Formation and Thermal Stability of Quasi Crystalline and Other Metastable Al-Rich Al-Mn Phases
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FORMATION AND ITIERMAL STABILITY OF QUASIC(RYSTALLINE PHASES* AND OTHER METASTABLE Al-RICH Al-Mn
M.X. Quan and B.C. Giessen, Materials Science Division, Barnett Institute, Northeastern University, Boston, MA 02115
ABSTRACT RSP of Al-rich Al-Mn alloys is known to produce several metastable phases, including (with increasing Mn content) supersaturated Al solid solution and two non-crystallographic phases, identified in the literature as an icosahedral phase (I) and a (probably) decagonal phase (T) with translational symmetry in one direction. The formation of these phases in the composition range from 8 to 30 at. pct. Mn has been studied in melt-spun and arc-furnace quenched alloys. The thermal transformation characteristics of all metastable phases were measured by DSC; transformation temperatures, enthalpies and activation energies were determined. At compositions from 17 to 20 at. pct. Mn, two transformations were observed. Noteworthy results include: smeared out (slow-onset) reactions, transformation temperatures dropping sharply toward lower Mn contents, transformation enthalpies rising steeply in the same range, and activation energies ranging from -40 kcal/gmol for the low-temperature process (at -763 K) to -80 kcal/gmol for the high temperature process (-883 K).
INTRODUCTION In 1984, Shechtman et al. [1] reported the finding of a metastable phase in rapidly solidified Al-rich Al-Mn alloys that exhibited noncrystallographic symmetry in TEN and was identified as having icosahedral symmetry.
Such
phases are
now
designated
quasicrystalline
and
a
large
body
of work has sprang up around them [2-51. The icosahedral phase was originally observed in Al,6Mn 14 alloys but was subsequently reported [2] to occur in better yields at -21 at. pct. Mn. This phase is designated I phase in the following. Subsequently, crystallographic
Bendersky phase
found
TEN
evidence
(designated T phase)
in
[6]
for
a
different
RSP Al-Mn alloys.
non-
T phase was
reported [6,71 to have a tenfold symmetry axis and to be quasicrystalline two
dimensions
while
being
periodic
(crystallographic)
in
the
in
third
dimension. The conditions under which either of these metastable phases forms are not yet clear and their thermal stabilities have not been reported. We report here the results of a DSC study of the thermal properties of Al-rich Al-Mn alloys produced by RSP under different conditions and include transformation temperatures, transformation enthalpies and activation energies of transformation for the metastable phases found in these alloys.
Communication No.280
Mat. Res- Soc. Symp. Proc. Vol. 5.
from the Barnett Institute
1986 Materials Research Society
242
EXPERIMENTAL Arc melted Al-Mn alloys (Al, 99.9+% from Leico and Mn 99.97% from Ventron) were quenched either by the arc-furnace hammer-and-anvil method [8], ("foils") or by melt spinning in inert-gas atmosphere ("ribbons"). Samples were examined by X-ray diffraction (XRD, Mo-Ka radiation), heated at variable T (5, 10, 20, 40, 80 K "s-1) in the Perki
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