Quasicrystals rapid solidification

As the first reported quasicrystals were metastable phases at room temperature produced by rapid solidification, they were consequently of poor quality. Stable quasicrystals have since been discovered that have revealed very high strucmral perfection, even comparable to single crystals. This discovery made it possible to apply conventional solidification techniques. The preferred method appears to be system-specific, as it depends on the temperature stability of the quasicrystalline phase. If the quasicrystal is only stable at elevated temperatures, for example, it can decompose into a crystalline phase if the melt is solidified slowly. If the phase is thermodynamically stable down to room temperature, as is the case for Al-Pd-Mn, quasicrystals can be grown with conventional cooling rates (e.g. 10°C/h). 

We also list three other alloy phase types of current interest that are not treated here in detail. Quasicrystals are alloy phases partially or completely lacking translational symmetry (see Quasicrystals) they form both equilibrium and nonequilibrium alloy phases. Metallic glasses lack crystalline symmetry entirely they are always metastable and generally require gas-phase deposition or rapid solidification to be retained, although in some cases their equilibration kinetics are so slow that they can be prepared in bulk from the melt (bulk metallic glasses). 

Rapid solidification processes are successfully used for A1 alloys to form a dispersion particles of intermetallic phases, which resist coarsening and strengthen the alloys at elevated temperatures. It has recently been shown that metastable intermetallic phases with a quasicrystalline structure, mainly of the icosahedral type, can also be produced by rapid solidification [23], As distinct from a crystalline state, translational long-range order is absent in quasicrystals, but there is rotational symmetry with 5-, 8-, 10- or 12-fold axes, which is forbidden in crystalline materials. The absence of translational symmetry in all three orthogonal directions is characteristic of the icosahedral structure [24],... 

Rapid solidification of TaioFe2oAl7o gives rise to a quasicrystal phase with icosahedral symmetry [1988Tsa, 1989Sri, 1993Kel], and it has a quasilattice constant of a = 460 pm [1989Sri]. 

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