Prakt. Met. Sonderband 30 (1999) 121 
. hieved However, melt drops of approximately the same undercooling level quenched on a solder-coated 
he me) copper substrate displayed a weakly segregated layer of greyish appearance adjacent to the sub- 
strate. Its composition 24 at.% Si was determined from EPMA. The beginning decomposition into 
the two-phase microstructure above the 20 pm thick interface layer is apparent and in regions more 
distant from the substrate a lamellar eutectic microstructure with spacing of A = 0.4 um was de- 
tected. Gas cooled samples which solidified from high undercooling levels of AT =~ 200 K display 
LESEN of much the same anomalous eutectic microstructure reported by previous authors (Fig. 3a, b). No sign 
* Su of residual lamellar eutectic microstructure was detected within the as-solidified highly undercooled 
iam samples. 
Haw an The quenched highly undercooled melt drops (AT = 235 K) again show a greyish Si-enriched layer 
ples (AT + adjacent to the substrate (Fig. 3c). The Si content of 25 at.% is slightly increased with increasing 
J um alone undercooling levels. Narrow regions of finely spaced lamellar microstructure with A = 60 to 100 
nm, which seemingly originate from the residual solidification in interdendritic regions were de- 
tected by TEM investigations. The upper part of the sample mainly displays a fine-grained segre- 
gated microstructure (Fig. 3d). Its equiaxed grain morphology resembles the anomalous eutectic 
microstructure, but the grains exhibit inferior atomic contrast. The composition difference between 
individual grains revealed by EPMA is much less than that of the equilibrium phases. 
The apparent difference in phase content of gas-cooled and quenched samples is inferred from the 
X-ray diffraction pattern shown in Fig. 4. The gas-cooled samples exhibit the reflections of the 
equilibrium fcc a-Ni and cubic p1-Ni3Si phases along with vanishingly small reflections of a minor 
fraction of the metastable hexagonal Ni; Sij; phase. The quenched samples of highly undercooled 
melts, on the other hand, show reflections of the supersaturated a-Ni solid solution phase superim- 
posed with the metastable hexagonal NiysSig phase. The lattice parameters of the metastable hex- 
agonal NisSis phase are a = 0.6698 nm and c = 2.8855 nm. The TEM electron nanodiffraction 
pattern confirm the hexagonal structure of the Ni»sSig phase obtained from the X - ray results. 
3.3 Phase transformations on DSC 
Undercooled quenched Nig ¢Si; 4 samples were subjected to an isochronal DSC heating with a rate 
of 10 K/s. The virgin curve displays an exothermic reaction near 500 °C, which is ascribed to the 
decay of the metastable phase (see Fig. 5). The endothermic peaks near 1020 °C and near 1120 °C 
are due to the B; — PB: transformation and the B, — PB; transformation, respectively. Whereas the 
peak near 1148 °C represents the melting of the eutectic 3 + a — L. On cooling the eutectic solidi- 
fication sets in at 1122 °C, i.e. with slight undercooling of 26 K. It is succeeded by the reverse 3 — 
Bz and B2 —; reactions. There was no transformation near 500 °C on cooling and no exothermic 
reaction in a repeated DSC run of the sample. That is, the metastable phase completely disappeared 
after heating. Resolidified DSC samples exhibit a coarse lamellar eutectic microstructure with 
spacings of A = 4.3 um. The DSC method was also used to check the eutectic nature of the alloys 
because it is of vital interest for assessing the solidification behaviour of the undercooled melts. 
fro 
autectic N- 
yr the sample 
| sample with 
sr of the mE" 
1d) the de-