e-beam melted icosahedral Al–Mn surface layers: Microstructures and phason defects
e-beam melted icosahedral Al–Mn surface layers: Microstructures and phason defects
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电子束熔化二十面体 Al-Mn 表面层：微观结构和相子缺陷
DOI：
10.1557/jmr.1989.1398
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发表时间：
1989
期刊：
影响因子：
--
通讯作者：
J. Knapp
中科院分区：
文献类型：
--
作者：
D. Follstaedt;J. Knapp
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 D _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 D _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 m >0. Thus the phason character identified here may apply to other aluminum-transition metal icosahedral phases as well.
