Influence of the mode of introduction of a reactive element on the high temperature oxidation behavior of an alumina‐forming alloy. Part III: The use of two stage oxidation experiments and in situ X‐ray diffraction to understand the oxidation mechanisms

The aim of this work was to investigate several different yttrium introduction routes to improve the high temperature oxidation resistance of a Fe‐20Cr‐5Al model alloy. Y₂O₃ sol‐gel coatings, Y₂O₃ metal‐organic chemical vapor deposition (MOCVD) coatings, yttrium ion implantation and yttrium as alloying element (0.1 wt.%) were the different methods of introduction of the reactive element. Both isothermal and cyclic oxidation tests showed that the surface introduction of yttrium or yttrium oxide did not drastically improve the oxidation behavior of the steel. Complementary experiments were performed to understand the lack of major beneficial effects of the so‐treated samples. Two stage oxidation experiments under 200 mbar ¹⁶O₂ and ¹⁸O₂ followed by secondary neutral mass spectrometry (SNMS) were performed to understand the alumina scale growth mechanisms, according to the introduction route of the reactive element. The results exhibited that the yttrium induced an increase of the inward transport of oxygen through the alumina scale compared to the untreated specimen. Nevertheless, the outward transport of aluminum was generally observed, except for the specimen containing Y as alloying element, which exhibited only a single¹⁸O peak close to the metal/oxide interface. Phase transformations during the oxidation at 1100°C were registered by in‐situ X‐ray diffraction (XRD). The untreated alloy was only covered by a thin layer of α‐Al₂O₃. For implanted specimens, yttrium was incorporated in Y₃Al₅O₁₂ and YAlO₃ phases. All the YAlO₃ is transformed into Y₃Al₅O₁₂ after less than 10 h. For the MOCVD or the sol‐gel coated samples, the primary formed YAlO₃ phase was progressively transformed into Y₃Al₅O₁₂. For the Fe‐20Cr‐5Al‐0.1Y alloy, no yttrium containing phases could be detected, even after 40 h of oxidation test at 1100°C.     

The effect of yttrium and thorium on the oxidation behavior of Ni-Cr-Al alloys

The effect of quaternary additions of 0.5% Y and 0.5 and 1.0% Th to a base alloy of Ni-10Cr-5Al on the oxidation behavior and mechanism was studied during oxidation in air over the range of 1000–1200°C. The presence of yttrium decreased the oxidation kinetics slightly, whereas the addition of thorium caused a slight increase. Oxide scale adherence was markedly improved by the addition of the quaternary elements. Although a number of oxides formed on yttrium-containing alloys, quantitative x-ray diffraction clearly showed that the rate-controlling step was the diffusion of oxygen through short-circuit paths in a thin layer of alumina that formed parabolically with time. Mixed oxides containing both aluminum and yttrium formed by the reaction of Y₂O₃ to form YAlOP₃ initially, and Y₃Al₅O₁₂ (YAG) after longer times. Although the scale adherence of the yttrium-containing alloy was considerably better than the base alloys, spalling did occur that was attributed to the formation of the voluminous YAG particles that grew in a mushroom-like manner, lifting the protective scale off the substrate locally. The YAG particles formed primarily at grain boundaries in the substrate in which the yttrium originally existed as YNi₉. This intermetallic compound reacted to form Y₂O₃, liberating metallic nickel that subsequently reacted to form NiO or NiAl₂O₄ spinel or both. The Y₂O₃ reacted with aluminum to ultimately form the YAG mushrooms. Thorium did not form any mixed oxides; the only oxide involving thorium was ThO₂, which existed as small particles at the oxide-metal interface. A highly beneficial effect of the thoria particles in reducing film spalling was observed. Scale spalling in the base alloy was attributed to void formation at the oxide-metal interface, the voids forming by condensation of excess vacancies from the Kirkendall effect associated with fast back-diffusion, of nickel into the substrate as aluminum was preferentially oxidized and diffused slowly outward. The mechanism of improved scale adherence in the quaternary alloys was the elimination of voids by annihilation of the Kirkendall vacancies at vacancy sinks introduced by the noncoherent interfaces between yttrium and thorium-containing intermetallics or oxides or both.This work is based on a portion of the dissertation of Arun Kumar in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Engineering, University of California, Los Angeles.Supported by NASA-Ames under grant No. NGR 05-007-352.     

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.     

