Theory Of Internal Oxidation Research Articles

High Temperature Intergranular Oxidation of Nickel Based Superalloy Inconel 718

AbstractIntergranular oxidation (IGO) of the Ni-based superalloy Inconel 718 was studied at 650 °C, 700 °C and 900 °C. The oxidized samples were characterized by X-ray diffraction and scanning electron microscopy. For all the studied temperatures, the external scale was mainly composed of Cr2O3, while the oxides along the grain boundaries were rich in Al and, to a minor extent, Ti. This was consistent with thermodynamic computations. The time evolution of the maximum depth of IGO was found to be parabolic with an apparent activation energy of 164 kJ/mol. The results of this study confirm with three temperatures that IGO kinetics can be described using an extension of the Wagner’s theory of internal oxidation, as recently suggested in the literature at 850 °C. According to this description, the mechanisms controlling the IGO kinetics of Inconel 718 are the aluminum diffusion in the alloy matrix and the oxygen diffusion along the interface between the alloy matrix and the oxidized grain boundary.

Non-Isothermal Oxidation Behaviors and Mechanisms of Ti-Al Intermetallic Compounds.

Non-isothermal oxidation is one of the important issues for the safe application of Ti-Al alloys, so this study aimed to illustrate the non-isothermal oxidation behaviors and the corresponding mechanisms of a TiAl-based alloy in comparison with a Ti3Al-based alloy. The non-isothermal oxidation behaviors of Ti-46Al-2Cr-5Nb and Ti-24Al-15Nb-1.5Mo alloys in pure oxygen were comparatively investigated with a thermogravimetry-differential scanning calorimetry (TGA/DSC) simultaneous thermal analyzer heating from room temperature to 1450 °C with a heating rate of 40 °C/min. When the temperature rose above 1280 °C, the oxidation rate of the Ti-46Al-2Cr-5Nb alloy sharply increased and exceeded that of the Ti-24Al-15Nb-1.5Mo alloy owing to the occurrence of internal oxidation. When the temperature was higher than 1350 °C, the oxidation rate of the Ti-46Al-2Cr-5Nb alloy decreased obviously due to the generation of an oxygen-barrier β-Al2TiO5-rich layer by a chemical reaction between Al2O3 and TiO2 in the oxide scale. Based on Wagner’s theory of internal oxidation, the reason for the occurrence of internal oxidation in the Ti-46Al-2Cr-5Nb alloy is the formation of the α phase in the subsurface, while no internal oxidation occurred in the Ti-24Al-15Nb-1.5Mo alloy due to the existence of the β phase in the subsurface with the enrichment of Nb and Mo.

Numerical analysis of high temperature internal corrosion mechanisms by the cellular automata approach

Aim of the study is to develop a simulation software which allows to predict the diffusion‐controlled transformation processes during oxidation and nitridation of metals and alloys. Internal oxidation and nitridation often results in a deep penetration of coarse and sometimes needle‐shaped precipitates that act as crack initiation sites, i.e., giving rise to an embrittlement of the surface layer. The method of cellular automata according to Chopard and Droz has been applied to simulate internal nitridation processes. The approach allows implementing, e.g., the diffusion‐blocking effect of the internal precipitates, and the mechanisms of nucleation and growth. As an example, TiN formation in Ni‐base alloys was simulated by treating nitrogen diffusion and precipitation separately and simultaneously. The progress of internal nitridation follows a parabolic rate law in agreement with Wagner's theory of internal oxidation and experimental results. Furthermore, the cellular automata approach is capable to predict the transition from internal precipitation to external scale formation and to implement the process of nucleation and growth of nitride and oxide precipitates.

Numerical Analysis of Internal Oxidation and Nitridation by the Cellular Automata Approach

In this study, the phenomena of internal oxidation and nitridation are modeled numerically by means of the cellular automata approach. As an example, TiN formation in Ni-base alloys was simulated by treating nitrogen diffusion and precipitation separately and simultaneously. It was shown that the concentration profiles of nitrogen diffusion are consistent with the analytical solution. The progress of internal nitridation follows a parabolic rate law in agreement with Wagner’s theory of internal oxidation and experimental results. Furthermore, the cellular automata approach is capable to predict the transition from internal precipitation to superficial scale formation and to implement the process of nucleation and growth of nitride and oxide precipitates.

