Materials Science

Beading a Band

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Science  13 Jan 2006:
Vol. 311, Issue 5758, pp. 149
DOI: 10.1126/science.311.5758.149a

Plastic deformation in semicrystalline or amorphous materials is often restricted to very thin shear bands. For metallic glasses, these bands are particularly important because the associated work-softening leads to a plastic instability in tension that limits the potential of these materials as structural materials. Despite the appearance of liquid-like features at fracture surfaces, there is some controversy over the local temperature rise at the bands.

Lewandowski and Greer use a fusible coating and find that the local temperature can reach as high as a few thousand Kelvin over a few nanoseconds. Specimens were coated with a thin film of tin, which formed hemispherical beads when heated. By measuring the half-width of the beads or their volumes, the authors were able to calculate the enthalpy required to form the beads, and thus the local temperature flux during specimen deformation. The same calculations, however, predict that the shear bands should be much larger than the 10 to 20 nm typically observed. Thus, the authors conclude that although local heating is important in understanding the changes that take place at the bands, the thickness of the bands is controlled by local structural changes, such as the formation of nanocrystals and voids. — MSL

Nat. Mater. 5, 15 (2006).

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