NPG Asia Materials featured highlight | doi:

Strong materials: Twin strength

Most metals and ceramics are polycrystals, meaning they are made up of grains. The size, shape and bonding of the grains have a great effect on the material’s physical properties.
Now, Lei Lu and co-workers¹ at the Institute of Metal Research in Shenyang, China and the Risø National Laboratory in Roskilde, Denmark have produced an unusually strong and ductile type of copper, by encouraging the formation of so-called ‘twin boundaries’ within the grains.

“A twin boundary is a special planar interface at which the arrangements of atoms on one side are mirror reflections of those on the other side,” says Lu. “The substantial benefits of twin boundaries are revealed in particular when the twin thickness (the mean spacing between neighboring twin boundaries) is reduced to the nanometer regime.”

Polycrystal materials become deformed when crystal lattice dislocations spread throughout the material. Twin boundaries are particularly good at stopping the spread of these dislocations, and can make the material more ductile, or capable of being hammered and drawn out without breaking.
Lu and co-workers made ‘nanotwinned’ copper foils by a pulsed electrodeposition technique. Their foils all had grains around 500 nanometres in diameter, but different twin thickness.

The researchers found that all of their samples were stronger than copper that had no twin boundaries. The maximum yield strength—about four times that of the untwinned copper—was found in samples with a twin thickness of 15 nanometres. Samples with even smaller twin thickness could be elongated uniformly by up to 30%. They also exhibited remarkable strain hardening – an increase in strength when strain was applied.

“The nanotwinned copper has both high strength and high ductility,” says Lu. “Moreover, it shows very good electrical conductivity. A promising potential application is to use it as interconnecting wires in integrated circuits where high mechanical strength and low electrical resistivity are desired.”

Lu suggests that the same concept could be applied to other materials.“Twins are not uncommon in nature, and appear in various metals and alloys with different crystallographic structures. Our nano-twinned copper joins a number of nanostructured metals that are being tailored for high performance.”
 

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.

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