Oxidation Behavior of Ni3Al and Fe3Al: II. Early Stage of Oxide Growth

Oxidation treatments of Ni₃Al and Fe₃Al were performed at room temperature in 0.2 atm O₂ for 5 min and at 300 and 500°C in air for 5 min, and 6, 50, 100, and 200 hr. The oxides were analyzed by XPS, AES, and SEM. A model explaining the initial stages of oxide formation is suggested. At room temperature and 300°C, islands of Al₂O₃ and NiO combined with NiAl₂O₄ formed on Ni₃Al. At 500°C, the Ni oxides grow laterally and cover the Al₂O₃ islands. Islands of Al₂O₃ and Fe₂O₃ mixed with Fe(Fe, Al)₂O₄ formed on Fe₃Al at room temperature. At 300 and 500°C the scale is composed of an outer layer rich in Fe oxides and an inner layer rich in Al oxides. During long time exposure, islands of Fe₂O₃ and Fe(Fe, Al)₂O₄ formed at the surface by diffusion of Fe cations through the alumina layer. The oxide growth on Fe₃Al reaches a steady-state regime after formation of the continuous alumina layer. At 300°C, the oxide formed on Fe₃Al is thicker than on Ni₃Al, whereas it is reverse at 500°C.     

Influence of Cr on the Oxidation of Fe3Al and Ni3Al at 500°C

The influence of chromium on the mechanical properties of the aluminides Fe₃Al and Ni₃Al has been studied extensively. In order to evaluate the role of Cr during the early stages of oxidation, Fe₃Al and Ni₃Al containing 2 and 4 at.% Cr were oxidized in dry air at 500°C for 6, 50, and 100 hr. The oxide scale on Fe₃Al consists of a layer of Fe₂O₃ mixed with FeAl₂O₄ on top of a continuous layer of (Al, Cr)₂O₃. Ni₃Al is covered with a mixed layer of (Al, Cr)₂O₃ and NiO/NiAl₂O₄ underneath a layer of NiO/NiAl₂O₄. Moreover, Cr induces the nucleation and growth of Fe₂O₃ and NiO particles at the oxide surface of Fe₃Al and Ni₃Al, respectively. This is due to enhanced cationic diffusion through the Cr-modified oxides. As a conclusion, additions of Cr up to 4 at.% are detrimental to the oxidation behavior of both aluminides at 500°C.     

The Effect of Cr Implantation on the Isothermal-Oxidation Behavior of a Ni3Al PM Alloy

The isothermal-oxidation behavior ofnonimplanted and Cr-implanted Ni₃Al obtainedby powder metallurgy (PM) was investigated at 585, 635,and 900°C for exposures up to 200 hr. For thenonimplanted material, the mass gain at 900°C was lower than thatfound at 635°C, whereas for the implanted materialthe oxidation rate increased with temperature, asexpected. Furthermore, Cr implantation significantlyreduced the oxidation rate with respect to thenonimplanted material, particularly at the lowertemperatures. Cross-sectional examination of theimplanted alloy revealed a heterogeneous attackcharacterized by a thin Al-rich oxide layer, occasionallydisrupted by isolated zones with a thicker three-layerscale, the morphology of which was similar to that ofthe nonimplanted alloy. The density of these zonesdecreased with increasing temperature. The beneficialeffect of Cr is associated with the rapid establishmentof a protective Al-rich oxide layer.     

Influence of Powder Particle Size on the Oxidation Behavior of a PM Ni3Al Alloy

The influence of particle size on the oxidationbehavior of Ni₃Al prepared by powdermetallurgy (PM) was investigated in the temperaturerange of 535 to 1020°C for exposures up to 200 hr.Four alloys were obtained, each one processed with a differentpowder particle size fraction (<25, 25-50, 50-100,and 100-200 m). For temperatures below 730°C,the scale consists of an outer NiO layer, a thindiscontinuous intermediate nickel layer, and an internaloxidation zone. The lowest oxidation rate corresponds tothe material with the smallest particle size. Thisresults from its higher grain-boundary density; the boundaries act as easy-diffusion paths foraluminum leading to the rapid formation of a continuousinner alumina layer. At temperatures above 730°C, athree layered scale is observed consisting of an outer NiO layer, an intermediate layer that,depending on temperature, consisted of a mixture ofnickel and aluminum oxides orNiAl₂O₄, and an inner layer ofAl₂O₃, which accounts for thehigher oxidation resistance. The oxidation attack is characterized byintrusions of the scale into the alloy, the intrusionnumber increasing as the particle size decreases. It isassumed that oxide particles and impurities present at the original particle boundaries facilitatealumina growth along these regions. Thus, the lowestoxidation rate for the highest temperature rangecorresponds to the largest particle-sizematerial.     

