This paper is a brief review of the extensive literature relating to the formation of protective α—Al2O3 scales on alloys at high temperature. Emphasis is placed on the proposed mechanisms of scale growth based on observations of scale morphologies and microstructures, inert-marker experiments and the distribution of oxygen isotope tracers within thermally-grown oxides. Attention is also given to the determination of ionic-transport mechanisms by electrochemical methods and to the effects of reactive elements such as yttrium in modifying ionic-diffusion processes.

The formation of aluminum oxide scales on high-temperature alloys

Two-step sintering (TSS) was applied on nanocrystalline zinc oxide (ZnO) to control the accelerated grain growth occurring during the final stage of sintering. The grain size of a high-density ( > 98%) ZnO compact produced by the TSS was smaller than 1 μm, while the grain size of those formed by the conventional sintering method was ∼ 4 μm. The results showed that the temperature of both sintering steps plays a significant role in densification and grain growth of the nanocrystalline ZnO compacts. Several TSS regimes were analyzed. Based on the results obtained, the optimum regime consisted of heating at 800°C (step 1) and 750°C (step 2), resulting in the formation of a structure containing submicrometer grains (0.68 μm). Heating at 850°C (step 1) and then at 750°C (step 2) resulted in densification and grain growth similar to the conventional sintering process. Lower temperatures, e.g., 800°C (step 1) and 700°C (step 2), resulted in exhaustion of the densification at a relative density of 86%, above which the grains continued to grow. Thermogravimetric analysis results were used to propose a mechanism for sintering of the samples with transmission electron micrographs showing the junctions that pin the boundaries of growing grains and the triple-point drags that result in the grain-boundary curvature.

Two‐Step Sintering of Nanocrystalline ZnO Compacts: Effect of Temperature on Densification and Grain Growth

Two commercial oxide dispersion strengthened alumina-forming FeCrAl alloys, Inco alloy MA956 and Kanthal alloy APM, were studied in order to look at the effect of reactive elements on their oxidation behaviour. MA956 has a distribution of Y2O3−Al2O3 particles, while APM has a ZrO2—AI2O3 distribution. Isothermal oxidation at 1000°C and 1200°C showed a reduced oxidation rate for both alloys compared to that of an undoped FeCrAl alloy. In short-term cyclic tests at 1200°C, both alloys exhibited excellent scale adhesion. Using scanning transmission electron microscopy with X-ray energy dispersive spectroscopy, both Y and Zr, respectively, were found to segregate to the oxide grain boundaries and the metal-scale interface after oxidation at 1000°C and 1200°C. These experimental observations are discussed with regard to a new theory to explain the reactive element effect.

The reactive element effect in commercial ODS FeCrAI alloys

The second stage sintering kinetics of powder compacts

Transformation and Microstructure Control in Boehmite‐Derived Alumina by Ferric Oxide Seeding

Alumina powders with varying iron oxide contents were prepared by coprecipitation. The powders were spheroidized by passing them through an oxygen-acetylene flame. The spheres were sized, annealed, and sintered in air and in N2 with 132 ppm O2. Isothermal studies were combined with constant-rate-of-heating studies to identify the mechanism of sintering and to calculate the diffusion coefficients. The contribution of surface diffusion during initial-stage sintering of Fe-doped Al2O3 was estimated by combining shrinkage and neck-growth data. The effect of Ti on the sintering rate of Fe-doped Al2O3 was also studied. Both Fe2+ and Ti4+ ions enhanced the sintering rate of Al2O3. A defect model for corundum is proposed to explain the sintering data for transition-metal-ion-doped Al2O3.

Effect of Iron Oxide on the Sintering Kinetics of Al2O3

Initial Sintering and Surface Diffusion in Al2O3

Aluminum oxide powders containing iron oxide were passed through a reducing oxygen acetylene flame. The resulting spherical particles of alumina coming out of the flame were quenched in water. The above process resulted in reduction of iron oxide dissolved in alumina and formation of large numbers of point defects. Quenching of the alumina spheres led to the quenching of large numbers of nonequilibrium point defects. The number of these quenched-in defects over the equilibrium concentration were estimated by using the sintering techniques. Activated sintering of alumina was observed in the case of aluminum oxide spheres containing quenched-in defects.

W. Raja Rao

Papers

Effect of Iron Oxide on the Sintering Kinetics of Al2O3

Initial Sintering and Surface Diffusion in Al2O3

Activated Sintering of Alumina by Quenching Point Defects

Anfangssintern und oberflaechendiffusion bei al2o3

Effect of iron oxide on the sintering kinetics of al2o3