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5. Conclusions
Three MG-Si samples underwent three different thermal histories. The first one was
untreated, the second one was treated at 600°C without a melting step and the last one
was melted at 1450°C for 2 hours, then annealed at 1200°C. Both the treatments
lasted for 17 hours. XRF chemical analysis and EPMA detected chemical and phase
composition. It was demonstrated that the first two samples have the same
microstructure. This implies that at 600°C no change occurs in the material.
Oxidation, segregation and chemical reactions are the mechanisms of the changes
in impurity level and displacement in the samples. The first acts as a slag refining
treatment. Segregation moves Fe and Ti to the center and the top of the mold. The
reactions are of two kinds. The first reaction consumes CaAl
2
Si
2
and FeSi
2
.
Al
6
CaFe
4
Si
8
, CaSi
2
and Si are supposed to increase, but the oxidation step did not
allow the creation of Ca-containing phases. In the second reaction, FeSi
2
undergoes a
transition from HT- to LT-structure. This decreases the percentage of FeSi
2
and
creates silicon precipitates. Experiments could have been carried on in an inert
atmosphere to limit compositional changes. Refining and segregation effects needed
to be discussed together for the annealed sample.
Final considerations about the effects of the annealing and refining on the
production of MCS are resumed in Table 5 and 6. Table 5 regards the impurity level
change for each intermetallic phase's relative percentage. Table 6 has been formulated
by a comparison with Table 1. The conclusions are that refining effects decrease both
reactivity and selectivity, whereas annealing increases selectivity but decreases
reactivity.
Table 5
: Variation in relative phase percentage after refining at 1450°C for 2 hours, followed
by annealing at 1200°C for 17 hours. Legenda: + = increase; 0 = unvaried; ÷ = decrease
Effect
FeSi
2
TiFeSi
2
Al
8
Fe
5
Si
7
CaAl
2
Si
2
Al
6
CaFe
4
Si
8
Annealing
÷
0
0
÷
+
Refining
+
+
+
÷
÷
Table 6
: Expected effects on MCS production after a melting step of 2 hours at 1450°C and
an annealing at 1200°C for 17 hours. Legenda: 0 = unvaried; P = moving towards +;
N = moving towards ÷
Effect
Phase
Reactivity
change
Selectivity
change
Annealing
•
Ca-ternary phase to Ca-quaternary
phase
•
Reduced FeSi
2
FeSi
2
P
P
TiFeSi
2
0
0
Al
8
Fe
5
Si
7
0
0
CaAl
2
Si
2
N
P
Al
6
CaFe
4
Si
8
N
P
OVERALL
N
P
Refining
•
Reduced Al and Ca content in MG-Si
FeSi
2
N
N
TiFeSi
2
N
N
Al
8
Fe
5
Si
7
P
N
CaAl
2
Si
2
N
P
Al
6
CaFe
4
Si
8
P
N
OVERALL
N
N
93