Materials such as glass or polystyrene are often called amorphous solids – literally ‘without shape’ - because their atoms are arranged randomly without the long-range repeating patterns seen in crystals. Scientists have also managed to make amorphous metal alloys known as metallic glasses, which promise great strength, elasticity and magnetic properties.

Now, Li Yi at the National University of Singapore, Carl Thompson at Massachusetts Institute of Technology and co-workers1 have developed a technique that can easily identify the best combinations of metals for forming metallic glass.

Fig. 1: Three dimensional metallic surfaces.

Amorphous solids are usually made by melting a material then cooling it very rapidly so that there is no time for crystals to form. This process, called quenching, is difficult with metals because they have relatively low melting points.

“It is very difficult to make glasses from metals compared to any other class of materials, such as semiconductors, ceramics and polymers,” says Thompson. “Despite intensive research, we still don’t know why some alloy compositions can be made into glasses and others cannot.”

To answer this question, the researchers devised a clever experiment to measure how tightly atoms are packed in a metallic glass. This is important in searching for long-lasting metallic glasses because if the atoms are close together they can’t move very much, making it harder for crystals to grow.

The researchers deposited a thin film of amorphous copper-zirconium (Cu-Zr) alloy onto tiny cantilevers. They then heated the cantilever until the Cu-Zr crystallized, causing the cantilever to bend upwards.

The experiment was repeated with several Cu-Zr alloys, containing different ratios of the two metals. In each case the cantilever bent by a different amount depending on the initial density of the amorphous phase. The researchers found that the density was strongly linked to the ability of the alloy to form a metallic glass in the first place.

“Our results provide new insight into the characteristics of liquid alloys that can most readily be quenched into the glassy state,” says Li. “We expect these new results, and the technique we developed to obtain them, will play a key role in solving the mystery of metallic glass formation.”