Massive Formation of Equiaxed Crystals by Avalanches of Mushy Zone Segments
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I.
INTRODUCTION
MANY common alloys solidify by forming an outer equiaxed zone, followed by a columnar zone and a more-or-less extended inner equiaxed zone.[1,2] The grains of the outer equiaxed zone form in contact with the mold wall by heterogeneous nucleation, and with the columnar dendritic zone by growth competition from the outer equiaxed zone.[3] In the interior of the casting, equiaxed crystals form by either heterogeneous nucleation[4] or fragmentation of dendrites from the columnar zone.[5–7] However, both mechanisms reveal same uncertainties. For instance, the origin of a heterogeneous site that reduces the energy barrier for nucleation is often unknown, especially for non-inoculated alloys. Conversely, the proposed criteria for the occurrence of fragmentation of dendrite arms[5,7] can hardly be applied due to the complex overall interdendritic flow caused by forced or natural convection or deformation of the dendrite skeleton. In the present study, observations are reported which show that relatively large areas of vertical columnar zones—especially at the upper part of the casting mold—may slide downward and form crystal avalanches. These avalanches consist of thousands of dendritic fragments from which equiaxed crystals grow. In the framework described above, this can be seen as substantial fragmentation and thus massive formation of equiaxed crystals.
A. LUDWIG, M. STEFAN-KHARICHA, A. KHARICHA, and M. WU are with the Montanuniversitaet Leoben, Department of Metallurgy, Chair for Simulation and Modelling of Metallurgical Processes, 8700 Leoben, Austria. Contact e-mail: [email protected] Manuscript submitted August 12, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
II.
EXPERIMENTAL PROCEDURE
A relatively large container (0.6 m tall, 0.4 m wide and 0.06 m thick) was filled with a 29.6 wt pct ammonium chloride—water solution. The given measures are internal dimensions. The lateral walls are made of brass, the bottom plate is made of aluminum, and the front and back walls are made of commercial PMMA (Polymethylmethacrylat) plates. The top of the cell was left open. The temperature of the brass walls was controlled via a circulation bath by applying an exponential cooling curve as T(t) = Ta exp( at) + Tinf. with Ta = 47 K, a = 0.033, and Tinf. = 279 K (6 °C). For the experiment reported in this work, we started with an alloy at a temperature of T0 = 325 K (52 °C), which was then cooled down to Tinf. = 279 K (6 °C) via the side walls. Note that the interdendritic eutectic in the ammonium-water system forms at around 253 K ( 15 °C). Thus, even by reaching the minimal cooling temperature some interdendritic melt in the mushy zone remains liquid. The ammonium chloride—water solution was prepared directly in the container by mixing ammonium chloride powder with distilled water at 325 K (52 °C). After the well-stirred alloying, we waited typically 1 hour to equilibrate the temperature field before cooling. During this time, the small, but continuous, heat loss through the front and back window resulted i
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