Nucleation-controlled microstructures and anomalous eutectic formation in undercooled Co-Sn and Ni-Si eutectic melts

Free Solidification of Undercooled Eutectics

“…30) We would like emphasize that this kind nucleation behavior can be confirmed not only in the Figure 1(c) reveals the surface microstructure of the binary Ni-18.7 at% Sn metallic eutectic solidified on the electromagnetic levitator (EML) at an undercooling of around ÁT ¼ 100 K, which consists of separate eutectic colonies with clear contacting boundaries as well. 33) Meanwhile, we have levitated and solidified other binary metallic eutectics, e.g., Ni-21.4 at% Si, Co-20.5 at% Sn, 34) Co-23.1 at% Si, Co-25.5 at% Ge, and Co-25.5 at% Sb eutectics at various melt undercoolings, and confirmed that copious nucleation occurs in all of these alloys when solidified containerlessly from low undercoolings to high undercoolings attainable. Different from a single-phase alloy with a dendritic morphology that can propagate throughout the entire sample from the seeding area, we never found a eutectic colony that grows throughout the entire bulk sample to reach the opposite side of the nucleation area.…”

“…In order to describe the occurrence of decoupled growth, we 34) recently presented a semi-quantitative model to consider the attachment kinetics of competing phases, in which a dimensionless undercooling is defined. It is worth noting that in this model, we considered the growth kinetics of eutectic phases separately, rather than treating two eutectic phases as a basic growth unit as in traditional eutectic theories.…”

“…Although we have investigated systematically the free solidification behavior of various undercooled eutectics from the viewpoints of nucleation and growth, there are still some open questions that are subject further clarification, as we 34) have addressed when discussing the origin of decoupled growth. Besides those questions, future work may be required to clarify other issues that are detailed as follows:…”

“…In our semi-quantitative model, 34) we defined a critical condition under which decoupled growth occurs when relating the melt undercooling and a time scale. Theoretically, decoupled growth can take place provided that the growth period is sufficiently long so that a nonfacetted phase may overgrow the competing facetted phase even at a very low undercooling.…”

“…However, this critical undercoolings upon the formation of anomalous eutectic in nonfacettedfacetted metallic eutectics are always around ÁT ¼ 50 K in practice, as we have referenced in many systems. 34) Further effort may be necessary to clarify whether or not there is a specified time scale for the occurrence of decoupled growth, which is essential in developing a quantitative model to interpret the state-of-art results. (3) On coarsening of anomalous eutectic.…”