Microstructural Peculiarities of Al-Rich Al-La-Ni-Fe Alloys

Abstract

The results of a comprehensive microstructural study of the ternary and quaternary phases in air-cast Al85Ni11−xFexLa4 (where x = 2 and 4 at. pct) polycrystalline alloys by means of optical microscopy, scanning electron microscopy, transmission electron microscopy, scanning transmission electron microscopy, energy-dispersive X-ray microanalysis, and X-ray diffraction are presented. It was found that these alloys contain several phases, namely, fcc-Al, Al11La3, Al3Ni1−xFex, Al9Ni2−xFex, Al8Fe2−xNixLa, and Al3.2Fe1−xNix, in the form of δ-layers in Al8Fe2−xNixLa particles. A high density of other defects in the quaternary Al8Fe2−xNixLa particles was found, and the formation of these defects could be caused by the dendritic type of growth of this phase. The crystal structures of all these defects were revealed by high-resolution scanning transmission electron microscopy together with atomic-resolution energy-dispersive X-ray microanalysis. The thermal stability of the ternary and quaternary phases was also studied. The Al8Fe2−xNixLa phase is metastable and undergoes an irreversible transformation: Al8Fe2−xNixLa → Al9Ni2−xFex + Al11La3. Based on these data, a sequence of solid-phase reactions in these alloys during cooling and heating is proposed.