Most metals are composed of atoms arranged in highly ordered crystalline structures. However, it is possible to make very stiff and strong ‘amorphous’ metals —known as bulk metallic glasses—with disordered atomic structure like common glass. Hai Yang Bai and co-workers at the Chinese Academy of Sciences in Beijing1 have created new bulk metallic glasses based on the rare earth metal thulium that has some excellent physical properties.

“Bulk metallic glasses have strength approaching the theoretical limit, offering potential applications as structural materials,” says Bai. “They are already in use for magnetic sensing, chemistry and microelectromechanical systems.”

Fig. 1: Samples of bulk metallic glass developed at the Chinese Academy of Science.

Amorphous metals are usually man-made alloys of more than one metal, because atoms of different sizes are able to bunch close together. In the past, such alloys had to be cooled very quickly to avoid crystallization, and so were limited to forming thin sheets or ribbons. More recent techniques allow slower cooling, and can build up much thicker samples (Fig. 1).

Bai and co-workers created their new bulk metallic glasses by melting aluminum, cobalt, yttrium and thulium under an electric arc. X-ray diffraction was used to confirm that the alloys were fully amorphous.

The alloy was very stiff and resistant to compression, even withstanding more than double the pressure of previous rare earth bulk metallic glasses. When subjected to sufficient pressure, the alloy shattered into many pieces like glass. Periodic strip patterns were seen on the surfaces of the alloys just before shattering, which indicated a controlled fracturing process.

The thulium-based alloy are believed to exhibit superior properties because of strong chemical interactions between their different components, which resist structural change. “Thulium has strong affinity with aluminium, cobalt and other transition metals,” says project scientist Wei Hua Wang. “The tightly bonded atoms are not easily deformed and so induce high strength.”

The study suggests that materials with many different properties can be fabricated from rare earth metals, by careful selection of ingredients. “The brittleness and cost of our new glass is not good for structural materials,” says Bai, “but with appropriate composition changes, they could have unique magnetic and mechanical properties.”