He-atom scattering was used to characterize the adsorption properties of Na and K on a faceted Al(111) surface, to gain information about the differences in the growth of alkali metals at different temperatures, and to study the interaction between the He atoms and the surfaces. At 100 K, both Na and K were observed to grow in a layer-by-layer mode on top of the surface, whereas at 300 K, the alkalis intermix with the substrate and do not grow in a layer-by-layer mode. For adsorption at 300 K, the He-atom scattering from facets indicated that the primary step direction on the surface changes from being parallel to [10] to being parallel to [2]. The He-atom scattering potentials of the alkali overlayers were in general quite smooth (evidenced by a small non-specular diffraction intensity) but were more corrugated for the intermixed structures compared to the on-surface structures and for Na compared to K. The former was attributed primarily to the larger in-plane metallic bonding for the on-surface structures.