On the phase transitions of the quasicrystalline phases in the Al-Cu-Fe-Co alloy

Microstructural change with Co content together with heat treatment was studied in rapidly solidified Al-Cu-Fe-Co alloys using X-ray diffractometry and transmission electron microscopy. With an increase in Co content in the as-cast Al₆₅Cu₂₀Fe_15-xCo_x alloys, the relative amount of the icosahedral phase drastically decreased and that of the decagonal phase increased. In the as-melt spun alloys, effects of Co content are critically changed at around 5 at% Co. For the less Co containing alloys (below 5 at%), the icosahedral phase proved to be the major phase, while the 5 at% Co containing alloy showed the coexistence of the icosahedral and decagonal phases. The 8 at% containing alloy showed the monolithic decagonal phase in the as-melt spun state. Unlike as-cast alloys, all the as-melt spun alloys showed no trace of the λ-phase. However, after heat treatment of ribbons, λ-phase newly appeared in the Al₆₅Cu₂₀Fe₁₀Co₃ and Al₆₅Cu₂₀Fe₁₀Co₅ alloys. The λ-phase showed orientation relationships with surrounding icosahedral or decagonal phases.