Organization of defects in the first cubic approximant of the quasicrystal Al₆Li₃Cu

Abstract

Transmission electron microscopy of the R phase, cubic approximant, of the quasicrystal Al₆Li₃Cu reveals a very characteristic microstructure of defects. It is well known that the R phase contains a large number of stacking faults; in the present work an organization of these defects has been detected by electron diffraction. When the density of defects is very high (distance between faults ∼10 Å) the pattern made by the fundamental and superlattice spots resembles the electron diffraction pattern of the quasicrystal. In the same samples (as-cast R-phase ingots) quasicrystalline zones are found among faulted crystallines areas. The cut-and-projection method is used to describe the faulted approximant. In this representation, an accumulation of defects produces an increase in the average slope of a selected band, the crystal selected band becoming closer to that of the quasicrystal. It seems to correspond fairly well with the evolution of the diffraction pattern observed in areas of increasing fault density.