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.