  The thermal stability of nanocrystalline (nc) Al has been studied by means of
positron lifetime spectroscopy and X-ray diffraction (XRD), prepared by
compacting nanoparticles under high pressures. Those nanoparticles were
produced by the flow-levitation (FL) method [1] and the arc-discharge (DC)
method. Effect of oxide at nanoparticle surface on structure stability was
investigated especially in this experiment. The positron lifetime results
reveal that vacancy clusters in grain boundaries are dominant positron traps
according to the high relative intensity of the positron lifetime {\tau}2 in
both samples. The mean grain size of the sample consisting of nanoparticles
with partially oxidized surfaces is almost unchanged after aging at 150 for 84
h, with that of the sample consisting of pure nc Al increasing. The partially
oxidized surfaces of nanoparticles hinder the growth of grain when aging at
150, vacancy clusters related to Al2O3 need longer aging time to decompose,
which implies that the oxide stabilizes the microstructure of the
nanomaterials. The effect is beneficial for nc materials to keep excellent
properties.
