PENTAGONAL PHASES AS A TRANSIENT STATE OF THE REVERSIBLE ICOSAHEDRAL RHOMBOHEDRAL TRANSFORMATION IN AL-FE-CU

From a transmission electron microscopy study, a transient state of th e icosahedral-to-rhombohedral transformation in Al-Fe-Cu has been char acterized as being constituted of an alternate mixing of two pentagona l structures. These pentagonal phases have been identified from high-r esolution electron microscopy and diffraction patterns. The linear pha son fields relating their structure to the icosahedral phase have been fully determined. Such a transient state results from the transformat ion of a modulated icosahedral phase, itself obtained from the icosahe dral state and precedes the formation of the crystalline multidomain s tructure of the rhombohedral phase. The interpretation proposed for th e formation of these pentagonal phases can be related to that given in a previous paper on the modulated icosahedral state, where the modula tion was found to result from the propagation of phason waves along fi vefold directions of the six-dimensional hyperspace.