Supercomputer Helps To Find Metallic Glass

By Roland Piquepaille

Before going further, what are metallic glasses? Unlike conventional metals, which have regular molecular structures, amorphous metals, or metallic glasses, exhibit disorganized structures. Because of this disorganization, they don't have the defects which are common in crystals, and which lead to corrosion or even rupture. Until now, these amorphous metals were created by melting and casting various alloys. Not anymore. The Pittsburgh Post-Gazette reports that a team of physicists led by a Carnegie Mellon University (CMU) professor is using a new computational method to simulate future alloys at the Pittsburgh Computing Center (PSC). For example, with the help of the supercomputers at PSC, the team discovered that adding small quantities of yttrium will lead to superstrong amorphous steel, before doing any physical experiment. In the next three to five years, this will bring to the market ship hulls that never rust and are invisible to magnetic detection. And amorphous aluminum will be incorporated into lighter planes and cars. Read more...

Here is how the Pittsburgh Post-Gazette introduces this new computational method.

This computational method, developed by a group led by Carnegie Mellon University physicist Michael Widom [and his group], potentially could be used to create and perfect additional amorphous metals and to develop other metallic alloys.

Though the computations can take a day to run on even the fastest computers, the method could help researchers winnow the possible combinations of elements being considered for new amorphous metals and predict which combinations are most likely to yield the material's desired structure.

Now, let's look at the difference between conventional and amorphous metals with a couple of pictures.

	In this figure, two different elements are arranged to form a crystalline lattice structure like that found in an ordinary metal. (Credit: Michael Widom and his group, CMU)
	Here is a figure of disorganized elements. In this simulated mixture of elements, the investigators have added small amount of the large element yttrium to prevent crystal formation and facilitate metallic glass production. (Credit: Michael Widom and his group, CMU)

Amorphous metals have been discovered fifty years ago. But they needed to be cooled very fast in order to keep their unusual properties and to avoid to return to a crystalline structure. Thanks to new additions to alloys, it's now possible to cool them like regular metals.

That has opened the possibility that amorphous metals might be produced in bulk so they could be used in structural materials. The key, Widom said, is to find elements that disrupt the crystallization process.

In the case of amorphous steel, that turned out to be a large atom, called yttrium. The other elements in this steel formula -- iron, boron, carbon, chromium and molybdenum -- favor crystal formation. But adding a small amount of yttrium, or one of the elements known as rare earths, destabilizes the crystal and preserves the glassy structure.

So, we'll get stronger and lighter cars and planes, but what kind of computer resources are needed to find a stable and interesting metallic glass?

Yang Wang, [a physicist] of the Pittsburgh Supercomputing Center, said it now takes about a day to simulate the cooling of 100 atoms of an alloy. Larger simulations would produce more reliable results, he noted, but would take much longer; a 1,000-atom simulation might take more than a month, for instance, on existing computers.

In addition to amorphous steels, Widom is investigating amorphous aluminum. The computational method also is useful for studying other types of metal alloys and thus far has generated recipes for more than 1,700 metal structures, most of which have yet to be analyzed.

For other details, you also can read this informative CMU press release, dated September 2, 2004 and named "New Computational Method Developed By Carnegie Mellon University Physicist Could Speed Design and Testing of Metallic Glass."

The research work will be published by the Physical Review B journal in a coming issue. However, here is a link to the preview of this paper named "Cohesive energies of Fe-based glass-forming alloys" (PDF format, 16 pages, 238 KB) and submitted on February 11, 2004.

Another paper on a related subject has been submitted to the Journal of Materials Research on July 28, 2004. Here is a link to the preview of this other paper named "Stability of Fe-based alloys with structure type C₆Cr₂₃" (PDF format, 11 pages, 126 KB).

Sources: Byron Spice, Pittsburgh Post-Gazette, September 6, 2004; and various Carnegie Mellon University web pages