A TEM study of the oxide scale development in Ni-Cr-Al alloys

In the present study the isothermal oxidation behaviours of Ni-10Cr-5Al, Ni-20Cr-5Al and Ni-30Cr-5Al alloys were investigated. The alloys were oxidised in air for 50 h at 1000 °C. Analytical transmission electron microscopy was used to characterize the morphology, structure and composition of the oxide scale. The oxide formed adjacent to the alloy was α-Al₂O₃ such that the higher was the Cr content of the alloy the easier was its formation. The Ni-30Cr-5Al alloy formed a complete layer of α-Al₂O₃ in the initial stages of oxidation through ‘oxygen gettering’ by Cr. A decrease in scale thickness and an increase in scale adherence were observed with an increase in Cr content from 10 to 30 wt.%.     

The effect of an Al2O3 dispersion on the adhesion of Al2O3 scales on aluminized Co-10% Cr alloys

The beneficial effect of dispersions of reactive-metal oxide particles on the adhesion of Cr₂O₃ and Al₂O₃ scales formed on heat-resisting alloys is wellknown. It has been shown that an Al₂O₃ dispersion in an alloy can improve the adhesion of a Cr₂O₃ scale, and it is of particular interest in assessing the various theoretical proposals for the effect to determine whether such a dispersion can affect the adhesion of an Al₂O₃ scale. In this investigation, a Co–10% Cr–1 % Al alloy was first internally oxidized to form an Al₂O₃ dispersion. This alloy was then aluminized so that on subsequent oxidation an Al₂O₃ scale developed. It was shown that the dispersion did indeed improve the scale adhesion. The implications of this result are discussed.     

Microstructure and cyclic oxidation behavior of hot dip aluminized coating on Ni-base superalloy Inconel 718

Ni-base superalloy In-718 was coated by hot-dipping into a molten bath containing Al-7wt.%Si. Cyclic oxidation experiments on bare substrate and aluminized alloy were conducted at 1100 °C, covering 240 cycles in static air. After hot dip treatment the coating layers consisted of two phases Al and FeAlSi were detected in the external topcoat to the aluminide/alloy substrate. After oxidation testing, a continuous alumina scale was detected on the surface of the aluminide layer. This coating shows better cyclic oxidation resistance for In-718 alloy than untreated substrate. Cr₂O₃ was found to be the primary oxide phase in the oxidation of bare In-718 alloy. The inward diffusion of Al in the aluminide layer was restricted by the interdiffusion zone. The NiAl phase constituent of the aluminide layer was similar through all of the testing. Only the γ phase could be found below the coating surface and in the subsurface region as aluminum was lost to form the oxide.     

Effect of internal oxidation pretreatments and Si contamination on oxide-scale growth and spalling

Internal oxidation pretreatments carried out in quartz capsule with a Rhines pack were found to have a profound effect on the subsequent oxidation behavior of alloys. Specimens of Co-15 wt.% Cr, Co-25 wt.% Cr, Ni-25 wt.% Cr, and Ni-25 wt.% Cr-1 wt.% Al were tested at 1100°C after pre-oxidation treatments. Even without the development of internal oxide particles, pretreated binary CoCr and NiCr alloys oxidized with significantly lower rates. Selective oxidation of chromium was observed on the non-Cr₂O₃-forming Co-base alloys, whereas on the Cr₂O₃-forming Ni-base alloys, elimination of base-metal oxide, reduction in the Cr₂O₃ growth rate, and better scale adhesion were found. These effects were more apparent with pre-oxidation temperatures greater than 1000°C and with longer pretreatment times. Contaimination of Si from the quartz is believed to be the cause.     

The Oxidation of TiB2 Ceramics Containing Cr and Fe

The oxidation behavior of TiB₂, TiB₂–0.5 wt.% Cr–0.5 wt.% Fe and TiB₂–1 wt.% Cr–1 wt.% Fe was studied at 800, 900, and 1000°C in static air. These ceramics oxidized rather rapidly and formed thick oxide scales. The oxidation rates of TiB₂-base ceramics were comparable to TiO₂ formation on pure titanium. The scale formed on TiB₂ consisted of TiO₂ and B₂O₃. For TiB–Cr–Fe ceramics, a small amount of Cr- and Fe-oxides was additionally formed. B₂O₃ formed during oxidation tended to evaporate because of its high vapor pressure, making oxide scales porous and fragile. The oxidation of the TiB₂-base ceramics appeared to be governed by the inward transport of oxygen via the highly porous oxide scale. The oxidation resistance of TiB₂–Cr–Fe ceramics was similar to or better than that of TiB₂.     