TEM studies of oxidized NiAl and Ni3Al cross sections

Cross sections of oxide scale/(Ni-Al) intermetallics were prepared by a new method and studied using primarily transmission electron microscopy (TEM). The cross sections were prepared by encasing an oxidized metal specimen sandwich in a low-melting-temperature zinc alloy. Observations of oxidized zirconium-doped -NiAl cross sections revealed crystallographic voids beneath an adherent Al₂O₃ scale. The oxide-metal interface was incoherent, but a high dislocation density in the metal near the interface suggested that a large tensile stress was induced by the attached oxide scale. A duplex Al₂O₃-NiAl₂O₄ scale formed on zirconium-doped and zirconium/boron-doped -Ni₃Al alloys. Additional results are presented involving oxidation mechanisms and oxide-metal interface structures.     

TEM investigations of the early stages of TiAl oxidation

The early stages of TiAl oxidation at 900°C and 1000°C in air have been investigated by transmission electron microscopy (TEM). The investigations revealed that at the beginning of oxidation, i.e., after 4 min, TiO₂ and Al₂O₃ grow in a preferential orientation on the -TiAl substrate. After 4 h of oxidation an oxide scale structure can already be found similar to that known from long-term oxidation. In addition, besides -Al₂O₃, the formation of a second aluminum oxide phase and of titanium nitrides is observed. The processes at the metal-oxide interface of oxidation in the early stages, consisting of a repeated cycle of Al₂O₃ formation, Al₂O₃ dissolution, outward migration of Al through the scale, and reprecipitation of Al₂O₃ in the outer scale, are described by a model. The four stages observed in the kinetics of TiAl oxidation are explained on the basis of the results obtained and the structure of the oxide scale.     

The influence of alloy microstructure on the oxide peg morphologies in a Co-10% Cr-11%Al alloy with and without reactive element additions

The addition of reactive elements such as yttrium or hafnium to heat-resisting alloys has marked beneficial effects on their high-temperature oxidation behavior, and in particular greatly improves the adhesion of the oxide scale to the substrate. In the case of alloys forming Al₂O₃ protective oxide scales, the improvement in adhesion appears to be related to the formation of intrusions of oxide into the metal—pegs—which hold the scale on. The morphology of these pegs is a function of alloy structure and of the reactive element. In particular, it appears that while the pegs of Hf-containing alloys grow as a result of a diffusion process, perhaps along the oxide-metal phase boundary, the pegs on Y-containing alloys seem to form too fast for this, and it is suggested that stresses established by the oxidation of the Co₃Y inclusions lead to the formation of porous short-circuit paths.     

Influence of the mode of introduction of a reactive element on the high temperature oxidation behavior of an alumina‐forming alloy. Part I: Isothermal oxidation tests

Different modes of introduction of yttrium have been tested with regard to the influence on the high temperature oxidation behavior of a FeCral alloy. Y₂O₃ sol‐gel coatings, Y₂O₃ metal‐organic chemical vapor deposition (MOCVD) coatings, implanted yttrium ions and yttrium as alloying element (0.1 wt.%) in the same Fe‐20Cr‐5Al alloy were oxidized at 1100°C in air under atmospheric pressure. Whatever the mode of introduction of the reactive element, the oxidation rates were not decreased compared to the oxidation rate of the blank specimen. The observation of the oxidized surface indicated that the alumina scale largely spalled from the blank alloy. Spallation was reduced for the Y₂O₃ sol‐gel coated, the Y₂O₃ MOCVD coated alloys and the yttrium ion implanted steels. The Y‐containing alloy did not exhibit any detachment of the oxide scale, indicating the best high temperature oxidation behavior, at least from the viewpoint of scale adherence.     

Scale growth process at 1473 K on unmodified and yttrium‐ or chromium‐implanted β‐NiAl

Early oxidation of unmodified and yttrium‐implanted or chromium‐implanted β‐NiAl intermetallic compound at 1473 K was studied using a combination of two‐stage‐oxidation exposure with ¹⁸O₂ as a tracer, SIMS elemental distribution analysis (depth profiling and imaging modes) and photoluminescence spectroscopy analysis of the scale phase composition. It was found that phase transformation of transient aluminium oxides, represented by θ‐ Al₂O₃ into stable and protective α‐Al₂O₃ occurs locally and is affected by implanted additions: Yttrium retarded while chromium appeared to accelerate it. Typical patch‐ and/or web‐like scale morphology of the growing scales was observed.     

