Approximants and Intermediate Stages during Transformation from a Quasicrystalline to a Crystalline Phase in an Al–Mn–Si Alloy

Abstract

In precipitate particles formed by heat treatment of industrially cast Al-(1.8–1.6wt.%)Mn alloys, a sequence of stages intermediate between the quasicrystalline I phase and the cubic α-AlMnSi phase have been found. Their electron diffraction patterns and images can be interpreted in terms of faults on {110}α planes, with a fault vector [{\bar 3}50]/16 or [{\bar 1}τ0]/2τ2, where τ = 1.618 is the golden mean. A lattice generated by such faults on every (110)α plane has as its unit cell a prolate rhombohedron (α′) that is complementary to the oblate rhombohedron (α) that describes the packing of the MacKay icosahedra in α-AlMnSi. Domains of this new lattice have been observed. Based on the two rhombohedra, a cubic Pa3 structure model (α′′) with cell dimension τ times α-AlMnSi and nearest-neighbour intericosahedral vectors along all threefold axes of the icosahedron is proposed as a second approximant to the I phase in the system, with α-AlMnSi as the first approximant. Calculations of diffraction patterns and Patterson sections are presented as support for this proposal.