Growth morphology of single-crystal grains obtained by directional crystallization of an Al-Cu-Fe alloy
Résumé
Quasicrystalline as well as crystalline faceted single grains of four phases have been obtained during directional crystallization of an Al-Cu-Fe alloy by the Bridgman technique. The monoclinic λ phase Al13(Cu,Fe)4 dominating at high temperatures formed single-crystal lamellas 0.5mm to 1mm thick. The second type of attractive morphological forms exhibiting flux dissolution terraces has been observed on spherical single crystals of β phase Al(Fe,Cu). Rectangular, hexagonal and octagonal shape of dissolution terraces can be seen in the positions of two-, three- and four-fold symmetry axes, respectively. Single quasicrystalline ψ phase Al6Cu2Fe exhibited the icosahedral symmetry with growth forms of a dodecahedron with pentagonal facets. The flux dissolution of β phase plays apparently an essential role in a peritectic reaction leading to quasicrystalline ψ phase formation. Polygonal single grains of ω phase Al7Cu2Fe exhibiting the tetragonal symmetry formed the fourth type of thermodynamically stable growth forms. Single grains of the ω phase crystallized in a form of pellets with an octagonal cross-section. The growth morphology of stable phases has been investigated by the scanning electron microscopy. The chemical composition of the growth forms described has been confirmed by the X-ray microanalysis using a scanning electron microscope whereas the phase composition has been determined by the electron selected area diffraction and X-ray powder diffraction.
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