118 Prakt. Met. Sonderband 30 (1999) 
1200 - 1 
1150 - 
Oo 1100 - 
= 1050 — 
1000 - 
950 - 
0= Lp 
14 16 18 20 22 24 26 28 
at.% Si 
Fig. 1: Ni-Si phase diagram according to (5). The dashed inserts are the high- 
temperature phase modifications predicted from quenching experiments and the 
metastable extensions of liquidus and solidus lines (4). Dash-dotted: Calculated 
To-lines (6) 
2. Experimental Methods 
Master alloys of Ni 21.4 at.% Si were prepared from 99.99 % pure Ni and 99.999 pure Si in an arc 
melting furnace on a water cooled copper crucible under Ar atmosphere. Samples of about 1.2 g 
mass, corresponding to spheres of 6 mm diameter, were investigated by means of electromagnetic 
levitation experiments. A comprehensive description of the type of levitation facility used is given 
elsewhere (7, 8).The samples were repeatedly molten in a ultrapure atmosphere and cooled by a gas 
stream permitting cooling velocities of ~ 10 K/s. The temperature of the sample was monitored by a 
two-colour pyrometer at a sampling rate of 50 Hz with an accuracy of < 3 K. The dendrite growth 
velocities were determined by measuring the time needed by the solidification front to sweep across 
the sample surface section of 1.0 x 1.0 mm’, which was projected onto a fast responding silicon 
photodiode. The equipment enables recording with sampling rates of 1.5 MHz. 
Melt drops of well-defined undercooling level were quenched onto a copper substrate, which was 
coated with tin solder in order to enhance the interfacial heat transfer. Immediately after nucleation 
the drops solidify with a preferred heat flow into the undercooled melt but a subsequent heat trans- 
fer into the substrate. This permits high cooling rates up to ~ 10° K/s at least in a layer adjacent to 
the chill substrate. 
The phase content of as-solidified specimens was analysed qualitatively by X-ray diffraction utilis- 
ing the Co-K,, line. The microstructure of as-solidified samples was revealed by optical microscopy 
and scanning electron microscopy (SEM). Electron probe microanalyses (EPMA) were performed 
in the energy-dispersive mode (EDX) and wavelength dispersive mode (WDX) in order to identify 
the local compositions of selected microstructure components. In addition, some sample cross sec- 
tions were subjected to ion beam thinning and investigated by transmission electron microscopy 
(TEM). Differential scanning calorimetric (DSC) investigations were conducted in order to reveal 
the transformation temperatures of quenched samples. 
NiS 
Nm