Move over titanium, the new titanium-gold alloy in town is four times harder than most steels.
Titanium is the leading material for artificial knee and hip joints because it's strong, wear-resistant and nontoxic, but the unexpected discovery by Rice University physicists shows that the gold standard for artificial joints can be improved with the addition of some actual gold.
The study describes the properties of a 3-to-1 mixture of titanium and gold with a specific atomic structure that imparts hardness.
"It is about 3-4 times harder than most steels," said lead scientist Emilia Morosan, adding "It's four times harder than pure titanium, which is what's currently being used in most dental implants and replacement joints."
Morosan 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 lead co-author Eteri Svanidze 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.
Morosan stated: "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 crystal structure and other structural properties.
"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 study appears in journal Science Advances.
