DECOMPOSITION OF ICOSAHEDRAL QUASI-CRYSTALS IN AL-CU-CR ALLOYS

The structural transformations occurring during isothermal heat treatm ent of splat-quenched Al85-xCuxCr15 (x = 0, 5, ...20), containing a la rge amount of the icosahedral phase, were studied by analytical electr on microscopy and X-ray diffraction. The as-quenched specimens contain only a small volume fraction of crystalline material (about 10% for x = 0 down to nearly 0% for x = 20), which transforms either to an equi librium crystalline phase (x = 0, 5) or to an icosahedral phase (x gre ater-than-or-equal-to 10) as a first reaction on heating. The icosahed ral phase itself decomposes to the crystalline equilibrium phases via a single-step eutectoid (x = 1 5, 20) or a two-step polymorphic transf ormation (x less-than-or-equal-to 10), the intermediate phases of whic h are either crystalline (unidentified) for x = 0 or quasicrystalline decagonal for x = 5 and 10. The existence of a decagonal phase has not previously been reported in this alloy system. The lattice parameters and the measured compositions of all identified phases are given.