All That Matters

A tough metallic glass. This Pd79Ag3.5P6Si9.5Ge2 glass did not fracture catastrophically even after undergoing significant bending. Credit: Maximilien E. Launey

Glasses are very useful structural materials, because they are strong. You can stand on a glass table without it breaking. But once you put too much stress on a glass it breaks. Steels on the other hand are much tougher — which means that they show a much stronger resistance to fracture: rather than suddenly breaking under stress, steels tend to deform first. This toughness of steel is one of the obvious reasons why steel skeletons instead of glasses are used for the construction of large buildings.

This conception of glasses not being very tough is now made history by Marios Demetriou and colleagues from the California Institute of Technology and The University of California, Berkeley. They have discovered a glass that is not only strong, but also tough as steel. It is a metallic glass, which means that it dominantly consists of metal atoms. Like other metallic glasses it isn’t transparent, but looks like a normal metal  — except that unlike crystalline metals its atomic structure is more or less random. The damage tolerance of this metallic glass, its combination of strength and toughness, is higher than any known material. “It has been usual to regard metallic glasses as damage sensitive, but over recent years it has become increasingly recognised that they can be tough. This new paper marks a further decisive shift in showing that metallic glasses can be very tough indeed,” says Lindsay Greer from Cambridge University, who studies the properties of metallic glasses.

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The past year has been a great year for science with major advances in several areas. Too many exciting results to mention here. Instead, to reflect about the past year I have chosen a representative paper for each month of the year that I hope can serve as an example of the great science going on in a number of research fields. Of course, this is a highly subjective and personal collection, and indeed there might be others worth mentioning. But the aim was also to provide a balanced overview of the year that covers a variety of topics.

Of course, if you have an exciting paper to add, please feel free to use the comments section below to let us know!

Anyway, enough said, here are some of my highlights from the past year:

Simulations of electronic excitations in an iron-based superconductor. Image by Oak Ridge National Laboratory via flickr.

JANUARY – iron-based superconductors

Since they were discovered in 2008, iron-based superconductors, the pnictides, have been one of the hottest topics in condensed matter physics. Part of their appeal stems from the fact that they are based on iron, which is a magnetic element. Normally, magnets and superconductivity exclude each other.

The iron-based compounds have a similar crystal structure as the so-called cuprates, which are the materials with the highest superconducting temperatures known. The mechanism for these high-temperature superconductors is unknown, and studying the iron-based superconductors may also be relevant to the understanding of the cuprates.

This paper published in Science shows for the first time that the electrons in the iron-based superconductors show a periodic arrangement that is different to the periodicity of the atoms in the crystal. Similar observations have been made in the cuprates, and their understanding is considered important to the mechanism of high-temperature superconductivity.

Chuang, T., Allan, M., Lee, J., Xie, Y., Ni, N., Bud’ko, S., Boebinger, G., Canfield, P., & Davis, J. (2010). Nematic Electronic Structure in the “Parent” State of the Iron-Based Superconductor Ca(Fe1-xCox)2As2 Science, 327 (5962), 181-184 DOI: 10.1126/science.1181083

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This USB flash drive used a bulk metallic glass for its casing. Credit: Liquidmetal Technologies.

This week I am attending the 2010 Materials Research Society Fall Meeting in Boston — one of the key meetings in materials science. One of the sessions is on bulk metallic glasses and their applications, which this year is a little special. It is organised in honour of the 50 year anniversary of the first demonstration of a metallic glass by William Klement, Ronald Willens and Pol Duwez from Caltech. Their paper on gold-silicon alloys was published in Nature on September 3^rd, 1960. (In addition to Duwez and colleagues, David Turnbull must be mentioned here as one of the pioneers with several key contributions to the field. For example in 1948 he demonstrated that metals can be considerably undercooled below their crystallisation temperature.)

Metallic glasses have since become very interesting for applications that include sports products, coatings, power transformers where they reach several ten thousands of metric tons annual production, sports equipment, bioimplants and others. At the same time research researchers still try to learn more why these glasses form in the first place.

Why are metallic glasses so special?

All metals prefer to form crystals, as  the metal atoms easily form structured bonds with other atoms. So easy in fact that even when the metal is melted some of that arrangement carries over into the liquid. This makes the formation of a crystal the preferred pathway once the melt is cooled down again. Glass on the other hand is amorphous, which means that the atoms are disordered and there is no long-range periodicity. This is not something metals prefer. Unlike the window glass made of silicon oxide. In comparison, metallic glasses are a very different animal.  […]