Abstract
Deviations from perfect icosahedral m35 point-group symmetry in icosahedral particles in rapidly solidified Al-3Cu-l·5Li-lMg and Al-3Cu-l·5Li-lMg-0·1Zr have been investigated experimentally by a combination of conventional and high-resolution transmission electron microscopy. The as-rapidly-solidified icosahedral particles are nearly spherical and are typically 0·1–0·2 um in diameter. Four types of deviation from perfect icosahedral symmetry are observed:
(1) Dark-field images show speckled contrast on a scale of approximately 20–50nm.
(2) Selected-area electron diffraction patterns show elongation and radial and circumferential displacement of individual diffraction spots, with the degree of distortion varying from one particle to another, and from one place to another, on a scale of about 100 nm inside a given particle. Unusual triangularly shaped electron diffraction spots from whole icosahedral particles result from superimposed local spot distortions.
(3) High-resolution images show displacive inhomogeneities such as fringe shifts, terminations and curvature, on a scale of approximately 10–40 nm.
(4) Energy-dispersive X-ray analysis shows chemical inhomogeneity on a scale of approximately 25–50 nm within individual icosahedral particles.
The scale of speckled contrast, diffraction spot distortion, and displacive and chemical inhomogeneities corresponds quite well to that expected from X-ray and electron diffraction experiments for an icosahedral quasicrystalline structure consisting of slightly misoriented micrograms.