Fine Structure Characterization of Martensite/Austenite Constituent in Low-Carbon Low-Alloy Steel by Transmission Electron Forward Scatter Diffraction

C.W. Li, L.Z. Han, X.M. Luo, Q.D. Liu, J.F. Gu*

Keywords:Fine structure; martensite; austenite (M/A) constituent; transmission electron forward scatter diffraction (t-EFSD)

Abstract: Transmission electron forward scatter diffraction and other characterization techniques were used to investigate the fine structure and the variant relationship of the martensite/austenite (M/A) constituent of the granular bainite in low-carbon low-alloy steel. The results demonstrated that the M/A constituents were distributed in clusters throughout the bainitic ferrite. Lath martensite was the main component of the M/A constituent, where the relationship between the martensite variants was consistent with the Nishiyama-Wassermann orientation relationship and only three variants were found in the M/A constituent, suggesting that the variants had formed in the M/A constituent according to a specific mechanism. Furthermore, the sigma 3 boundaries in the M/A constituent were much longer than their counterparts in the bainitic ferrite region. The results indicate that transmission electron forward scatter diffraction is an effective method of crystallographic analysis for nanolaths in M/A constituents.

Lay description Martensite/Austenite (M/A) constituents have considerable effect on the strength and impact toughness of low-carbon low-alloy steels. The microstructure and mechanical properties of steels can be controlled by studying the formation and evolution of M/A constituents, which is a research topic of great interest. However, the fine structure of M/A constituents has not been accurately described owing to the limitations of conventional experimental methods. Transmission electron forward scatter diffraction (t-EFSD), a recently developed technique, can be used to characterize the crystallographic relations of different phases. Compared with backscattered electron diffraction (EBSD), t-EFSD shows significantly higher resolution in crystallographic analysis since the incident electron beam only affects a small volume in the thin film. Further, t-EFSD shows higher efficiency than selected area electron diffraction (SAED) in transmission electron microscopy (TEM) owing to regular scanning of the incident electron beam. T-EFSD was introduced in the present study to analyse the fine structure of the M/A constituents in low-carbon low-alloy steel. The results demonstrated that lath martensite was the main component of the M/A constituent, where the martensite variants complied with the Nishiyama-Wassermann (N-W) orientation relationshipsuggesting that the variants formed in the M/A constituent according to a certain variant selection rule. The present work indicates that t-EFSD can be an effective method of crystallographic analysis for M/A constituents.