Phase transitions in quasicrystals — the example of decagonal Al–Co–Ni

The numerous phase transformations of quasicrystals observed can be separated into two distinct classes, the transitions from quasicrystals-to-quasicrystals and from quasicrystals-to-crystals of periodic approximants. The formation of modulated quasicrystals driven by chemical ordering represents a member of the first class. The transformation of a quasicrystal to orientationally twinned domain structures of approximants belongs to the latter class. aracteristics of some class-one transformations with special emphasis on their experimental evidence are discussed on the example of phase transformations in the ternary alloy system Al–Co–Ni. State-of-the-art techniques for acquiring in situ X-ray diffraction data at high temperature of quasicrystalline material undergoing a phase transition are presented. Methods suitable for accurate and reproducible determination of structural changes of specimens in thermodynamic equilibrium are shown.