Evidence for structural disorder in the icosahedral phase

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Abstract
We report transmission electron microscopy results on quasicrystalline samples of Al–25 wt% Mn and Al-38 wt% Mn-5 wt% Si melt-spun flakes. The selected-area diffraction patterns (SADPs) from ‘off-axis’ orientations show not only sharp diffraction maxima but also a diffuse ring, invariant of sample thickness, indicating that it does not arise from amorphous surface oxide. Over-exposed SADPs taken from axial orientations also showed the weak diffuse ring. We suggest that our results indicate the presence of disordered material within the icosahedral phase. Models for the icosahedral phase which involve an assembly of icosahedral clusters inherently contain interstitial voids, which may contain disordered material. Thus our results support these models rather than those derived from space-filling filings, or multiple twinning.