Below 1000°C the oxidation of nickel cannot be controlled by the diffusion of ions through the bulk crystal lattice of the pure oxide, because the measured oxidation rates are several orders of magnitude faster than would be predicted on this basis. Short-circuit diffusion through oxide grain boundaries or dislocations has usually been held responsible, but there has hitherto been no proper quantitative confirmation of this mechanism. We report measurements of the oxide scale thickness and oxide grain size as a function of time during the oxidation of high-purity nickel in the temperature range 500–800°C. All the oxidation experiments were carried out in pure oxygen at a pressure of one atmosphere. The measured parabolic oxidation rate constants have been compared with those calculated from grain boundary diffusion data obtained in our previous work, using a grain boundary diffusion model for the oxidation process. The quantitative agreement between measured and calculated oxidation rates shows c...

A quantitative demonstration of the grain boundary diffusion mechanism for the oxidation of metals

http://www.paulchefurka.ca/Thermo%20Reading%20List/Martyushev&Seleznev%202006%3b%20MEPP%20in%20physics,%20chemistry%20and%20biology.pdf

Maximum entropy production principle in physics, chemistry and biology

Steels for bearings

Metastable single‐phase, NaCl‐structure, polycrystalline Ti0.5Al0.5N alloy films have been shown to exhibit much better high‐temperature (750–900 °C) oxidation resistance than polycrystalline TiN films grown under similar conditions. The Ti0.5Al0.5N alloys, ≂3 μm thick, were deposited at temperatures between 400 and 500 °C on stainless‐steel substrates by dc magnetron sputter deposition in mixed Ar+N2 discharges with an applied negative substrate bias Vs of either 0 or 150 V. Oxidation in pure O2 initially occurred at a rate that varied parabolically with time. The oxide overlayers consisted of two partially crystalline sublayers, the upper one Al‐rich and the lower one Ti‐rich, with no measurable N concentrations in either. Inert‐marker transport experiments showed that oxidation proceeded by the simultaneous outward diffusion of Al to the oxide/vapor interface and inward diffusion of O to the oxide/nitride interface. The oxidation rate constant K increased with oxidation temperature Tox at a rate much h...

Oxidation of metastable single‐phase polycrystalline Ti0.5Al0.5N films: Kinetics and mechanisms

In this review the role of stress in the high temperature oxidation of metals is examined. In the introductory sections the Wagner theory of oxide growth and its modification to account for short circuit diffusion are outlined. The direct influence of oxide growth stresses on defect diffusion is then examined and shown to be a significant factor when constrained volume changes accompany the growth of the oxide layer. Methods for evaluating growth stresses are critically reviewed and the possible origins of such stresses considered in detail. The response of the oxide layer to thermally and mechanically induced stresses is considered in the last part of the review. Under tensile conditions, through-thickness cracks can readily be produced but may also heal with fresh oxide if imposed strain rates are relatively low. Spallation of the oxide is not usually found under tensile conditions but occurs frequently under compressive stresses by either a ‘buckling’ or ‘wedginq’ process. Detailed descriptions...

Stress effects in high temperature oxidation of metals

Various theoretical dendrite and cell spacing formulas have been tested against experimental data obtained in unsteady- and steady-state heat flow conditions. An iterative assessment strategy satisfactorily overcomes the circumstances that certain constitutive parameters are inadequately established and/or highly variable and that many of the data sets, in terms of gradients, velocities, and/or cooling rates, are unreliable. The accessed unsteady- and steady-state observations on near-terminal binary alloys for primary and secondary spacings were first examined within conventional power law representations, the deduced exponents and confidence limits for each alloy being tabularly recorded. Through this analysis, it became clear that to achieve predictive generality the many constitutive parameters must be included in a rational way, this being achievable only through extant or new theoretical formulations. However, in the case of primary spacings, all formulas, including our own, failed within the unsteady heat flow algorithm while performing adequately within their steady-state context. An earlier untested, heuristically derived steady-state formula after modification, 
$$\lambda _1 = 120\left( {\frac{{16X_0^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} G_0 (\varepsilon \sigma )T_M D}}{{(1 - k)m\Delta H G R}}} \right)^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} $$

