Ultrahigh hardness on a face-centered cubic metal

Highlights: • Nanotwinned surfaces are developed using plastic deformation at room temperature. • The hardness on nanotwinned surfaces is about three times that of their pristine counterparts. • Novel nanotwinned structure is confirmed by transmission electron microscopy. - Abstract: Nanotwinned (NT) surfaces are developed on a face-centered cubic (fcc) metal with ultrahigh hardness under cyclic loading using plastic deformation at room temperature. The hardness on NT surfaces remains constant at 7.9 and 8.5 GPa indented at 1 and 7 N under 0–100 cycles respectively, which are about three times that of their pristine surfaces. This is different from the NT metals and nonmetallic materials, on which the hardness is about two times that of their pristine counterparts. Moreover, NT metals usually consist of randomly oriented twin and grain boundaries, making it difficult to control the uniform mechanical property. Here, novel nt structure is proposed on an fcc metal, in which all the twin boundaries are along (-1-11) orientation, forming bundles of nanotwins to several micrometers in length.