COST 507 - Repositório Aberto da Universidade do Porto

An experimental program was thus initiated during COST 507 Round II, in order to
clarify this situation. Attention has been focused mainly on the isopleth containing 4
wt% Si. The results of the annealing experiments by Thornton at Alean International
[96Tho] and the Smith calorimetrie work of Hayes and Robinson at UMIST
[96Hay/Rob] are summarised in Fig 1. A few studies have also been made on the 4 wt
% Mn isopleth, as shown in Fig 2. These two figures also show the calculated phase
boundaries resulting from the present assessment. The more complete study on the 4
wt% Si section does indeed indicate that the liquidus temperatures measured by Phillips
for the separation of the cubic-alpha and beta phases are too low by some 20 - 30 K.
There is also fairly convincing evidence that the manganese level in the ternary eutectic
involving fee, cubic-alpha and silicon is somewhat lower than the value of c. 0.8 wt%
suggested by Phillips. Mn levels in the liquid near this eutectic measured by Zakhorov
et al [88Zak/GuI] and Simensen [96Sim] suggest that the Mn content in the liquid is
between 0.5 and 0.7 wt%.
The position of the line for the co-existence of cubic-alpha, beta and liquid is also not
well-defined. Pratt and Raynor [51Pra/Ray] extracted relatively large crystals of the
beta-phase by cooling alloys to 660 - 700 °C, annealing for 1 hour and quenching.
However, Simensen [96Sim] has repeated this experiment with an alloy containing 6
wt% Mn, 4 wt% Si, using annealing times of 1 day, and found crystals of cubic-alpha
containing a little beta, surrounded by alpha (see Fig 1). This indicates that although
beta can be the phase first formed, as found by Pratt and Raynor (1 hr anneal), the
equilibrium phase is cubic-alpha. Redford et al have suggested that the cubic-alpha
phase is not in equilibrium with the liquid above c. 7.5 wt% Mn and 1023 K.
Phillips[43Phi, 59Phi, 61 Phi] has given data for four invariant points in the system,
detailed in Section 6 below. Clearly if the liquidus temperatures from which these are
derived are somewhat in error, there will be some uncertainty in these values.
Recent data for the mutual solid solubility of Mn and Si in fee Al are also available
[94Kol/Sig].
We must therefore conclude that although the general form of the Al-Mn-Si ternary is
well-established, the details of the system at Mn levels above 1 wt% are still subject
to appreciable uncertainty and we are in the unhappy situation of not being able to rely
with any certainty on the main corpus of experimental phase diagram information for
this system [43Phi, 59Phi, 61Phi].
3 Phase Models
The cubic alpha phase, called here C alpha, has a variable composition around
Al I7 Mn 4 Si 2 and is formed at relatively low Mn and Si contents. This has been modelled
with a range of silicon content using the sublattice description
Al 16 Mn 4 Si (Al,Si) 2
which corresponds to an essentially constant manganese content of 30.0 wt%, and a
silicon content which could vary from 3.8 to 11.4 wt%. This is an important phase in
the quaternary Al-Fe-Mn-Si system, since the Mn in this phase can be replaced up to
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