Crystallographic Stability of Metastable Phase Formed by Containerless Processing in REFeO3 (RE: Rare-Earth Element)

Undercooling a melt often facilitates a metastable phase to nucleate preferentially. Although the classical nucleation theory shows that the most critical factor for forming a metastable phase is the interface free energy, the crystallographic stability is also indispensable for the phase to be frozen at ambient temperature. In compound materials such as oxides, authors have suggested that the decisive factors for forming a critical nucleus are not only the free energy difference but also the difference of the entropy of fusion between stable and metastable phases. In the present study, using REFeO3 (RE: rare-earth element) as a model material, we investigate the formation of a metastable phase from undercooled melts with respect to the competitive nucleation and crystallographical stabilities of both phases

Phase Selection in Undercooled Y 3 Al 5 O 12 Melt

Two kinds of solidification paths from Y 3 Al 5 O 12 melt has been reported; one is stable Y 3 Al 5 O 12 (YAG) garnet, the other is metastable YAlO 3 perovskite (YAP) and subsequent YAP+Al 2 O 3 eutectic. The reason for this, however, has been puzzled. The effect of cooling rate on this phase selection was addressed under containerless condition using an aero-acoustic levitator. A high-speed video camera (HSV) enabled us to directly observe the recalescence behavior. As the cooling rate increased from 15 to 350 K/s, the solidification of a metastable YAP and YAP+Al 2 O 3 eutectic, a monophasic YAG, and an amorphous phase were successively obtained. At around the critical cooling rate of approximately 50 K/s for the formation of YAP and YAG, simultaneous recalescence of YAP and YAG was observed by HSV, and the sample obtained contained both the metastable YAP and stable YAG. The nucleation rate of YAG corresponds with that of YAP at the critical cooling rate and the growth velocity of YAP, which first nucleated in the undercooled melt, was slow enough for YAG to nucleate in the remaining undercooled melt, resulting the simultaneous recalescence. In general, the metastable phase nucleates at the higher cooling rate than the stable phase. However, in this system, the higher nucleation barrier of YAG than that of YAP led to the nucleation of YAG at the higher cooling rate

Metastable Phase Formation from Nd-Dy-Fe-B Undercooled Melt

Abstract Nd 10-x Dy x Fe 85 B 5 (x = 0-3) alloy samples were melted and then solidified in the containerless state of a drop tube at oxygen partial pressure of 10 -1 Pa. The calculated cooling rate of the spherical sample was over 10 3 K/s. The Nd 10 Fe 85 B 5 sample consists of the Nd 2 Fe 17 B x metastable phase together with the α-Fe dendrite. The metastable phase was partially decomposed into small grains of Nd 2 Fe 14 B and α-Fe phases by a solid state decomposition reaction. The substitution of Dy for Nd in the range from 10 to 20 atomic percent was effective to suppress the primary formation of the α-Fe dendrite and to promote the formation of the RE 2 Fe 17 B x metastable phase. When the substitution rate of Dy increased to 30 atomic percent, a large amount of the α-Fe dendrite was formed because an oxide layer of rare earth elements was generated at the sample surface due to the easy oxidization tendency of Dy