Cyclic Oxidation Behavior of Electrodeposited Ni3Al–CeO2Base Coatings at 850°C

The cyclic-oxidation behavior of electrodeposited pure, nano CeO₂ (9–15 nm)- and micron CeO₂ (5 μm)-modified Ni₃Al coatings on Fe–Ni–Cr substrates have been investigated at 850°C for periods up to 1000 hr. All the coatings showed good oxidation resistance in the early stage, but decreased oxidation protection during the intermediate stage of exposure. The formation of slow-growing γ-Al₂O₃ scales provided the coatings with good oxidation resistance in the early stage. However, a high volume fraction of pores in the coatings decreased oxidation resistance in the intermediate stage by forming oxidized channels between the pores. After long-term exposure, however, the pores in the coatings were healed by coating–substrate interdiffusion, and an improvement in cyclic-oxidation resistance was observed. The CeO₂ particles in the coating affected the stability of the protective oxide scale and the pore-healing behavior. The most significant effect was from nano size CeO₂ particles, which improved the stability of the protective oxide scale but retarded the rate of pore healing.     

Effect of Cr,Co, and Ti additions on the high-temperature oxidation behavior of Ni3Al

The oxidation behavior of Ni₃Al+2.90 wt.% Cr, Ni₃Al+3.35 wt% Co, and Ni₃Al+2.99 wt.% Ti alloys was studied in 1 atm of air at 1000, 1100, and 1200°C. Isothermal tests revealed parabolic kinetics for all three alloys at all temperatures. Cyclic oxidation for 28 two-hour cycles produced little spallation at 1000°C, but caused partial spallation at 1100°C. Especially, at 1200°C severe spallation in all three alloys was observed. Although additions of Cr, Co, or Ti to Ni₃Al alloys slightly increased the isothermal-oxidation resistance, the additions tended to decrease the cyclic-oxidation resistance. The major difference in the oxidation of the three alloys compared with the oxidation of pure Ni₃Al alloys was the existence of small -Al₂O₃ particles in the middle of the -Al₂O₃ scale and the formation of irregularly shaped Kirkendall voids at the alloy-scale interface.     

Oxidation Behavior of Cast Ni3Al Alloys and Microcrystalline Ni3Al+5% Cr Coatings with and without Y Doping

Ni₃Al+5% Cr and Ni₃Al+5% Cr+0.3% Y (wt.%) microcrystalline coatings were produced using a close-field, unbalanced magnetron-sputter deposition (CFUMSD) technique. Isothermal and cyclic-oxidation tests were carried out to assess the oxidation resistance of the coatings. The results showed that Al₂O₃ formed on the coatings as the main oxidation products, with the formation of - and -Al₂O₃ scales at 900 and 1200°C, respectively. The spallation resistance of the Al₂O₃ scales formed on the coatings was superior to the oxide scales formed on cast Ni₃Al. After oxidation, interfacial voids were observed on the oxide–metal interface of the cast alloy while no voids were found on the coating surfaces. On the basis of the enhancement of Al diffusion, because of the high density of grain boundaries in the coatings, oxidation mechanisms were proposed.     

Cyclic oxidation behavior of Ni3Al-0.1B base alloys containing a Ti,Zr, or Hf addition

Ni ₃Al and Ni₃Al-0.1B, with and without additions of about 2% Ti, Zr, or Hf were subjected to a thermal cycling oxidation test in pure flowing oxygen at atmospheric pressure at temperatures cycled between 400 and 1300 K. The scales formed on Ni₃Al and Ni₃Al-0.1B spalled repeatedly, resulting in a considerable mass loss of the specimen. The Ti addition to Ni₃Al led to a repeated scale spollation, whereas Ti added to Ni₃Al-0.1B resulted in a very adherent scale, although the oxidation kinetics were linear and the formation of deeply penetrating Al₂O₃ along the alloy grain boundaries took place. The scales were very adherent on alloys containing Zr and Hf. This was attributed to the so-called keying mechanism, because uneven penetration of Al₂O₃ into the alloy took place, leading to irregularly shaped scale/alloy interfaces. ZrO₂ and HfO₂ particles were incorporated into the Al₂O₃ layer and protrusions, and some of them were formed ahead of the Al₂O₃. The shape of these particles was not stringerlike as found with other alloys. The Ti, Zr, and Hf additions tended to decrease the density of voids formed at the scale/alloy interface, but the extent of the change seems to be insufficient to support the vacancy-sink mechanism. The Hf addition was found to be most effective in forming a protective scale.     

