COST 507 - Repositório Aberto da Universidade do Porto

3.2 Determination of the physical properties
The thermal conductivity was measured by a comparative steady-state method using a
commercial apparatus (TCFCM model of Holometrix Inc., Cambridge, USA). The cylindrical
sample (25mm in diameter and 28mm long) was sandwiched between two identical
reference samples from electrolytic iron. The measurements were carried out as function of
temperature in air during heating and subsequent cooling cycle. Furthermore the thermal
conductivity of two Al-Si-based alloys was measured at constant temperature (250°C and
300°C) as a function of time. Due to the high values of the thermal conductivity, the
measurements on ΑΙ-based materials are very difficult and It is assumed that the
experimental error exceeds the usual range of ±5%.
Hardness was determined at room temperature using the Brinell method. The
measurements of hardness as a function of the aging time at constant temperature (250°C
and 300°C) were carried out on KS1295 and KS281.1 specimens. Hardness was always
measured on samples before and after measurements of their thermal conductivity.
The specific heat capacity was determined by means of a differential scanning calorimeter
(DSC) using a "Perkin-Elmer-DSC-2" apparatus in the temperature range of 50-500°C. For
these measurements discs of 6mm in diameter and 1mm thick were machined. The
inaccuracy was below 3%.
Electrical resistivity measurements were carried out by means of the four point technique in
the temperature range from 20°C to 400°C. The measurements were made on rods (1.5mm
in diameter and 15mm long) during heating and subsequent cooling (0.5-2K/min) in vacuum
of 2*E-5 Torr.
4 Results and Discussion
4.1 Thermodynamic analysis
Computional analysis was performed using the Thermo-Calc software package [85Sun/Jan]
and the recently developed COST507 data base for light metal alloys [94Ans]. For two Al-
Si-based alloys (KS1275.1 and KS1295) the following diagrams were prepared:
isopleth section (with varying Si-content): Fig.1 a and 1 b, resp.,
phase amount (in mole%) as a function of temperature: Fig.2a and 2b, resp.
The Isopleth sections show relevant phase equilibria as a function of Si content. In both
alloys four solid phases are present: (Al) solid solution, Mg 2 Si and AI 2 Cu (the Θ phase)
precipitates as well as the Si diamond phase. From these diagrams it can be seen that an
addition of Cu reduces the solidus temperature to 508 C C for KS1295 compared to 543°C for
KS1275.1. The AI 2 Cu phase dissolves in KS1295 at 480°C while it dissolves at 360°C in
KS1275.1.
- 106 -