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Figure 7
: (a)
Segregation and solidification sketch compared to the material extracted for
chemical analysis (white areas) and the material collected for metallographic analysis, or
discarded by cutting (black areas). The horizontal and the inclined arrows show the
movement of impurities caused by segregation effects, whereas the vertical arrows show the
effects of directional solidification. (b) Concentration profiles of impurity elements: results from
XRF analysis of the annealed sample at 1200°C.
4.3 Chemical reactions: annealing effect
4.3.1 Reaction between intermetallics
The first one is a proposed mechanism by Margaria [8] which involves several
intermetallic phases in MG-Si. The second is a structural transition from a metastable
to a stable lattice structure of the FeSi
2
phase. This mechanism was confirmed by
several experiments and previous works [9-13]. According to Margaria [8], an
annealing at 900°C decreases the quantity of CaAl
2
Si
2
and increases the amount of
Al
6
CaFe
4
Si
8
according to the equilibrium:
3CaAl
2
Si
2
+ 4FeSi
2 =
Al
6
CaFe
4
Si
8
+ 2CaSi
2
+ Si
(2)
Unfortunately the present work cannot prove that this reaction is happening. The
equilibrium reaction should increase the content of quaternary phase after an
annealing. However, the quantity of Ca lost during the melting step was higher than
expected. Therefore the quaternary phase is noticed in lower amounts in the annealed
sample. CaAl
2
Si
2
might have not formed. If it did, the annealing would start the
reaction until the total consumption of CaAl
2
Si
2
. CaSi
2
was not found in the annealed
sample. The chemical composition of the analysed silicon does not fall in the CaSi
2
existing range in the graph traced by Margaria [15].
4.3.2 FeSi
2
transition
Si precipitates are formed in areas close to the FeSi
2
phase. This compound undergoes
a transition from FeSi
2.4
(high-temperature structure) to FeSi
2
(low-temperature
structure) according to the equilibrium:
FeSi
2.4
= FeSi
2
+ 0.4Si
(3)
This reaction increases the lattice parameters of this compound. Dilatation of
lattice induces stresses in the phase and causes cracks. Cracks created pass through
the silicon matrix and all the common intermetallic phases except TiFeSi
2
, as shown
Slag layer
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