COST 507 - Repositório Aberto da Universidade do Porto

2.1 Solid solubility measurements
The COST507 effort on investigating the solid solubility of selected alloying elements
in aluminium has been a collaborative project between Pechiney CRV and SINTEF
Materials Technology. The systems targeted are Al-Mn, Al-Mn-Si, Al-Mn-Fe, and Al-
Mn-Fe-Si. Pechiney CRV have been responsible for the casting and heat treatment of
the alloys, as well as determining the Mn content by way of the Thermo Electric Power
(TEP) technique [81Bou, 89Bor]. SINTEF has measured all the elements in solid
solution by using the SIBUT dissolution technique [80Sim, 84Sim] combined with
pertinent analysis techniques. In this fashion the measurement of the Mn content has
been duplicated using two fundamentally different techniques, which ensures reliable
results. The results of these investigations have prompted a réévaluation of the
reliability of the work of [43 Phi] in the solid state. Due to extremely slow reaction
rates in the Mn containing systems, up to 6000 hrs homogenisation time was required
to reach equilibrium. This is longer than can be explained by a small diffusivity of Mn
in solid Al. It is therefore probable that nucleation is sluggish and that the driving
forces for precipitation are small. This could in tum help to explain the discrepancy
between the COST507 assessment and the commonly accepted diagrams in all systems
containing Mn.
The experimental tie line determinations have been conducted on the same samples
used in the solvus measurements. By measuring the composition, type and amount of
particles left on the filter after dissolution we are able to combine this with the
corresponding solid solution composition to form an experimental tie line. Amongst
the results of this work on the Al-Mn-Fe system, it was discovered that the Al 12 Mn
phase is stable at 550°C in the pure Al-Mn system. However, an Fe content as low as
20ppm was sufficient to stabilise the Al 6 (Mn,Fe) phase, and bring the sample into the 3
phase field Al + Al^Mn + Al 6 (Mn,Fe). It is believed that as little as 100 ppm will be
sufficient to completely surpress the formation of Al^Mn. These results are in
contradiction to previous beliefs based on the work of Schaefer et al [86Sch] which
suggests that this phase decomposes at temperatures above 512°C, but in agreement
with unpublished work conducted at the CNRS Vitry [96Har].
2.2 Determination of invariant points in Al-Mn-Si
Due to problems in fitting all the invariant points on the liquidus surface
simultaneously, we have experimentally redetermined the eutectic point [93Sim]. Our
coordinate for the eutectic is lower in Mn than that of Phillips, which already belongs
to the lower spectrum of values quoted in the literature. We do, however, feel
confident that this measurement is correct since the optimiser in Thermo-Calc seems to
draw this point to lower Mn values. Our values have been obtained by performing area
analysis in the microprobe in regions of the sample occupied by the eutectic structure.
In this way the Mn content should be overestimated if regions which have solidified
preeutectically formed part of the analysing volume.
The delta-AlMnSi phase has also been investigated from the point of view of
determining the position of the invariant point comprising alpha, silicon and delta in
addition to the melt. Our results contradict those reported by [76Mon], who claims the
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