The effect of Si additions on the high temperature oxidation of a ternary Ni-10Cr-4Al alloy in 1 atm O2 at 1100 °C

The oxidation of four Ni-10Cr-ySi-4Al alloys was studied at 1100 °C to examine the effects of Si additions (from 2 to 6 at.%) on the behavior of the alloy Ni-10Cr-4Al. Addition of 2 at.% Si prevented completely nickel oxidation, but could not form alumina scales. Larger Si additions produced alumina only over part of the alloy surface (about 20% with 4 at.% Si and 30% with 6 at.% Si), but could not prevent completely the internal oxidation of Al. The results are interpreted by extending to quaternary alloys the mechanism of the third-element effect already proposed for ternary alloys.     

The oxidation of three Ni-6Si-xAl alloys in 1 atm O2 at 1000 °C

The oxidation of three ternary Ni-6Si-xAl alloys containing 6, 10 and 15 at.% Al and of the corresponding binary Ni-Al alloys has been studied at 1000 °C under 1 atm O₂ to examine the effect of different Al additions on the behavior of ternary Ni-Al-Si alloys containing 6 at.% Si. Of the three binary Ni-Al alloys only Ni-15Al was able to form external alumina scales. Conversely, all the three ternary alloys formed an innermost layer of alumina directly in contact with the alloy following very similar and approximately parabolic kinetics after a short faster initial stage due to transient formation of NiO. Thus, the presence of silicon is very effective to reduce the critical Al content needed to form exclusive alumina scales with respect to binary Ni-Al alloys. The third-element effect due to silicon is interpreted on the basis of an extension of Wagner’s criterion for the transition from the internal to the external oxidation of the most reactive component in binary alloys.     

The oxidation of two ternary Ni-Cu-10 at.% Al alloys in 1 atm of pure O2 at 800-900 °C

The oxidation of the ternary alloys Ni-45Cu-10Al and Ni-30Cu-10Al has been studied at 800-900 °C under 1 atm O₂. The presence of 10 at.% Al reduces significantly the oxidation rate of the corresponding Cu-Ni alloys during the initial oxidation stages, even before the establishment of a complete Al₂O₃ layer. The weight of individual sample of the two ternary Ni-Cu-10Al alloys at 800 °C increases more rapidly than at 900 °C during the initial oxidation stage. As oxidation proceeds, the weight gain at 800 °C slows down to a degree that the total weight gain after 24 h oxidation at 800 °C is less than that at 900 °C. Due to a faster formation of the Al₂O₃ layer, which suppresses earlier the further oxidation of Cu and Ni, the external region of the scales grown on Ni-45Cu-10Al contain much less Cu and Ni oxides than those grown on Ni-30Cu-10Al. The transition from the internal oxidation to the selective external oxidation of the most reactive component Al in Ni-Cu-Al alloys is favored by higher values of the Al content, of temperature and of the Cu/Ni ratio.     

The nature of the third-element effect in the oxidation of Fe-xCr-3 at.% Al alloys in 1 atm O2 at 1000 °C

The oxidation of an Fe-Al alloy containing 3 at.% Al and of four ternary Fe-Cr-Al alloys with the same Al content plus 2, 3, 5 or 10 at.% Cr has been studied in 1 atm O₂ at 1000 °C. Both Fe-3Al and Fe-2Cr-3Al formed external iron-rich scales associated with an internal oxidation of Al or of Cr+Al. The addition of 3 at.% Cr to Fe-3Al was able to stop the internal oxidation of Al only on a fraction of the alloy surface covered by scales containing mixtures of the oxides of the three alloy components, but not beneath the iron-rich oxide nodules which covered the remaining alloy surface. Fe-5Cr-3Al formed very irregular external scales where areas covered by a thin protective oxide layer alternated with others covered by thick scales containing mixtures of the oxides of the three alloy components, undergrown by a thin layer rich in Cr and Al, while internal oxidation was completely absent. Conversely, Fe-10Cr-3Al formed very thin, slowly-growing external Al₂O₃scales, providing an example of third-element effect (TEE). However, the TEE due to the Cr addition to Fe-3Al was not directly associated with a prevention of the internal oxidation of Al, but rather with the inhibition of the growth of external scales containing iron oxides. This behavior has been interpreted on the basis of a qualitative oxidation map for ternary Fe-Cr-Al alloys taking into account the existence of a complete solid solubility between Cr₂O₃ and Al₂O₃.     