Internal oxidation processes under non-ideal conditions

Wagner’s classical theory of internal oxidation is concerned with a single oxidant diffusing into an alloy to precipitate particles of a single compound which do not themselves affect significantly the diffusion process. This paper is concerned with some of the complexities which arise when this ideal description is inapplicable. The simultaneous precipitation of multiple chromium compounds in heat-resistant alloys exposed to two or more of oxygen, sulphur, carbon and nitrogen is shown to be in qualitative agreement with theoretical prediction: discrete precipitation zones form and widen according to parabolic kinetics.When lamellare or acicular precipitates grow parallel to the oxidant diffusion direction, the aligned precipitate-matrix interfaces can provide rapid diffusion pathways, accelerating the reaction rates. The effect is demonstrated by molybdenum sulphide needles in Ni-Mo alloys, Al2O3 rods in austenitic and ferritic alloys, lamellar and Widmanstatten Cr2N in nickel base alloys and lamellar Cr23C6 in both austenitic and ferritic alloys. In many cases, the formation of the aligned precipitates can be described as a cellular phase transformation at the reaction front.

Internal nitridation of nickel-base alloys. Part II : Behavior of quaternary Ni-Cr-Al-Ti alloys and computer-based description

Whereas in Part I of this study the process ofinternal nitridation was described for binary andternary alloys within the Ni-Cr-Al-Ti system, this partfocuses on quaternary Ni-Cr-Al-Ti alloys, which are similar to commercial Ni-base alloys used inhigh-temperature applications regarding their chemicalcompositions. These alloys can simultaneously form twodifferent nitride-precipitation zones consisting of TiN and AlN. In order to quantify thenitridation process, thermogravimetric measurements inan oxygen-free nitrogen atmosphere in the temperaturerange 800-1100°C were carried out and supplemented by extensive microstructural studies. Whilesingle-nitride internal nitridation can easily bedescribed by Wagner's theory of internal oxidation,modeling of the more complex internal-precipitationreactions that involves more than one nitride requires anumerical treatment of both the diffusion and thethermochemical processes in the alloy. For this purpose,a computer simulation was developed in which the commercial thermodynamic software ChemApp iscombined with a finite-difference diffusion calculation.It was shown that this calculation technique can beapplied successfully to quantitatively describe the internal-nitridation process of theNi-Cr-Al-Ti model alloys used in this study.

Internal-carburizing behavior of Ni-V, Ni-Cr, and Ni-3Nb alloys

Ni-2V, Ni-5V, Ni-12V, Ni-10Cr, Ni-20Cr, andNi-3Nb alloys were carburized in 1.5 v/oC3H6 (bal. H2) over therange 700-1000°C. Carburization of Ni-5V, Ni-12V,Ni-20Cr, and Ni-3Nb obeyed the parabolic rate law. Ni2V and Ni-10Cr, however, formed onlythin carbide scales upon carburizing. Carburizationrates decreased with increased vanadium content fromNi-5V to Ni-12V for all exposure conditions. V4C3 formed throughout the reaction zones of Ni-12V.Cr3C2 formed in the surfaceregions and Cr7C3 formed withinthe interior of Ni-20Cr. NbC precipitated in Ni-3Nbunder all conditions. The precipitate morphology changed with temperature and distance from thegas-metal surface. V4C3 andCr3C2 particles were generallysmall and spheroidal near the surface of Ni-12V andNi-20Cr, respectively, increasing slightly in size with distance from the surface and withincreasing temperature. The vanadium and chromiumcarbides formed intergranular networks toward thereaction fronts. The NbC precipitates were generallylarge and became Widmanstatten at increasing distancewithin the carburized zone of Ni-3Nb. Expressions forthe diffusion coefficient of carbon in nickel from themeasured permeabilities and carbon solubility data were determined. Solubility products weredetermined for all of the carbides formed and found tobe large in comparison with the product of theactivities of the precipitate elements. Wagner's theory of internal oxidation was shown to be anapproximation to the carburization kinetics attemperatures of 900°C or higher.

The Effect of Supersaturation on the Internal Oxidation of Binary Alloys

According to the theory of Bohm and Kahlweit ofthe internal oxidation of binary A-B alloys, theparabolic rate constant for the formation of reasonablystable internal BOν oxides as well as theconcentrations of O and B at the oxidation front arecontrolled only by the degree of supersaturationnecessary for the nucleation of new oxide particles. Theeffects of this factor on the previous parameters arecalculated for various values of the solubility product ofthe oxide and of the diffusion coefficients of O and B.Moreover, an alternative procedure for the calculationof the critical degree of supersaturation behind the precipitation front required for oxideprecipitation, which is a function of the concentrationof the reactants at the internal oxidation front, isproposed. A simple modification of Wagner's theory of internal oxidation is also presented, andits results are compared with those of the treatment byBohm and Kahlweit. Finally, the limitations of the twomethods are examined.

