Free Solidification of Undercooled Eutectics

Abstract

The traditional eutectic solidification theories always stress the role of eutectic growth and morphology selection. However, when a eutectic alloy solidifies freely from an undercooled state, the first step commences from nucleation and then follows eutectic growth. This paper reviews the free solidification behavior of various undercooled eutectic alloys from the viewpoints of nucleation and growth. It is realized that an independent eutectic colony should be the basic unit when discussing eutectic solidification rather than a bulk sample, which urges us to reexamine the previous discussions and conclusions that were based on bulk solidification in which the role of nucleation was frequently ignored. Regarding to the eutectic growth after nucleation, particular emphasis is devoted to shedding light upon the physical mechanisms of the anomalous eutectic formation when the eutectic is undercooled beyond a certain critical undercooling. The interface attachment kinetics of terminal phases in a eutectic reaction is of great importance in yielding decoupled growth between a mobile interface and a sluggish interface. Further growth behavior after decoupled growth occurs is investigated when the effect of crystallization heat on solidification is considered. Future work is briefly directed to improve current understanding on the free solidification from undercooled states.

Microstructural verification of the theoretically predicted morphologies of the NiAl–Cr pseudo-binary alloy systems and NiAl–Cr eutectic structure modification by Mo addition

“…As the growth velocity slows, due to recalescence released heat in front of the solid-liquid interface, the lamellar spacing increases and the fine lamellas become coarser at the end of solidification. Similar coarsening has been reported by other works as well [28,35,36]. For the alloys F, G and H, no rod or fibrous like Cr solidification has been observed.…”

“…In such manner, alternating A2-B2 phase configurations are formed. Similar phenomena have been observed in various research efforts dealing with regular and anomalous eutectic solidification [25][26][27][28][29][30][31]. Further recalescence causes temperature increase in the remaining liquid phase and eutectic growth commences.…”

NiAl-Cr-Mo Medium Entropy Alloys: Microstructural Verification, Solidification Considerations, and Sliding Wear Response

“…The alterations of the eutectic morphology observed in this system are also attributed to recalescence phenomena: As soon as temperature rises above the T0-B2 point, the undercooling for eutectic growth is considerably high. High undercooling favors rod-or hive-like (commonly referred as fibrous) eutectic morphologies [47]. Taking into consideration the postulates of Li and Kuribayashi [47], according to which the eutectic constituent commences at the central areas of the remaining liquid, the hive-like eutectic morphology observed in the central areas of the remaining liquid observed in the present effort are, hence, expected.…”

Sunflower-like Solidification Microstructure in a Near-eutectic High-entropy Alloy

“…According to Li and Kuribayashi [22] when discussing the free solidification behavior of undercooled eutectics, a single eutectic colony shall be the basic unit rather than the bulk. This methodology was adopted here to study the eutectic structure.…”

“…To the best knowledge of the authors, the most similar structures are only seen in some NiAl-Cr(Mo) hypo-eutectic alloys, [26,30] which are also essentially equiatomic ternary alloys doped with small amounts of alloying elements such as Mo, Ti and rare-earth elements. To achieve the universal sunflowerlike solidification microstructure as seen in Figure 1(a), all the following four conditions need to be satisfied simultaneously: (1) the existence of copious nucleation sites for the primary phases; [22] (2) the almost isotropic growth of the primary phase; (3) the eutectic lamellae growing normal to the phase boundary of the primary phase, hence forming a radial pattern; and (4) the primary phase decomposing to form copious discontinuous particles at lower temperatures. Certainly, it is highly demanding to obtain the perfect sunflower-like microstructure, in terms of the alloy compositions [2,21] and solidification conditions.…”