The oxidation of two ternary Fe-Cu-10 at.% Al alloys in 1 atm of pure O2 at 800-900 °C

The oxidation of two ternary Fe-Cu-Al alloys containing 10 at.% Al (Fe-65Cu-10Al and Fe-30Cu-10Al) has been studied at 800-900 °C under 1 atm O₂. Under all conditions both alloys show an initial faster stage during which Fe-65Cu-10Al corrodes more rapidly at 800 °C than at 900 °C, while Fe-30Cu-10Al follows nearly identical kinetics at both temperatures. As oxidation proceeds, a continuous alumina layer is eventually established on the surface of the two alloys, thus decreasing significantly their oxidation rates. Altogether, the Fe-rich alloy Fe-30Cu-10Al oxidizes slightly faster than the Cu-rich alloy Fe-65Cu-10Al at both temperatures. The possible reasons for the decrease in the critical Al content needed to form external alumina scales for the Cu-rich alloy in comparison with binary Cu-Al alloys are examined.     

Oxidation of a quaternary three-phase Cu-20Ni-20Cr-5Fe alloy at 700-900 °C in 1 atm of pure O2

The oxidation of a quaternary Cu-Ni-Cr-Fe alloy containing approximately 20 at.% Ni, 20 at.% Cr and 5 at.% Fe, balance Cu (Cu-20Ni-20Cr-5Fe), was studied at 700-900 °C in 1 atm of pure oxygen. The alloy is composed of a mixture of three phases, where the lightest α phase with the largest Cu content forms the matrix, while the other two, much richer in Cr, form a dispersion of isolated particles. At variance with the ternary three-phase Cu-20Ni-20Cr alloy examined previously, which was unable to form protective chromia scales over the alloy surface even after an extended period of oxidation, the present alloy formed complex external scales containing mixtures of the oxides of the various components plus a deep internal region containing a mixture of alloy and oxide phases. With time, a very irregular and thin but essentially continuous chromia layer formed at the bottom of the mixed internal oxidation region, producing a gradual decrease of the oxidation rate. Thus, the addition of 5 at.% Fe to Cu-20Ni-20Cr alloy is able to decrease the critical Cr content required to form the most stable oxide and promotes the formation of a continuous chromia scale under a lower Cr content in spite of the simultaneous presence of three different phases.     

An investigation on oxidation wear mechanisms of Ti-46Al-7Nb-0.7Cr-0.1Si-0.2Ni intermetallic-based alloys

The results of investigation on oxidation wear mechanism of Ti-46Al-7Nb-0.7Cr-0.1Si-0.2Ni-based intermetallic alloy are presented. Oxidation was carried out in air at temperatures: 900 °C, 925 °C and 975 °C taking into account the micro-geometry of surfaces being heated and oxidized. It was determined that the rise of surface roughness to Ra = 5.8 μm definitely reduces scale of chipping in higher temperature (975 °C). Investigation of the material structure of the specimen and chemical composition of oxidation products was performed. Possibilities of an increase in the heat resistance of the tested alloy by means of application of high roughness surfaces were shown.     

High-temperature oxidation behavior of Nb-Si-Cr alloys with Hf additions

The oxidation behavior of two alloys from the Nb-Si-Cr system containing hafnium has been investigated under isothermal and cyclic conditions. Nb-20Si-20Cr-(5,10)Hf alloys (composition in atomic percent) were exposed to air for 24 and 168 h over a range of temperatures from 700 °C to 1400 °C. A gravimetric method was used to determine the oxidation kinetics; weight gain per unit area as a function of temperature or time. Computed isothermal sections of the quaternary Nb-Si-Cr-Hf phase diagrams were used for alloy selection. XRD, SEM and EDS were used to characterize the phases present in the oxidation products and the alloys. Oxidation experiments revealed extremely good oxidation resistance at 700 °C and 800 °C and above 1200 °C under isothermal conditions for both alloys. Partial pesting was observed when the samples were exposed to 800 °C. Complete oxide formation was observed above 1000 °C for 5Hf and above 900 °C for 10Hf up to 1200 °C. Beneficial effects have been observed with the addition of 10Hf to the alloy compared to 5Hf at 700 °C, 1200 °C and 1300 °C resulting in a reduction of weight gain per unit area.     

