AR+-ION-IRRADIATION-INDUCED PHASE-TRANSFORMATIONS IN AN AL70CO15NI15 DECAGONAL QUASI-CRYSTAL

Phase transformations in the Al70Co15Ni15 decagonal quasicrystal (DQC), induced by 120 keV Ar+ ion irradiation at room temperature, have been studied by means of transmission electron microscopy. The transformation behaviour is dependent on the dose, the dose rate and the thickness of the specimen. In the Ar+ -ion-affected depth the ordered DQC phase transformed into disordered DQC and a b.c.c. phase when the dose is equal to or less than 5 x 10(13) Ar+ cm-2. Then the b.c.c. phase was ordered to a CsCl-type structure when the dose was increased to 10(14)-5 x 10(15) Ar+ cm-2. However, the ordered CsCl structure transformed back to disordered b.c.c. phase with further increase in the irradiation dose and dose rate. This disordering-ordering-disordering transformation sequence is explained by the Dienes-Potter ordering rate expressions.