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 the icosahedral-to-rhombohedral transformation in Al-Fe-Cu has been characterized as being constituted of an alternate mixing of two pentagonal structures. These pentagonal phases have been identified from high-resolution electron microscopy and diffraction patterns. The linear phason fields relating their structure to the icosahedral phase have been fully determined. Such a transient state results from the transformation of a modulated icosahedral phase, itself obtained from the icosahedral state and precedes the formation of the crystalline multidomain structure of the rhombohedral phase. The interpretation proposed for the formation of these pentagonal phases can be related to that given in a previous paper on the modulated icosahedral state, where the modulation was found to result from the propagation of phason waves along fivefold directions of the six-dimensional hyperspace.