RHOMBOHEDRAL-ICOSAHEDRAL REVERSIBLE PHASE -TRANSITION OF THE ALLOY AL63.5FE12.5CU24

A reversible phase transformation between icosahedral and rhombohedral structures has been studied by transmission electron microscopy. Some transient states have been characterised by x-ray diffraction using a 4 circles goniometer on a synchrotron beam line. The microcrystalline rhombohedral phase is stable at low temperature; above 675-degrees-C it transforms into an icosahedral phase via an intermediate pentagonal structure. The icosahedral phase, metastable at low temperature, tran sforms between 500 and 675-degrees-C into the rhombohedral phase via d ifferent transient states: modulated icosahedral phases and pentagonal approximants. These intermediate states have been interpreted as resu lting of shear waves of phason type, propagating along the basis vecto rs of a 6-D hyper cubic lattice and polarized along the perpendicular space. The chemical composition variations associated to these shear w aves and predicted by the theory have been experimentally observed.