Quasicrystals, crystals and multiple twins in rapidly solidified Al---Cr---Si, Al---Mn---Si and Al---Mn---Cr---Si alloys

Abstract

The coexistence of quasicrystals and rational approximant structures (RAS) has been observed in melt-spun Al₈₀Cr₁₄Si₆, Al₈₀Mn₁₄Si₆ and Al₇₅Mn₁₀Cr₅Si₁₀ alloys. The presence of a b.c.c. α -AlMnSi phase in Al---Mn---Si and α -AlMnSi(Cr) phase in Al---Mn---Cr---Si has been seen. A multiple twinning around an irrational axis of the RAS has been reported in an aggregate of fine size cubic crystallites in all three alloys. Selected area diffraction patterns show that the crystalline aggregate symmetry is linked to the icosahedral point group symmetry (m35). Various ways of expressing the twin relationship in the cubic crystalline aggregates have been discussed. The thermal stability of the icosahedral phase at high temperatures reveals that the icosahedral phase in Al---Mn---Si and Al---Mn---Cr---Si alloys transforms to α-AlMnSi at temperatures of 690 and 670 K, respectively. In Al---Cr---Si alloy, heating to a high temperature (615 K) leads to the transformation of the icosahedral phase into a new metastable phase having an ordered cubic structure equivalent to a-AlMnSi. The occurrence of multiple twinning leading to icosahedral symmetry in the as-spun Al---Cr---Si alloy is presumably due to this metastable phase.