Quasicrystals in an aluminium alloy matrix and the transformation to alpha-AlMnSi via intermediate stages

During heat treatment of an industrially produced Al high-Mn alloy, small particles of the quasicrystalline icosahedral AlMnSi phase were precipitated, with a definite orientation relation to the Al matrix. Upon further annealing, the particles coarsen and transform, through intermediate stages into the cubic alpha-AlMnSi phase. The intermediate stage is characterized by faults on {110}(a)lpha planes, with fault vectors of type delta=1/16[5(3) over bar0$]] as revealed by diffuse streaks in electron diffraction patterns. The fault vector component normal to the fault plane produces displacement of diffraction spots relative to the alpha-AlMnSi reflections. The faults produce a local arrangement of MacKay icosahedra that can be described by a rhombohedron (alpha') which is complementary to the rhombohedron which constitutes the primitive unit cell of the bcc packing of MacKay icosahedra in alpha-AlMnSi. A model for a composite defect, built from three {100} faults is constructed.