杉山 明

The microstructure evolution in ductile cast iron with magnesium addition was observed in situ by using X-ray radiography (two-dimensional observation) and time-resolved tomography (three-dimensional observation) in the BL20XU of a synchrotron radiation facility, SPring-8 (Hyogo, Japan). In the two-dimensional observation, graphite nodules nucleated in the melt and floated up immediately after nucleation. The floating was terminated by engulfment of graphite nodules into austenite dendrites. The radiography indicated that the average floating distance was shorter than the dendrite arm spacings in the 100-μm-thick specimen. Because the short distance could be influenced by the sample confinement, time-resolved tomography was performed by using a pink X-ray beam in the BL28B2 of SPring-8. Graphite nodules that nucleated in the melt (probably on magnesium–oxygen–sulfur inclusions in the melt) floated and were engulfed by austenite dendrites within several seconds, even in the bulk specimen. Although the average distance in the bulk specimen was approximately twice as large as that in the 100-μm-thick specimen, floating after nucleation and engulfment into austenite dendrites within a short duration were observed commonly from both techniques. The sequence of nucleation and engulfment had a critical effect on the number and size of the graphite nodules.

© 2018 Trans Tech Publications, Switzerland. Time-resolved and in-situ observations using synchrotron radiation X-rays were performed to observe solidification of cast iron (CE=4.5, 0.02mass%Mg). Morphology of graphite particles was influenced by specimen holder material. In the Al2O3 holder, graphite particles were spheroidal at the beginning and then deviated from the spheroidal shape. In addition, the coupled eutectic solidification of austenite and graphite occurred at the final stage. In contrast, the divorced eutectic solidification, in which graphite particles and austenite dendrites independently grew, was selected until the end of solidification in MgO holder. Spheroidal graphite particles were engulfed by austenite. Consequently, typical microstructure observed in ductile cast iron was reproduced in the in-situ observation. The results suggested that oxygen potential, which was determined by Al2O3 or MgO (specimen holder) in the observations, could be an important factor for the selection of eutectic growth mode and graphite morphology.