INFLUENCE OF THE ANNEALING ATMOSPHERE ON THE FORMATION OF AL-CU-FE QUASI-CRYSTALS

Icosahedral samples of nominal composition Al63.5Cu24Fe12.5, which normally grow a rhombohedral approximant crystal, were submitted to heat treatments of various durations, The atmosphere in contact with the surface of the specimens was either dry air or secondary vacuum in order to test whether the formation of the icosahedral phase is sensitive or not to the nature of the atmosphere, Materials annealed in air showed a very thin surface layer of oxide which contained hydrogen, Beneath this oxide layer, small quantities of oxygen and hydrogen were also detected within the underlying lattice, This contamination was sufficient to shift the composition of the samples out of the stability domain of the icosahedral phase and has induced formation of a significant amount of the cubic phase. In contrast, the rhombohedral compound no longer appeared. This behavior goes along with the great sensitivity of the icosahedral phase stability to a sharply defined electron to atom ratio.