 ultimately proved its utility in the unsteady regime, and so it is recommended for purposes of predictions for general terminal alloys. For secondary spacings, a Mullins and Sekerka type formula proved from the start to be adequate in both unsteady- and steady-state heat flows, and so it recommends itself in calibrated form, 
$$\lambda _2 = 12\pi \left( {\frac{{4\sigma }}{{X_0 (1 - k)^2 \Delta H}}\left( {\frac{D}{R}} \right)^2 } \right)^{{1 \mathord{\left/ {\vphantom {1 3}} \right. \kern-\nulldelimiterspace} 3}} $$

 for future predictions.

Prediction of dendrite arm spacings in unsteady-and steady-state heat flow of unidirectionally solidified binary alloys

A critical appraisal of theory and experiments for both isothermal and forced velocity pearlite is presented. It is concluded for binary systems that both the theoretical models for volume diffusion and boundary diffusion control are well-advanced and adequate for the purposes of experimental test. However, some ambiguity remains in the boundary diffusion model with respect to the thermodynamics of the boundary ”phase” region, so it is still not possible to predict absolute rates of transformation. The theoretical problem for ternary pearlites is also well understood, although rigorous theory seems intractable. A new perturbation procedure for definition of the optimal steady-state spacing is presented and amplified for both isothermal and forced velocity pearlite, and for both volume and boundary diffusion models. In terms of the critical spacing Sc for isothermal pearlite and the spacing at minimum undercooling Sm for forced velocity pearlite the predicted stability points are as follows: {fx2777-1} For isothermal pearlite these perturbation results correspond closely to the state of maximum entropy production rate while for forced velocity pearlite the correspondence is also satisfactory. A detailed analysis of the data leads us to reaffirm the author’s conclusions that the eutectoid reactions in Cu-12 pct Al and some related ternary alloys reported by Asundi and West are controlled by volume diffusion and that the eutectoid reaction in Al-78 Zn reported by Cheetham and Ridley is controlled by boundary diffusion. We conclude further after careful analysis that the pearlite reaction in Fe-0.8 C is controlled for the higher temperatures by volume diffusion of carbon in austenite. We are also led to state that the pearlite transformations in Fe-C-Mn and Fe-C-Ni occur for the most part in a nopartition regime and are therefore controlled by volume diffusion of carbon in austenite, while the transformations in Fe-C-Cr and Fe-C-Mo, being forced by thermodynamics to sustain partition of chromium and molybdenum, are controlled by phase boundary diffusion of the latter elements. nt]mis|M. P. PULS, formerly Postdoctoral Fellow, Department of Metallurgy and Materials Science, McMaster University, Hamilton, Ontario, Canada

The pearlite reaction

Oxidation of metals by short circuit and lattice diffusion of oxygen

The available data on the solubility of niobium carbide and niobium carbonitride in plain carbon and alloyed austenite has been analyzedvia dilute solution thermodynamics with a view to establishing a consistent set of interaction parameters for predicting austenite + niobium carbonitride equilibria. The computation algorithm includes the prediction of phase mass fractions as a function of alloy composition and temperature between 900° and 1300 °C (tie lines). Analogous ferrite equilibria are included.

J. S. Kirkaldy

Papers

Prediction of dendrite arm spacings in unsteady-and steady-state heat flow of unidirectionally solidified binary alloys

The pearlite reaction

Oxidation of metals by short circuit and lattice diffusion of oxygen

Solubility of niobium carbide and niobium carbonitride in alloyed austenite and ferrite

Theory of decomposition of eutectoids assuming local equilibrium and phase boundary diffusion