A Better Way to Find Bulk Metallic Glass
Jenn U posted on September 21, 2016 |
When metal alloys are melted, the atoms lose their ordered structure and become amphorous, as seen above. Most metals alloys will snap back to their rigid crystal structures when cooled back down but bulk metallic alloys will retain the random amorphous structure even in the solid state. (Image courtesy of the Vlassak Group/Harvard SEAS.)

When metal alloys are melted, the atoms lose their ordered structure and become amphorous, as seen above. Most metals alloys will snap back to their rigid crystal structures when cooled back down but bulk metallic alloys will retain the random amorphous structure even in the solid state. (Image courtesy of the Vlassak Group/Harvard SEAS.)

Plastics are the material of choice for many manufacturers due to their ability to be easily molded to nearly any shape as well as their ready availability and relatively low price. However, they tend to have lower strength and durability compared to other materials in similar applications as well as lacking the conductivity and other desirable properties.

Enter bulk metallic glasses, also known as amorphous metals – alloys that lack the crystalline structure of most metallic solids. Like other glasses, metallic glasses soften and flow upon heating, making them well-suited to injection molding applications.

Until recently, however, bulk metallic glasses have been difficult to identify and expensive to create. These alloys are created from three or more different metals, and the only way to identify an amorphous alloy was to mix the alloy, cool it under the right conditions and then physically determine whether its structure is crystalline. In addition to this, most known metallic glasses include expensive elements, such as gold and palladium.

Fortunately, researchers at Harvard, Duke, and Yale can now predict which combinations of metals will form amorphous alloys prior to actually synthesizing them. This is thanks to careful examination of the combinations of metals’ crystalline properties.

When cooled, most alloys will crystallize, forming a single crystal structure throughout the material. Others, however, have a variety of crystal patterns which could potentially be formed.  In amorphous metals, these potential patterns have similar formation energies, and compete to form simultaneously within the alloy.


Predicting the Formation of Bulk Metallic Glass

The research team identified the “critical size” that a crystalline area must reach in order to produce crystalline growth throughout the metal and predicted that simultaneous formations of many crystalline nuclei can prevent any one nucleus from reaching this critical size. This prevents the formation of any one crystal structure, and forces the alloy to remain amorphous in its solid state. 

The team created and analyzed a database of potential crystal structures of alloys, then tested the predictions resulting from the analysis. The experiments found that the analysis was 73 percent accurate.

(Image courtesy of the Vlassak Group/Harvard SEAS.)
(Image courtesy of the Vlassak Group/Harvard SEAS.)
As a result of this method, hundreds of candidates for metallic glasses have been identified.  Many of these can be created from only two metals, unlike previously known metallic glasses - in fact, the team identified 17 percent of binary alloy systems as candidates for metallic glass formation. This lends credence to the potential for bulk metallic glasses to become more accessible for a variety of applications, including electronics, nuclear reactor engineering, medical instruments and even sports equipment.

The research team has published their findings in the journal Nature Communications.

For another materials story, check out these sub-zero self-healing composite materials.

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