COST 507 - Repositório Aberto da Universidade do Porto

after heat treatment was submerged in cold water. Precise lattice parameters and
standard deviations were obtained by a least squares refinement using Guinier Huber
CrKoci or Fe-Kot] X-ray powder data at room temperature employing an internal
standard of 99.9999% pure Ge (ace = 0.5657906 nm). The microstructures of the
alloys in the as-cast and annealed condition were studied by optical microscopy on
surfaces prepared by SiC-grinding and polishing the resin mounted alloys with
diamond pastes down to 1/4 μπι grain size. A CAMEBAX SX50 wavelength
dispersive spectrograph was used for proper identification of the phases and
precipitates. Quantitative analyses were performed comparing the Al-Κα, Fe-Ka and
ΜηΚα emissions of the three elements in the alloys with those from AI2O3 and/or
elemental Al, Fe, Μη as reference materials and applying the PAP correction
procedure [85Pou]. An annealed alloy with composition Al 7] 3 Fe 2 8 7(4) (in at.%) defined
by wet chemical analysis served as internal standard. The experimental parameters
employed were 20 kV acceleration voltage, 15 to 20 nA sample current and
spectrometer crystals such as TAP for the AlKa and LiF for the FeKoc and MnKct
radiation.
3 Results and Discussion
3.1 Binary Systems
The binary systems are accepted from [90Mas]. Some controversy concerns the true
melting behaviour of Fe 4 Aln (earlier 'FeAlj') for which [86Len] claimed congruent
melting. A recent critical assessment of the Al-Mn binary system is due to [87McA].
Isothermal reactions, solidus and liquidus values have been reanalysed by means of
precise Smith thermal analysis for a series of seven selected Al-Mn alloys including
also the metastable reactions on cooling (for further details on this joint research see
the COST-507 report by the research group of F. Hayes in Manchester-UMIST). The
crystallographic data and ranges of existence for all of the binary equilibrium phases
pertinent to the phase diagram are listed in Table 1.
3.2 Solid Phases
Phase relations in the Al-Fe-Mn ternary are characterizsed by the formation of
extended solid solutions, which at 550 °C tend to extend from the binary transition
metal aluminides far into the ternary system at a constant aluminium content thus
reflecting considerable Fe/Μη atom exchange. Whereas the large solid solubility in
(Fe x Mn|. x )Al 6 and its temperature dependence (increasing iron content with decreasing
temperature) have been corroborated by various research teams (see i.e. [92Ran]),
there is little known on the ternary mutual solubility of the various iron and manganese
aluminides.
We determined the concentration dependence of the unit cell dimensions at 550 °C for
(Fe x Mn|. x )Al 6 , (Fei. x Mn x ) 4 Ali 3 , (Fe.. x Mn x ) 2 Al5 and for (Fe x Mni_ x ) 4 Aln as a function of
the Fe/Μη atom exchange. Compositions richer in iron generally show smaller unit cell
volumes suggesting a ratio of the atom radii R Ic /RMn < 1 · The solubility limits derived
from X-ray analyses are in good accordance with those obtained from quantitative
ΕΡΜΑ evaluation for two- or three-phase alloys. The results of the EPM analysis
determine the vertices of the various three-phase equilibria as well as the solid phase
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