Effect of pre-oxidation on the hot corrosion of CoNiCrAlYRe alloy

The effect of pre-oxidation on the resistance to hot corrosion was examined by corroding the CoNiCrAlYRe alloy at 900 °C in molten Na₂SO₄. Preoxidized specimens featured strong adhesion of oxide scale with uniform multi-layered structure. The time of pre-oxidation was crucial for controlling Al content sufficient for subsequent hot corrosion. However, direct corrosion yielded a defective and non-protective oxide scale, which allowed detrimental penetration of sulfur into substrate. Sulfur migrating along phase boundary was trapped by yttrium to diminish slightly sulphidation. Thus, two advantages of proper pre-oxidation treatment were presented, as keeping repairing for Al₂O₃ scale and inhibiting sulfur penetration.     

Mechanism of isothermal oxidation of the intel-metallic TiAl and of TiAl alloys

The oxidation behavior of Ti36Al, Ti35Al-0.1C, Ti35Al-1.4V-0.1C, and Ti35 Al-5Nb-0.1C (mass-%) in air and oxygen has been studied between 700 and 1000°C with the major emphasis at 900°C. Generally an oxide scale consisting of two layers, an outward- and an inward-growing layer, formed. The outward-growing part of the scale consisted mainly of TiO₂ (rutile), while the inward-growing part is composed of a mixture of TiO₂ and -Al₂O₃. A barrier layer of Al₂O₃ on TiAl between the inner and the outer part of the scale was visible for up to 300 hr. Under certain conditions, the Al₂O₃ barrier dissolved and re-precipitated in the outer TiO₂ layer. This shift leads to an effect similar to breakaway oxidation. Only the alloy containing Nb formed a longlasting, protective Al₂O₃ layer, which was established at the metal/scale interface after an incubation period of 80–100 hr. During this time, Nb was enriched in the subsurface zone up to approximately 20 w/o. The growth of the oxide scale on TiAl-V obeyed a parabolic law, because no Al₂O₃ barrier layer formed; large Al₂O₃ particles were part of the outward-growing layer. A brittle 2-Ti₃Al-layer rich in O formed beneath the oxide scale as a result of preferential Al oxidation particularly when oxidized in oxygen. Oxidation in air can lead also to formation of nitrides beneath the oxide scale. The nitridation can vary between the formation of isolated nitride particles and of a metal/Ti₂AlN/ TiN/oxide, scale-layer system. Under certain conditions, nitride-layer formation seemed to favor protective Al₂O₂₃ formation at the metal/scale interface, however, in general nitridation was detrimental with the consequence that oxidation was generally more rapid in air than in oxygen.     

Selective carbide oxidation and internal nitridation of the Ni-base superalloys IN 738 LC and IN 939 in air

Ni-base superalloys contain beside other phases relatively large blocky MC carbides of the type (Ta, Nb, Ti, W)C, which oxidize much faster than the / matrix. The large volume increase during oxidation and the oxide formation at the carbide-oxide interface shift the corrosion products outward. High shear stresses between the Cr₂O₃ scale and the carbide oxidation products lead to scale cracking favoring internal corrosion processes in this area. The formation of Al₂O₃ in the subscale is accompanied by a volume increase and tensile stresses in the outer Cr₂O₃ scale. This causes scale cracking and gives nitrogen a chance to enter the metal and form the most stable nitride, TiN beneath the Al₂O₃ subscale.     

Effect of Fe and partial pressure of oxygen on the formation and phase transformation behavior of Al2O3 scale

The effect of oxygen partial pressure on the phase transformation of Al₂O₃ scale on various Fe–Al alloys with and without very thin (～100 nm) Fe coating was investigated. Fe‐coating on Fe–Al alloys can effectively suppress metastable Al₂O₃ formation, but little effect was observed when the samples were oxidized in a low partial pressure of oxygen. Under the low ${\rm P}_{{\rm O}_{{\rm 2}} } $ atmosphere, metastable to stable α‐Al₂O₃ scale phase transformation on both Fe‐coated and non‐coated Fe–Al alloys was significantly delayed. The lattice spacing of α‐Al₂O₃ scale formed in air decreased with increasing alloy Al content. Further decrease in the lattice spacing of α‐Al₂O₃ scale was observed when the alloy was oxidized in low ${\rm P}_{{\rm O}_{{\rm 2}} } $ . The results obtained clearly indicated that the formation of Fe₂O₃ or Fe³⁺ in metastable Al₂O₃ accelerated the metastable to stable α‐Al₂O₃ scale transformation.     

