Materials Science

A Sizeable Break

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Science  26 Aug 2005:
Vol. 309, Issue 5739, pp. 1302
DOI: 10.1126/science.309.5739.1302e

Metals and alloys containing nanocrystalline-sized grains are of interest because of their superior strength, wear resistance, and superplasticity, which is the ability to deform a material beyond its usual breaking point. When nano-structured metals are defect-free, they also show reasonable tensile elongations in addition to their enhanced strength. However, as the grain size decreases, the mechanism of plastic deformation changes from one that is dislocation-mediated to one that is grain boundary- mediated; it is not known if the failure mechanism changes from ductile to brittle, which might limit the applicability of these materials.

Li and Ebrahimi examined nanocrystal-line nickel and nickel-iron alloys with grain sizes above and below the critical size, respectively. In tensile testing, the Ni specimen showed significant necking before fracture, indicative of ductile behavior. Examination of the fracture surface showed matching concave features on both halves, which is consistent with the formation of microvoids during deformation. In contrast, the NiFe alloy showed little necking, indicative of a much lower toughness. The fracture surface showed a cup and cone pattern, or a series of voids and protrusions. The authors attribute this cup and cone pattern to the meandering of the path, and hence the fracture to the breakage of atomic bonds rather than cavity growth. — MSL

Adv. Mater. 17, 1969 (2005).

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