The effect of aluminum as an alloying addition or as an implant on the high-temperature oxidation of Ni-25Cr

The oxidation behavior of aluminum-implanted Ni-25Cr and Ni-25Cr containing 1 wt.% Al has been studied at 1000°C and 1100°C in oxygen. As did Y alloying addition or Y-implantation, 1 wt.% Al added to Ni-25Cr prevented nodular formation of Ni-containing oxides, improved spalling resistance of the scale upon cooling to a similar degree, and eliminated the formation of large voids between the alloy and the scale at the oxidation temperature. However, the Al addition did not alter the rate of growth of the Cr₂O₃ scale, nor did it change the growth direction. Al-implantation produced no effect even when the maximum concentration and depth of penetration were adjusted to be identical with those of the yttrium in the Y-implanted alloy. The implications of these results concerning the reactive element effect are discussed.     

The kinetics and morphological development of oxide scales on cobalt-iron alloys (0–70 wt.% Fe) at 1200°C

The oxidation kinetics of Co-Fe alloys containing up to 70 wt.% Fe have been determined at 1200°C in oxygen at pressures ranging from 10^–4 to 1 atm. These kinetics, which were parabolic, resulted from metal diffusion through the scales as the rate-controlling process. The scales were either one-phase cobalto-wustite (CoFe)O or two-phase, containing growth of spinel cobalt ferrite (CoFe)₃O₄ in the cobalto-wustite matrix. The particular shape of the relationship, exhibiting a maximum for the parabolic oxidation constant vs. the iron alloy composition, is interpreted as being caused by the combined opposing effects of more rapid metal diffusion in cobalto-wustite and diffusional resistance of the spinel growths in cobalto-wustite with increasing iron alloy content.     

TEM studies of cross sections of oxidized Fe-25Cr-6Al-La alloy

The oxidation behavior of the alloy Fe–25%Cr–6%Al-RE (rich in lanthanum) was investigated by means of TEM analysis. The results show that after 2 hr oxidation of the alloy, in pure oxygen at 1200° C, La precipitated in the oxide scale at the top of -Al₂O₃ grains and at the grain-boundary regions in the form of tiny particles of hexagonal La₂O₃. These tiny particles prevented aluminum from diffusing toward the surface and suppressed lateral growth of the oxide scale. The rare-earth constituents accelerated the internal oxidation of the alloy during thermal cycling between 1200° C and room temperature. They appeared as particles of aluminum oxide and lanthanum oxide. Particles of cubic La₂O₃ precipitated in the alloy matrix near the oxide scale-metal interface in a direction parallel to grain boundaries.     

Oxidation behavior of a Ni3Al PM alloy

The oxidation behavior of a Ni₃Al powder-metallurgical (PM) alloy doped with boron was investigated by means of discontinuous isothermal tests in the temperature range of 535° to 1020°C for exposures of up to 150 hr. The oxidation kinetics were characterized by a sharp decrease in the oxidation rate at about 730°C which is associated with a change in the oxidation mechanism. Below 730°C, the scale exhibited an outer NiO layer and an internal-oxidation zone consisting of a fine dispersion of alumina in a diluted Ni-Al solid solution. Between these two layers a very thin layer of nickel could be observed. Above 730°C, a three-layered scale was observed consisting of an outer NiO layer, an intermediate layer that depending on temperature consisted of a mixture of nickel and aluminum oxides or NiAl₂O₄, and an inner layer of Al₂O₃, which accounts for the higher oxidation resistance. Oxidation at the higher temperatures resulted in extensive void formation at the scale/metal interface which led to poorly adherent scales. It is worth noting that at the early oxidation stage the scale was characterized by planar interfaces. Roughening of the air/scale and, specially, the scale/metal interfaces after long exposures at the low-temperature range or after short times at higher temperatures could be related to the formation of the inner Al₂O₃ layer at the grain boundaries which favor oxygen penetration through the grain interior.     

Effect of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> content on the oxidation behavior of Fe-Cr-Al-based ODS alloys

A study was conducted to investigate the cyclic oxidation behavior of two oxide dispersion strengthened (ODS) Fe-Cr-Al based alloys containing 0.17 wt.% and 0.7 wt.% Y₂O₃. The alloys were oxidized in air for 100 h at 1200°C based on a 24 h cycle period. X-ray diffraction (XRD) and analytical transmission electron microscopy (TEM) were used to characterize the structure, morphology, and composition of the oxide scales. Both alloys formed highly adherent and continuous layers of α-Al₂O₃ exhibiting a morphology indicative of inward scale growth. The role of Y₂O₃ was to promote adherence by segregating to the grain boundaries within the oxide. Concurrently, Y₂O₃ generated micro-porosity resulting in a scale of comparatively higher thickness in the alloy with 0.7 wt.% Y₂O₃.     

