MICROSTRUCTURE OF HIGHLY SUPERCOOLED Ni-Sn EUTECTIC ALLOY 
Z.G. Ranko”, University of Banja Luka, Department of Mechanical I 
Engineering, Yugoslavia of 
M.C. Flemings, Department Material Science and Engineering, of 
Massachusets Institute of Technology, Cambridge, USA sho 
spa 
ABSTRACT Pow 
Several samples of Ni-32,5% Sn eutectic alloy weighing approximately eon 
0,8 to 1,0 gram were levitated and supercooled by various amounts den 
prior to nucleation of the solid. Different methods of ranid quenching SH 
were used. The highest supercooling abserved was about 135 deg. It the 
should be noted that supercooling of samples was generally more is 
difficult than that of other alloys. The influence of supercooling of 
and cooling rate on the structures of Ni-8n eutectic alloy was The 
studied. The main object of the investigation was to determine the COM) 
structure and solidification mechanism of supercooled Ni-Sn eutectic sup 
alloy. den 
und 
INTRODUCTION abo 
the 
Previous work done studied the effects of supercooling using different YıV 
methods. T.Z. Kattamis and M.C. Flemings (4) investigated the Sup 
solidification behavior of supercooled eutectic Ni-Sn. Their samvles anoı 
were cooled in fused silica crucibles. Slow cooling rates after Bot! 
nucleation (about 5°C) were obtained by allowing the melt to solidify rat 
without special means of heat extraction, the faster rates were sol: 
obtained by placing a water-cooled chill at the top of the melt mic; 
immediately after recalescence to achieve supercoolings of 240°C. Sch 
They explained the phenomenon of branching growth by postulating eute 
that the first solid formed is a highly supersaturated a solid has 
solution of the eutectic composition which grows dendritically and inve 
subsequently decomposes to (a + n), probably with an intermeaiate erit 
(a + liquid) stage. Its approximate morpholoay near the end of orie 
recalescence is a dendrite-like structure. At the end of recalescence, rel: 
some liquid remains and this solidifies as normal lamellar eutectic the 
growing from the decomposed dendrite or "anomalous" eutectic. took 
Powell and Hogan (20) supercooled samples of Ag-Cu alloy with copper the 
contents up to 32% and obtained supercooling of 80-90°c. They Crys 
* former visiting scientist at MIT, USA 
Z21 8