title: Segregation stabilizes nanocrystalline bulk steel with near theoretical strength.
abstract: Grain refinement through severe plastic deformation enables synthesis of ultrahigh-strength nanostructured materials. Two challenges exist in that context: First, deformation-driven grain refinement is limited by dynamic dislocation recovery and crystal coarsening due to capillary driving forces; second, grain boundary sliding and hence softening occur when the grain size approaches several nanometers. Here, both challenges have been overcome by severe drawing of a pearlitic steel wire (pearlite: lamellar structure of alternating iron and iron carbide layers). First, at large strains the carbide phase dissolves via mechanical alloying, rendering the initially two-phase pearlite structure into a carbon-supersaturated iron phase. This carbon-rich iron phase evolves into a columnar nanoscaled subgrain structure which topologically prevents grain boundary sliding. Second, Gibbs segregation of the supersaturated carbon to the iron subgrain boundaries reduces their interface energy, hence reducing the driving force for dynamic recovery and crystal coarsening. Thus, a stable cross-sectional subgrain size <10  nm is achieved. These two effects lead to a stable columnar nanosized grain structure that impedes dislocation motion and enables an extreme tensile strength of 7 GPa, making this alloy the strongest ductile bulk material known.
authors.affiliation: Max-Planck Institut für Eisenforschung, Max-Planck-Strasse 1, D-40237 Düsseldorf, Germany and Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, D-37077 Göttingen, Germany.
authors.forename: Yujiao
authors.initials: Y
authors.lastname: Li
authors.affiliation: Max-Planck Institut für Eisenforschung, Max-Planck-Strasse 1, D-40237 Düsseldorf, Germany.
authors.forename: Dierk
authors.initials: D
authors.lastname: Raabe
authors.affiliation: Max-Planck Institut für Eisenforschung, Max-Planck-Strasse 1, D-40237 Düsseldorf, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Herbig
authors.affiliation: Max-Planck Institut für Eisenforschung, Max-Planck-Strasse 1, D-40237 Düsseldorf, Germany.
authors.forename: Pyuck-Pa
authors.initials: PP
authors.lastname: Choi
authors.affiliation: Max-Planck Institut für Eisenforschung, Max-Planck-Strasse 1, D-40237 Düsseldorf, Germany and Akita University, Tegata Gakuencho, Akita 010-8502, Japan.
authors.forename: Shoji
authors.initials: S
authors.lastname: Goto
authors.affiliation: Max-Planck Institut für Eisenforschung, Max-Planck-Strasse 1, D-40237 Düsseldorf, Germany.
authors.forename: Aleksander
authors.initials: A
authors.lastname: Kostka
authors.affiliation: Suzuki Metal Industry Co. LTD., Narashino, Chiba 275-8577, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Yarita
authors.affiliation: Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, D-37077 Göttingen, Germany.
authors.forename: Christine
authors.initials: C
authors.lastname: Borchers
authors.affiliation: Max-Planck Institut für Eisenforschung, Max-Planck-Strasse 1, D-40237 Düsseldorf, Germany and Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, D-37077 Göttingen, Germany.
authors.forename: Reiner
authors.initials: R
authors.lastname: Kirchheim