Role of Fe substitution and quenching rate on the formation of various quasicrystalline and related phases

摘要

We have investigated Fe substituted versions of the quasicrystalline (qc) alloy corresponding to Al_(65)Cu_(20) (Cr, Fe)_(15) with special reference to the possible occurrence of various quasicrystalline and related phases. Based on the explorations of various compositions it has been found that alloy compositions Al_(65)Cu_(20)Cr_(12)Fe_3 and Al_(65)Cu_(20)Cr_9Fe_6 exhibit interesting structural phases and features at different quenching rates. At higher quenching rates (wheel speed approx 25 m/sec) all the alloys exhibit icosahedral phase. For Al_(65)Cu_(20)Cr_(12)Fe_3 alloy, however, both the icosahedral and even the decagonal phases get formed at higher quenching rates. At higher quenching rate, alloy having Fe 3 at percent exhibits two bcc phases, bed (a = 8.9 A) and bccII (a = 15.45 A). The orientation relationships between icosahedral and crystalline phases are: Mirror plane || (001)_(bcc I) and (351)_(bcc II), 5-fold || (113)_(bcc II) and 3-fold || (110)_(bcc II). At lower quenching rate, the alloy having Fe 6 at percent exhibits orthorhombic phase (a = 23.6 A, b = 12.4 A, c = 20.1 A). Some prominent orientation relationships of the orthorhombic phase with decagonal phase have also been reported. At lower quenching rate (approx 10 m/sec), the alloy (Al_(65)Cu_(22)Cr_9Fe_6) shows the presence of diffuse scattering of intensities along quasi-periodic direction of the decagonal phase. For making the occurrence of the sheets of intensities intelligible, a model based on the rotation and shift of icosahedra has been put forward.