This study deals with the influence of various mechanical loadings (fatigue, creep, creep-fatigue) on oxygen diffusion in a particular system, oxidizing nickel. A distinction between the behavior of the oxide layer and underlying nickel was noted during the first step of oxidation at 550°C, in PO2=1 atm. Mechanical loading causes a decrease of the oxygen mobility through the oxide scale (factor of 103). The oxide thicknesses on nickel undergoing mechanical loadings are different than for an unloaded sample, due to distinct contributions of the oxygen and nickel fluxes in the growing oxide. In the substrate, the ingress of oxygen becomes easier with a constant tensile load (creep). The intergranular-oxygen diffusion coefficient, Di, is increased by a factor of 102 with respect to other samples. In creep, oxygen diffusion takes place along grain boundaries of a structure with smaller grains than in unstrained Ni. A short fatigue period during creep-fatigue decreases the sensitivity of nickel to intergranular-oxygen diffusion.

The oxidation of a chromium PVD coating on pure iron by a continuous 5-kW CO2 laser beam in pure oxygen at 700°C for 20 min was compared with classical furnace oxidation. Laser oxidation induces faster oxidation kinetics, especially at the beginning of oxidation, without modifying the oxide nature (Cr2O3) and morphology. Oxygen-isotopic-exchange tests show that oxygen grain-boundary diffusion does not depend on the oxidation conditions, at least after 12 min oxidation. The effec of the laser treatment is discussed with respect to oxide nucleation, metastable-oxide formation, and oxide-formation-entropy evolution. The oxidation kinetics follow a parabolic law, and the oxide-growth mechanism was attributed to countercurrent oxygen and chromium diffusion along grain boundaries. Oxygen diffusion occurred by oxygen interstitials. The oxidation constant calculated from grain-boundary diffusion in the chromia scale is smaller than the experimental oxidation constant, suggesting the presence of particular short circuits (e.g., microcracks).

Two distinct iron substrates (pure iron and iron containing 0.3 wt% of carbon) have been protected by chromium using physical vapor deposition. The oxidation behaviour of the Cr coating in oxygen at 700 °C depends on the iron grade, due to the influence of carbon. The study shows that carbon can be dissipated into the surrounding atmosphere, depending on its stability in the substrate and on the nature of the atmosphere (confined or dynamic oxidizing atmosphere). Carbide precipitation in the coating, near the oxide interface, decreases the oxidation rate. There is no large influence of the nature of the substrate on oxygen diffusion into the outer part of the Cr2O3 layer. Oxygen diffuses preferentially via interfaces, with a diffusion coefficient of 10−14 cm2s−1. In the chromium coating, there is a significant difference in oxygen penetration, depending on the grade of iron. Carbon incorporation into the coating is greater with the pure iron substrate than with the substrate made of iron containing 0.3 wt% carbon, and the presence of carbon in the coating decreases the oxygen diffusion, particularly along grain-boundaries: thus Dgb(O → CrFe) = 6.2 × 10−13cm2s−1 and Dgb(O → CrFe + C) = 2.2 × 10−12cm2s−1.

To know the high temperature corrosion of Inconel 601 by melted oxides, the behaviour of this alloy under pure oxygen, then with TeO2 and finally in a complex environment of melted oxides with temperature gradient, was characterized. In melted oxide environment, two condensation zones appeared, one where the phases rich in molybdenum provoke generalized corrosion, the other where the compounds rich in Te, Cs, Na and/or B provoke localized corrosion. Covering these zones induces catastrophic corrosion.

In order to profit from the good tribological properties of chromium oxide, a chromium deposit on low alloy steel was made using electrolysis, then it was oxidized either using a continuous CO2 laser surface treatment at the constant temperature of 800 and 600-degrees-C, or in a furnace under partial oxygen pressure of 1 atm. A morphological and analytical comparison was realized on the Cr2O3 layers obtained in the furnace or by laser. Then, in order to understand better the diffusional mechanisms brought into play, during oxidation in the furnace of an electrolytical chromium deposit, a kinetic, then a diffusional study of the oxygen in the chromium deposit was made. The Cr2O3 layers obtained by laser are always thicker, more homogeneous and denser than the ones obtained in the furnace.

