Three-Dimensional Growth of Coherent Ferrite in Austenite: A Molecular Dynamics Study

doi:10.1007/s40195-019-00889-0

Abstract

Abstract:

Coherent second phase often exhibits anisotropic morphology with specific orientations with respect to both the second and the matrix phases. As a key feature of microstructure, the morphology of the coherent particles is essential for understanding the second-phase strengthening effect in various industrial alloys. This letter reports anisotropic growth of coherent ferrite from austenite matrix in pure iron based on molecular dynamics simulation. We found that the ferrite grain tends to grow into an elongated plate-like shape, independent of its initial configuration. The final shape of the ferrite is closely related to the misfit between the two phases, with the longest direction and the broad facet of the plate being, respectively, consistent with the best matching direction and the best matching plane calculated via the Burgers vector content (BVC) method. The strain energy calculation in the framework of Eshelby’s inclusion theory verifies that the simulated orientation of the coherent ferrite is energetically favorable. It is anticipated that the BVC method will be applicable in analysis of anisotropic growth and morphology of coherent second phase in other phase transformation systems.

Key words: Coherent, ferrite, Anisotropic, growth, Molecular, dynamics, simulation, Burgers, vector, content, Eshelby's, inclusion, theory

Zhi-Peng Sun, Fu-Zhi Dai, Ben Xu, Wen-Zheng Zhang. Three-Dimensional Growth of Coherent Ferrite in Austenite: A Molecular Dynamics Study[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(6): 669-676.

Figures/Tables 4

Fig. 1 Initial configuration of two initial embryos (designated by P), defined in both ferrite and austenite phases with an N-W OR: P1 a-c and P2 d-f. a, d Contain the dichromatic pattern of overlapped atoms projected from \([1\bar{1}0]_{\gamma }\), while b, e from \([112]_{\gamma }\). In these figures, FCC and BCC atoms are represented with solid and hollow symbols, respectively. Symbols with different shapes stand for atoms in different layers of the crystal plane. In d, the dichromatic pattern exists only in the region of the embryo for clarity. c, f Show the overall configuration of the simulation cell, where the atoms in the left half region are removed for clarity. The crystal structures in c, f are identified by the adaptive common neighbor analysis in OVITO [49]. Green: FCC atoms; Blue: BCC atoms; Red: fixed boundary atoms; White: interfacial atoms

Fig. 2 MD snapshots illustrating the process of ferrite growth with the initial configuration P1 a-d and P2 e-h; i, j zoomed pictures of atoms inside the dashed boxes in d, h, respectively, showing the detailed interfacial structure of the habit plane. Atoms with different colors have the same meaning with that in Fig. 1

Fig. 3 Comparison between a the final morphology of the ferrite in the present MD simulation, b the \(\alpha\) ellipsoid with the unit displacement calculated via the BVC method

Fig. 4 Variation of strain energy density with respect to different orientations of the ellipsoid ferrite by the rotation around a\({v}_{ 3}\) (with the rotation angle \(\theta\)) and \({v}_{1}\) (with the rotation angle \(\phi\)); b\({v}_{ 3}\) and \({v}_{2}\) (with the rotation angle \(\psi\))

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