Oxide-alloy Interface Research Articles

Oxide adherence of Fe-20Cr-4Al alloys with small amounts of sulfur and reactive elements (Y,Hf)

Oxide adherence of Fe–20Cr–4Al alloys with small amounts of sulfur, yttrium and hafnium was studied in air for 360 ks at 1,373, 1,473 and 1,573K by mass change measurements, X-ray diffraction, scanning electron microscopy and electron probe microanalysis. After oxidation at 1,373K, spalling of oxide scales on 7ppmS, 53ppmS and 1,300ppmS alloys was recognized. However, spalling of oxide scales on the other alloys was not observed. After oxidation at 1,573K, spalling of the oxide scales on the alloys with sulfur increased roughly with increasing contents of sulfur, and spalling of oxide scales on the alloys containing yttrium was scarcely recognized, however, oxide scales on all of the alloys containing hafnium spalled at the entire surface. Oxide adherence on the alloys may relate to morphologies of oxide scales and oxide–alloy interface, size and distribution of chromium sulfide, Y3Al5O12 and HfO2 particles at the oxide–alloy interface and temperature of oxidation.

Cyclic-Oxidation Behavior of Fe–20Cr–4Al Alloys with Small Amounts of Sulfur at High Temperatures

The cyclic-oxidation behavior of Fe–20Cr–4Al alloys with 3, 35, 53, 104, and 171 ppm sulfur was studied in oxygen at 1273, 1373, 1473, 1573, and 1673 K by mass-change measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron-probe microanalysis (EPMA). The cyclic oxidation consisted of an 18.0 ksec exposure that was repeated up to five times. Amounts of spalled oxides of the alloys with 35 and 53 ppm sulfur increased after one and two oxidation cycles, and then decreased after three or more oxidation cycles at 1473 and 1573 K. On the other hand, spalling of oxide scales on the alloys with 3 and 171 ppm sulfur was scarcely recognized after all cycles used in this study. SEM observations of both sides of the spalled oxides indicated that spallation was related to the morphology of the surface oxides and to the morphology and volume of cavities at the oxide–alloy interface.

Second-harmonic spectroscopy of interband excitations at the interfaces of strainedSi(100)−Si0.85Ge0.15−SiO2heterostructures

We have applied optical second-harmonic generation to the spectroscopy of strained $\mathrm{Si}(100)\ensuremath{-}{\mathrm{Si}}_{0.85}{\mathrm{Ge}}_{0.15}\ensuremath{-}{\mathrm{SiO}}_{2}$ heterostructures, covering the full range of the fundamental critical-point interband transitions. By analyzing spectra from samples with different thicknesses of the alloy layer ranging from 4 to 28 nm, we were able to resolve different interband excitations localized at the two buried interfaces of this system. The contribution of the substrate-alloy interface to the spectra consists of bulklike, ${E}_{1}$- and ${E}_{2}$-type critical-point transitions, consistent with a pseudomorphic structure of this interface. The dominating excitations at the alloy-oxide interface, comprised of a broad resonance centered around 3.6 eV, have no equivalent in the bulk, and were also observed at $\mathrm{Si}(100)\ensuremath{-}{\mathrm{SiO}}_{2}$ interfaces. These transitions are assigned to alloy atoms without ${T}_{d}$ symmetry at the boundary between the ${\mathrm{Si}}_{0.85}{\mathrm{Ge}}_{0.15}$ layer and the ${\mathrm{SiO}}_{x}$ transition region.

High-resolution SIMS and analytical TEM evaluation of alumina scales on?-NiAl containing Zr or Y

High-resolution SIMS and TEM have been used to evaluate growth processes and interfacial segregation occurring in α-Al2O3 scales grown at 1200°C on β-NiAl containing zirconium or yttrium.18O/SIMS shows that the extent of aluminum diffusion occurring during α-Al2O3 growth is reduced by the presence of these alloying elements, which are seen by SIMS imaging as oxide particles within the scale. STEM/EDS of the same oxide scales show that zirconium and yttrium also segregated to the oxide-alloy interface to the extent, respectively, of ≈0.15 and ≈0.07 of a monolayer and to oxide grain boundaries (≈0.2 monolayer). The complementary information provided by SIMS, TEM, and STEM provides a better understanding of the role of “reactive elements” in modifying scale-growth processes.

