Nucleation and Dendritic Growth in Undercooled Melts
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C050Si5O. They allow for a detailed study of solute and disorder trapping during rapid solidification of deeply undercooled metals. These mechanisms are crucial for the solidification of metastable supersaturated solutions and disordered intermetallic phases. Metastability is also present in grain refined materials. The excess free energy of the undercooled melt can be used by the system to build up very fine grains in diameter of 1 ýtm or even less. Finally, experiments will briefly be introduced in which metallic melts of C080Pd 20 are undercooled to, or even below, their magnetic transition temperatures. The liquid metal behaves as a material with ferromagnetic order. EXPERIMENTAL DETAILS Samples having a mass of about 1g were prepared from the constituents of
purity better than 99.99% by premelting in an induction furnace into spheres of about 6mm. Undercooling conditions were established by the application of the melt fluxing and the electromagnetic levitation technique. The experiments were performed under high-purity environmental conditions. Temperatures were measured contactlessly by pyrometry. For further details of the experimental methods and a review of the application of containerless processing in the study of undercooled melts see reference [2]. NUCLEATION OF QUASICRYSTALLINE AND METASTABLE PHASES Nucleation into a specific crystallographic phase is characterized by an activation energy AG* to form a nucleus of critical size in the undercooled melt. The nucleation barrier arises from the interfacial energy (7,1 between the crystal nucleus and the undercooled melt. According to classical nucleation theory [3] and to the negentropic model by Spaepen [4] for the estimation of cysl, AG* reads: AG*=
3 16ra . - f(0)
3iAG
a, =a
with
si
AS1
T
(1)
SN7,'
f(0) is a catalytic potency factor in the case of heterogeneous nucleation, AG the Gibbs free energy difference between the solid and the liquid phase, ASf the entropy of fusion, NA the Avogadro number, Vm the molar volume, T the temperature, and (xs a factor depending on the structure of the nucleus with numerical values axs=0.71 for a bcc, and xs=0.86 for an fcc or hcp structure. Hence, the barrier for nucleation, i.e. the interfacial energy Gsl, depends on the structure of the nucleus. cysl is smaller for systems with similar short-range order of the undercooled melt and the nucleus are. Already Frank has pointed out that an icosahedral short-range order should be energetically favored in undercooled melts of metals and metallic alloys [5]. Hence, the energy barrier between an undercooled melt and a nucleus of a crystallographic phase with polytetrahedral symmetry should be small in comparison to crystalline phases. The interfacial energy is often determined by measuring the maximum undercooling attainable for a melt [6]. Undercooling experiments on alloys which 16
form quasicrystalline phases have been performed using the electromagnetic levitation technique [7]. Figure 1 shows two temperature-time profiles obtained from experiments on an A1
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