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Posted: Sep 20, 2011

Metallic glass: Not so random after all

(Nanowerk News) Metallic glasses are alloys that are as moldable as plastic but tougher than any known pure metal. The amorphous internal structure that gives metallic glasses their remarkable properties consists of an assembly of small and large atoms packed as tightly together as possible. However, this chaotic distribution of elements also makes metallic glasses prone to mechanical problems like sudden shattering.
Researchers from Zhejiang University in China with co-workers in the US have now discovered a new way of understanding metallic glasses that may make their fabrication easier than ever ("Long-Range Topological Order in Metallic Glass"). Using a combination of computer simulations and X-ray synchrotron radiation, the researchers showed that the organization of a ceriumľaluminum (CeľAl) metallic glass is not entirely accidental Ś instead, these glassy states retain a crystal 'memory' that can be exposed by pressure-induced transformations.
"For a long time, glassy materials were considered to have only short- or medium-range order on scales of a few nanometers," says Jian-Zhong Jiang from the research team. "Our work demonstrates that even with amorphous alloys, long-range topological order could exist."
The researchers subjected a CeľAl metallic glass ribbon to pressures hundreds of thousands times greater than atmospheric levels to uncover the secrets of its amorphous structure. Powerful X-ray beams revealed an initial broad signal, characteristic of disorganized systems. Yet as the researchers increased the pressure up to 25 GPa, the glass abruptly transformed into a single-crystal material with a face-centered cubic orientation Ś a surprising observation as truly amorphous substances yield only randomly directed crystals under pressure.
The team postulated that this hidden long-range order originates during fabrication of the metallic glass, a phenomenon they successfully reproduced using sophisticated molecular dynamic simulations. Their computations showed that while pure cerium and aluminum crystallize into cubic arrangements, the size mismatch between the two atoms leads to 'amorphization' of the alloy. The application of extreme pressure then collapses the larger atomic volume of cerium, allowing the preferred cubic crystallinity to reemerge.
Jiang notes that the CeľAl system may aid in the discovery of 'perfect' glasses that combine the predictability of crystal states with robust amorphous packing.
Source: Tokyo Institute of Technology
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