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Metallic Glass Research - Prof. Todd C. Hufnagel

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Thứ hai, 16 Tháng tư 2007

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Metallic Glass Research - Prof. Todd C. Hufnagel
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Prof. Todd C. Hufnagel (more information! )
Department of Materials Science and Engineering
Johns Hopkins University
Background

Most metals and alloys are crystalline; that is, their atoms are arranged in a regular, ordered pattern that extends over long distances (hundreds or thousands of atoms). These regions of ordered atomic arrangement we call crystals. The regular arrangement of atoms in a crystalline material can be directly viewed using a transmisson electron microscope (TEM). For instance, Figure 1 shows the atomic-scale structure of a zirconium-based alloy. Many of the important properties of engineering alloys can be explained in terms of this sort of crystalline order, or, in many cases, in terms of defects in the crystal structure.

Figure 1: High resolution TEM image of a crystalline zirconium alloy. The bright spots are long rows of atoms, viewed end-on. Notice how these spots are very regularly arranged, proving that this material has a crystalline structure.

Metallic glasses, in contrast, are alloys that are noncrystalline or amorphous; that is, there is no long-range atomic order. Figure 2 below shows a high resolution TEM image of a metallic glass; notice that in this case, the spots are more or less randomly arranged. This tells us that there are none of the long rows of atoms seen in Figure 1, and we conclude that the material is indeed amorphous.

Figure 2: High resolution TEM image of an amorphous zirconium alloy. In contrast to Figure 1, the spots are randomly arranged, which tells us that this material is noncrystalline.

Making amorphous solids is nothing new. Many common materials, including oxide glasses (such as ordinary window glass) and most polymers, are amorphous. It is quite unusual, however, for a metallic material to be amorphous. The trick to making a metallic glass is to cool the a metallic liquid (which has a disordered structure as well) down so rapidly that there is not enough for the ordered, crystalline structure to develop. In the original metallic glasses (developed some forty years ago), the required cooling rate was quite fast - as much as a million degrees Celsius per second! More recently, new alloys have been developed that form glasses at much lower cooling rates, around 1-100 degrees per second. While still fairly rapid, it is slow enough that we can cast bulk ingots of these metallic alloys, and they will solidify to form glasses. An example is shown below.

Figure 3: A wedge of an amorphous Zr-Ti-Cu-Ni-Al alloy, produced by casting into a copper mold.

From an engineering point of view, our interest in metallic glasses stems from their unique structure. Since the structure of a material determines its properties, you might expect that a material with an unusual structure might have interesting properties. This is certainly true of metallic glasses. For instance, metallic glasses can be quite strong yet highly elastic, and they can also be quite tough (resistant to fracture). Even more interesting are the thermal properties; for instance, just like an oxide glass, there is a temperature (called the "glass transition temperature") above which a metallic glass becomes quite soft and flows easily. This means that there are lots of opportunities for easily forming metallic glasses into complex shapes.

Source: http://www.jhu.edu/~matsci/people/faculty/hufnagel/background.html

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