Hardening up: New alloy four times as tough as titanium

A chance discovery in a physics lab at Rice University has turned up an ultra-hard material that could usurp the titanium commonly used in today's knee and hip replacements. Scientists have found that by melting gold into the titanium mix they can produce a non-toxic metal that is four times harder than titanium itself, raising the prospect of more durable, longer lasting medical implants.

Emilia Morosan, a professor of physics at Rice University, was carrying out experiments on a magnetic material made from nonmagnetic elements, more specifically, a titanium-gold mix with a one-to-one ratio. Part of her team's process in developing new compounds like this one is to grind it up into powder so that it can be X-rayed, which helps them identify things like its composition, structure and purity.

"When we tried to grind up titanium-gold, we couldn't," she says. "I even bought a diamond-coated mortar and pestle, and we still couldn't grind it up."

It proved a tough nut to crack, but Morosan and her team carried out a series of tests to work out how hard this compound really was, along with a few other titanium-gold compounds that had been used as comparisons in their earlier work. Part of this mix was one alloy containing three parts titanium to one part gold, which had been formed at high temperature.

Preparing the compound at high temperatures, as it turns out, creates an almost purely crystalline form of the beta version of the alloy, with four times the hardness of titanium. The researchers point out that the compound is actually not a new one, nor is it difficult to make, but they are the first to come across its impressive properties.

They say that the reason for this is the high temperature at which they had cooked up the material. When prepared at lower temperatures, they say they atoms arrange themselves in a cubic structure as the alpha form of the so-called titanium-3-gold, with a hardness similar to regular titanium. It seems, therefore, the scientists that had previously assessed its hardness were working with materials consisting of this alpha arrangement of atoms.

"[Beta titanium-3 gold] is about three to four times harder than most steels," says Morosan. "It's four times harder than pure titanium, which is what's currently being used in most dental implants and replacement joints."

The researchers say that material could lend itself particularly well to use in medical implants, as it is made of titanium and gold, which are up there with the more biocompatible materials and are commonly used for that reason. But testing showed their titanium-3-gold to be even more biocompatible and wear-resistant than pure titanium. They the team is exploring whether treating it with chemicals can make it even harder again.

The research was published in the journal Science Advances.

Source: Rice University