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Metallic Glass Research - Prof. Todd C. Hufnagel

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16/04/2007

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Metallic Glass Research - Prof. Todd C. Hufnagel
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Nanometer-scale voids in shear bands

Metallic glasses are alloys with highly disordered, amorphous structure, in contrast to the ordered, crystalline structure of conventional alloys. Because of this difference in structure, metallic glasses deform in unusual ways. In particular, at high stresses the deformation can be concentrated into narrow bands, called shear bands.

The micrograph shows a shear band (indicated by dashed lines) in a metallic glass. The variation in contrast is due to variations in the atomic density; in particular, the bright spots are regions of low atomic density. Most of these are due to the random nature of the structure, but quantitative analysis of the image reveals that some (colored in red) are actually nanometer-scale voids that result from the coalescence of excess free volume when the deformation ceases. This kind of microstructural analysis can lead to important insights about how deformation occurs in these novel materials.

Structure of metallic glasses from resonant x-ray scattering

Studying the structure of disordered materials such as metallic glasses is a significant challenge that requires sophisticated techniques. By making use of the resonant scattering effects at energies close to x-ray absorption edges, we can determine the atomic environments around elements of interest in our materials.

The figure shows the short-range atomic order around Zr atoms in two metallic glasses, with and without a small amount of tantalum as an alloying element. The presence of Ta enhances the order in the second-nearest atomic shell (r=5-7 Å) without affecting the nearest-neighbor shell (r=2-4 Å)-a most interesting and unexpected effect!

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