Nanocrystalline Fe-C alloys produced by ball milling of iron and graphite

A series of nanocrystalline Fe-C alloys with different carbon concentrations (x(tot)) up to 19.4 at.% (4.90 wt.%) are prepared by ball milling. The microstructures of these alloys are characterized by transmission electron microscopy and X-ray diffraction, and partitioning of carbon between grain boundaries and grain interiors is determined by atom probe tomography. It is found that the segregation of carbon to grain boundaries of alpha-ferrite can significantly reduce its grain size to a few nanometers. When the grain boundaries of ferrite are saturated with carbon, a metastable thermodynamic equilibrium between the matrix and the grain boundaries is approached, inducing a decreasing grain size with increasing x(tot). Eventually the size reaches a lower limit of about 6 nm in alloys with x(tot) > 6.19 at.% (1.40 wt.%); a further increase in x(tot) leads to the precipitation of carbon as Fe3C. The observed presence of an amorphous structure in 19.4 at.% C (4.90 wt.%) alloy is ascribed to a deformation-driven amorphization of Fe3C by severe plastic deformation. By measuring the temperature dependence of the grain size for an alloy with 1.77 at.% C additional evidence is provided for a metastable equilibrium reached in the nanocrystalline alloy. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.