Reconstruction of three-dimensional ferrite-austenite microstructure and crystallographic analysis in an early stage of alpha-gamma phase transformation in Fe-Mn-low C alloy
For the development of advanced steels, control of austenite formation in the phase transformation upon heating is essentially important since microstructure in high strength steel comprising martensite and bainite is greatly influenced by the amount and morphology of austenite grains at high temperature. Crystallographic relationship between ferrite(a) and austenite(g) specifies coherency of a/g interface and influences the behavior of austenite nucleation and growth during the phase transformation. In our work, the crystallographic relationship between a and g in an early stage of the phase transformation upon heating in low carbon steel was investigated by 3D EBSD analysis using FIB serial sectioning technique. In order to investigate three-dimensional a-g microstructure, the sample was produced by quenching from a-g temperature region (730 °C) and the crystal orientation of transformed g was determined from the martensite using the analysis method of parent grain orientation, which was proposed by Humbert et. al[1]. Crystallographic analysis in the 3D microstructure has provided lots of information about the orientation relationship and grain boundary plane between a and g. Most of the initial g grains are transformed at the grain corner in a matrix. The transformed g grain holds the K-S relationship or those close to it with two or three adjacent a grains and the boundary plane is in the vicinity of {011}a (// {111}g). Under in situ heating EBSD observation of g grain growth at 790 °C, the migration of such grain boundaries is significantly restricted. On the basis of these findings, the behavior of g formation during the a-g phase transformation will be discussed.
[1] M. Humbert, H. Moustahfid, F. Wagner and M. J. Philippe: Scripta Metall. Mater., 30 (1994), 377
Dr. Kengo Hata
Advanced Technology Research Laboratories
Nippon Steel & Sumitomo Metal Corporation (NSSMC)
1-8 Fuso-Cho, Amagasaki, Hyogo Prefecture 660-0891
Japan
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