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

This graduate textbook presents a comprehensive, unified treatment of the materials science of deformation as applied to solid Earth geophysics and geology. The deformation of Earth materials is presented in a systematic way covering elastic, anelastic and viscous deformation. Advanced discussions on relevant debates are also included to bring readers a full picture of science in this interdisciplinary area. This textbook is ideal for graduate courses on the rheology and dynamics of solid Earth, and includes review questions with solutions so readers can monitor their understanding of the material presented. It is also a much-needed reference for geoscientists in many fields including geology, geophysics, geochemistry, materials science, mineralogy and ceramics.

Deformation of Earth Materials: An Introduction to the Rheology of Solid Earth

The product of the grain boundary diffusion coefficient and width, D′δ, has been measured directly for the diffusion of 63Ni along grain boundaries in high-purity NiO. These measurements have been made at an oxygen pressure of 1 atm in the temperature range 500–800°C, and also at an oxygen pressure of 7 × 10−6 atm at 700°C. At po 2 = 1 atm, D′δ is given by D′δ = 3·0× 10−8 exp {−1.78(eV)/kT} cm3 S−l. The width of the boundary, δ, is estimated to be 7 × 10−8 cm and in this region near the boundary the concentration and mobility of Ni vacancies are greater than in the bulk. The measured values of D’δ confirm quantitatively that the rate of oxidation of Ni to NiO at 500–800°C is controlled by the diffusion of Ni along grain boundaries in the NiO scale.

The diffusion of 63Ni along grain boundaries in nickel oxide

For a given rate of heat input into a prograde metamorphic sequence the extent of overstep of reaction temperature (disequilibrium) depends on the slowest of three sequential steps: (a) surface detachment of reactant minerals, (b) transport of material to the site of mineral growth, and (c) nucleation and growth of the product mineral. We have developed analytical expressions which enable determination of the rates of mineral dissolution and growth and of advective and diffusive mass transport during metamorphism. The dissolution and growth steps are linear functions of the driving force (−Δ G) of the overall reaction while diffusion may take place either through a grain boundary fluid film or through the disorganized grain boundary itself.

Rate and mechanism in prograde metamorphism

The effect of small amounts of yttrium (up to 1 wt. %) and hafnium (up to 1.5 wt.%) on the oxidation behavior of Co-Cr-Al alloys in the temperature range 1000–1200°C for times up to 1000 hr in air has been studied. The major portion of the study has been concerned with Co-10Cr-11Al base alloys. Both isothermal and cyclic tests have been carried out; the cycle used consisted of 20 hr at temperature, followed by cooling to room temperature. Both additions reduce the overall oxidation, Hf somewhat more so than Y. In part, this is due to the improved adhesion between scale and alloy reducing scale spallation at temperature, and in part due to possible modification of the Al2O3 grain size. The former factor is far more critical under thermal cycling conditions. Under isothermal conditions the oxidation rate increases with increasing Hf content with all but the 1.5 wt.% alloy oxidizing more slowly than the Hf-free alloy; increase in Y content has the reverse effect. Under thermal cycling conditions the 0.3 and 1.0 wt.% Hf alloys show the lowest overall weight gain. Metallographic evidence suggests that the improved scale adhesion is due principally to a pegging mechanism; the active elements promote the growth of intrusions of Al2O3 into the alloy. However, if the intrusions are too large, they can act as initiators of scale failure.

The oxidation behavior of CoCrAI systems containing active element additions

A technique is presented to calculate grain‐boundary widths and grain‐boundary diffusion coefficients in metals and nonmetals using grain growth, sintering, diffusion and/or creep data. High‐purity metals yield boundary widths of the order of a few atomic diameters, whereas ionic materials yield ``effective boundary widths'' orders‐of‐magnitude wider.

R. L. Coble

Papers

Grain‐boundary diffusion and boundary widths in metals and ceramics