Prakt. Met. Sonderband 30 (1999) 117 
Metastable Phase Formation in the Undercooled Eutectic Niss6Siz;1.4 Alloy 
Monika Leonhardt, Wolfgang Loser, Hans-Giinther Lindenkreuz 
Institut fiir Festkorper- und Werkstofforschung Dresden, 
Postfach 27 00 16, D-01171 Dresden, Germany 
Abstract 
Non-equilibrium solidification processes in bulk undercooled eutectic Nig Siz; 4 melts are investi- 
gated with respect to metastable phase and microstructure formation. Melt undercooling levels up to 
AT ~ 250 K have been achieved by an electromagnetic levitation technique. The in situ observation 
of the recalescence events was accomplished by a fast responding silicon diode. Levitated drops of 
controlled undercooling prior to solidification were quenched onto chill substrates. A transition 
from coupled eutectic to primary dendrite growth beyond a critical undercooling level AT. > 30 to 
50 K was revealed from double-recalescence events. Quenched samples displayed a featureless 
zone of the metastable hexagonal high-temperature phase Ni,sSig, which replaces the equilibrium 
phases. The metastable dendrites gradually decompose into an anomalous eutectic microstructure 
on slow post-solidification cooling. 
1. Introduction 
Phase selection processes under rapid solidification conditions do not solely depend on alloy com- 
position but on process parameters such as cooling rate, temperature gradient or melt undercooling 
prior to solidification, too (1,2). For eutectic NisgeSizi4 a gradual transition from lamellar to 
anomalous eutectic microstructures has been detected beyond a critical melt undercooling level of 
AT = 31 K which is complete at AT =~ 150 K. But, undercooling did not yield any metastable phase 
in addition to the a-Ni and B-NisSi equilibrium phases (3). On the other hand, two hexagonal non- 
equilibrium phases, a high-temperature modification Ni3Si-h and Ni,sSis, have been reported for the 
Ni-Si system from quenching experiments (4). A Ni-Si phase diagram section relevant for the so- 
lidification of undercooled melts, which also involves the predicted non-equilibrium phases is 
shown in Fig. 1 (5). 
Since rapid solidification processes are hardly accessible to direct observation it is difficult to reveal 
the solidification pathways. Direct evidence for metastable phase formation could be inferred from 
in situ observation of growth kinetics of bulk undercooled melts, which can display multi-step re- 
calescence processes. The identification of the metastable phases in as-solidified bulk samples, e. g. 
by X-ray techniques, is intricate because of their immediate decomposition on slow cooling. A cor- 
relation of recalescence behaviour and structure of the metastable phase can be accomplished by 
special quenching methods. 
In this paper, undercooled eutectic Ni-Si melts are studied by an electromagnetic levitation tech- 
nique allowing for in situ observation of the solidification process along with quenching of under- 
cooled samples on chill substrates. In that way the primary solidification mode and microstructure 
formation mechanisms are elucidated. Results are compared with existing metastable Ni-Si phase 
diagram calculations and solidification models.