INFLUENCE OF COHERENT AUSTENITE TWIN BOUNDARIES ON THE VARIANT SELECTION OF BAINITIC FERRITE IN Fe-C-Mn-Si STEELS
WANG Xixia, GUO Hui, WANG Ding, BAI Yin, YANG Shanwu, HE Xinlai
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083
Cite this article:
WANG Xixia, GUO Hui, WANG Ding, BAI Yin, YANG Shanwu, HE Xinlai. INFLUENCE OF COHERENT AUSTENITE TWIN BOUNDARIES ON THE VARIANT SELECTION OF BAINITIC FERRITE IN Fe-C-Mn-Si STEELS. Acta Metall Sin, 2012, 48(4): 385-392.
Abstract To develop higher performance steels the requirements of the control of the microstructure is increasingly enhanced. The crystallographic orientation characteristics in the microstructure attract more and more attention because the mechanical properties are usually determined by the grain boundary density, especially high angle grain boundary density. So far researchers have conducted comprehensive investigation of the variant selection during the phase transformation. To decrease the nucleation barrier, the new phase usually chooses a specific orientation (variant) to decrease the interfacial energy between the parent and new phases. The variant selection occurs on the grain boundaries has been well studied. The nucleation and variant selection on austenite twin boundaries, however, are seldom reported, especially in the low carbon steels in which austenite can hardly be retained to the room temperature. In this work, the variant selection of bainitic ferrite on coherent austenite twin boundaries was studied using electron back scattering diffraction (EBSD) in Fe-C-Mn-Si steels with 0.05%C or 0.4%C (mass fraction). The orientation relationship between bainitic ferrite and austenite is close to K-S relationship in both steels. It was observed that the variant pairs with the similar crystallographic orientation nucleated on both sides of austenite twin boundaries. The twin grain boundaries were erased after the bainitic ferrite variants grew up. Crystallographic analysis showed that no more than three pairs of variants could be formed on one austenite twin boundary, the habit planes of which were all parallel to the twin boundary. As a result, the bainitic ferrite variant nucleated first would grow and expand along the twin boundary. In 0.05C steel, only one pair of variants was observed on the austenite twin boundary because the first nucleated bainitic ferrite variant pair grew fast due to the low carbon content which covers the twin boundary very soon and leaves no chance for the other variant pairs to nucleate on this twin boundary. In 0.4C steel, all three pairs of variants could be formed on one twin boundary because the higher carbon content slowed down the growth of first nucleated bainitic ferrite and more variant pairs could be nucleated on the twin boundary.
