COST 507 - Repositório Aberto da Universidade do Porto

Table 2 : Alloy compositions
Alloy
atom%
mass%
Al
Si
Zn
Λ1
Si
Zn
1
2
3
6
7
S
9
10
11
12
13
76.46
72.73
60.82
39.13
84.83
79.89
11.73
90.19
58.51
55.84
53.28
4.40
9.08
18.69
1.02
5.04
9.97
0.05
4.71
1.00
4.66
10.64
19.14
18.19
20.49
59.85
10.13
10.14
88.22
5.10
40.49
39.50
36.08
60.01
57.61
46.81
21.13
74.02
69.57
5.20
83.93
37.11
35.70
35.10
3.59
7.49
14.98
0.57
4.58
9.04
0.02
4.56
0.66
3.10
7.30
36.40
34.90
38.21
78.30
21.40
21.39
94.78
11.51
62.23
61.20
57.60
Alloys 6 and 9 were kept at 360 °C for 7 h before oven cooling to room temperature;
the other alloys were annealed for 5 h at 400 °C and maintained a week at 265 °C
(1,2,3,11,12,13) or 240 °C (7,8) or 175 °C (10) before oven cooling. The hope of
retaining stable structures at room temperature was vain, as successive DSC
experiments at 5 °C/min still showed endothermal dissolution effects at low
temperatures (fig. 3). As expected, the specimens 7, 8 and 10 with low Zn
concentrations do not undergo a transformation around 277 °C. Due to complications
arising from the existence or metastable phases and to the fact that Cp­measurements
are very time consuming and tedious, it was decided to carry them out at selected
temperatures only.
A Perkin­Elmer thermal analyser DSC 7 series 1020 was used in a constant
temperature environment. The recommendations given by Höhne et al. in their book on
differential scanning calorimetry [96Hoh] were observed and the following procedure
was adopted in order to obtain a high accuracy of the Cp­data :
­ The discontinuous 3step technique which measures heats was employed, as it needs
no correction for thermal lag after precise temperature calibration.
­ Steps of 20 °C were chosen at a rate of 5 °C/min. In order to check reversibility, at
least three heating and cooling cycles under argon atmosphere were performed after
establishment of isothermal steady state conditions.
­ AI was used as reference sample, as the alloy and calibrant should have similar heat
capacities and similar thermal conductivities. Cp­data for Al were calculated according
to a formula given in Dinsdale's paper "SGTK data for pure elements" [91 Din]. A
critical assessment of the thermodynamic properties of AI was also published by Desai
[87Des]. Specific heat values of interest proposed by Dinsdale and Desai are
confronted in table 3. They agree within 0,5 % except at 450 °C. Also are given some
Cp­data on Si and Zn according to [91 Din].
Table 3 : Cp in J/gK
50 °C
100 °C
150°C
250 °C
350 °C
400 °C
SGTE
0.91604
0.94297
0.96649
1.00954
1.05180
1.07346
Al
Desai
0.91367
0.94186
0.96485
1.00543
1.04987
1.07546
%
0.25
0.12
0.17
0.41
0.18
0.08
Si
SGTE
0.73656
0.86572
0.87721
Zn
SGTE
0.39156
0.44023
0.45037
129 ­