Studies concerning the effect of nitrogen on the oxidation behavior of TiAl-based intermetallics at 900°C

The oxidation behavior of the titanium aluminides Ti-50Al and Ti-48Al-5Nb has been investigated in Ar+20%O₂ and in air at 900°C. Thermogravimetric studies in combination with structural analyses using optical metallography, SEM/EDX and X-ray diffraction show a marked influence of nitrogen on the composition and growth rate of the oxide scales. For a more detailed study concerning the effect of nitrogen on the scale-growth kinetics, thermogravimetrical analyses were carried out during which the gas atmosphere was changed from air to Ar–O₂, and vice versa, without intermediate cooling of the specimen. The results show, that nitrogen adversely affects the formation of the initially formed alumina scale and that it enhances the growth rate of the rapidly growing Ti-rich oxide. This effect was observed in both alloys investigated, although the thermogravimetric results at first sight indicated an opposite effect for the Nb-containing alloy. This apparent contradiction is caused by internal oxidation which occurs in this alloy during exposure in Ar–O₂.     

The combined effect of refractory coatings containing reactive elements on high temperature oxidation behavior of chromia-forming alloys

The high temperature oxidation behaviors of chromia-forming alloys (F17Ti and Fe-30Cr alloys) have been studied at 1273 K under isothermal conditions and at 1223 K under cyclic conditions, in air under the atmospheric pressure. To extend the oxidation lifetime, coatings have been applied onto the alloy surfaces. Al₂O₃ and Cr₂O₃ films doped with Sm₂O₃ or Nd₂O₃ were prepared via the metal-organic chemical vapor deposition technique. Single Cr₂O₃, Al₂O₃, Nd₂O₃ and codeposited Cr₂O₃-Nd₂O₃, Al₂O₃-Nd₂O₃, Al₂O₃-Sm₂O₃ coatings drastically improved the chromia-forming alloy high temperature oxidation behavior, since they decreased the oxidation rate and enhanced the oxide scale adhesion. Results showed that a critical amount of reactive element (Nd or Sm) in chromia or alumina coatings (11-18 at.%) was needed to observe the most effective effect. The fast precipitation of NdCrO₃ or NdTi₂₁O₃₈ and the segregation of reactive elements at the chromia grain boundaries slowing down outward cation transport and consequently blocking the chromia grain growth, was supposed to be the main reasons of the beneficial effect ascribed to the reactive elements in chromia scales.     

Effect of Y2O3 additions on the plasticity of sintered Cr2O3

We have carried our constant strain-rate compression tests on polycrystalline Cr₂O₃ and Cr₂O₃ doped with 0.09 wt. % Y₂O₃ to establish whether there exists an effect of Y₂O₃ on the plasticity of Cr₂O₃. This study is motivated by previous work on the oxidation of alloys containing reactive-element additions. In that work, it has been observed that the addition of oxygen-active elements, such as Y to alloys that form Cr₂O₃ or Al₂O₃ oxide layers upon exposure at high temperature, strongly enhances the adhesion of the oxide layer to the base alloy as compared with alloys without reactive-element additions. We have found that at 1200°C (1) chromia exhibits limited plasticity at high temperatures, and (2) the presence of Y in the oxides does not enhance plasticity compared with addition-free oxides.     

High-Temperature Oxidation of Ni Coated with La2O3 by Atomic-Layer Chemical-Vapor Deposition (ALCVD)

The effects of superficial (30–100 nm) La₂O₃ surface coatings on the oxidation kinetics of Ni from 700 to 1100°C in air and the oxide morphology of the NiO scales have been investigated. The parabolic rate constant is lower than for uncoated Ni by a factor of 5 to 10. The oxide morphology changes with the La₂O₃ coatings: The oxide scale consists of an outer fine-grain layer with an inner region of coarser, but still equiaxed, grains. SIMS shows that the majority of the La remains at the surface where a highly oxygen-defective spinel, La₂Ni₄O₇, was found by TEM. Two-stage oxidation followed by SIMS profiling reveals that the oxide growth occurs inside the scales.     

Oxygen Transport during the High Temperature Oxidation of Pure Nickel

The high temperature oxidation of nickel has been investigated in air under atmospheric pressure in the temperature range 600–900°C. The oxidation kinetic curves deviate from the parabolic law for temperatures over 800°C. The observation of scale morphologies and the use of two stage oxidation experiments under ¹⁶O₂/¹⁸O₂ atmospheres showed that oxygen transport through the NiO scale had to be taken into consideration during the oxidation process. Despite the main outward diffusion of Ni species through the oxide scale, the inward oxygen diffusion at lower temperatures (<800°C) or the oxygen transport, probably as molecular species, via pores or micro-cracks were found to play a major role in the formation of duplex oxide scales, made of small equiaxed oxide grains at the metal/oxide interface overgrown by larger columnar grains at the gas/oxide interface. Oxygen diffusion coefficients into thermally grown NiO scales were determined and compared to the values of Ni diffusion coefficients from the literature.     

