On the new structural phases in Al₆₅Cu₂₀Cr₁₅ quasicrystalline alloy

Abstract

The quasicrystalline (qc) alloy Al₆₅Cu₂₀Cr₁₅, unlike its Ru- and Fe-bearing counterparts like Al₆₅Cu₂₀Ru₁₅ and Al₆₅Cu₂₀Fe₁₅, is a metastable phase. This qc alloy has been shown to possess several structural variants and curious structural characteristics. We have investigated the qc alloy Al₆₅Cu₂₀Cr₁₅ with special reference to the possible occurrence of new structural variants. TEM exploration of the as-quenched qc alloy has indeed revealed the existence of several new phases. These are (i) body-centered cubic (bcc) (a = 12.60 Å, disordered) and simple cubic (s.c.) (a = 12.60 Å, ordered), which are the 1/1 approximants of the primitive icosahedral phase (i phase); (ii) a twice order-induced modulated cubic phase (bcc, a = 25.20 Å) which has been shown to correspond to 1/1 approximant of the ordered i phase [i.e., face-centered icosahedral (FCI)]; and (iii) real crystalline bcc (a = 8.90 Å) and face-centered cubic (fcc) (a = 17.98 Å) phases possessing a specific orientation relationship with the icosahedral matrix phase. Tentative structural models showing the interrelationships between the bcc/fcc phases have been outlined.