Ferrite precipitating around the graphite nodules shaping the typical bull’s-eye microstructure could grow under negligible partitioning local equilibrium or under paraequilibrium conditions, as both imply that ferrite inherits the composition of the parent austenite. The first mechanism has been rejected by other researchers by means of simple calculations of the silicon spike width necessary for local equilibrium conditions to take place. Nevertheless, experimental analyses are necessary to verify this conclusion. In this study, transmission electron microscopy has been used to assess the presence of a silicon spike in front of the growing ferrite interface. The outcome allowed the authors to confirm that a paraequilibrium mode governs the transformation, supporting the conclusions of previous calculations. In addition, some issues about ferrite growth modeling are discussed.
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The authors would like to thank the Sánchez and Piccioni Company for providing them its facilities to carry out the casts and Dr. Tolley (CAB-CNEA) for his help with the TEM measurements. L.N. García, F.D. Carazo, and R.E. Boeri are members of CONICET and would like to thank the institution for the economic support for their respective researches. L.N. García would like to especially thank the SECITI (Secretaría de Estado de Ciencia, Tecnología e Innovación del Gobierno de San Juan) and the INP school for the economic support for her stay at the CIRIMAT laboratory at Toulouse, France. Finally, L.N. García would like to deeply thank Prof. Jacques Lacaze for transmitting his knowledge about Thermo-Calc software and for enriching the discussions of this research.
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Manuscript Submitted July 19, 2019.
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García, L.N., Carazo, F.D. & Boeri, R.E. About Equilibrium Mode Ruling Ferritic Transformation in Low-Alloy SGI. Metall Mater Trans A 50, 5585–5593 (2019). https://doi.org/10.1007/s11661-019-05485-6