High-resolution electron microscopy study of particle dispersion and precipitation in a nanostructured Al–2%Fe alloy

Al–Fe alloys have drawn attention of researchers for many decades since Fe is the most common impurity in Al that, because of its low solid solubility, forms brittle aluminides that affect the mechanical properties. Thus, it is interesting to provide microstructure control of these intermetallics and achieve high strength, thermal stability, and high electrical conductivity. High-pressure torsion (HPT) has been applied to an Al–2%Fe alloy with two different eutectic phases containing Al₆Fe and Al₃Fe intermetallics, to refine, disperse and partially dissolve them. The microstructure after HPT processing at different levels of imposed strain was observed using aberration-corrected high-resolution scanning/transmission electron microscopy (HR-S/TEM) to perform a detailed analysis of the dispersion of intermetallic phases and their orientation relationships with the Al matrix. The precipitates from the supersaturated solid solution achieved by post HPT aging, responsible for age-hardening were also studied in detail. Fragmentation of secondary Al₃Fe and Al₆Fe phases occurs down to sizes in the nano-scale and a fine dispersion of particles with coherency with the matrix in lattice planes with similar spacing (low misfit strain) was observed even after high straining by HPT. Semi-coherent Al₆Fe is the dominant precipitate in the peak-aged condition and phase transformation of the particles from Al₆Fe to Al₃Fe occurs with aging. The Al₃Fe particles loose coherency as they grow in size. It was possible to explain the age hardening effect with respect to the coherency and orientation relationship of the Fe-containing particles in the nano-scale, by a similar mechanism for Al alloys with miscible elements, which has not been reported for ultrafine-grained Al–Fe alloys to date.