Decoupled versus coupled growth dynamics of an irregular eutectic alloy

Abstract

We present an experimental study of irregular growth patterns observed in real time during thin-sample directional solidification of a faceted/nonfaceted eutectic alloy, namely, the transparent 2-amino-2-methyl-1,3-propanediol (AMPD)-succinonitrile (SCN) system. The body-centered cubic SCN crystals are nonfaceted, while monoclinic AMPD crystals grow as faceted needles. At low velocities (<0.3 µms -1 ), a decoupled growth regime is observed, during which the tip of the AMPD crystals grows ahead of the SCN-liquid interface. At intermediate velocities, an unsteady coupled-growth regime takes place, with intermittent pinning of triple SCN-AMPD-liquid junctions, and frequent noncrystallographic branching. At higher velocities (>1 µms -1 ), two-phase fingers form.Many irregular eutectic alloys of industrial importance are such that one of the solid phases is faceted (e.g., silicon, germanium, graphite), while the other one (e.g., aluminum, austenite)is nonfaceted [1][2][3]. In spite of extensive research, the growth of such faceted/nonfaceted (or f/nf) eutectics is still poorly understood. The term irregular refers to the disordered microstructures left frozen in the solid behind the growth front, whereas regular eutectics grow with fully nonfaceted solid-liquid interfaces. The growth dynamics of regular eutectics is determined by solute diffusion in the liquid, and local equilibrium at the solid-liquid interfaces, and at the triple-contact lines (trijunctions) between the liquid and the two solids [4]. During directional solidification (DS) at velocity V in a temperature gradient G, steady periodic two-phased patterns can then form in a quasi-planar geometry at a temperature close to the eutectic point. The diffusive coupling between the growing entities (e.g., lamellae or rods) extends over distances much larger than the interphase spacing l -hence the coupledgrowth appellation. In contrast, the unsteadiness of the growth dynamics of f/nf eutectics is attributable to a nonlinear growth kinetics of the faceted interfaces. A facet can remain macroscopically immobile irrespective to the diffusion field over a finite range of undercoolings ("blocked" facet). The mobility of a facet is tributary of active sources of

In-situ investigation of the solidification dynamics in an irregular eutectic alloy

“…We used a model organic transparent alloy system and in-situ observations to investigate the growth dynamics of irregular eutectic alloys. V-ramp imposed by the RDS experiments enabled us to further investigate the various microstructures obtained at different velocities during DS experiments [3] at the same solidification front. Observing all the three previously reported microstructures at the same solidification front confirms that for a given alloy composition, both the microstructure and the growth dynamics, i.e.…”

“…Despite their practical importance, scientific studies on irregular eutectics remain essentially descriptive, and the simultaneous growth of faceted and nonfaceted crystals in an irregular eutectic alloy has been rarely addressed with the aim of highlighting the solidification processes that would not be specific to the chemistry of the alloy. In a recent paper [3], we showed that a faceted/nonfaceted transparent alloy, i.e. Amino-methyl-propanediol/Succinonitrile (AMPD-SCN) at near-eutectic concentration, can be used as an experimental model system for exploring the growth dynamics of irregular-eutectic microstructures in situ.…”

Mikroyapı Gelişiminin Temellerini Gerçek Zamanlı Olarak İncelemek için Mikroskop ve Katılaştırma Ekipmanı Tasarımı

“…Bu nedenlerden ötürü, Mikroyapı Evrim Laboratuvarında (MEL), katılaşma dinamiklerini gerçek zamanlı incelemek için optik mikroskop kullanılmıştır. Bu yöntem halihazırda çoğunluğu Amerika ve Avrupa'da bulunan birçok üniversitede ve araştırma merkezinde kullanılmaktadır [6,7,[14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]. Bu yöntemin yaygın olarak kullanılma sebepleri ise (i) Optik mikroskop ve katılaşma sisteminin diğer yöntemlerden çok daha ucuz ve kolay erişilebilir olması, (ii) Numune eninin X-ışını nanotomografisine göre onlarca kat daha büyük olması ve böylelikle istatiksel olarak geçerli gözlemlerin ve ölçümlerin yapılabilmesi (iii) Elde edilen çözünürlüğün X-ışını mikro-tomografiden çok daha iyi ve nanotomografi seviyelerinde olması ve (iv) Hem organik hem de metalik numunelerin incelebilmesidir.…”