Kosuke Nagashio

In situ identification of the metastable phase during solidification from the undercooled YFeO3 melt by fast x-ray diffractometry at 250Hz

A time-resolved x-ray diffraction (XRD) experiment at 250Hz using a synchrotron radiation source was carried out during the containerless solidification of ReFeO3 (Re=Y and Lu) in order to identify the metastable phase in situ. The metastable phase solidified primarily from the undercooled YFeO3 melt finally transformed to the stable orthorhombic YFeO3 phase during the short period of recalescence (∼0.035s). Although the metastable phase could not be detected in the as-solidified sample by the powder XRD, the in situ diffraction pattern of the metastable phase in the YFeO3 system was consistent with that of the metastable hexagonal LuFeO3 phase.

Real-time x-ray observation of solidification from undercooled Si melt

Grain refined microstructure is often obtained spontaneously in the solidification of metals and semiconductors from the undercooled melt without any external forces. Although it has been reported that the grain refinement is mainly caused by the fragmentation of the dendrites, the dynamic process of the fragmentation of dendrites has not been fully understood because only the microstructure after the solidification has been analyzed. Here, we present a time-resolved two-dimensional x-ray diffraction experiment on the solidification of Si from the undercooled melt. The number of diffraction spots observed at low undercoolings (ΔT<100K) did not increase at the plateau stage, while the diffraction pattern at medium undercoolings (100K<ΔT<200K) changed from the spots with the tail to rings with the lapse of time. Both this result and high speed video imaging suggested that the high-order arms of the dendrites mostly detached from the main stems because nucleation could not be expected at the melting point af...

Dynamic process of dendrite fragmentation in solidification from undercooled Si melt using time-resolved x-ray diffraction

The spontaneous dendrite fragmentation in solidification from undercooled Si melt was analyzed by time-resolved two-dimensional x-ray diffractometry. For the sample solidified at ΔT=261K, several spots appeared at 1ms after recalescence and the subsequent transition from spots to rings occurred within ∼25ms, which suggests that the fragmentation occurred after recalescence but just at the initial stage of the plateau period. Although the present experiment supported that the driving force for the fragmentation is a capillarity effect at the plateau period, the time scale for the fragmentation differed from the current quantitative prediction.

Real-time x-ray diffraction of metastable phases during solidification from the undercooled LuFeO3 melt by two-dimensional detector at 1 kHz

In-situ identification of metastable phases formed from the undercooled LuFeO3 melt under controlled oxygen partial pressure Po2 was studied by x-ray diffraction measurements at a synchrotron radiation source. Real-time observation of the formation and growth of individual phases during the single recalescence of Lu3Fe2O7 and LuFe2O4 phases at Po2 of 1u2009×u2009103u2009Pa has been revealed by a high speed imaging system at 1u2009kHz. The obtained diffraction pattern of the metastable phase in the LuFeO3 system was consistent with that of the metastable and stable phases reported in the Lu-Fe-O system.