Oxidation of an amorphous Ni77P23 alloy in air and dry oxygen at 300 °C

The oxidation behaviour of an amorphous Ni77P23 alloy, prepared by electrodeposition, was investigated in air and in dry oxygen at 300 °C. The morphology, structure and composition of the oxide developed on the alloy were examined by X-ray photoelectron spectrometry, secondary-ion mass spectrometry (SIMS), and by high resolution transmission electron microscopy of ultramicrotomed sections. With successful preparation of extremely thin ultramicrotomed sections, between 5 and 10 nm thick, the microstructure of the oxide grown on the alloy and the oxide-alloy interface were revealed clearly, at atomic scale resolution, for the first time. SIMS depth profiling analysis has shown that the oxide films, grown in air and dry oxygen, consist mainly of NiO. However, the oxide growth rate in air was found to be considerably less than that in dry oxygen. High resolution cross-sectional transmission electron microscopy has shown that the difference in oxidation rates appears to be due to the distinctly different mosaic structure of the oxide films grown in air and dry oxygen which, in turn, affects the length of easy paths for Ni2+ diffusion during oxide growth.

The effect of thermal annealing on the adherence of Al2O3-films deposited by low-pressure, metal-organic, chemical-vapor deposition on AISI 304

Thin alumina films, deposited at 280°C by low-pressure, metal-organic, chemical-vapor deposition on stainless steel, type AISI 304, were annealed at 0.17 kPa in a nitrogen atmosphere for 2,4, and 17 hr at 600, 700, and 800°C. The effect of the annealing process on the adhesion of the thin alumina films was studied using a scanning-scratch tester, type SST-101, developed by Shimadzu. The best mechanical properties were obtained with unannealed samples. After thermal annealing the critical load decreased, proportional to annealing time and/or temperature. This effect was probably due to the presence of a high thermal stress and to preferential segregation of sulfur near the oxidealloy interface.

Oxydation des alliages TiZr sous air et sous oxygéne. III: Particularités morphologiques de l'interface substrat/oxyde de l'alliage Ti 52Zr 48

Oxidation of the Ti 52Zr 48 alloy at 720°C leads to a morphological peculiarity: undulation of the oxide-alloy interface. This phenomenon, which can be linked neither to preferential oxidation of the metal nor to preferential oxidation at boundary grains, has been ascribed to the relaxation of strains by an α+ β two-phase substrate. This hypothesis is supported by measurements of the concentration profiles of titanium, zirconium and oxygen using Castaing's electronic microprobe. In conclusion, there are four conditions which the TiZr alloys have to fulfil for the alloy interface to be undulated.

POSAP analysis of the oxide-alloy interface in stainless steel

A recently built position-sensitive atom probe (POSAP) was used to visualize the 3D atom-by-atom microstructures of the oxide-alloy interface of stainless steel. The oxide was formed in oxygen at a pressure of 10 -4 Torr on the clean field-evaporated surface of a type 316 stainless steel. The oxidation was carried out at temperatures of 350°C and 475°C for 5 min. The composition of the oxide and the microstructure of the interface were determined by the POSAP, and were compared with the oxide film on the ferritic phase of a duplex stainless steel formed at 600°C. The oxide films formed at 350°C and at 475°C consist of mostly iron oxide, whereas in the topmost part of the film at 475°C, chromium oxide is predominant. The oxide on the duplex stainless steel at 600°C consists mainly of chromium oxide. The interface is very sharp at 300°C. It becomes slightly rough at 475°C, and a rough interface is observed at subnanometer scale at 600°C. The oxide film tends to grow only in the region just above where molybdenum does not exist.

High-temperature oxidation behaviour in the Zr-Al system

The oxidation behaviour of phases in the Zr-Al system has been studied in dry oxygen at elevated temperatures and isothermally. The oxidation of α-Zr-1 mass% Al, Zr 3Al, Zr 2Al, ZrAl and Zr 2Al 3 obeys a parabolic rate law, while the oxidation of ZrAl 2 and ZrAl 3 obeys a logarithmic rate equation. Zr 3Al, Zr 2Al, ZrAl, Zr 2Al 3 and ZrAl 2 are selectively oxidized to give tetragonal and monoclinic ZrO 2 as the oxidation product. During this process aluminium escapes oxidation by diffusion into the bulk of the alloy and forms there Zr 2Al, Zr 5Al 3, Zr 2Al 3, ZrAl 2 and ZrAl 3 respectively, as a secondary product of oxidation. The new phases formed near the oxide alloy interface of higher aluminium content than the matrix phase, are the adjoining phases in the phase diagram of the Zr-Al system.