Isothermal oxidation behaviour of Nb-W-Cr Alloys

A study of the effect of Cr content on the microstructure and isothermal oxidation behaviour of four alloys from the Nb-Cr-W system has been performed. Selection of specific alloy compositions has been based on the ternary isothermal sections. Oxidation experiments were conducted in air at 900 and 1300 °C for 24 h under isothermal conditions. Weight gain per unit area as function of the temperature has been used to evaluate the oxidation resistance. The phases present in the alloys and the oxide scales were characterized by XRD, SEM, and EDS. Microstructure consists of Nb solid solution and NbCr₂, Laves phase. The oxidation kinetics follows a parabolic behaviour at 1300 °C; the addition of 30% Cr resulted in the significant reduction of the parabolic oxidation rate. At 900 °C, alloys with higher Cr content exhibit higher oxidation rates in comparison to alloys with lower Cr content. The oxidation products are a mixture of CrNbO₄ and Nb₂O₅ and the amount of each oxide present in the mixture is related to the intermetallic phase content and the oxidation temperature. The characterization results delineate the effect of the Cr content on the oxidation mechanisms of these alloys that represent a promising base for high-temperature alloy development.     

Effect of silicon on oxidation of Ni-15Al alloy

1　INTRODUCTIONChromium, aluminum, and silicon can formsatisfactory protective scales on Ni based alloys.Chromium is expensive and not suitable for use attemperatures above 1 000℃ due to the evaporationof CrO3. It has also been well established that theincorporation of Si in many alloy systems has abeneficial effect on their oxidation resistance[1, 2].In addition, silicon is abundant and cheap. More over, Si has one of the largest solubility in Ni3Alwhere it …     

The Oxidation of Two Ternary Ni–Cu–5 at.%Al Alloys in 1 atm of Pure O2 at 800–900°C

The oxidation of a Ni-rich and a Cu-rich single-phase ternary alloy containing about 5at.% aluminum has been studied at 800 and 900°C under 1atm O₂. The behavior of the Ni-rich alloy is similar to that of a binary Ni–Al alloy with a similar Al content at both temperatures, with formation of an external NiO layer coupled to the internal oxidation of aluminum. The Cu-rich ternary alloy shows a larger tendency to form protective alumina scales, even though its behavior is borderline between protective and non-protective. In fact, at 800°C, after an initial stage of fast reaction during which all the alloy components are oxidized, this alloy is able to develop a continuous layer of alumina at the base of the scale which prevents the internal oxidation of aluminum. On the contrary, at 900°C the innermost alumina layer undergoes repeated rupturing followed by healing, so that internal oxidation of Al is only partly eliminated. As a result, the corrosion kinetics of the Cu-rich ternary alloy at 900°C are much faster than at 800°C and very similar to those of pure copper and of Al-dilute binary Cu–Al alloys. Possible reasons for the larger tendency of the Cu-rich alloy to form external alumina scales than the Ni-rich alloy are examined.     

The Effect of Si Additions on the High-Temperature Oxidation of a Ternary Ni–10Cr–4Al Alloy in 1 ATM O2 AT 900–1000 °C

