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Atomic-scale investigation of ε and θ precipitates in bainite in 100Cr6 bearing steel by atom probe tomography and ab initio calculations

Authors
Journal
Acta Materialia
1359-6454
Publisher
Elsevier
Volume
61
Issue
20
Identifiers
DOI: 10.1016/j.actamat.2013.08.051
Keywords
  • ε Carbide
  • Cementite
  • Atom Probe Tomography
  • Ab Initio
Disciplines
  • Medicine
  • Physics

Abstract

Abstract Carbide precipitation during upper and lower bainite formation in high-carbon bearing steel 100Cr6 is characterized using transmission electron microscopy and atom probe tomography. The results reveal that both ε and θ carbides precipitate in lower bainite isothermally held at 260°C and only θ precipitates form in upper bainite isothermally held at 500°C. ε and θ precipitate under paraequilibrium condition at 260°C in lower bainite and θ precipitates under negligible partitioning local equilibrium condition in upper bainite at 500°C. In order to theoretically study ε and θ precipitation and the ε→θ transition in bainite, thermodynamic calculations have been carried out using ab initio techniques. We find that ε and θ carbides in ferrite have almost identical thermodynamic stability, and hence have similar formation probability. In austenite, however, cementite formation is clearly preferred: it is favored by 5kJmol−1 at room temperature and still by 4kJmol−1 at 500°C. Hence, the thermodynamic predictions agree well with the atom probe tomography results.

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