Properties of alloy precursors and the resulting superconducting YBa2Cu3O7−δ-Ag composites

A significant obstacle to the widespread application of high-temperature superconductors are the poor mechanical properties of these oxides. A novel processing technique was investigated that attempts to circumvent this problem using ductile alloy precursors to superconducting composites. Alloys of composition Y-2Ba-3Cu-Ag were prepared by arc- and induction melting the metals. A variety of mechanical tests show that alloys with 90 wt % Ag are ductile, and differential thermal analysis indicates a solidus temperature of 703 °C. Using thermogravimetric analysis, the oxidation rate of Y-2Ba-3Cu-90 wt % Ag alloys was studied. This rate does not follow that which is expected based upon the theory of internal oxidation, nor is there a transition to external oxidation upon lowering the partial pressure of oxygen. The superconducting properties of the oxide in the resulting composite are similar to those of polycrystalline samples, with a Tc of 90 K. Superconductivity is achieved only when the alloy is oxidized above the solidus temperature, where the presence of a liquid phase enables the oxides to be concentrated and connected.

Carburization kinetics of heat-resistant steels

A series of 21 commercial austenitic heat-resistant alloys was carburized in H2-C3H6 atmospheres which were reducing to Cr2O3 and SiO2 and which provided a carbon activity of one. Reactions were carried out at 900⩽T⩽1100°C and the resulting depth of carburization was measured metallographically. All alloys showed parabolic carburization kinetics after an initial brief period, at low temperatures, of more rapid reaction. The apparent activation energies for carburization are discussed in terms of carbon solubilities and diffusion coefficients. It is concluded that Wagner's theory of internal oxidation is quantitatively applicable to simple alloys but not to alloys which contain additions of reactive elements. The effectiveness of additions of niobium, aluminum, titanium, and silicon is demonstrated and is discussed in the context of Wagner's theory.

Etude de l'oxydation interne dans les alliages a base de cuivre, a l'aide de la diffusion centrale des rayons X

Central diffusion of X-rays may give data on oxide particles which are formed in alloys undergoing an internal oxidation. The size of the particles, their nature and their volume fraction may thus be measured. Moreover, analysis of the tails of diffusion curves permits to determine the fraction of solute which has not been oxidized. For copper based alloys, with different added elements such as Be, Al or Mg, we show that, as soon as the front has passed, the formed particles remain stable; later annealings have no effect on their size and their nature. Cu-Ti seems, however, to behave differently in this respect, but in this alloy, the particles become more rapidly incoherent. At last, we obtain always after oxidation an non-negligible part of the solute which remains in solid solution. This agrees with the Theory of internal oxidation.

On the theory of internal oxidation and sulphation of alloys

On the assumption that the internal precipitate is finely dispersed and equilibrated with the immediate surroundings, the matrix diffusion profiles and precipitate distribution have been obtained in closed form for systems of arbitrary constitution. The predictions are in good agreement with Nakao's observations of MnS precipitation in Fe-Mn alloys (1). Résumé En supposant que la précipitation interne est infiniment petite et, de plus, en équilibre avec le milieu voisin, le proftl de diffusion dans la matrice et la distribution du précipite ont été obtenus sous forme d'intégrales, pour des diagrammes ternaires arbitraires. De telles prédictions concordent bien avec les observations expérimentales faites par Nakao, au sujet de la précipitation du MnS dans les alliages fer-manganèse (1).

On the theory of internal oxidation and sulphation of alloys

On the assumption that the internal precipitate is finely dispersed and equilibrated with the immediate surroundings, the matrix diffusion profiles and precipitate distribution have been obtained in closed form for systems of arbitrary constitution. The predictions are in good agreement with Nakao's observations of MnS precipitation in Fe-Mn alloys (1). Résumé En supposant que la précipitation interne est infiniment petite et, de plus, en équilibre avec le milieu voisin, le proftl de diffusion dans la matrice et la distribution du précipite ont été obtenus sous forme d'intégrales, pour des diagrammes ternaires arbitraires. De telles prédictions concordent bien avec les observations expérimentales faites par Nakao, au sujet de la précipitation du MnS dans les alliages fer-manganèse (1).