An investigation on the transformation of the icosahedral phase in the Al-Fe-Cu system during mechanical milling and subsequent annealing

Abstract Icosahedral quasicrystalline material in the Al-Fe-Cu system was mechanically milled in an attritor ball mill (Szegvari attritor) for 1, 3, 6 and 10 h in dry air, at a speed of 400 rev min−1 and with a ball-to-powder ratio of 20 to 1. Structural transformations and the consequent phase evolutions during mechanical milling and subsequent heat treatments were studied by X-ray diffraction, differential thermal analysis (DTA) and transmission electron microscopy techniques. After milling for 1 h, the evolution of disordered B2 phase (a=0.29 nm) was observed to coexist with the parent icosahedral phase, whereas a microstructure consisting of nanosized B2 particles distributed in an amorphous matrix was observed after further milling (3–10 h). However, no sharp peak corresponding to the phase transformation was identified in DTA, but the microstructure of the powder milled for 10 h after the DTA experiment was found to transform to a mixture of icosahedral and B2 phases, where B2 appears to be the major phase in contrast with that in as-cast material. Isothermal heat treatment of powder mechanically milled at 850°C for 10 and 20 h led to complete transformation to a single B2 phase with a high degree of long-range ordering. The implication of these transformations will be discussed with reference to their relative stabilities among the competing phases during milling and subsequent heat treatment.