Morphological instability of a planar oxide-alloy interface for inward oxide growth

The morphological stability of an oxide layer during selective oxidation of a binary alloy is investigated theoretically for the case of inward oxygen diffusion. According to the result of a linear stability analysis, a roughening of a planar oxide-metal interface can occur if interdiffusion in the alloy is rate determining. The wavelength of the most unstable mode is evaluated taking into account interface smoothing via volume and interface diffusion. For parabolic scale growth, the wavelength increases with time according to the power law t 1 4 for volume diffusion and t 1 6 for interface diffusion. The theoretical results are compared with recent experiments on the oxidation of γ′-Ni 3Al at low oxygen partial pressure where protrusions of the oxide into the metal were found in the initial stage of oxidation. The observed smoothing of the interface in the course of further oxidation is interpreted as a transition from metal-to oxygen-diffusion controlled growth.

Ondulation de l'interface métal/oxyde dans le cas des alliages Ti-Zr

Oxidation of Ti-Zr alloys containing 47,5 atomic % zirconium, at 720 o C and below 100 mbar of air, allows us to show the undulation of alloy-oxide interface. These undulations appear only for particular conditions of temperature and thickness. This process (interface undulation) could be the «visual» translation of the stresses accomodation. These stresses are recorbed by two crystallographic phases (α and β) exhibiting different mechanical properties

Oxidation of Fe25Cr4AlPd and Fe25Cr4AlRu alloys

Fe25Cr4Al alloys containing Pd and Ru additions were oxidized in air at 1090°C. Scanning electron microscopy, dispersive x-ray analyses, and x-ray mapping were used to characterize the morphologies of the scales formed on these alloys. The results showed that Pd tended to segregate at the oxide-alloy interface as discrete and platelike particles during elevated temperature oxidation of the Pd-containing FeCrAl alloys. These Pd particles appeared to improve the adherence of the alumina scale. However, no improvement of the scale adherence of Ru-containing FeCrAl alloys was observed as outward diffusion of Ru cations to the external alumina scale led to the formation of RuO 2 within this scale.

SEM and EDX analyses of the scales formed on Fe—24Cr—5V—Pd and Fe—24Cr—5Mo—Pd alloy systems

AbstractSEM and EDX analyses were used to characterize the morphologies of the scales formed on the Fe—24Cr—5V—Pd and Fe—24Cr—5Mo—Pd alloy systems in air at 900°C. SEM observations revealed that porous and non‐protective FeCr2O4 scales were formed on the Fe—24Cr—5V—Pd alloy system. Addition of Pd has no influence on the oxidation performance of the Fe—24Cr—5V alloy. However, thin and adherent Cr2O3 scales were formed on the Fe—24Cr—5Mo—Pd alloy system. The EDX analyses showed that Pd was enriched at the oxide—alloy interface of the Fe—24Cr—5Mo—3Pd alloy. This Pd‐enriched layer provided a fast diffusion path for Cr to form Cr2O3 oxide scales.

Selective oxidation of Zr 2Al 3

The oxidation of Zr 2Al 3 in dry oxygen in the temperature range 875–965 K obeys a parabolic rate law. An activation energy of 270 kJ mol −1 has been estimated. During the oxidation, aluminium diffuses from the oxide layer into the Zr 2Al 3 bulk, thus forming the ZrAl 2 phase very close to the oxide-alloy interface. The excess zirconium is therefore selectively oxidized, giving tetragonal and monoclinic ZrO 2 containing no Al 2O 3.

AES study of amorphous Fe 100− xB x ( x = 14–25 at.%) alloys

Auger electron spectroscopy in conjunction with argon ion sputtering was used to study the sputtering behaviour of melt-spun amorphous Fe 100− x B x ( x = 14–25 at.%) alloys. Measurement of equilibrium surface composition using the low energy peak of iron Fe(47 eV, MVV) indicated an enrichment of boron in the sputter altered layer in all alloys for argon ion energies of 1 keV and 3 keV. The corresponding sputtering yield ration ( Y Fe Y B ) had a value between 1.3 and 1.6. The surface composition, calculated by using the high energy iron peak (651 eV, LMM), indicated depletion of boron in the sub-surface region in comparison to that measured by using the low energy peak. It appears that binding effects rather than mass play a significant role in determining the preferential sputtering behaviour of these alloys and the possible sputtering mechanisms could involve cascade sputtering and/or surface segregation. The plot of surface composition or the sputter yield ration versus boron atom fraction in the bulk showed a break at 18 at.% boron. This is attributed to the known structural differences in FeB amorphous alloys. Analysis of native oxide films on these alloys showed enrichment of boron near the oxide-alloy interface and the film thicknesses were approximately in the range 4–6 nm. It was inferred from an analysis of the low energy Auger peaks that the oxide films consisted of Fe 3+, Fe 2+ and B 3+ species.

