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 or thorhombic with the lattice parameters a = 2.41 nm, b = 3.32 nm and c = 2.40 nm (experimental error: +/- 2%). Selected-area electron diffrac tion patterns from regions containing differently oriented grains of t his crystalline phase with a common c axis show tenfold symmetry. The phason strain relating the approximant to the quasicrystal is given. S imulated electron diffraction patterns calculated by applying this pha son strain to the diffraction the patterns of decagonal Al78Mn22 are i n good agreement with the experimental ones. Furthermore, a structural description of the new phase is obtained from the Fourier transformat ion of the simulated electron diffraction patterns. The new phase is o ften found to occur together with Al3Mn, with a fixed orientation rela tionship. The structural relationship of these two phases is discussed .