Phase transitions in quasicrystals - the example of decagonal Al-Co-Ni

The numerous phase transformations of quasicrystals observed can be separat ed into two distinct classes, the transitions from quasicrystals-to-quasicr ystals and from quasicrystals-to-crystals of periodic approximants. The for mation of modulated quasicrystals driven by chemical ordering represents a member of the first class. The transformation of a quasicrystal to orientat ionally twinned domain structures of approximants belongs to the latter cla ss. The characteristics of some class-one transformations with special emphasis on their experimental evidence are discussed on the example of phase trans formations in the ternary alloy system Al-Co-Ni. State-of-the-art technique s for acquiring in situ X-ray diffraction data at high temperature of quasi crystalline material undergoing a phase transition are presented. Methods s uitable for accurate and reproducible determination of structural changes o f specimens in thermodynamic equilibrium are shown. (C) 2000 Elsevier Scien ce B.V. All rights reserved.