Liquid Al₂O₃-CaO-MgO inclusion entrapment at delta ferrite intercellular boundaries during solidification

The behaviour of liquid Al₂O₃-CaO-MgO inclusions at the δ ferrite/melt interface in aluminium killed and calcium treated steel has been observed in situ using a confocal scanning laser microscope equipped with a gold image furnace, Movement of inclusions parallel to the solid/melt front was observed (a) during solidification rates below 1 μnm s^-1 or with nearly static fronts, (b) during remelting of the front or (c) when inclusions at the solid/melt interface were pushed by a newly arriving liquid inclusion. Upon sliding, the inclusions tended to become entrapped in the intercellular regions of the front. After entrapment, a critical solidification velocity for pushing/engulfment was measured and compared with corresponding experimental results for pushing/engulfment at planar fronts. Engulfment at the intercellular boundaries occurred at solidification velocities larger than a critical velocity V_cr = 9.91 × 10^-9R^-1/2 in the present study. The result follows the same trend with respect to inclusion size as that found for planar interfaces, but indicates that the critical velocity is ∼20%. lower. This is attributed to the observation that engulfment at intercellular regions involves replacing a section of the intercellular boundary with the inclusion, which decreases the surface energy change owing to engulfment compared with engulfment at planar fronts.