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Time-resolved X-ray imaging of dendritic solidification for pure Fe and carbon steels with sufficient spatial and time resolutions has been developed for the first time by overcoming essential problems in low contrast between solid and liquid phases and in high melting temperatures. Static observation showed that the solid/liquid interface in pure Fe specimen was determined by the absorption contrast at photon energy ranging from 16 to 30 keV. In addition, the phase contrast was also observed in the vicinity of the interface. Dynamic observation showed that cellular growth in pure Fe specimen was observed at a growth velocity up to 400 mm/s. Feasibility observation was also performed for two different carbon steels (0.0025 mass% C and 0.45 mass% C). Growing dendrites were observed in-situ at a growth velocity up to 500 mm/s. This study proves that the developed imaging enabled to observe solidification phenomena in-situ for various kinds of steels.KEY WORDS: in-situ observation; radiography; synchrotron radiation; dendrite.of carbon steels by X-ray imaging will be concluded. Based on the requirements, the paper explains an X-ray imaging developed by this study and demonstrates in-situ observation of solidification for pure Fe. Feasibility observations for carbon steels were briefly presented. Preparation of X-ray Imaging Requirement for Transmission X-ray ImagingImportant parameters for developing X-ray imaging using the absorption contrast are intensity of transmission X-ray through a specimen and contrast between different phases. One estimates the intensities through Fe-C alloys and pure Fe for building X-ray optics and for choosing dimension of specimen. In general, intensity of transmission X-ray, I, is expressed by (m/r) and r are the mass X-ray absorption coefficient and the mass density, respectively. The suffix, i, indicates constituent element species. r¯is average density and w i is mass fraction of the constituent element, i. The mass X-ray absorption coefficient, (m/r), which is a function of photon energy, is uniquely defined by the element.The linear X-ray absorption coefficient of Fe-C alloys is given byThe mass X-ray absorption coefficient of C is 0.044 m 2 /kg at 20 keV while that of Fe is 2.57 m 2 /kg at 20 keV. 27) Since the mass fraction of C is mostly less than 0.01 in conventional carbon steels, the mass X-ray absorption coefficient and the density of Fe dominantly determine the linear X-ray absorption coefficient of the Fe-C specimen.Conventional carbon steels contain Mn as well. The mass X-ray absorption coefficient of Mn is 2.25 m 2 /kg at 20 keV. 27) Since the partition coefficient of Mn in Fe matrix is approximately 0.75, difference in Mn concentration between solid and liquid phases is estimated to be 0.1-0.3 mass% for a specimen with average Mn concentration of 0.3-0.9 mass%. Change in the linear X-ray absorption coefficient is evaluated by the same procedure as for Fe-C system. The solute redistribution gives only 0.01-0.03% change in the linear X-ray absorption coefficient. The ch...

Dendrite arm fragmentation is considered in solidification structure tailoring. Time-resolved and in situ imaging using synchrotron radiation X-rays allows the observation of dendrite arm fragmentation in Fe–C alloys. Here we report a dendrite arm fragmentation mechanism. A massive-like transformation from ferrite to austenite rather than the peritectic reaction occurs during or after ferrite solidification. The transformation produces refined austenite grains and ferrite–austenite boundaries in dendrite arms. The austenite grains are fragmented by the liquid phase that is produced at the grain boundary. In unidirectional solidification, a slight increase in temperature moves the ferrite–austenite interface backwards and promotes detachment of the primary and secondary arms at the δ–γ interface via a reverse peritectic reaction. The results show a massive-like transformation inducing the dendrite arm fragmentation has a role in formation of the solidification structure and the austenite grain structures in the Fe–C alloys.

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