COST 507 - Repositório Aberto da Universidade do Porto

3. Results
3.1. Industrial Al-Si based alloys
Table 4 summarises the results of Cp-determinations on alloys KS-4 and KS-5 as well
as their repeatability (the maximum Öeviation is ± 0.2 %). Originally four temperatures
were selected : 50, 150, 250 and 450 °C; at the latter, the alloy had been kept for an
hour before the actual measurements, but results had to be discarded, as they did not
give a unique value on heating and on cooling. Also indicated are the Cp-results of
Jaroma-Weiland [96Jar] on the same alloys, with extrapolations to 50 °C. She used
sapphire as reference material in conjunction with the normal continuous 3step
procedure. The reproducibility of the two independent determinations, i.e. the
deviation of the mean values of the results of an instrument from the total mean value
[96 Höh] lies within ± 1.5 %. The agreement is very good, as Jaroma-Weiland et al.
evaluate the inaccuracy (band) of their measurements to lie below 3 % [94Jarl. The
present Cp-data may be connected linearly between 50 and 250 °C. The higher Si
content of KS-5 is responsible for lower Cp-values.
Tal ble 4 : Cp inJ/gK
KS-4
50 °C
150 °C
250 °C
Present study
repeatability
±
0.879
0.2 %
0.928
0.1 %
0.9715
0.1 %
Jaroma-Weiland
reproducibility
±
0.889
1.1 %
0.928
-0%
0.985
1.4%
KS-5
Present study 0.852
repeatability ± 0.1 %
Jaroma-Weiland 0.865
reproducibility ± 1.5 %
0.900
0.2 %
0.901
0.2 %
0.944
0.2 %
0.954
1 %
3.2 Al-Si-Zn alloys
Having gained confidence in the adopted measuring technique, the Cp-data on the
ternary alloys were determined on heating each specimen three times over the
temperature intervals 40 - 60 and 390 - 410^C (340 - 360 °C in the case of alloys 6
and 9). The mean values (Cpexp in J/gK), often obtained on two different specimens,
are given in table 5; their reproducibility is estimated to be the same as for the
industrial alloys (± 1.5 %). The results are consistent with the compositions of the
alloys : increasing Si and especially Zn concentrations decrease the Cp-values. For
comparison, the Cpadd-data derived according to the additivity rule are indicated; the
values for the pure elements are taken from table 3 [91 Din]. At 50 °C, the calculated
values are more important than Cpexp. The alloys 7, 8 and 10 in the Al rich corner
possess Cp-values at 400 °C which correspond to additivity within the experimental
inaccuracy.
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