Nanopore formation in B2 iron aluminide by vacancy clustering

Nanopores which have several tens nanometer in diameter are formed by vacancy clustering near surfaces after annealing at 723 K of rapidly-solidified B2 FeAl. The nanopore formation was confirmed by SEM and AFM after the heat treatment, and by TEM during in-situ heating experiments. Pore shape depends on surface orientation. This is well consistent with morphology and crystallography of pores created inside with pore surfaces faceting toward {100} planes Pore density seems to be increased with an increase in Al content, corresponding to the composition dependence of excess vacancy concentration determined by density and lattice constant measurements. The pore densities near surfaces are predicted by a simple vacancy cluster model using the vacancy concentrations in the ribbons, being in good agreement with the observed ones.