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e-beam melted icosahedral Al–Mn surface layers: Microstructures and phason defects

Published online by Cambridge University Press:  31 January 2011

D. M. Follstaedt  and
J. A. Knapp
D. M. Follstaedt
Affiliation:
Sandia National Laboratories, Albuquerque, New Mexico 87185-5800
J. A. Knapp
Affiliation:
Sandia National Laboratories, Albuquerque, New Mexico 87185-5800
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Abstract

The microstructure of icosahedral Al(Mn) in melt-quenched surface alloys has been characterized with TEM. Single-phase alloys with 20 at. % Mn show angular misorientations (up to ∼7°) across internal boundaries radiating from the nucleation centers of grains. Lowering the Mn content results in fcc Al at grain boundaries and between branches of the icosahedral grains; the presence of Al-rich liquid around growing grains apparently eliminates the misorientations seen with 20 at. % Mn. The grains have a dense distribution of dislocation-like defects, but have sufficient orientational order to exhibit well-defined diffraction contours. Weak reflections in diffraction patterns from small areas (0.7 μm) of grains exhibit deviations from exact icosahedral symmetry positions which were analyzed to evaluate a phason strain tensor and characterize misordering defects in the quasicrystalline structure. A negative matrix element (m ≍–0.05) for a tensor axis along a twofold icosahedral axis accounts for most of the distortions in these patterns and those from other melt-quenched icosahedral Al(Mn) and Al(Mn, Si) alloys; this dominant element lowers the symmetry to D2. This axis is nearly perpendicular to the growth direction, which suggests that the misordering is due to rapid growth during the quench; however, the D2 distortion may also result from an elastic instability predicted for icosahedral phases. Distortions in Al(Cr, Ru) can be fit with a similar tensor but with m>0. Thus the phason character identified here may apply to other aluminum-transition metal icosahedral phases as well.

Type
Articles
Information
Journal of Materials Research , Volume 4 , Issue 6 , December 1989 , pp. 1398 - 1416
Copyright
Copyright © Materials Research Society 1989

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References

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