Max-Planck-Institut für Eisenforschung GmbH

Atomic-scale mechanisms of deformation-induced cementite decomposition in pearlite

Pearlitic steel wires can exhibit tensile strengths higher than 5 GPa and even up to 7 GPa after severe plastic deformation, where the deformation promotes a refinement of the lamellar structure and cementite decomposition. However, a convincing correlation between deformation and cementite decomposition in pearlite is still absent.

In the project, a local electrode atom probe LEAP 3000 was used to characterize the microstructural evolution of pearlitic steel, cold-drawn with progressive strains up to 5.4. Transmission electron microscopy was also employed to perform complementary analyses of the microstructure. Both methods yielded consistent results. The overall carbon content in the detected volumes as well as the carbon concentrations in ferrite and cementite were measured by atom probe. In addition, the thickness of the cementite filaments was determined. In ferrite, we found a correlation of carbon concentration with the strain, and in cementite, we found a correlation of carbon concentration with the lamella thickness. Direct evidence for the formation of cell/subgrain boundaries in ferrite and segregation of carbon atoms at these defects was found. Based on these findings, the mechanisms of cementite decomposition are discussed in terms of carbon–dislocation interaction.

Fig.: (a) Part of the 3D carbon atom map with an isoconcentration surface at 7 at.% C for wires at _ = 5.4. (b) and (c) Front and side views, respectively, of the atom maps for the ROI taken from (a). The yellow, red and gray dots represent C, Mn and Si atoms, respectively. (d) 1D concentration profiles for the three elements shown in (b) and (c) (fixed count of 5000 atoms) along the direction (marked with the black arrows) perpendicular to the carbon-enriched boundary. — Fig.: (a) Part of the 3D carbon atom map with an isoconcentration surface at 7 at.% C for wires at _ = 5.4. (b) and (c) Front and side views, respectively, of the atom maps for the ROI taken from (a). The yellow, red and gray dots represent C, Mn and Si atoms, respectively. (d) 1D concentration profiles for the three elements shown in (b) and (c) (fixed count of 5000 atoms) along the direction (marked with the black arrows) perpendicular to the carbon-enriched boundary.

Max-Planck-Institut für Eisenforschung GmbH

Fig.: (a) Part of the 3D carbon atom map with an isoconcentration surface at 7 at.% C for wires at _ = 5.4. (b) and (c) Front and side views, respectively, of the atom maps for the ROI taken from (a). The yellow, red and gray dots represent C, Mn and Si atoms, respectively. (d) 1D concentration profiles for the three elements shown in (b) and (c) (fixed count of 5000 atoms) along the direction (marked with the black arrows) perpendicular to the carbon-enriched boundary.

Max-Planck-Institut für Eisenforschung GmbH

Fig.: (a) Part of the 3D carbon atom map with an isoconcentration surface at 7 at.% C for wires at _ = 5.4. (b) and (c) Front and side views, respectively, of the atom maps for the ROI taken from (a). The yellow, red and gray dots represent C, Mn and Si atoms, respectively. (d) 1D concentration profiles for the three elements shown in (b) and (c) (fixed count of 5000 atoms) along the direction (marked with the black arrows) perpendicular to the carbon-enriched boundary. — Fig.: (a) Part of the 3D carbon atom map with an isoconcentration surface at 7 at.% C for wires at _ = 5.4. (b) and (c) Front and side views, respectively, of the atom maps for the ROI taken from (a). The yellow, red and gray dots represent C, Mn and Si atoms, respectively. (d) 1D concentration profiles for the three elements shown in (b) and (c) (fixed count of 5000 atoms) along the direction (marked with the black arrows) perpendicular to the carbon-enriched boundary.

Max-Planck-Institut für Eisenforschung GmbH

Fig.: (a) Part of the 3D carbon atom map with an isoconcentration surface at 7 at.% C for wires at _ = 5.4. (b) and (c) Front and side views, respectively, of the atom maps for the ROI taken from (a). The yellow, red and gray dots represent C, Mn and Si atoms, respectively. (d) 1D concentration profiles for the three elements shown in (b) and (c) (fixed count of 5000 atoms) along the direction (marked with the black arrows) perpendicular to the carbon-enriched boundary.

Max-Planck-Institut für Eisenforschung GmbH

Publication Reference

Y. Z. Chen, A. Herz, Y. J. Li, C. Borchers, P. Choi, D. Raabe, and R. Kirchheim, "Nanocrystalline Fe–C alloys produced by ball milling of iron and graphite," Acta Materialia 61 (9), 3172-3185 (2013).

MPG.PuRe

Yujiao Li, Dierk Raabe, Michael Herbig, Pyuck-Pa Choi, Shoji Goto, Aleksander Kostka, Hiroshi Yarita, Christine Borchers, and Reiner Kirchheim, "Segregation stabilizes nanocrystalline bulk steel with near theoretical strength," Physical Review Letters 113 (10), 106104 (2014).

MPG.PuRe

Michael Herbig, D. Raabe, Yujiao Li, Pyuck-Pa Choi, Stefan Zaefferer, and Shoji Goto, "Atomic-scale quantification of grain boundary segregation in nanocrystalline material," Physical Review Letters 112, 126103 (2014).

MPG.PuRe

Y. J. Li, P. Choi, C. Borchers, Y.Z. Chen, S. Goto, D. Raabe, and R. Kirchheim, "Atom Probe Tomography characterization of heavily cold drawn pearlitic steel wire," Ultramicroscopy 111 (6), 628-632 (2011).

MPG.PuRe

Y. J. Li, P. Choi, C. Borchers, S. Westerkamp, S. Goto, D. Raabe, and R. Kirchheim, "Atomic-scale mechanisms of deformation-induced cementite decomposition in pearlite," Acta Materialia 59 (10), 3965-3977 (2011).

MPG.PuRe

Y. J. Li, P. Choi, S. Goto, C. Borchers, D. Raabe, and R. Kirchheim, "Evolution of strength and microstructure during annealing of heavily cold-drawn 6.3 GPa hypereutectoid pearlitic steel wire," Acta Materialia 60, 4005-4016 (2012).

MPG.PuRe

Yujiao Li, Pyuck-Pa Choi, Shoji Goto, Christine Borchers, Dierk Raabe, and Reiner Kirchheim, "Atomic scale investigation of redistribution of alloying elements in pearlitic steel wires upon cold-drawing and annealing," Ultramicroscopy 132, 233-238 (2013).

MPG.PuRe

Yujiao Li, Aleksander Kostka, Pyuck-Pa Choi, Shoji Goto, Dirk Ponge, Reiner Kirchheim, and Dierk Raabe, "Mechanisms of subgrain coarsening and its effect on the mechanical properties of carbon-supersaturated nanocrystalline hypereutectoid steel," Acta Materialia 84, 110-123 (2015).

MPG.PuRe