Online ISSN : 1347-5320
Print ISSN : 1345-9678
ISSN-L : 1345-9678
Concurrent γ-Phase Nucleation as a Possible Mechanism of δ-γ Massive-like Phase Transformation in Carbon Steel: Numerical Analysis Based on Effective Interface Energy
Masato YoshiyaManabu WatanabeKenta NakajimaNobufumi UeshimaKoki HashimotoTomoya NagiraHideyuki Yasuda
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2015 Volume 56 Issue 9 Pages 1467-1474


Effective interface energies of various homo- and hetero-interfaces of iron were calculated with an aid of phase-field modeling, taking into account geometric constraints by competition among grains or interfaces. Calculated effective interface energies for δ/γ, δ/δ, and γ/γ interfaces are 0.56, 0.44 and 0.37 J/m2, respectively. Using two simple geometric models for nucleation on or off an interface in the matrix, the optimal shape of a nucleus at a given radius and undercooling, a critical radius and an energy barrier for nucleation for each possible circumstance were numerically calculated. It is found that, although the energy barrier for γ-phase nucleation in homogeneous δ-phase matrix is more than three orders of magnitude greater than that for homogeneous solidification of δ-phase, the γ nucleation on a δ/δ grain boundary in the solidifying matrix suppresses the energy barrier, increasing a nucleation rate. Furthermore, it is found that the γ-phase nucleation on an existing γ nucleus halves undercooling needed with smaller critical radius. This suggests that, once γ nucleation is initiated, then following γ nucleation is promoted by doubled driving force, enabling multiple γ nucleation as in chain reaction. These findings are sufficient to explain experimentally observed phenomena during the δ-γ massive-like phase transformation even if other factors such as solute re-distribution or transformation is neglected.

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© 2015 The Japan Institute of Metals and Materials
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