Reconstruction of the Three-dimensional Ferrite–austenite Microstructure and Crystallographic Analysis in the Early Stage of Reverse Phase Transformation in an Fe–Mn–C Alloy

Abstract

We have investigated the crystallographic orientation relationship between ferrite and austenite and the interfacial planes between them of a low-carbon steel formed by the transformation from ferrite to austenite upon heating. Three-dimensional investigation using EBSD measurement with FIB serial sectioning technique is carried out on samples quenched from an early stage of the reverse transformation. The prior austenite orientation of martensite in the quenched microstructure is determined based on an analysis of the variants in the Kurdjumov-Sachs (K-S) relationship and the three-dimensional ferrite-austenite microstructure is successfully reconstructed. The crystallographic analysis on the three-dimensional microstructure has revealed that the nucleus of reverse-transformed austenite maintains the K-S relationship or close to it with two or three adjacent ferrite grains. The significance for reverse-transformed austenite to select these specific orientation relationships was estimated through a statistical assessment. In addition, the influence of crystallographic relationship between ferrite and austenite to nucleation and growth of reverse transformation is discussed.

Three-dimensional EBSD Analysis and TEM Observation for Interface Microstructure during Reverse Phase Transformation in Low Carbon Steels

“…Growth during Reverse Transformation The microstructure change of the Fe-0.2C alloy owing to ferrite to austenite transformation from 730 to 840°C was observed using in situ EBSD measurement in the same way as the previous study 31) to confirm the repeatability. The grain-growth of austenite with increasing temperature of the sample is indicated in Figs. …”

“…30,31) The orientation of the reverse transformed austenite satisfies the orientation relationships close to the K-S relationship (within 6°) with one or two ferrite grains. Furthermore, another neighboring interface is still close to the K-S relationship within 12°.…”

“…The typical local areas including a martensite region and the adjacent ferrite grains were investigated using 3D EBSD measurement in the sample quenched from 730°C. The procedure for the 3D reconstruction of the crystal structure is similar to that in the previous paper, 31) and one example is indicated by a series of cross sectional maps in Fig. 3.…”

“…The details of the analysis method were described previously. 31,[38][39][40][41][42][43][44] In our previous study, 31) the misorientation between the reconstructed austenite grain and each surrounding ferrite grain was investigated. In the present analysis, we focus on the characteristics of crystallography of interfaces itself.…”

“…[20][21][22][23][24][25][26][27][28][29][30][31] Lischewski et al observed the α→γ phase transformation in the C-Mn steel containing microalloying elements such as Niobium by in situ electron backscattering diffraction (EBSD) measurement, and analyzed the orientation relationship between the parent ferrite and transformed austenite grains. 28,29) They found that a considerable number of transformed austenite grains are related to two adjacent mother grains by the approximate K-S relationship and proposed that the variant selection rule is due to the preferential nucleation of the K-S variant with low interfacial energy at a grain boundary.…”