The effect of nanostructure on the oxidation of NiAl

Bulk nanostructured NiAl samples with a grain size of 104 nm have been obtained through cryomilling of NiAl intermetallic feedstock powder and subsequent sintering via SPS. Oxidation testing of these conventional and nanostructured NiAl samples reveals that while the conventional samples have oxidation rates on the order of 10−11 g2/cm4/s across all tested temperatures, nanostructured samples demonstrate decreasing oxidation rates with temperature, as low as 6.78 × 10−13 g2/cm4/s. This decrease in oxidation rate is attributed to an earlier transition through the metastable aluminum oxide phases, resulting in a stable slow growing α-Al2O3 phase earlier in the oxidation tests. In-situ X-ray diffraction (XRD) during high temperature oxidation testing has shown that lattice strain at the surface of the nanostructured samples is substantially higher than conventional, and that the decrease of this lattice strain with temperature is significant. It is believed that the relief of this lattice strain stimulates the earlier θ-α transition seen in the nanostructured samples.