En vue de contribuer à la compréhension des processus de corrosion à haute température de l’Inconel 601 par des oxydes fondus, le comportement de cet alliage a d’abord été caractérisé sous oxygène pur, puis en présence de l’oxyde de tellure TeO2 et, enfin, dans un milieu complexe d’oxydes fondus, en présence d’un gradient de température. Le tellure a une action agressive très marquée et provoque une corrosion localisée. En milieu oxydes fondus, deux zones de condensation apparaissent, l’une où les phases riches en molybdène provoquent une corrosion généralisée, l’autre où les composés riches en Te, Cs, Na et/ou B provoquent une corrosion localisée ; un recouvrement de ces zones induit une corrosion catastrophique.

En vue de bénéficier des bonnes propriétés tribologiques de l’oxyde de chrome, un dépôt de chrome a été effectué par voie électrolytique sur un acier faiblement allié, puis oxydé soit au moyen d’un traitement de surface par laser CO2 continu, à température constante à 800 et 600°C, soit en four, sous pression partielle d’oxygène de 1 atm. Dans un premier temps, une comparaison morphologique et analytique a été réalisée sur les couches de Cr2O3 obtenues en four ou par laser. Dans un deuxième temps, afin de mieux comprendre les mécanismes diffusionnels mis en jeu lors de l’oxydation en four d’un dépôt électrolytique de chrome, une étude cinétique puis diffusionnelle de l’oxygène dans le dépôt de chrome a été effectuée. Cette étude montre que les couches de Cr2O3, obtenues par laser, sont toujours plus épaisses, plus homogènes et plus denses que celles obtenues en four.

Nickel-base alloys, such as Astroloy, used for aeronautical turbine disks, are sensitive to time-dependent cracking in environments containing oxygen. The “mosaic” structure of the alloy consisting γ′ islands (200 nm average size) surrounded by the γ-phase (100 nm thick) induces complex oxidation phenomena. Various analytical approaches allow the delineation of all the steps from segregation to oxidation occurring on the surface of such a duplex structure. The protection of Astroloy by its outer oxide layer against oxygen penetration was studied also, using alternative16O2then18O2oxidation. In association with STEM studies, it is shown that the outer oxide scale is not a real barrier against oxygen penetration and that inner precipitation of chronium (+ aluminium and titanium)-enriched oxides, takes place especially in the γ structure.

In order to improve the wear resistance of chromium coating, Cr2O3 scale can be formed by laser oxidation. The objective of this work consisted of determining the differences in the oxidation mechanism between a classical oxidation and a laser oxidation. The chromium coatings were first deposited on a Z32CDV13 steel by an electrochemical technique, but due to their bad protective character, the chromium deposition was then carried out by physical vapour deposition. Oxidation treatments were performed for short times between 600 and 800°C. It was verified that, in all cases, the oxidized layer is thin and consists of Cr2O3. In the case of classical oxidation, a parabolic law is observed, indicating that the oxide growth is controlled by a diffusional process. The combination of many characterization techniques showed the complexity of the diffusional phenomena; oxygen diffusion in the oxide layer and in the underlying chromium coating, with lattice and intergranular diffusion combined with interdiffusion between the steel substrate and the chromium coating. Nevertheless, it was shown that the chromia scale grows predominantly by anionic diffusion. It appears that laser oxidation induces the formation of a slightly thicker oxide scale and to a carbon concentration in the scale more important than in case of classical oxidation.

In order to improve the wear resistance of a PVD chromium coating on a Z32CDV13 steel, a Cr2O3 surface layer was developed by laser oxidation. A comparaison between classical and laser oxidation is done to clarify the diffusion laws which govern such an oxidation process. Oxidation treatments took place in 1 atm pure oxygen between 600 and 800-degrees-C. For laser oxidation, a 5 kW continuous CO2 laser beam was used in oxidizing atmosphere (1 atm p(O2)) at a constant temperature (600-800-degrees-C). In all cases the oxidized layer is thin (100-300 nm) and made of Cr2O3. In case of classical oxidation, a parabolic law is observed, indicating that the oxide growth is controlled by diffusion. The complexity of such diffusion processes is studied especially with the help of O-18(2)-O-16(2) alternative oxidations together with SIMS analyses. A bulk diffusion parameter of oxygen through the oxide scale of nearly 10(-14) cm2/s is then calculated. The study also shows interdiffusion phenomena between the substrate and the chromium coating and a specific influence of carbon.

