Hirarchical structures and strength in cold-drawn pearlitic steel wire

Deformation, as one of the major methods to improve the (specific) strength of metals, can be combined with phase transformation to improve the strength of nanometals to an ultrahigh level close to the theoretical strength in single crystals. This is demonstrated by the analysis of the microstructural evolution, strengthening mechanisms and strength–structure relationships in a cold-drawn pearlitic steel with a structural scale in the nanometer range and a flow stress up to about 3.5 GPa. Structural parameters including the interlamellar spacing, the dislocation density in the ferrite lamellae and the cementite decomposition, have been analyzed and quantified by scanning electron microscopy, transmission electron microscopy and high resolution electron microscopy for wires cold drawn up to a strain of 3.68. Three strengthening mechanisms, boundary strengthening, dislocation strengthening and solid solution hardening, have been analyzed based on the microstructural analysis. The individual and combined contributions, of these mechanisms to the wire strength have been estimated and good agreement has been found between the measured flow stress and values estimated based on an assumption of linear additivity of the three strengthening mechanisms. Mechanisms behind the higher strength of about 6.4 GPa in the wires drawn to higher strains and to a finer microstructural scale is also discussed.