Vacancies and atomic processes in intermetallics – From crystals to quasicrystals and bulk metallic glasses†

A review is given on atomic vacancies in intermetallic compounds. The intermetallic compounds cover crystalline, quasicrystalline, and bulk metallic glass (BMG) structures. Vacancies can be specifically characterized by their positron lifetimes, by the coincident measurement of the Doppler broadening of the two quanta emitted by positron―electron annihilation, or by time-differential dilatometry. By these techniques, high concentrations and low mobilities of thermal vacancies were found in open-structured B2 intermetallics such as FeAl or NiAl, whereas the concentrations of vacancies are low and their mobilities high in close-packed structure as, e.g., Li 2 -Ni 3 Al. The activation volumes of vacancy formation and migration are determined by high-pressure experiments. The favorable sublattice for vacancy formation is found to be the majority sublattice in Fe 61 Al 39 and in MoSi 2 . In the icosahedral quasicrystal Al 70 Pd 21 Mn 9 the thermal vacancy concentration is low, whereas in the BMG Zr 57 Cu 15.4 Ni 12.6 Nb 3 Al 10 thermal vacancies are found in high concentrations with low mobilities. This may determine the basic mechanisms of the glass transition. Making use of the experimentally determined vacancy data, the main features of atomic diffusion studies in crystalline intermetallics, in quasicrystals, and in BMGs can be understood. Manfred Fahnle and his group have substantially contributed to the theoretical understanding of vacancies and diffusion mechanisms in intermetallics.