A chromium coating of 25 mu m depth was physically deposited on two iron substrates differing mainly in their respective carbon content: pure iron and iron containing 0.3 wt% of carbon (Fe0.3C). After oxidation, the oxidized layer was thin and consisted mainly of Cr2O3. In the case of Cr on Fe0.3C, carbon was located in the deeper part of the oxide, and Al showed a strong enrichment both in the very outer part of the oxide scale and in its inner zone. With the help of successive oxidations in O-16(2), then O-18(2), it was observed that the carbon-enriched zone in the oxide layer on the Cr coating of the Fe0.3C sample behaves as a good barrier against oxygen inner diffusion. On pure iron, the carbon-enriched layer does not prevent oxygen diffusion into the Cr coating and both a cationic and an anionic mechanism occur for the growth of the oxide layer. The oxygen diffusion parameter in Cr2O3 is the same for the two substrates: 10(-14) cm(2) s(-1).

Gamma-gamma' nickel base alloy, as Astroloy is sensitive to environment for fatigue crack propagation at 750-degrees-C. Two main oxides are obtained, especially enriched with Ni, Al (+Cr) in the outer zone and with Al, Cr in the inner one. The oxide scale growth is mainly controled by oxygen bulk diffusion, expect in the case of rolled sample where a linear diffusion of oxygen can also intervene for the growth of inner oxide layer. The inner oxide scale grows 10(3) times faster than the outer one. On account of the different diffusivities of metal and oxygen together with the dissolution of oxygen and formation of suboxides, (containing Ni, Cr in gamma phase and Al, Ti in gamma' islands) at the oxide scale-alloy matrix interface, an accumulation of mechanical constraints occurs in the alloy during oxidation. With a model based on ''a solutal elastic effect when the concentration of solute varies'' [1] a depth corresponding to a maximum stress was estimated between 3.4 x 10(-8) cm to 1154 x 10(-8) cm in front of the oxide scale-alloy matrix interface. Local enrichment in oxygen on induced defects (vacancies, holes) or precipitates then disturb the 180 diffusion profile versus alloy's depth.

Les alliages susceptibles de posséder une longue durée de vie à haute température sous atmosphère oxydante forment essentiellement une couche interne soit de Cr2O3 soit de Al2O3.

During the laser surface treatment, oxidation can take place even with the protection of helium gas. The CW-CO2 laser melting of the surface does not induce a large distortion of the crystallographic orientation which remains (1 0 0). A misorientation of 3–4 degrees between the melted zone and the unaffected base alloy was measured by Laue diffraction and ion channelling studies. Surface chemical analysis was carried out by X-ray photoelectron and Auger electron spectroscopies in order to obtain information on surface segregation during either the laser cooling, or after annealing up to 850 °C, in PO2=5 × 10−9 Torr. The latter causes segregation of sulphur and/or nitrogen to the surface. This segregation of sulphur and nitrogen is modified between the untreated zone and the treated surfaces (thermal affected zone and melted zone). For instance, on the contrary of nitrogen, the diffusion of sulphur is increased on the melted surface and the activation energy for this diffusion is very dependent upon the defect nature. A difference of nearly 65–70 kJ mol−1 is calculated for the enthalpy for the surface segregation of sulphur ΔHseg, between the treated and untreated surface. This difference is only 37 kJ mol−1 for nitrogen. In both cases the absolute ΔHseg values are smaller for the treated zone than for the base alloy. A structural study by low electron diffraction (LEED) which gives information on the structure of the surface when clean and in the presence of sulphur and/or nitrogen, shows also the formation of edges, terraces and steps on the treated surfaces. These results are mainly associated with the difference in the content and nature of the defects between the different zones, as observed by scanning electron microscopy analysis or channelling study. Finally a comparison between the mechanical properties of the different zones is made on account of the observation of a linear change of the microhardness on the thermally affected surface. In the thin outer surface zone which is treated by the laser, the stresses associated with the cooling or the isothermal annealing are calculated using formula which associate the stress and the radius of curvature of the sample (from the misorientation which was analysed by ion channelling between the different surface domains). The influence of such an isothermal stress is believed to increase the percentage of vacancies in the melted zone and therefore to modify the activation energy for the diffusion of a substitutional element, such as sulphur. The influence of the elastic energy which is kept during cooling, for example according to the analysis of Evans and Lobb [12], is also discussed.

