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.

Deformation‐Induced Martensite: A New Paradigm for Exceptional Steels

“…The highest total wire drawing strain ( ε = 6.52) leads to a very high tensile strength of 7 GPa. [ 13 ] …”

“…[ 35 ] This results in a carbon-supersaturated Fe matrix and segregation of carbon atoms to grain boundaries stabilizing the nanosized Fe grain structure. [ 13,14,32,36 ] For the same samples, Li el al. reported a transition from a lamellar pearlite to a nanocrystalline microstructure during wire drawing at very high drawing strains.…”

“…[ 11,12 ] To this end, cold-drawn pearlitic-steel wires have been very successful, reaching an ultrahigh tensile strength of up to ca. 7 GPa, [ 13 ] making them one of the world's strongest bulk materials. Pearlitic steels are used, for example, in large constructions such as suspension bridges.…”

“…The high strength is associated with the refi nement of the originally lamellar eutectoid body-centeredcubic (bcc) α-Fe (ferrite) + Fe 3 C (cementite) structure of the pearlite, which leads to a nanocomposite that is stabilized by carbon segregation to the α-Fe grain boundaries. [ 13,14 ] With ongoing structure refi nement, more and more carbon must be accommodated inside the ferrite due to the dissolution of the cementite. [ 15,16 ] As a consequence, the concentration of carbon in the α-Fe exceeds the equilibrium solubility limit by far.…”

“…reported a transition from a lamellar pearlite to a nanocrystalline microstructure during wire drawing at very high drawing strains. [ 13 ] The refi nement in grain size leads to an increase in tensile strength following the Hall-Petch law. In contrast, the present study focuses on the accommodation of C in the Fe matrix.…”