Mechanism of the Fe3(B,C) and Fe23(C,B)6 solid-state transformation in the hypoeutectic region of the Fe-C-B system

• Published in: Acta materialia Vol. 119; pp. 80 - 91
• Main Authors: Lentz, Jonathan, Röttger, Arne, Theisen, Werner
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.10.2016
• Subjects: Boron; Computer simulation; Electron back scatter diffraction; Fe-C-B; Mathematical models; Melts; Microstructure; Phases; Solid state phase transformation; Steel; Ternary alloys; Transformations; X-rays; Ternary alloys; Boron; Solid state phase transformation; Steel; Fe-C-B
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2016.08.009

This study investigates the microstructural mechanisms involved in the solid-state transformation of the Fe3(B,C) → Fe23(C,B)6 phases in the hypoeutectic region of the iron-carbon-boron (Fe-C-B) system. We analyzed the influence of different initial microstructural characteristics on the Fe3(B,C) → Fe23(C,B)6 transformation with regards to the matrix phase, matrix C content, B/(C + B) ratio, and agglomeration of the parental Fe3(B,C) phase. We performed thermodynamic calculations using the CALPHAD method, validated by laboratory melts with varying B/(B + C) ratios. These laboratory melts were then microstructurally characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and wavelength-dispersive X-ray spectroscopy (WDS). We particularly focused on solid-state transformation of borides and carboborides of type M3(C,B) and M23(C,B)6 in the hypoeutectic region of the ternary system Fe-C-B, investigated via both in situ and ex situ XRD measurements. It was found that the solid-state transformations are influenced by enriched B inside the eutectic structure, a result of solidification. This increased B content is not reduced in solid state due to the kinetic limitations of B and C inside the hard-phase structure. Thus phase stability is subject to local equilibria depending on the local C and B concentration of the hard-phase structure. In this process the Fe23(C,B)6 phase also forms a shell-like structure surrounding the Fe3(B,C) and Fe2B phases. [Display omitted]