The oxidation of four Ni–10Cr–ySi–4Al alloys has been studied in 1 atm O₂ at 900 and 1000 °C to examine the effects of various Si additions on the behavior of the ternary alloy Ni–10Cr–4Al, which during an initial stage formed external NiO scales associated with an internal oxidation of Cr + Al, later replaced by the growth of a chromia layer at the base of the scale plus an internal oxidation of Al. The addition of 2 at.% Si was able to prevent the oxidation of nickel already from the start of the test, but was insufficient to form external alumina scales at 1000 °C, while at 900 °C alumina formed only over a fraction of the alloy surface. At 1000 °C the addition of 4 at.% Si produced external chromia scales plus a region of internal oxidation of Al and Si, a scaling mode which formed over a fraction of the alloy surface in combination with alumina scales also by oxidation at 900 °C. Conversely, the presence of about 6 at.% Si produced external alumina scales over the whole sample surface at 900 °C, but only over about 60 % of the alloy surface at 1000 °C. The changes in the oxidation modes of the ternary Ni–10Cr–4Al alloy produced by Si additions have been interpreted by extending to these quaternary alloys the mechanism of the third-element effect based on the attainment of the critical volume fraction of internal oxides needed for the transition to the external oxidation of the most-reactive-alloy component, already proposed for ternary alloys.     

Cyclic oxidation behaviour of Ti3Al-based alloy with Ni-Cr protective layer

The influence of a thin 80Ni-20Cr (at.%) protective coating on the cyclic oxidation of a Ti-24Al-11Nb (at.%) alloy based on Ti₃Al at 600 and 900 °C in air was investigated using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction analysis (XRD). The results of the oxidation tests showed that deposited Ni-Cr layer provides an improved oxidation resistance due to the formation of protective oxide scale which barriers the outward Ti diffusion into the scale. In some extent surface formation of the nitride layer also prevents diffusion of alloying elements from the matrix. Although oxidation at 900 °C is faster than that at 600 °C, a remarkable reduction in mass gain of the alloy with protective coating was observed. The thickness of oxide scale on the coated samples is approximately two times less than that formed on the uncoated samples treated under the same exposure conditions (120 h).     

Criteria for the formation of protective Al2O3 scales on Fe-Al and Fe-Cr-Al alloys

The conditions for the formation of external alumina scales on binary Fe-Al alloys and the nature of the third-element effect due to chromium additions have been investigated by studying the oxidation at 1000 °C in 1 atm O₂ of a binary Fe-10 at.% Al alloy (Fe-10Al) and of two ternary Fe-Cr-10 at.% Al alloys containing 5 and 10 at.% chromium (Fe-5Cr-10Al and Fe-10Cr-10Al, respectively). An Al-rich scale developed initially on Fe-10Al was subsequently replaced by a multi-layered scale containing mixtures of Fe and Al oxides plus a large number of Fe-rich oxide nodules: internal aluminum oxidation was essentially absent from this alloy. Addition of 5 at.% chromium to Fe-10Al did not suppress the formation of nodules, but they were eventually healed by the growth of an alumina layer at their base, resulting in a significant reduction of the oxidation rate. Finally, the alloy with 10 at.% Cr formed continuous external alumina scales without any Fe-rich nodule. Thus, the addition of sufficient amounts of chromium to Fe-10Al produces a third-element effect as expected. However, the process found in this alloy system does not involve a prevention of the internal oxidation of Al. Instead, it shows a transition from the growth of mixed Fe- and Al-rich external scales directly to an external Al₂O₃ scale formation. An interpretation of this kind of mechanism involving a third-element is presented along with a prediction of the critical Al contents required to produce the various possible scaling modes on binary Fe-Al alloys.     

Effect of lamellar microstructure on oxidation kinetics of Fe3Al sintered by hot isostatic pressing

The present paper focuses on the investigation of the relationship between microstructure of Fe₃Al prepared by hot isostatic pressing (HIP) and kinetics of alumina layer formation during oxidation at 900 °C, 1000 °C and 1100 °C. As prepared HIPed Fe₃Al sample reveals lamellar microstructure with inhomogeneous Al distribution which originates from the preliminary mechanical activation of Fe-Al mixture. At 900 °C, Fe₃Al oxidation is characterized by selective growth of very rough alumina layer containing only transient aluminium oxides. In addition to these transient oxides, α-Al₂O₃ stable phase is formed at 1000 °C. At the highest temperature (1100 °C), continuous and relatively smooth alumina layer mainly contains fine crystallites of α-Al₂O₃. The initial lamellar structure and phase inhomogeneity in as-HIPed Fe₃Al samples are supposed to be the main factors that determine observed peculiarities after Fe₃Al oxidation at 900 °C and 1000 °C.     

