Abstract
Formation of microstructure defects at the phase boundaries in TRIP steels was investigated with the aid of microstructure analysis on a TRIP steel crystal, which was grown by the Bridgman technique. The microstructure studies comprised scanning electron microscopy (SEM), X-ray diffraction (XRD), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and transmission electron microscopy with high resolution (HRTEM). Initial XRD measurements revealed that the crystals under study consist of austenite and ferrite with extremely strong preferred orientations. Subsequent XRD pole figure measurements and EBSD scans have shown that the orientation relationship between austenite and ferrite can be described by the Nishiyama-Wassermann model. For a detailed description of the microstructure of the Bridgman crystal, the orientation distribution of crystallites within the individual phases was investigated using the XRD reciprocal space mapping and the rocking curve measurements. These experiments have shown that the density of microstructure defects is much lower in ferrite than in austenite. The direct information about the defect structures at the phase boundaries between austenite and ferrite was obtained from the TEM micrographs, which revealed complicated micro-twin structures at the boundaries between the neighbouring phases. HRTEM discovered very narrow stripes of ferrite embedded in austenite that were regarded as a source of the microstructure defects in austenite.
Highlights
The transformation induced plasticity (TRIP) plays an important role in the microstructure based design of modern steels with high yield strength, see, e.g., [1,2,3]
With the aid of the complete pole figures calculated from the electron backscatter diffraction (EBSD) measurements (Figs. 2 and 3), the mutual orientation of ferrite and austenite was assessed to correspond to the Nishiyama-Wassermann model [8, 9]
This orientation relationship is better visible in the pole figures that were calculated from the EBSD measurements than in the pole figures that were directly measured using X-ray diffraction (XRD)
Summary
The transformation induced plasticity (TRIP) plays an important role in the microstructure based design of modern steels with high yield strength, see, e.g., [1,2,3]. We describe the formation of microstructure defects at the phase boundaries in a TRIP steel crystal which was grown by a Bridgman technique. The principal crystallographic orientations of austenite and ferrite with respect to the sample surface were 〈33-1〉 and approximately 〈001〉, respectively, as obtained from the simulation of the pole figures for various crystal orientations.
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