No abstract

No abstract

A review and evaluation of the role of internal oxidation in the oxidation of alloys is presented and five alloy types represented by Ag-In, Cu-Be, Ni-Cr, Nb-Zr and Cu-Zn-Al are considered in detail. Internal oxidation is a diffusion-controlled process for which the kinetics may be predicted from solutions of the diffusion equation. Except for oxidations involving the formation of extremely stable solute oxide precipitates or oxidations at temperatures allowing only minimal solute mobility, the experimentally measured kinetics are in agreement with the calculated rates. The radius of a spherical internal oxide precipitate is expected to vary directly with the depth at which the precipitate is formed and inversely with the oxygen solubility at the external surface. Upon exceeding a critical solute content in a given binary alloy system, the occurrence of internal oxidation of the solute is replaced by the formation of an external scale of the solute metal oxide. A reduction in the oxygen content at the surface or the introduction of deformation into the surface of the alloy can reduce the solute content required for the transition from internal to external oxidation. In most cases, the prevention of internal oxidation by the formation of a compact surface layer of the solute oxide will result in a reduction of the oxidation rate of an alloy. When internal oxidation of a solute occurs in combination with external scale formation, the morphology of the precipitates in the microstructure of the oxidized alloy is determined both by the precipitation conditions and the mode of external scale formation. Internal oxide precipitates can affect both the mechanism and kinetics of external scale formation.

No abstract

Experimental data on the oxidation kinetics of SiC‐containing diborides of Zr and Hf in the temperature regime of 1473–2273 K are interpreted using a mechanistic model. The model encompasses counter‐current gas diffusion in the internal SiC depleted zone, oxygen permeation through borosilicate glass channels in the oxide scale, and boundary layer evaporation at the surface. The model uses available viscosity, thermodynamic and kinetic data for boria, silica, and borosilicate glasses, and a logarithmic mean approximation for compositional variations. The internal depletion region of SiC is modeled with CO/CO2 counter diffusion as the oxygen transport mechanism. Data reported for pure SiC in air/oxygen, for ZrB2 containing varying volume fractions of SiC, and for SiC–HfB2 ultra‐high temperature ceramics (UHTCs) by different investigations were compared with quantitative predictions of the model. The model is found to provide good correspondence with laboratory‐furnace‐based experimental data for weight gain, scale thicknesses, and depletion layer thicknesses. Experimental data obtained from arc‐jet tests at high enthalpies are found to fall well outside the model predictions, whereas lower enthalpy data were closer to model predictions, suggesting a transition in mechanism in the arc‐jet environment.

The d‐c polarization technique according to Wagner was applied for the first time at elevated temperatures for the determination of σ ⊖ and σ ⊕ in the Zr0.85Ca0.15O1.85 and Th0.85Y0.15O1.925 electrolytes over a range of oxygen activity and temperature. The applicability of the technique and an improved method for data analysis are demonstrated. Experimental difficulties and limitations are discussed. The total conductivity of Zr0.85Ca0.15O1.85 was determined over a range of oxygen activity and temperature.

No abstract

No abstract