70 
in 
su 
5 
2 
Fig. 8 - Photomicrograph of Ni-32,5% Sn eutectic specimen 
supercooled 100 deg. Magnification 80 times. Pale 
regions are irregular eutectic. Note dendrite- 
like morphology of these regions. ca; 
Su}; 
The foregoing observations are most readily explained by assuming £o1 
that initial growth is of a supersaturated singlephase a solid solution aa 
(of eutectic composition) (4), which phase grows dendritically during The 
recalescence. At some point during or near the end of recalescence, que 
this single-phase dendrite then partially decomposes because it is ext 
highly supersaturated with respect to tin. The decomposition is such Thi 
that the second-phase forming (tin-rich n phase) is distributed in a 
closely spaced network through the dendrite to minimize required COD 
diffusion distance during the transformation. The dendrite must either 
decompose directly to the o + n structure, or decompose first o + a 
network of liquid, with subsequent nucleation and growth of n through- 
out this liquid network. At the end of recalescence some liquid 
remains and this solidifies as normal lamellar eutectic growing from 
the decomposed dendrite or "irregular" eutectic. According to this 
model, the fraction of the structure that freezes dendritically (and 
hence the fraction of irregular eutectic) should increase with 
increasing supercooling. ~ 
Fraction of solid was calculated from the following equation: 
2% Cohn om) 
S r- 
Fig. 9 shows fraction of solid as a function of supercooling. About 
3906