Detection and analysis of fine particles acting as the nucleation site of aluminum grains using FIB-TEM

The paper describes a technique development using focused ion beam and transmission electron microscopy to find and analyze fine particles which acted as the heterogeneous nucleation substrate for aluminum grains. An aluminum alloy was melted, sheared intensively by a melt conditioning unit, and cast by a laboratory scale horizontal twin roll caster. The intensively sheared alloy showed low segregation of solute elements on the surface as well as at the centerline, probably by enhanced nucleation of aluminum grains from fine oxide particles. In order to verify the possibility of the heterogeneous nucleation of aluminum grains, several techniques using focused ion beam and transmission electron microscopy were tried to find any oxide particles in the grains. Finally, a technique using the lifting-out of a lamella combined with the serial milling by focused ion beam detected a fine particle at the center of one aluminum grain and transmission electron microscopy confirmed the particle as an oxide. The intense nucleation of aluminum grains on fine dispersed oxides in the intensively sheared aluminum alloy can induce uniform distribution of solute elements and consequently minimize the severe centerline and surface segregations.