Icosahedral phase

Fig. 8. Electron micrograph showing crystallization of icosahedral phase from glassy Pd—U—Si alloy.

Fig. 29. Icosahedral phase in electrodeposited Mn-Al alloys (a) bright field image (b) electron diffraction pattern showing 5-fold symmetry. Reproduced from Grushko et al. [126] by permission of Elsevier.

Icosahedral quasicrystals. Many icosahedral phases (both stable and metastable) have been identified in several systems, mainly Al-based but also Mg-based, Ti-based, etc. 

An important result is also that many approximants of these quasicrystalline phases have similar conduction properties. For example the crystalline a-AlMnSi phase with a unit cell size of about 12 A and 138 atoms in the unit cell has a conductivity of about 300(Qcm) at low temperature [7,9]. The conductivity has the same defect and temperature dependence as that of the AlCuFe and AlPdMn icosahedral phase. There is, to our knowledge, no experimental result on the optical conductivity of this a-AlMnSi phase, but it is very likely that it is similar to that of AlCuFe and AlPdMn icosahedral phase. 

Pauling countered the claim of Schechtman et al. that the x-ray powder diffraction pattern of the icosahedral phase of MnAle could not be indexed with any Bravais lattice by showing that the patterns of the quasicrystals could be explained by twinning [80-94]. Pauling s explanation was based on the idea that the truncated icosahedron shown in Figure 8 is the basic structural unit of the icosahedral phase of MnAl6. According to him,... 

Sc considerably lowers the glass forming ability of Al alloys compared to rare earth metals (Ce). No amorphous phase was formed in rapidly solidified binary Al-Sc alloys. An unstable quasicrystalline icosahedral phase was first found in the alloy A185Sci5. Substitution of Ce for Sc in Al91Ce9 xScx alloys caused a transfer to a mixed amorphous-nanocrystalline and then to nanocrystalline structure. In Al85Nii0Ce5 xScx alloys a mixed amorphous-nanocrystalline structure appeared in the alloy with x=5 at a lowered solidification rate (increased ribbon thickness). 

The icosahedral phase can be prepared from glassy alloys of the composition Pd,Q0 for x = 20 at.% by annealing between 480 and 540°C (Poon et... 

There are many examples which show that the strength and hardness of an alloy can be increased by a factor of 4-7 if the grain size is decreased into the nanometer range. One such example is the rapidly solidified Al96Cr3Ce]COjc x = 1-2%) alloy containing as a main component a nanosized quasicrystalline icosahedral phase [35]. A similar strengthening is found in many metals such as TiAl [36], Cu [37], and other... 

Crystal lattices have symmetry elements such as rotation axes, mirror planes, inversion points, and combinations of these. A crystalline lattice has translation symmetry, except for quasi-crystals or icosahedral phases, which have lattices with point symmetry elements only. Glasses are amorphous solids that do not have any symmetry element in their lattices. 

Figure 56. Diffraction pattern of the icosahedral phase of quenched Al-Mn alloy exhibiting fivefold symmetry (Courtesy of G. Van Tendeloo)...

Fig. 68. (a) Bright-field electron micrograph and (b) selected-area electron diffraction pattern of a rapidly solidified AljjMneCe alloy. The numbers 1, 2, 3 and 4 in (b) represent the d 11000), (111100), (211111) and (221001) reflection rings, respectively, of the icosahedral phase. 

Fig. 70. (a) High-resolution TEM image and (b-d) nanobeam dififiaction patterns taken from the icosahedral phase field in a rapidly solidified AI94 5Cr,Ce, Co, s alloy. Patterns (b), (c) and (d) were taken Ifom the regions A, B and C, respectively, with a diameter of I nm. 

Figure 105 summarizes the changes in the as-quenched phase and the atomic diameter in the Zn55Mg4oRs alloys with the R elements. We note a systematic change dependent on the atomic size of the R elements. The as-quenched structure exhibits the amorphous phase when it contains La, Ce, Pr or Eu, and the icosahedral phase wdien it contains Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu or Y. It seems that the atomic size of the R element is an important factor for the formation of amorphous and icosahedral phases. The alloys containing an R element with an atomic diameter larger than 0.366 nm, except Yb, form... 

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