Transient alumina transformations on Ni–Cr–Al alloys: nanocrystallization and Cr concentration impacts

Ni-Cr-Al coarse-grained (CG) or nanograined (NC) alloys with 5 wt.% Al but various Cr concentrations (10 wt.% and 30 wt.%) were prepared via casting and magnetron sputtering, respectively. An Ar atmosphere was utilized to significantly mitigate the initial oxidation rates of Cr and Al in the four ternary alloys at 1000 o C, thereby enabling the capture of transient alumina transformation events by transmission electron microscopy (TEM). It was found the initially formed Cr 2 O 3 particles serve as templates, facilitating the direct thermal growth of isostructural α-Al 2 O 3 . These findings provide valuable insights into the effect of nanocrystalline structure and Cr content on oxidation kinetics of the Ni-Cr-Al ternary alloys under a conventional air atmosphere.