Thermal stability and oxidation behavior of Al-containing nanocrystalline powders produced by cryomilling

Al-containing nanostructured coatings provide excellent protection from high temperature corrosion. Aluminum oxide scales generally provide better oxidation resistance and yield lower oxidation rates than other oxide scale compositions. In this study, nanocrystalline 316L stainless steel containing 6 wt.% Al was synthesized using cryogenic milling (cryomilling). Complete alloying was obtained after 32 h of milling and the average grain size was found to be 7 nm. High temperature thermal stability and oxidation kinetics of the alloyed powders were examined. The powder demonstrated good grain growth stability at 500 degrees C, at which point, the powders had been heat treated for 120 h and the average grain size was found to be 11.4 nm. The oxidation kinetics of the powder were studied for 48 h at 500, 800, and 1,000 degrees C, respectively. For comparison, conventional 316LSS powder was also tested. Nanocrystalline 316LSS-6 wt.% Al showed lower weight gain than the conventional 316LSS powders. During the oxidation of nanocrystalline 316LSS-6 wt. % Al at 500 degrees C, protective aluminum oxide scale formed at the surface. At 800 degrees C and 1,000 degrees C, most of the nanocrystalline 316LSS-6 wt.% Al particles showed completed outer aluminum oxide scale. However, at 800 and 1,000 degrees C, some particles showed growth of chromium oxide scale underneath the aluminum oxide scale. In those samples, Al depletion was also observed due to a non-homogenous distribution of Al during cryomilling. The activation energy of the oxidation reaction was calculated and was found to be affected by the enhancement of the grain boundary diffusion in nanostructured particles.