In situ observation of metastable rare-earth iron garnet formed at the melt/substrate interface by splat quenching

The Nd substitution limit in the NdxSm3-xFe5O12 garnet can be metastably extended from x = 0.375 up to x = 2.0 by splat quenching combined with containerless processing. To understand the rapid solidification process at the melt/substrate interface, a droplet with NdxSm3-xFe5O12 composition (x = 0.43) was dropped by free fall on a Si wafer having relatively high thermal conductivity. The sequence of dropping, impacting, spreading, and solidification of the droplet was monitored using an infrared high-speed video camera through the Si wafer from the bottom. When the droplet impinged on the Si wafer, an amorphous layer was formed at the initial impact. Then, the metastable garnet nucleated on it, resulting in the first recalescence. Subsequently, the second recalescence of the perovskite, which is stable above the peritectic temperature and identified as a high-temperature phase, was observed similar to 3 ms later. This time lag may be ascribed to the considerably low growth rate of the metastable garnet and the difficulty of nucleation of the perovskite on the metastable garnet. This is the first time that the phase transition at the melt/substrate interface during rapid quenching was observed in situ.