Evidence for a new quasicrystalline transition state in Al65Cu20Fe15 alloy

Abstract

In Al65Cu20Fe15 alloy icosahedral quasicrystals are formed of extraordinary perfection. On the other hand, there are indications that the quasicrystalline phase is only stable at temperatures above about 800°C. Here we report on a transmission electron microscopy study of a new phase which represents a transition stage between the icosahedral quasicrystalline (IQ) phase and a crystalline phase. It can be described by a modulation of the IQ phase along one of the five-fold axes (A5) of the original IQ phase.The icosahedral phase in Al65Cu20Fe15 was obtained by arc melting the high purity elements and subsequent slow cooling. The material was then heat treated at 700°C for 24 hours and again slowly cooled to room temperature. During this treatment the regular IQ phase transformed into a phase characterized by the occurence of translation periodicity along one of the five-fold axes of the original IQ phase. This is demonstrated by a commensurate arrangement of electron diffraction spots only along one of the original A5 (Fig. 1). High-resolution electron microscopy (HREM) corroborates this observation (Fig. 2).