Prakt. Met. Sonderband 38 (2006) 181 
2 AISi18 Figure 4a depicts a reconstructed 3D image obtained from 2D sectioning of an AlSi7Sr 
sample. The fibrous morphology of the modified eutectic is clearly observed, showing Si 
, where fibres of about 0.2-0.4um in diameter and some micrometers in length (~3-5um) oriented 
nnected in many different directions. These fibres are mostly interconnected by nodes of some 
different nanometres in cross section (~0.05-0.1um?). The intersection of 3 different grains in Figure 
several 4b is separated by Si-free grain boundary zones of about 1um width. Complete 
ion of a interconnection and similar orientation of fibrous Si is observed in each grain. Similar Si 
bserved orientation is observed in grains A and B, while Si fibres in grain C are oriented mostly 
(y-axis). perpendicular to those of grain A and B. In this grain (C), the Si fibres become thinner with 
um long increasing distance from the grain boundary 
ected to 
um J 4m 
tion: Fig. 4: 3D reconstruction of the Si structure in as cast AISi7Sr: a) fibrous structure in three 
iL different grains, b) detail showing the grain dependent orientation of the Si fibres. 
e 3a) is 
ed. The 
that of 
ites are 
Fig. 5: Si-Al,O3 fibre hybrid network extracted by dissolution of a-Al in AlSi7/Al,03/20s in 
a) as cast and b) detail of Si plates in a fibre free zone, c) solution treated sample. 
Figure 5 shows SEM micrographs of the structures extracted by dissolution of the a-Al 
from AISi7/Al,03/20s in the as-cast (Fig. 5a) and solution treated at 540°C/4h (Fig. 5c) 
conditions. The as cast composite presents a connected Si structure of the matrix together 
with the Al,Os-fibres, where Si platelets bridge the alumina fibres. During solution 
um treatment, Si coarsens into Si-particles sticking to the Al,O; fibres. Like in the unreinforced 
te AISi7 alloy, an interconnected structure of lamellar Si eutectic is observed also in fibre free 
zones in the as cast condition (Fig. 5b). The isolated and coarsened Si particles are 
removed from the composite after deep etching of samples in the T4 condition.