Scientists have found that an alloy, consisting of titanium and gold, is about three to four times harder than most steels and could be used for stronger, more biocompatible medical implants.
Titanium is the leading material for artificial knee and hip joints because it is strong, wear-resistant and nontoxic.
"It is about 3-4 times harder than most steels," said Emilia Morosan, from Rice University.
"It's four times harder than pure titanium, which is what's currently being used in most dental implants and replacement joints," said Morosan.
The atomic structure of the material - its atoms are tightly packed in a "cubic" crystalline structure that is often associated with hardness - was previously known.
However, Morosan and former graduate student Eteri Svanidze are first to document the remarkable properties of the ultra-hard "beta" form of the compound.
"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," said Morosan.
"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," she said.
"When we tried to grind up titanium-gold, we couldn't. I even bought a diamond (coated) mortar and pestle, and we still couldn't grind it up," she said.
Researchers 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.
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.
Researchers also performed other comparisons with titanium. For biomedical implants, for example, two key measures are biocompatibility and wear resistance.
Since 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.
Tests by colleagues at the University of Texas MD Anderson Cancer Centre determined that the new alloy was even more biocompatible and wear-resistant than pure titanium.
