Transformation of the decagonal quasicrystalline phase to a B2 crystalline phase in the Al-Cu-Co system by high-energy ball milling

Samples of a decagonal quasicrystalline phase, located in the Al-Cu-Co system and synthesized by a slow cooling technique, have been mechanically milled in a high-energy planetary ball mill for 10, 20 and 30h. The milled powders, as well as powders that had been annealed (after milling) for times ranging from 30 to 150min at 600C, were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. A phase transformation from the decagonal phase to a B2 crystalline phase during highenergy ball milling is reported here for the first time. Powders milled for more than 10h contained predominantly the B2-type crystalline phase with a lattice parameter of 0.29nm. This crystalline phase was found to be quite stable after milling for 30h and also on subsequent annealing at 600C. These experimental results lend support to an earlier suggestion that the decagonal phase in Al-Cu-Co is actually less stable than the B2 phase at low temperatures.