Multiply twinned oxide-free Al nanoparticles, with diameters of 2 nm t
o 50 nm, were obtained from metastable theta-Al2O3 particles by an ele
ctron beam irradiation technique using a high-resolution transmission
electron microscope on a room-temperature stage. The phenomenon of for
mation, the morphology and the multiply twinned structures of the Al n
anoparticles were investigated by the in situ observation method. It i
s clear that the morphology of polyhedral multiple twinned Al nanopart
icles were mostly decahedra, and that the decahedra were categorized i
nto pentagonal, spherical, ellipsoidal, and lamellar morphology. (C) 1
998 Acta Metallurgica Inc.