High tensile ductility in a nanostructured metal

Nanocrystalline metals—with grain sizes of less than 100 nm—have strengths exceeding those of coarse-grained and even alloyed metals1,2, and are thus expected to have many applications. For example, pure nanocrystalline Cu (refs 1–7) has a yield strength in excess of 400 MPa, which is six times higher than that of coarse-grained Cu. But nanocrystalline materials often exhibit low tensile ductility at room temperature, which limits their practical utility. The elongation to failure is typically less than a few per cent; the regime of uniform deformation is even smaller1,2,3,4,5,6,7. Here we describe a thermomechanical treatment of Cu that results in a bimodal grain size distribution, with micrometre-sized grains embedded inside a matrix of nanocrystalline and ultrafine (<300 nm) grains. The matrix grains impart high strength, as expected from an extrapolation of the Hall–Petch relationship. Meanwhile, the inhomogeneous microstructure induces strain hardening mechanisms8,9,10,11 that stabilize the tensile deformation, leading to a high tensile ductility—65% elongation to failure, and 30% uniform elongation. We expect that these results will have implications in the development of tough nanostructured metals for forming operations and high-performance structural applications including microelectromechanical and biomedical systems.