An electron-microscopic investigation of the transformation behaviour of Al6Mg4Cu quasi-crystalline alloy

In the melt-spun icosahedral Al₆Mg₄Cu alloy, non-uniform grain size distribution and overlapping grains giving rise to periodic Moire patterns were noted. On heating, this phase was observed to transform through the formation of nuclei, predominantly at the grain boundaries, but occasionally also inside the grains. In the early stages, some semicoherent phase appeared to form but transformed eventually to the equilibrium phase, with the formation of holes. Lattice imaging of the partially transformed alloy revealed both the icosahedral and transforming phases to be periodic. The present observations on structure and transformation behaviour seem to be consistent with a model based on pentagonal chains present in the liquid and as-quenched alloys.