Pressure induced seemed phase transition in metallic glass along with abnormal thermal expansions - Dr. Dawei Fan

FEBRUARY 2, 2015

A team of scientists including Dr. Dawei Fan from HPSTAR, discovered a seemed first-order phase transition in Ce-based metallic glass along with abnormal thermal expansion at high pressure. This work indicate that pressure will be a very effective method for investigating the structure or improving performance of metallic glasses for future applications.The related work is published on Nature Communications (“Hierarchical densification and negative thermal expansion in Ce-based metallic glass under high pressure”, DOI:10.1038/ncomms6703).

Metallic glasses (MGs) are new comers to the glass family. They are different from the traditional network forming glasses due to metallic bonds in their structure. Besides the disorder structure can actually improve some properties of metallic glasses. Metallic glasses can therefore combine the advantages and avoid many of the problems of normal metals and glasses, having superior strength and durability, which makes metallic glasses has wide commercial applications for decades. It's well known that polymorphism is very common phenomena in metal, or  alloy. Even amorphous ice, silica, and silicon were reported to exhibit "polyamorphism” in the glassy state. These lead researchers also try to find "polyamorphsim" in metallic glasses since 2007.

High pressure is an extremely powerful tool for exploring new structure or properties of materials. Using diamond anvil cell, a research team including Dawei Fan from HPSTAR  investigated the structural evolution in a Ce-based metallic glass (Ce₆₅Co₂₅Al₁₀) with pressure at room temperature (RT) and near the glass transition temperature by synchrotron X-ray diffraction. The Ce-based MG shows low-medium-high densification processes with pressure at both temperatures. A relatively abrupt and discontinuous change in volume happened on the sample around ~50, 000 times atmospheric pressure (5 gigapascals) at 390 Kelvin. This was obvious attributed to “a possible weak first-order phase transition.” Furthermore, an abnormal negative thermal expansion shows on Ce-based MG from 0.6 to 9 gigapascals. “It is pressure which  play the dominate role on the polyamorphic transformations and the abnormal thermal expansion,” explained by the authors.

"High-pressure and/or high-temperature is very routine way to study the struture changing in minerals, which may also have the same "structure-effect" in metallic glasses," said the co-author Dawei Fan, a geochemistry scientist from the Deep earth and Extreme Physics group of HPSTAR.

Caption: Pressure leads to densified process in Ce-based metallic glass.

“Our findings may be helpful to understand the nature of the near-Tg polyamorphic transformations in the MG system, and high pressure and/or high temperature may be very potential means for researchers to design metllic galsses with better properties for applications," said Dr. Dawei Fan. " We are looking forward to finding other mysterious structure in the MG family in next work."