Development of quasicrystal morphology in gas-atomized icosahedral-phase-strengthened aluminum alloy powders

Abstract

The microstructures in gas-atomized powders and in blind-die-compacted material from three Al-Cr-Mn-Co-Zr alloys have been studied using X-ray diffraction and electron microscopy. The materials consist of a mixture of FCC Al and icosahedral quasicrystalline phases, but three different phase distributions are observed which depend on the powder particle size, and hence on the cooling rate. The finest particles contain a nano-composite mixture of equiaxed Al grains and fine (<200 nm) quasicrystalline dispersoids. Coarser particles exhibit large cellular dendritic Al grains with thin films of Co- and Cr/Mn-rich phases at the cell boundaries and coarser (up to 2 µm) quasicrystals at the particle surfaces. The largest particles contain coarse (up to 5 μm) radial quasicrystalline growths within the particle. The same microstructures were observed for each alloy, but the volume fractions of the microstructural types varied with the alloy content. These effects are explained on the basis of competing nucleation and growth phenomena during solidification of the atomized droplets. Since these microstructures are retained in the bulk material following blind-die-compaction, they could have a profound influence on the mechanical and other properties of materials made from such powders. The importance of such effects for alloy and process design are discussed.