| The "reactive element effect", which pertains to the slowing of the growth kinetics of protective alumina scales on Al2 O3-forming alloys with the addition of reactive elements, such as yttrium, has been recognized for some time. However, the mechanisms underlying the effect are still the subject of debate. In this work, two model experiments were developed in order to determine the effect of yttrium on oxygen grain boundary transport in polycrystalline alpha-alumina.;A monolithic experiment on alumina/Ni composites was developed first. The grain boundary transport of oxygen in undoped and 500 ppm Y 3+-doped alumina/Ni composites was studied in the temperature regime of 1100∼1500°C by monitoring the oxidation of a fine, uniform dispersion of Ni marker particles (0.5 vol%). The annealing treatments were carried out in a high purity O2 atmosphere (>99.5%). The Ni particles, which were visibly oxidized to nickel aluminate spinel, were used to determine the depth of oxygen penetration. The thickness of the reaction zone was measured as a function of heat-treatment time and temperature, and a comparison of the oxidation rate constants and activation energies for undoped and Y3+-doped alumina was made. The results indicate that the presence of Y3+ slows oxygen grain boundary transport in alumina by a variable factor of from 15 to 3 in the temperature regime of 1100∼1500°C. The values of the activation energy for undoped and Y3+-doped alumina were determined to be 430+/-40 and 497+/-8 kJ/mol, respectively.;A novel wedge-shaped dual-layer sample was designed later in order to investigate the effect of Y3+ on oxygen grain boundary transport in alumina without the existence of Ni, which was inspired by the results from monolithic experiments. More specifically, anion transport behavior was extracted from a relationship between the depth of a wedge-shaped alumina top layer and that of an oxidation zone of an alumina/Ni substrate, the latter containing a fine, uniform dispersion of Ni marker particles (0.5 vol%). Mathematical models were used to obtain rate constants in both the alumina top layer and the alumina/Ni substrate. By comparing the data between samples in which Y3+ was altered, an oxygen diffusivity ratio was determined. It was found that the presence of yttrium slows oxygen grain boundary diffusion in alumina by a factor of 5 up to 1300°C and increases the corresponding activation energy from 407+/-20 to 486+/-34 kJ/mol, which are in good agreements with data acquired from monolithic experiments.;The fabrication methods in this work focused the study on the effect of yttrium on oxygen transport in alumina, circumventing complications in oxidation of alloys. The self-consistent results led to robust estimates of the kinetic parameters. The methods could be used to investigate the effect of other REs on oxygen grain boundary diffusion in alumina and then reveal the effectiveness of each reactive element. |
