A NEW CRYSTALLINE APPROXIMANT OF THE DECAGONAL PHASE IN AL-MN ALLOYS

A new approximant of the decagonal phase has been found in an Al-22 at.% Mn alloy by electron microscopy. The structure is C face-centred orthorhombic with the lattice parameters a = 2.41 nm, b = 3.32 nm and c = 2.40 nm (experimental error: +/- 2%). Selected-area electron diffraction patterns from regions containing differently oriented grains of this crystalline phase with a common c axis show tenfold symmetry. The phason strain relating the approximant to the quasicrystal is given. Simulated electron diffraction patterns calculated by applying this phason strain to the diffraction the patterns of decagonal Al78Mn22 are in good agreement with the experimental ones. Furthermore, a structural description of the new phase is obtained from the Fourier transformation of the simulated electron diffraction patterns. The new phase is often found to occur together with Al3Mn, with a fixed orientation relationship. The structural relationship of these two phases is discussed.