Oxidation of a chromia-forming nickel base alloy at high temperature in mixed diluted CO/H2O atmospheres

Corrosion of a chromia-forming nickel base alloy, Haynes 230^®, has been investigated under impure helium containing a few Pa of CO and H₂O at 900 °C. It has been found that this alloy reacts simultaneously with CO and H₂O. Oxidation by CO has been revealed to occur mainly in the first hours. CO diffuses through the scale via short-circuit pathways and oxidizes Al, Cr and Si at the oxide/metal interface. Kinetics of CO oxidation has been investigated and several rate limiting steps are proposed. In the long term, H₂O is the major oxidant of chromia-forming nickel base alloys in impure helium.     

The third-element effect in the oxidation of Ni–xCr–7Al (x = 0, 5, 10, 15 at.%) alloys in 1 atm O2 at 900–1000 °C

The oxidation in 1 atm of pure oxygen of Ni–Cr–Al alloys with a constant aluminum content of 7 at.% and containing 5, 10 and 15 at.% Cr was studied at 900 and 1000 °C and compared to the behavior of the corresponding binary Ni–Al alloy (Ni–7Al). A dense external scale of NiO overlying a zone of internal oxide precipitates formed on Ni–7Al and Ni–5Cr–7Al at both temperatures. Conversely, an external Al₂O₃ layer formed on Ni–10Cr–7Al at both temperatures and on Ni–15Cr–7Al at 900 °C, while the scales grown initially on Ni–15Cr–7Al at 1000 °C were more complex, but eventually developed an innermost protective alumina layer. Thus, the addition of sufficient chromium levels to Ni–7Al produced a classical third-element effect, inducing the transition between internal and external oxidation of aluminum. This effect is interpreted on the basis of an extension to ternary alloys of a criterion first proposed by Wagner for the transition between internal and external oxidation of the most reactive component in binary alloys.     

High temperature oxidation of Fe-Al and Fe-Cr-Al alloys: The role of Cr as a chemically active element

Good high-temperature corrosion resistance of Fe-Al alloys in oxidizing environments is due to the α-Al₂O₃ film which is formed on the surface provided temperature is above 900 °C and the Al-content of the alloy exceeds the critical value. Ab initio calculations combined with experiments on Fe-13Al, Fe-18Al, Fe-23Al and Fe-10Cr-10Al alloys show that the beneficial effect of Cr on the oxidation resistance is significantly related to bulk effects. The comparison of experimental and calculated results indicates a clear correlation between the Fe-Cr chemical potential difference and the formation of the protective oxide scales.     

Oxidation performance of Mo3Si with Al additions

The Mo₃Si alloys with different aluminum contents were fabricated by the arc-melting and drop-casting technique, heat treated and then exposed to air at 700, 800, 900 and 1000 °C in order to assess their oxidation behavior. Line scan studies led to the assumption that the oxide scale thermally grown at 1000 °C was composed of SiO₂ which was located closer to the alloy matrix and Al₂O₃ around the outer surface of the oxidized sample, while the Mo oxide volatilized at this oxidation temperature. The results also showed that the unalloyed sample (Mo₃Si) underwent a pest reaction in a short time of exposure, while the sample with 16 at.% Al exhibited the best oxidation behavior, which could be attributed to the formation of SiO₂ and Al₂O₃ in the oxide scale.     

Comparison of the oxidation of high-sulfur Ni-25Cr-5Al alloys in as-cast and as-sputtered states

To understand the effect of sulfur on the oxidation of nanocrystalline (NC) alloys, a high-sulfur alloy having a chemical composition similar to a coarse-grained (CG) cast alloy of Ni-25Cr-5Al-1S (wt.%) was fabricated using magnetron sputtering. The oxidation of the two alloys in isothermal and cyclic conditions in air at 1000 °C shows that the alumina scale formed on the cast alloy was susceptible to spallation, whereas the alumina scale on the sputtered alloy was intrinsically adhesive. The result indicates that the nanocrystallization of alloys helps to eliminate the detrimental “sulfur effect” on oxidation, through minimizing the segregation of sulfur to the scale/alloy interface.