Electron diffraction and HREM image characteristics from quasicrystalline phases in alloys of Al-Mn and Al-Mn-Si

Abstract

An investigation of the quasicrystalline phases in rapidly cooled alloys of Al₈₆Mn₁₄ and Al₇₄Mn₂₀Si₆ is carried out. The Al-Mn-Si alloy shows three different kinds of phase, the icosahedral phase, the crystalline α-type phase and a third phase which resembles the decagonal T phase found in alloys of Al-Mn. Both kind of compounds (Al-Mn and Al-Mn-Si) have icosahedral diffraction patterns along similar zone axes whose lowest order diffracted vectors are different in magnitude. In the past, these extra rings of reflections were associated with the presence of superstructures. The HREM images obtained along the five-fold axis display image contrast features of similar characteristics when the specimen thickness is increased. These kind of images also show high atomic density features which are curved and modulated in intensity. These features can be seen along the five-fold, three-fold and two-fold axis. Diffraction patterns along the main icosahedral axis give rise to spots with characteristic morphology. A qualitative insight on the nature of these effects can be obtained from simulated images of density waves in a field of phasons and phonons. The Fourier transform of these images can also give a qualitative understanding of some features in the experimental diffraction patterns.