Ryohei Ozaki

Solidification rate in rapid conduction cooling

The ability of rapid conduction cooling to produce new non-equilibrium phases is examined with particular attention on solidification rate. By matching the rate of solidification calculated numerically from the heat flow balance with the crystal growth rate calculated by a kinetic equation, the kinetic undercooling for a given experimental condition is estimated. A limiting condition of cooling is presented in which the cooling rate is so large that the crystal growth cannot cover the whole sample body before it is cooled down to a low temperature where the growth rate of the crystal is negligibly small.

Crystal growth of the primary silicon in an Al-16 wt % Si alloy

Studies on the crystallographic growth habit of primary silicon crystals in an Al-16 wt% Si alloy were carried out by X-ray micro focus Laue analysis and ECP (electron channelling pattern) analysis.The plate-like primary silicon crystals grow by the same mechanism as that for germanium dendrites, i.e. the TPRE (twin plane re-entrant edges) mechanism.The spherical primary silicon crystal in sodium treated melts is composed of several pyramidal grains with tops at the centre of the sphere. Many of these grains have a twin relation to each other. The sodium enriched regions are found at the boundaries of these pyramidal silicon grains.The external surfaces of the spherical primary crystals exhibit regular crystal facets. The surface facets are most frequently parallel to {111} plane but there are also some facets parallel to other less densely packed planes such as {100}, {211} and so on.

The morphology of α-Fe crystals which grow in the amorphous Fe80(C1−xBx)20 alloys

Crystallization behaviour of amorphous Fe80(C1−xBx)20 alloys, obtained by splat-cooling technique, for x values ranging from zero to unity has been investigated mainly by transmission electron microscopy. The crystalline phase which first appeared in the amorphous matrix was α-Fe for all alloys studied. However, the morphology of α-Fe phase changed from a spherical shape for low x values to a watch-glass shape for intermediate x values and to dendritic for large x values. The nucleation of α-Fe crystals was homogeneous for low x samples while preferred nucleation on edges and surfaces was noted for samples with higher x values. The final volume fraction of α-Fe phase before the appearance of the second crystalline phase increased with the increase in x.

Crystallographic orientation relationships between primary silicon and aluminium crystals

Crystallographic orientation relationships between the primary silicon crystal and the aluminium crystal heterogeneously nucleated on the silicon surface in hypereutectic Al-Si alloys, were studied by the micro-focus X-ray diffraction analysis. The apparently random orientation relationships obtained by X-ray analysis have been classified into simple relationships by taking the twinning in the primary silicon crystals into consideration. The epitaxial relationships between silicon and aluminium crystals in untreated alloys, and that in sodium-treated alloys, are found to be distinctly different.