Highly elastic metallic glass brings bounce to hardened materials

A bulk metallic glass (BMG) ball (left) outbounces its steel cousin

Bulk metallic glasses (BMGs) are artificial materials that boast superior strength and hardness to conventional metals due to a jumbled arrangement at the atomic level. By tweaking this recipe just a little, researchers have been able to produce a bouncy material with the highest degree of elasticity of any BMG, something they say could come to form everything from new drill bits and body armor to meteor-resistant satellite casings.

Natural metals feature an organized crystalline structure at the atomic level, where atoms are neatly arranged on top of each other to form three-dimensional lattice structures. Because of this, when force is applied the layers of atoms can move past one another and the metals can be bent out of shape.

Conversely, BMGs are able to withstand larger forces while retaining their shape due to a disorganized arrangement of the atoms within. This is achieved through first rapidly heating the material to extreme temperatures to stimulate the atoms into a frenzied arrangement, and then very quickly cooling the material to freeze them in place.

In developing their flexible new material, scientists at the University of Southern California heated a powdered iron composite to precisely 630° C (1,166° F) and then rapidly cooled it. In doing so, they appear to have happened upon a sweet spot for BMGs, resulting in a material with just the right amount of structure.

"It has almost no internal structure, like glass, but you see tiny regions of crystallization," says Veronica Eliasson, lead author of the paper. "We have no idea why a small amount of crystalline regions in these bulk metallic glasses makes such a big difference under shock loading."

The team has labeled its new material SAM2X5-630, and says that it can endure heavy impacts without deforming, even when pushed beyond its elastic limits. The researchers found that the Hugoniot Elastic Limit for the material, which denotes the maximum shock it can withstand without irreversibly deforming, to be 11.76 ± 1.26 GPa. This is much higher than stainless steel at 0.2 GPa, and even the tungsten carbide used in military armor at 4.5 GPa.

"In particular, the fact that the new materials performed so well under shock loading was very encouraging and should lead to plenty of future research opportunities," says Eliasson.

The team's research was published in the journal Nature Scientific Reports. You can see SAM2X5-630 outbounce steel in the video below.

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