120 Prakt. Met. Sonderband 30 (1999)
a-Ni solid solution phase (3). The decrease of dendrite solidification velocity at highest achieved
undercooling levels could be ascribed to the temperature dependent diffusion coefficient in the melt
(9) in the same way as in the theory of eutectic solidification (10, 11).
3.2 Microstructure and phase analysis of as-solidified samples
Structure and composition of the respective phases cannot be inferred from in situ measurements of
recalescence behavior but must be determined in the as-solidified samples itself. From the equilib-
rium phase diagram (5) (compare Fig. 1) we expect a two-phase microstructure of a-Ni and ßı-
Ni3Si (or ß3-Ni3Si) for the eutectic melt composition Ni-21.4 at.% Si. Indeed, no trace of additional
metastable phase was observed in as-cast ingots. Gas cooled moderately undercooled samples (AT =
70 K) exhibit the lamellar two-phase microstructure with lamellar spacings of A ~ 1.0 pm along
with areas of an anomalous eutectic microstructure.
af +I
EN] mss
AN AT
ARLE
_ sn]
Fig. 3a-d: Cross-sectional SEM-micrographs of gas-cooled and substrate-quenched eutectic Ni-
21.4 at.% Si undercooled drops. (a) fine-grained anomalous eutectic microstructure near the sample
surface and (b) coarse anomalous eutectic microstructure in the centre of a gas-cooled sample with
AT = 200 K; (c) substrate-quenched sample with AT = 235 K showing a narrow layer of the me-
tastable phase (greyish appearance) in the vicinity of the substrate contact (bottom) and (d) the de-
composed two-phase microstructure in 1 mm distance from the melt-substrate interface
