COST 507 - Repositório Aberto da Universidade do Porto

appearance of this phase at 13.5 wt% Si. Our measurements indicate that this phase is
not precipitated in equilibrium at Si contents below 20wt%, however the phase may be
readily nucleated at lower Si contents under non equilibrium solidification conditions
[93Sim].
2.3 Growth of Large Crystal
In order to establish the enthalpy of the two important ternary phases, alpha and beta,
in the Al-Mn-Si system, a program was initiated at SINTEF to grow large crystals of
these phases. Since these are peritectic phases, they could not be made simply by
mixing appropriate amounts of Al, Mn and Si and having them react at high
temperature in the liquid state. On the other hand the reaction kinetics in the solid state
forms a serious barrier to achieving single phase samples with a homogenous
composition on a practical time scale, at lower temperatures. The only possible
approach was therefore to grow the alpha and beta crystals in the two phase liquid +
alpha (or beta) region. Subsequently the samples were quenched to room temperature
to minimise the problem of nucleation of other phases on the existing crystals during
solidification. After dissolution of the aluminium matrix we were left with millimetre
sized crystals of the desired phases [96Sim]. Attempts to measure the heat of formation
of these phases are underway at the Université Paris Sud.
A spin-off result of this investigation has been to question the results reported by Pratt
and Raynor [51Pra]. In the round I assessment of the Al-Mn-Si system, heavy
emphasis was placed on the results of Pratt and Raynor, with regard to the stability of
the beta-AlMnSi phase. The present experimental investigation has, however, not
succeeded in reproducing this earlier work. An attempt to grow the beta phase
according to the same procedure as was employed by Pratt and Raynor [51 Pra] was
unsuccessful. Unfortunately this experiment resulted in a beta core surrounded by an
alpha shell, indicating that the beta phase was the primary phase but that the holding
temperature was in the liquid + alpha phase region [96Sim]. This observation has lead
to some doubt as to the reliability of [51 Pra], which is currently being incorporated in
the reassessment of Al-Mn-Si [97Ran].
3 Calculations
Even though most of the work involved was completed in round I of the COST507
action the completion of our evaluation of the Al-Fe-Si [93Sei] and Al-Mn-Si [93Kol]
systems took place during round II. The Al-Mn-Si system has been subject to extensive
experimental and computational réévaluation during the course of round II as reported
by Rand et al [97Ran]. No detail will therefore be given in this report.
In the case of the Al-Fe-Si system, the assessment by Seiersten [93Sei] remains as the
accepted COST507 evaluation. The work is therefore detailed here.
3.1 The Al-Fe-Si Assessment
The central part of the assessment is based on experiments by Takeda and Mutuzaki
[40Tak]. This is the only complete determination of the phase diagram, and later
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