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Constrained minimal-interface structures in polycrystalline copper with extremely fine grains

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Science  13 Nov 2020:
Vol. 370, Issue 6518, pp. 831-836
DOI: 10.1126/science.abe1267

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Locking in nanoscale strength

Metals with nanometer-sized crystal grains are super strong, but they do not generally retain their structure at higher temperatures. This property undermines their high strength and makes their use in applications challenging. Li et al. found a minimum-interface structure in copper with 10-nanometer-sized grains that, when combined with a nanograin crystallographic twinning network, retains high strength to temperatures just below the melting point. This discovery suggests a different path forward for stabilizing nanograined metals.

Science, this issue p. 831

Abstract

Metals usually exist in the form of polycrystalline solids, which are thermodynamically unstable because of the presence of disordered grain boundaries. Grain boundaries tend to be eliminated through coarsening when heated or by transforming into metastable amorphous states when the grains are small enough. Through experiments and molecular dynamics simulations, we discovered a different type of metastable state for extremely fine-grained polycrystalline pure copper. After we reduced grain sizes to a few nanometers with straining, the grain boundaries in the polycrystals evolved into three-dimensional minimal-interface structures constrained by twin boundary networks. This polycrystalline structure that underlies what we call a Schwarz crystal is stable against grain coarsening, even when close to the equilibrium melting point. The polycrystalline samples also exhibit a strength in the vicinity of the theoretical value.

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