Oxidation characteristics of some Fe-40CrPd and Fe-40CrRu alloys in air at 900°C

Fe-40Cr-1Pd, Fe-40Cr-3Pd, Fe-40Cr-1Ru, and Fe-40Cr-3Ru alloys were oxidized in air at 900°C. The isothermal oxidation behavior of the alloys investigated was determined by using thermogravimetric (TG) techniques. Furthermore, the morphologies of the scales formed on the alloys were investigated by means of scanning electron microscopy, energy dispersive x-ray analyzer, and x-ray mapping. The TG results showed that 0.92 and 3.2 wt.% Pd additions to the Fe-40Cr alloy lead to a substantial decrease in the oxidation rate. However, the addition of 0.90 wt.% Ru to the Fe-40Cr alloy has little influence on the oxidation rate while a 3 wt.% Ru addition results in a slight decrease in the oxidation rate. The improved oxidation resistance of the Pd-containing Fe-40Cr alloys is attributed to Pd, which segregates to the oxide-alloy interface during elevated temperature oxidation and facilitates Cr diffusion.

Relation between impurities and oxide-scale growth mechanisms on Ni-34Cr and Ni-20Cr alloys. II. Influence of sulphur

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.

Change of oxidation kinetics in the Zr-Al system

The oxidation of ZrAl 2 in dry oxygen in the temperature range 883–1038 K obeys a logarithmic rate law, while the oxidation of Zr 3Al, Zr 2Al, ZrAl and Zr 2Al 3 follows a parabolic rate law. An activation energy of 115 kJ mol −1 is estimated for the oxidation of ZrAl 2. The reaction starts by chemisorption of oxygen and rapid formation of a ZrO 2 film. Subsequently, the reaction slows down and proceeds further at a low rate. Aluminium escapes oxidation and diffuses to the ZrAl 2 bulk, forming the ZrAl 3 phase at the oxide-alloy interface. The remaining zirconium is therefore selectively oxidized. Due to the higher content of aluminium in ZrAl 2 than in the Zr-Al phases mentioned, aluminium accumulates very quickly at the oxide-alloy interface, releasing a correspondingly small amount of zirconium. Thus, the reaction is rapid as long as there is enough zirconium available for oxidation; subsequently, the reaction rate decreases abruptly.

Interfacial segregation during oxidation in O2 at 1173 K of CeO2-coated Fe-20Cr alloys

The interfacial segregation of sulphur and carbon during the oxidation in 1 torr O2 at 1173 K of Fe-20Cr alloy, which was either free of Ce, alloyed with 0.078 wt.% Ce, or sputter-coated with a 4 nm-thick CeO2 layer, was studied using polyatomic SIMS. Oxidation periods were up to 19 hr. During oxidation, sulphur and carbon accumulated at the alloy-oxide interface region of both uncoated and coated alloys. The amount of segregated sulphur did not vary appreciably with time, whereas carbon increased with time. The total amount of segregants was similar for both uncoated and coated alloys, although the scales formed on the sputter-coated alloy maintained adhesion and were about 10 times thinner than those on the uncoated alloys.

Microstructural aspects of the scales formed on FeMnAl and FeMnAlCr alloys in SO 2/O 2 atmospheres at elevated temperatures

Fe-30Mn-8Al and Fe-30Mn-8Al-2Cr alloys were exposed to mixed oxidant gaseous environments containing both oxygen and sulfur dioxide at 900°C. The microstructure of the reaction products formed on these alloys were examined by scanning electron microscopy (SEM) and x-ray mapping techniques. The scales formed on the Fe-30Mn-8Al alloy exposed to oxidizing/sulfidizing environments consist of duplex oxides structure with an outer (FeMn) 2O 3 layer and an inner α-Al 2O 3 layer at the oxide-alloy interface. This alloy does not undergo severe degradation by sulfidation after prolonged exposure in SO 2/O 2 gas mixture for 24 h. On the other hand, addition of Cr to the Fe-30Mn-8Al alloy leads to a substantial change in scale morphology. The results show that Cr addition promotes the formation and growth of the external fibrous (Mn, Cr) S sulfide and internal MnS sulfide within the substrate alloy.