DECOMPOSITION OF ICOSAHEDRAL QUASI-CRYSTALS IN AL-CU-FE ALLOYS

The icosahedral phase in melt-spun Al-Cu-Fe alloys can be either stable or in a metastable state. Two modes of decomposition of the metastable icosahedral phase were observed: Discontinuous decomposition of the icosahedral phase proceeds by migration of a reaction front into the icosahedral phase as observed in A177Cu13Fel0 by a peritectoid reaction, in Al70Cu20Fe10 by a polymorphic reaction and in Al65Cu20Fe15 by precipitation. Continuous decomposition as observed in Al60Cu30Fe10 proceeds by development of "phason strains" inside the icosahedral phase without any reaction front separating the icosahedral phase from its rational approximants. All these decomposition reactions are thermally activated and their kinetics are controlled by diffusion.