosan, professor of physics and astronomy, of chemistry and of materials science and nanoengineering at Rice. "We published a study not long ago on titanium-gold, a 1-to-1 ratio compound that was a magnetic material made from nonmagnetic elements. 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 and other structural properties.

Eteri Svanidze (left) and Emilia Morosan. Credit: Jeff Fitlow/Rice University
"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's 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. 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 said 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.

Explore further: Combined titanium and gold create first itinerant antiferromagnetic metal

More information: Science Advances, DOI: 10.1126/sciadv.1600319 , http://advances.sciencemag.org/content/2/7/e1600319

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Noodle_Naut
5 / 5 (2) Jul 20, 2016
This could be ideal for high-end knives. It also does not sound like manufacturing would be very time consuming. It would have been nice to see an actual picture of the stuff. Is it gold or silver colored? Does it have a nice shine or is it more matte?
691Boat
5 / 5 (1) Jul 20, 2016
So the plan is to dilute a metal that is $3.80/kg by 1/4 with a metal that is $43,000/kg? Pretty sure the insurance companies would be in favor of that. haha!
Grallen
5 / 5 (2) Jul 21, 2016
If it increases the life of the product and prevents the need for further "maintenance" yeah, they would be all over it.
rrrander
not rated yet Jul 21, 2016
Not seeing the need for such strength for medical implants over titanium, tantalum. Maybe lightweight, long implants like permanent stents.
WBHobbs
5 / 5 (1) Jul 21, 2016
The surfaces of hip and knee joints are subject to wear, and the replacement cost in terms of dollars and pain is high. I would think this material could have a use there.
antialias_physorg
3 / 5 (2) Jul 21, 2016
Hmm, interesting...we once worked together with a company that 3D prints titanium hip replacements (using 3D laser sintering printing technique from a powder)
I wonder if the crystalline structure described in the article would form under those conditions given an adequate powder mix.

Lifetime of such implants is a consideration. For older patients it's not much of an issue (expected lifetime of a hip implant is on the order of 20-25 years). But for younger patients - of which there are an increasing number - any increase in implant lifetime could potentially mean one less big operation during their life.

Also consider that while a titanium implant is more costly than the regular kind (and a gold/titanium one would surely be even more costly) the cost of the operation itself is on the order of 20-30 times as much. So there is a real potential for overall cost reduction.
Steelwolf
4 / 5 (2) Jul 23, 2016
I can see this going far, insurance companies will spring for more expensive items up front if they know that they will have to do multiple replacements later with cheaper materials.

On the knife idea, unless it was molded to the knife form in the first place I do not see any way for it to be shaped, since they could not grind it to a dust even with diamond surfaces, I certainly would not be able to grind it with shop grade equipment. So unless it was cast completely finished it would not be viable for use. Even using a laser for cutting/shaping could affect the cubic structure. Also, super hard items tend to be fairly fragile, so brittleness in the knife would have to be taken into consideration. Of course, if one did use a laser for some shaping and surface treatment, I suppose it would drop back to the alpha configuration, which is probably tougher, so that may make for an overall stronger material, hard inside, slightly more ductile outer holding very hard, tough edge.

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