Date

2014

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Materials Science and Engineering

First Adviser

Harmer, Martin P.

Other advisers/committee members

Rickman, Jeffrey M.; Webb III, Edmund

Abstract

The growth of protective alumina scales in Al2O3-forming alloys can be affected by the addition of reactive elements, such as Hf4+, which has been considered one of the most effective dopants to slow down the scale growth rate. While a number of theories concerning the "reactive element effect" have been proposed, a full explanation of this phenomenon is not yet available. The overall objective for this study was to conduct a systematic series of model experiments in order to elucidate the effect of HfO2 on oxygen grain boundary transport in alumina. The key questions to focus in the current work are: How do the doping levels of HfO2 / oxidizing temperature affect oxygen grain boundary diffusion in alumina?First part of this work investigated the effect of doping levels of HfO2 on oxygen grain boundary transport in alumina, which contains uniformly distributed Ni metallic particles.The doping levels spanned the solubility limit ranging from 100ppm to 2000ppm. The plot of the ratio kundoped/kdoped (grain-size corrected) as a function of dopant level clearly shows two behavior regimes: namely a regime I that encompasses doping levels below and near the solubility limit and a regime II where second-phase HfO2 particles were well present in the microstructure. A clearer understanding of the influence of HfO2 doping on the transport behavior can be achieved by plotting the data with respect to the fractional grain boundary coverage (f), as opposed to overall HfO2 content. The linear relationship can be rationalized with a site-blocking model, in which the Hf4+ ions obstruct the diffusive paths at the grain boundary.The second part of the work is focused on the temperature dependence of the oxidation kinetics in HfO2 doped Al2O3. The activation energy and rate constant ratio plot from our work and alloys studies indicated that multiple diffusion mechanisms might be operative at grain boundaries owing to boundary transitions that modify local structure and chemistry. Resutls of ARM characterization of samples oxidized at 1150°C have revealed that boundary structures that differ from those observed in samples oxidized at 1250°C and 1400°C. Specifically, new types of boundary structures are present at the lower temperature that are more atomic rough structure, and exhibit high energy facet planes such as {0 0 0 6} and {2 -1 -1 3} different from relative low energy facet plane such as {2 -1 -1 0} and {1 0 -1 2} for samples oxidized at higher temperature. These high-energy facet planes maybe occupied with high Hf segregation level and resulted in a much better oxidation resistance. The current results highlight the significant role of complexion transition in the oxidation area, which is traditionally being neglected.

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