An Fe–17Cr–13Ni (wt-%) single crystal alloy was melted using a continuous wave (CW) CO2 laser. The surface segregation of sulphur and nitrogen at high temperature in vacuum is investigated on the various surface zones (base alloy, heat affected zone, and melted zone) using Auger electron spectroscopy (AES). Below ~700°C, cosegregation of sulphur and nitrogen is observed, while above 700°C, only sulphur segregates on the surface on all three zones. The segregation of sulphur or the cosegregation of sulphur and nitrogen induce, on the three zones, the segregation of chromium in the first atomic plane. The enthalpy for the surface segregation of sulphur ∆HsegS is found to be −164±10 kJ mol−1 for the base alloy. The values of ∆HsegS are about 40% lower for the treated zones. The result for the base alloy is in agreement with the segregation enthalpy calculated from the reported data on adsorption and dissolution of sulphur, i.e. ∆HsegS ≍−157 kJ mol−1. The pseudodiffusion parameters for sulphur and nitrogen are determined. The activation energy for nitrogen diffusion is the same for all three zones and is in agreement with literature data (~−180 kJ mol−1). For sulphur, the energies for diffusion are highly dependent on the laser treatment. The activation energy for diffusion is maximum in the melted zone: ~ 146 kJ mol−1; the values in the untreated and heat affected zones are 75 and 85 kJ mol−1, respectively.MST/1175

The oxidation of presulphidized Ni-Cr alloys has been studied by taking into account the influence of the two distinct oxidation mechanisms described in part I of this article. Sulphur enters the Cr2O3 scale (in Ni-34Cr alloys) mainly as S2− species, which at high temperatures increases the V‴Cr content, and hence the oxidation kinetics. Sulphur is randomly distributed in the scale, except at the inner oxide-alloy interface, where intergranular microsulphides are analyzed in the oxide-scale zone. In the case of NiO, NiCr2O4, Cr2O3 oxide multilayers (in a Ni-20Cr alloy), sulphur in the S2− state is distributed in the oxide layers or at Si-precipitate interfaces. Such a distribution leads to crack formation, especially during cooling.

After melting a FeCr12 alloy by CO2 laser, various zones are obtained: heat affected zone, melted zone and base alloy. Analytical studies (by Auger spectrometry) show that the segregation and diffusion of sulfur, during later annealing, are increased at the level of the laser treated zones (with a transition temperature towards 520-degrees-C). In the same way, evolutions in nitrogen content, going as far as precipitation, are shown. Interpretations take into account the structure evolutions after laser treatment and/or during segregation annealing.

AbstractThe fusion of single crystals of Fe–17Cr–13Ni and Fe–17Cr–14·5Ni–2·3Mo (wt-%) using a continuous wave CO2 laser does not induce significant crystallographic reorientation; the orientation remains predominantly (100) with misorientations of 3 or 4°. During the laser treatment oxidation can take place, even under helium gas protection. The composition and the in-depth distribution of the oxidised elements (iron oxides and chromium oxide) throughout the oxide layer are studied for the melted zone and for the non-treated surface, using electron spectroscopy for chemical analysis (ESCA). The two main modifications under discussion are the formation of an outer chromium oxide and the surface enrichment of sulphur and molybdenum during oxidation. The oxide thickness and composition are dependent on the heat dissipation rate during the laser treatment. When the energy dissipation is slow the oxide layer becomes thicker (up to 35 nm) and Cr2O3 is observed in the outer region of the layer on the heat affected zone and on the non-treated surface. This is not observed on the melted surface area, which is probably because of the vaporisation of chromium. When the alloy contains molybdenum, Cr2O3 is also found in the outer region of the oxide layer, even on the melted surface.MST/1174

AbstractAfter melting the (100) surface of a FeCrNi single crystal by a CWCO2 laser, a quasi‐epitaxial regrowth of the melted zone takes place2 within a misorientation of ∼4°. Three distinct areas are obtained: unaffected, heat affected and melted surfaces. On all areas, the segregation of sulphur alone occurs at high temperature, while there is a cosegregation of sulphur and nitrogen at T < 700°C. The thermodynamic interpretation of the segregation results is reported.For the surface segregation of sulphur, no interactions are noted with other alloy elements, except with chromium on the unaffected surface. The difference between the free enthalpies of segregation of sulphur on the untreated and treated surfaces is associated with the evolution of the defect content.When sulphur and nitrogen cosegregate on the surface of the untreated alloy, a repulsive coefficient of interaction between sulphur and nitrogen (i.e. βSN) of the order of 20 ± 5 kJ mol−1 is calculated. For the laser‐treated surfaces, the chromium content increases on the surface. Then, the regular solid solution model is applied to the CrNS ternary system. The calculated βSN coefficient is now 45 ± 5 kJ mol−1 and agrees with the experimental data obtained from the experimental evolution of ΔG versus YN (with ΔG = free energy of segregation of S and data YN = nitrogen concentration on the surface). More complex interactions are also discussed, especially for nitrogen, on account of the site competition between N and S and the influence of the vacancy content on the treated surfaces.