Prakt. Met. Sonderband 30 (1999) 119 
3. Solidification kinetics of undercooled Ni-Si melts 
3.1. Time-temperature characteristics of recalescence processes 
The levitated melt drops were initially superheated up to 300 K with respect to the liquidus tem- 
perature and then cooled by the gas stream. The solidification starts by spontaneous nucleation if a 
certain undercooling level is achieved on cooling. Alternatively, it has been triggered at a controlled 
undercooling level. The typical recalescence behaviour of Ni-Si samples with different undercool- 
ing levels is presented in Fig. 2. 
1.580 — 
| i, Pu 
1100 — 
So 
» - 
i 1050 — 
= - Undercooling 
; © 
a - - = 30K 
1000 — 
- 55K 
- - 105K 
Sina 950 A — rrr 
about 1.2 g 
ctromagnetic 10 ; 100 
used is given Time [ms] 
oled by a gas Fig. 2: Temperature-time characteristics of recalescence 
onitored by a events of eutectic Ni-21.4 at.% Si samples with undercooling 
ndrite growth levels of (a) AT = 30 K (b) AT = 55 K and (c) AT = 105 K 
Seep across and (d) AT = 225 K showing the transition from double- 
ding silicon recalescence to single-recalescence behaviour 
a. which Was 
ter mucleation A transition from single- to double-recalescence events beyond a critical undercoolings of AT > 
qt heat trans ATc ~ 30 K prior to solidification occurred. The plot at AT = 55 K clearly exhibits the double re- 
er adjacent 10 calescence with a thermal arrests time of order of 10 ms at AT = 15 K. With increasing AT, the 
N slope of the temperature-time characteristics of the primary recalescence process is increased, the 
action uti plateau length between the first and second recalescence step is diminished. Finally the plateau dis- 
il microscopy appears near AT ~ 150 K and single-recalescence events are observed beyond that level. 
re performed There is a moderate increase of the solidification velocity up to 1.5 m/s near AT ~ 220 K and the 
fr 10 identify velocity again decreases at higher AT. The calculated eutectic growth velocity increases with melt 
ple cross undercooling and then slows down with increasing undercooling level after passing a maximum 
n neo near AT = 200 K (6). The experimentally observed behaviour differs from calculated characteristics 
nder 10 16V of both a-Ni dendrite and coupled eutectic growth mechanisms. It could rather explained by den- 
drite growth of an ordered phase, which can exhibit substantially smaller growth velocities than the 
15i