Oxidation and intergranular disintegration of the aluminides NiAl and NbAl3 and phases in the system Nb-Ni-Al

The oxidation behavior is very different for an aluminide with a wide homogeneity range such as -NiAl than for a line compound such as NbAl ₃.Oxidation of -NiAl at temperatures 1273 K leads to a slow-growing -alumina layer. The metal phase beneath the scale remains as -NiAl; however, cavity formation is observed. The cavity formation may be favored by sulphur surface segregation. Oxidation of NbAl ₃ at temperatures 1273 K initially leads to -Al ₂O₃,but the Al depletion causes the formation of Nb ₂ Al beneath the oxide layer. Cracking of the Al ₂O₃ layer opens Nb ₂ Al to the atmosphere, which oxidizes rapidly to Nb ₂O₅ and NbAlO₄.After consumption of the Nb ₂ Al, a layer of Al ₂O₃ formed again on the NbAl ₃ phase, but failure of the alumina and the fast growth of the other oxides occur as a repeated process. Thus, NbAl ₃ exhibited rapid linear oxidation kinetics. Multiphase alloys in the system Nb-Ni-Al generally behave better than NbAl ₃,and the low oxidation rates of -NiAl can be approached. In the temperature range below 1273 K, with a maximum at 1000 K, both NiAl and NbAl ₃ show the pest phenomenon, an intergranular disintegration. Preceding the disintegration, oxygen diffuses into the grain boundaries of the material and Al ₂O₃ is formed at the grain boundaries, beginning from the surface region. NiAl is susceptible only in a very limited range of oxygen pressures and temperatures, whereas NbAl ₃ is much more susceptible.     

Oxidation Behavior of Ti--50Al Intermetallics with Thin TiAl3 Film at 1000°C

Isothermal oxidation tests at 1000°C in air indicate that the Ti--50Al alloy with about 8 m TiAl₃ layer on the surface can resist the oxidation for 10 hr. From the FESEM and EPMA/EDS results, the rapid oxidation behavior is attributed to the formation of oxide nodules through the protective Al₂O₃ and TiAl₂ layers on the outer surface. Upon increasing the oxidation time at 1000°C, the size and the number of oxide nodules increase. After 3 hr of oxidation at 1000°C, a laminated layer is formed in between the oxide nodule and substrate, which consists of two nearly parallel phases. The EDS results suggest that these two phases are Ti--Al--O compounds. After 20 hr oxidation, the oxidation nodules and laminated layers disappear and a complex oxide scale is formed which is similar to the bare Ti--50Al oxidized at 1000°C.     

A Comparative Study of High Temperature Corrosion of Al2O3, SiO2 and Al2O3–SiO2 Forming Alloys in a Na2SO4–NaCl Atmosphere

The present paper is designed to provide a summary of our study on the high temperature corrosion of Al₂O₃, SiO₂ and Al₂O₃–SiO₂ forming alloys in the gas phase and liquid phase of Na₂SO₄–NaCl system by comparing their corrosion resistance at 1000 °C. The obtained results show that the alumina-forming alloy experiences severe internal corrosion in the gas phase compared to in the liquid phase due to oxide cracking. This results in an increase in the inward diffusion and/or penetration of constituents of the salts and oxygen to form internal Al-oxide and Cr-sulfides. In the liquid phase, however, the formation of yttrium sulfide beneath a continuous double oxides layer of Al₅Y₃O₁₂ and Al₂O₃ may be related to the high affinity of yttrium for sulfur. On the other hand, it is apparent from the cross-sectional observations that a SiO₂ and Al₂O₃–SiO₂ forming alloys form a continuous and dense oxides layer, and demonstrate a high resistance against internal oxidation and corrosion in both corrosive environments.     

On the transient oxidation of a Ni-15Cr-6Al alloy

Stages in the development of a protective -Al₂O₃ scale on a Ni-15Cr-6Al (wt.%) alloy have been examined. It is shown that prior to the formation of a continuous -Al₂O₃ layer, a transient stage of oxidation occurs that consists of a rapid uptake of oxygen with conversion of a thin surface layer of alloy to predominantly spinel and the subsequent development of a discrete layer of Cr₂O₃. It is also shown that during the transient period of oxidation metastable phases of aluminum oxide are formed which transform to -Al₂O₃ upon incorporation into the external oxide scale.     

