Influence of codoping with Hf and La on grain-boundary transport in alumina

The role of reactive elements (RE) is an important topic in understanding the oxidation behavior of high-temperature alloys. In this work, the influence of codoping alumina with two different RE elements (500 ppm Hf + 500 ppm La) was studied. The kinetics of oxygen grain-boundary (GB) transport were studied at 1400 degrees C using metallic nickel particles as markers. The results were compared with data obtained on the corresponding singly doped compositions; alumina-500 ppm La, and alumina-500 ppm Hf. The results showed that singly doping with La did not have any benefit compared to undoped alumina, whereas singly doping with Hf resulted in a slowing of transport by a factor of similar to 7. The behavior of the codoped sample was very similar to that of the singly doped Hf composition. For all the studied compositions, atomic scale characterization using high-angle annular dark-field scanning transmission electron microscopy and atom probe tomography (APT) revealed strong segregation of the dopant ions to the alumina grain boundaries. In the codoped sample, APT revealed evidence of oxygen excess and aluminum depletion at the GB core.