STRUCTURAL RELAXATION OF THE MELT-SPUN ICOSAHEDRAL PHASE AND A NEW-TYPE OF APPROXIMANT IN AL-CU-RU ALLOYS

The formation range and thermal stability of the icosahedral phase produced by melt spinning were investigated in ternary Al Cu-Ru alloys by X-ray diffraction and transmission electron microscopy. The stable face-centred icosahedral (FCI) phase is formed in a wide region where the condition for the valence electron-to-atom ratio e/a almost-equal-to 1-75 is satisfied. Shifts of diffraction peaks to larger angles were observed during an annealing study of the melt-spun Al64Cu23Ru13 sample. The decrease in the quasilattice constant is considered to result from atomic ordering in the FCI phase. A new type of stable approximant (1/0 cubic type) was found to exist near the composition Al71Cu7Ru22. This structure has been determined to be simple cubic with a lattice constant a = 7.7 angstrom. The H3U structure is proposed for this approximant.