COST 507 - Repositório Aberto da Universidade do Porto

Table 2:
Results of the DSC measurements and comparison
Thermo-Calc (T.C.) predictions.
with
corresponding
A357 + Cu
A357 + Cu + Ag
A357 + Cu + Sm
DSC
T.C.
DSC
T.C.
DSC
T.C.
Tslart [°C]
562.83
557
558.42
Heating
run
Tpeakdl [°C]
Tpeak(2) [°C]
Tend [°C]
AHf [mJ/mg]
573.24
605.52
609.06
277.67
619.2
570.39
610.88
613.2
294.21
616.4
570.02
614.03
615.93
218.35
619.9
Cooling
run
Tstart [°C]
Tpeakdl [°C]
Tpeak(2) [°C]
Tend [°C]
ΔΗ 8 [mJ/mg]
604.62
603.63
567.52
561.83
­224.19
619.2
610.08
607.3
567.44
557.63
­138.19
616.4
614.74
612.29
566.39
558.94
­164.96
619.9
3.2 Thermodynamic Analysis
Computational thermodynamics was performed with the Thermo­Calc software
package [85Sun] and the recently developed COST 507 database for light
alloys [94Cos]. The isopleth section (with varying Si content) for alloy
A357+Cu+Sm is shown in Fig.3. Sm precipitates as the rhombohedral phase
below 250° C. An important feature of this diagram is that addition of Sm
lowers the liquidus temperature to 420° C compared to 540­560° C in the alloys
without Sm.
(AlWSm^MgjSi
(AI)»(Sm)*Mg,Si*Si
8. 300
E
200
(Alfr+Sm+MOíSi+Si+AljCij­e
100
0 2 6 8 10 12 14
Weight Percent Si
Fig. 3: Isopleth section for A357+Cu+Sm alloy.
16 18 20
Scheil solidification diagrams have been constructed, with the assumptions
that no diffusion is taking place in the solid and that local equilibrium exists at
the solid / liquid interface during solidification. Fig. 4 depicts the fraction of
liquid phase as a function of temperature during solidification for the four alloys
investigated. The kinks in the curves are due to the formation of solid phases
((Al), Si, Mg 2 Si, Θ) during solidification. From these diagrams, the temperature
range for solidification can be determined. This range is of the order of 70° C
for all alloys. Fig. 5 depicts the content of the remaining liquid phase during
142 ­