Effect of yttrium on the stability of aluminide-yttrium composite coatings in a cyclic high-temperature hot-corrosion environment

A composite coating of aluminide-yttrium has shown excellent corrosion resistance in a cyclic high-temperature hot-corrosion environment. To understand the effect of yttrium on the stability of the composite coating, the specimens were prepared with various coating parameters of Y thickness, sequence of post heat treatment and surface condition before Y-ion plating. Performance of the composite coating was evaluated by isothermal oxidation and cyclic high-temperature hot corrosion. Isothermal-oxidation-test results show that the Y in the composite coating helps to form a thick and dense Al₂O₃ scale which is ductile and resistant to thermal stress. The Y in Al₂O₃ may act as a donor which leads to an increase in concentration of interstitial oxygen and, thus, increases in oxidation rate. The presence of Y₂O₃ and (Y, Al) O-type compounds in grain boundaries of Al₂O₃ and boundaries between the Al₂O₃ and NiAl effectively prohibits the fast diffusion of oxidants (such as O and S) and Al along grain boundaries. Consequently, it may induce slow diffusion through the matrix, and thus the corrosion resistance of the composite coating under cyclic hot corrosion increases substantially.     

Simultaneous Internal Oxidation and Nitridation of Ni–Cr–Al Alloys

A series of Ni–Cr–Al alloys was subjected to thermal cycling to 1100°C in air for up to 260 1-hr cycles. All alloys exhibited poor corrosion resistance. Repeated scale spallation led to subsurface alloy depletion in aluminum and, to a lesser extent, chromium. This caused transformation of the prior alloy three-phase structures (-Cr+-NiAl+-Ni) to single-phase -nickel solution. Destruction of the external scale allowed gas access to this metal, which was able to dissolve both oxygen and nitrogen. Inward diffusion of the two oxidants led to development of a complex internal-precipitation zone: Al₂O₃ and Cr₂O₃ beneath the surface, followed by Al₂O₃, then AlN, then AlN+Cr₂N, and, finally, AlN alone in the deepest region. This distribution is shown to reflect the relative stabilities of the precipitates and the higher permeability of nitrogen. Diffusion-controlled kinetics were in effect initially, but mechanical damage to the internal-precipitation zone led to more rapid gas access and approximately linear kinetics in the long term.     

Influence of Yttrium-Alloying Addition on the Oxidation of Alumina Formers at 1173 K

The oxidation behavior of three commercial Fe–Cr–Al alloys, Kanthal APM, Kanthal A1, and Kanthal AF (containing alloying additions of yttrium), has been investigated during isothermal exposures in air at 1173 K. After an initial transient stage, a diffusional process appears to predominantly control the oxidation kinetics of both alloys. During the transient stage, relatively important mass gains have been registered and the presence of yttrium does not seem to have a significant effect on the oxidation rate. On the contrary, the reactive element markedly influences the parabolic oxidation rate and the composition of the oxide scale. In situ X-ray diffraction (XRD) shows that yttrium promotes the transformation of transition alumina into -Al₂O₃, leading to the formation of a more protective oxide scale.     

Influence of Al-La Cocementation on the Oxidation Behavior of Ti3SiC2-Base Ceramic

Al–La thermal diffusing was conducted on Ti₃SiC₂-base ceramic by the pack-cementation method. The microstructure, phase and oxidation resistance of the diffusion layer were characterized. The complete aluminide coatings have not been obtained. Al and La penetrated into the Ti₃SiC₂ substrate quickly, distributing into the whole interior of the specimen after cementation at 1100°C for 4 hr. Al existed as a solid solution and dispersed particles of AlLa₃. Oxidation of cemented Ti₃SiC₂ at 1100°C in air for 20 hr formed a single continuous Al₂O₃ layer with a small amount of TiO₂ grains on the outer layer. Compared with Ti₃SiC₂, the parabolic rate constant of the cemented Ti₃SiC₂ was decreased by two orders of magnitude, which means that the cementation treatment remarkably improves the oxidation resistance of Ti₃SiC₂. Al distributed in Ti₃SiC₂ acted as a reservoir to supply enough Al for the formation of a continuous Al₂O₃ scale during the oxidation process.