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

Journal Article · Wed Nov 01 00:00:00 EST 1989 · Journal of Materials Research; (USA)

DOI:https://doi.org/10.1557/JMR.1989.1398· OSTI ID:5046330

Follstaedt, D M; Knapp, J A ^[1]

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1. Sandia National Laboratories, Albuquerque, New Mexico 87185-5800 (US)

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 {similar to}7{degree}) 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 {mu}m) of grains exhibit deviations from exact icosahedral symmetry positions which were analyzed to evaluate a phase strain tensor and characterize misordering defects in the quasicrystalline structure. A negative matrix element ({ital m}{approx}{minus}0.05) for a tensor axis along a two-fold 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 {ital D}{sub 2}. This axis is nearly perpendicular to the growth direction, which suggests that the misordering is due to rapid growth during the quench; however, the {ital D}{sub 2} 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 {ital m}{gt}0. Thus the phason character identified here may apply to other aluminum-transition metal icosahedral phases as well.

• DOE Contract Number: AC04-76DP00789
• OSTI ID: 5046330
• Journal Information: Journal of Materials Research; (USA), Vol. 4:6; ISSN 0884-2914
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
ALUMINIUM BASE ALLOYS
MICROSTRUCTURE
MANGANESE ALLOYS
ELECTRON BEAMS
ION BEAMS
MELTING
MIXING
MODIFICATIONS
QUENCHING
SURFACES
SYMMETRY
TRANSMISSION ELECTRON MICROSCOPY
ALLOYS
ALUMINIUM ALLOYS
BEAMS
CRYSTAL STRUCTURE
ELECTRON MICROSCOPY
LEPTON BEAMS
MICROSCOPY
PARTICLE BEAMS
PHASE TRANSFORMATIONS
360102* - Metals & Alloys- Structure & Phase Studies