Analytical Electron-Microscopy Study of the Breakdown of α-Al2O3 Scales Formed on Oxide Dispersion-Strengthened Alloys

Alumina scales formed during cyclic oxidation at 1200°C on three Y₂O₃–Al₂O₃-dispersed alloys: Ni₃Al, -NiAl, and FeCrAl (Inco alloy MA956) were characterized. In each case, the Y₂O₃ dispersion improved the -Al₂O₃ scale adhesion, but in the case of Ni₃Al, an external Ni-rich oxide spalled and regrew, indicating a less-adherent scale. A scanning-transmission electron microscope (STEM) analysis of the scale near the metal–scale interface revealed that the scale formed an ODS FeCrAl showed no base metal-oxide formation. However, the scale formed on ODS Ni₃Al showed evidence of cracking and Ni-rich oxides were observed. The microstructures and mechanisms discussed may be relevant to a thermal-barrier coating with an Al-depleted aluminide bond coat nearing failure.     

High-temperature oxidation of rhodium

The oxidation kinetics of Rh were measured in air at 1 atm. in the temperature range 600–1000°C. The oxidation weight gain proceeds logarithmically at the lower temperatures (600°C, 650°C) followed by a transition to power law behavior at the higher temperatures (800°C). The logarithmic growth kinetics result from thickening of a hexagonal Rh₂O₃ scale. The transition from logarithmic to power law growth kinetics occurs in the range 700–800°C, and reflects thickening of hexagonal and orthorhombic Rh₂O₃ scales. Above 800°C, the growth kinetics result from thickening of a predominately orthorhombic Rh₂O₃ scale. At 1000°C the oxide becomes volatile, leaving the metal surface exposed.     

Oxidation resistance of sputtered Ni3(AlCr) nanocrystalline coating

Isothermal and cyclic oxidation resistance at 1000°C in air were investigated for a cast Cr-containing Ni₃Al-base alloy and its sputtered nanocrystalline coating. The results indicated that both the cast Ni₃Al alloy and its sputtered coating exhibit excellent isothermal oxidation resistance as a result of the formation of Al₂O₃ scales. However, the cast alloy possesses very poor cyclic oxidation resistance because of the spallation of the initially formed Al₂O₃ scale during cooling and subsequent formation of NiO. On the contrary, the sputtered Ni₃(AlCr) nanocrystalline coating exhibits very good cyclic oxidation resistance due to the significant improvement of the adhesion of Al₂O₃.     

Chromium transport through Cr2O3 scales. II. Changes in scale morphology during high vacuum treatment of oxidized chromium specimens

Chromium specimens oxidized at 1200 and 1300° O to give Cr₂O₃ scales with varying thicknesses have been high vacuum annealed for extended periods at temperature. During the high vacuum anneal chromium is transported through the scale and evaporates from the scale surface. Initially the rate of chromium evaporation decreases with time as a result of recrystallization and densification of the scale. On extended high vacuum treatment the rate of chromium evaporation again increases and major changes in scale morphology takes place. The outer scale surfaces develop hollows in the oxide grains while the grains protrude from the scale at the inner surfaces. The morphological changes are interpreted in terms of differences in diffusion rates along grain boundaries and through the lattice and resultant variations in surface energy along the surfaces.     

The effect of preoxidation on the sulfidation of Ni-20Cr (2–5)Al Alloys

Ni-20Cr alloys with 2, 3.5, and 5 wt.% Al have been preoxidized up to 100 hr at 1000°C in dry H₂, in H₂/23% H₂O and in air and subsequently exposed to an H₂/5% H₂S atmosphere at 750° C. During the preoxidation treatment different types of oxide scales were formed which affect the sulfidation protection in different ways. Optimum results were obtained for alloys with 3.5 and 5 wt.% Al after 20 hr exposure to dry H₂ at 1000°C. A thin Al₂O₃ scale is formed which decreases the sulfur attack by more than one order of magnitude. Preoxidation conditions for Ni-20Cr-2Al alloys in H₂ and for Ni-20Cr-2Al and Ni-20Cr-3.5Al in H₂/H₂O were observed to be less effective. No improvement was found for preoxidation in air or for Ni-20Cr-5Al alloys preoxidized in H₂/H₂O.