Grain Boundary Transport Mechanisms of Oxidation Behaviors in Alumina

Abstract Ni-based superalloys are resistant to high temperature oxidation due to the formation of a protective alumina scale known as a thermally grown oxide (TGO). The growth of the TGO in thermal barrier coatings (TBCs) is responsible for the spallation failure of the coatings. It is important to study grain boundary (GB) transport in alumina to understand the mechanisms of the growth of the alumina scale. This work studied the grain boundary transport in alumina from two aspects. The first was to study the effect of oxygen partial pressure on the grain boundary transport in alumina. During the formation, there is a large oxygen partial pressure gradient across the alumina scale. But how the grain boundary transport varies under different oxygen partial pressure inside the alumina scale is still under debate. The second was to study the effect of Hf-doping on the grain boundary transport in alumina at different temperatures. It is well known that the addition of reactive elements, such as Hf and Y, can effectively increase the oxidation resistance of the alumina forming alloys. But the mechanism by which Hf-doping has a beneficial effect on slowing down grain boundary transport in alumina is still unclear.In the first part of this study, grain boundary transport was measured in alumina under oxygen partial pressure ranging from 10-10 to 10-18 at 1400°C. Dense polycrystalline alumina samples containing nickel aluminate spinel marker particles were reduced and the reduction rates were measured at different oxygen partial pressure. The results showed that the dependence of the grain boundary transport on oxygen partial pressure is mainly due to the driving force, which is the gradient of oxygen partial pressure across the alumina. The point defect model which is most consistent with the measured results is one that assumes ionic diffusion determines the overall transport and the concentration of ionic defects is determined by impurities. The results of this study confirm that transport in a formed alumina scale is controlled by extrinsic defects and not by intrinsic defects as suggested by other researchers.The second part of this study uses the oxidation of Ni marker particles to measure the influence of Hf4+ on the grain boundary transport in polycrystalline alumina. At temperatures above 1250°C, 500ppm Hf doping retarded the transport by a factor of 8, whereas the retardation increased to a factor of 20 below 1200°C. The GB structures in samples subjected to oxidation heat-treatments at 1400°C and 1150°C were studied using aberration-corrected high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) imaging. The results show that Hf4+ strongly segregated at alumina grain boundaries and occupied Al3+ sites. There was a large frequency of GBs exhibiting {2-1-10} facets at 1400°C. Whereas at 1150°C, there was a large proportion of non-faceted GBs. Together with the previous results of the effect of Hf concentration on grain boundary transport in alumina, the results showed that the effect of Hf-doping on slowing down grain boundary transport in alumina is consistent with a site-blocking mechanism. These results taken together indicate a possible occurrence of a grain boundary complexion transition taking place in the current system. This study has revealed novel aspects of the oxidation behavior in Hf-doped alumina, and also suggested a complex influence of dopants and temperature on the GB structures and kinetic properties.