DROP TUBE SOLIDIFICATION OF AL-CU-FE QUASI-CRYSTALLINE PHASES

Droplets of quasicrystal forming alloys with average atomic compositio n Al63Cu25Fe12 have been produced by a drop tube solidification proces s which allows large liquid undercoolings and leads to special microst ructures. Approximate cooling laws for this drop tube process are firs t derived as a function of droplet diameter and cooling gas properties . Depending on these cooling conditions it is shown that the droplets contain either two (i + beta) or three (i + beta + lambda) major phase s, with a predominant icosahedral i-phase whose (metastable) compositi on Al76Cu15Fe18 differs noticeably from the reported equilibrium value . An annealing treatment at 1073K transforms these multiphased droplet s into an Al63Cu25Fe12 Single phase with a crystalline approximant i'- structure of higher density. It is concluded that the composition of ( quasi)crystalline Al-Cu-Fe phases strongly depends on the solidificati on path and solid cooling rate involved in the elaboration process and that the phase diagram to be considered is often a metastable one.