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.     

Isothermal oxidation behavior of Ni3Al-0.1B base alloys containing Ti,Zr, or Hf additions

The isothermal oxidation behavior of Ni₃Al-0.1B and of this alloy containing additions of approximately 2% Ti, Zr, or Hf was studied in purified oxygen at atmospheric pressure over the temperature range 1300 to 1500 K and for periods up to 400 ks. Ni₃Al was also studied similarly for comparison. The oxidation of Ni₃Al and Ni₃Al-0.1B resulted in an extensive formation of geometric voids on the substrate surface leading to poorly adherent scales. The addition of B to Ni₃Al did not change the oxidation behavior much, however, both the activation energy of oxidation and the relative thickness of the Al₂O₃ layer in the scale were increased. The addition of Ti led to the formation of adherent scales at 1300 K, but the oxidation was accelerated after an initial period at 1300 and 1400 K. At higher temperatures the scale was protective, and the parabolic rate constant, k_p, decreased, however, the scale adherence was impaired by the formation of interfacial voids. The addition of Zr resulted in very adherent scales at all temperatures, but at the same time it increased k_p considerably. The addition of Hf also resulted in very adherent scales at all temperatures and decreased k_p except at 1300 K. The improved scale adherence can be accounted for by the keying effect, since the additives resulted in roughened scale/alloy interfaces. A complex of oxide particle and an associated void was found on the exposed surface of the Hf-containing alloy. This supports the vacancy-sink mechanism.     

Oxidation behavior of Ni2Al-0.1B containing 2Cr

Ni ₃ Al-0.1B containing 2% Cr was subjected to a thermal cycling oxidation test in pure flowing oxygen at atmospheric pressure at temperatures cycled between 400 and 1300 K. Scales formed on the alloy spalled repeatedly after several tens of cycles, resulting in a considerable mass loss of mass of the specimen. The isothermal oxidation behavior of the alloy was also studied at temperatures of 1300, 1400, and 1500 K in the same oxidizing atmosphere. Characteristic breakaway oxidation caused by extensive scale blistering was observed at 1300 and 1400 K after a protective period of about 50 ksec, whereas the oxidation at 1500 K followed a parabolic law without any significant blistering. Particularly at 1300 K, the alloy grain boundaries provided favorable sites for blistering. The influence of Cr may be explained by the modification of mechanical property of the scale.     

High-temperature oxidation behavior of pure Ni3Al

The high-temperature oxidation behaviour of pure Ni₃Al alloys in air was studied above 1000°C. In isothermal oxidation tests between 1000 and 1200°C, Ni₃Al showed parabolic oxidation behavior and displayed excellent oxidation resistance. In cyclic oxidation tests between 1000 and 1300°C, Ni₃Al exhibited excellent oxidation resistance between 1000 and 1200°C, but drastic spalling of oxide scales was observed at 1300°C. When Ni₃Al was oxidized at 1000°C, Al₂O₃ was present as -Al₂O₃ in a whisker form. But, at 1100°C the gradual transformation of initially formed metastable -Al₂O₃ to stable -Al₂O₃ was observed after oxidation for about 20 hr. After oxidation at 1200°C for long times, the formation of a thick columnar-grain layer of -Al₂O₃ was observed beneath a thin and fine-grain outer layer of -Al₃O₃. The oxidation mechanism of pure Ni₃Al is described.     

Carburization Behavior of Electrodeposited Ni3 Al–CeO2-Base Coatings on Fe–Ni–Cr Alloys

The cyclic carburization of electrodeposited pure and CeO₂-dispersed Ni₃Al intermetallic coatings on Fe–Ni–Cr alloys has been investigated at 850 and 1050°C for periods up to 500 h in a reducing 2%CH₄–H₂ atmosphere. At 850°C, all Ni₃Al-base-coating samples showed excellent carburization resistance and slow mass increases due to the formation of a thin γ-Al₂O₃ scale and a low carbon activity (a _c = 0.73). At 1050°C and a high carbon activity (a _c = 3.21), all coatings are superior to the uncoated Fe–Ni–Cr alloy in terms of carburization resistance. A thin α-Al₂O₃ scale slowly formed on all Ni₃Al coatings effectively blocked the carbon attack. The addition of CeO₂ particles in the Ni₃Al coatings significantly mitigated the cracking of the α-Al₂O₃ scale and the resultant internal oxidation and carburization. For all coatings, Ni-rich particles were found to be formed on the α-Al₂O₃ scale during oxidation, which had led to the deposition of catalytic coke.     

Oxidation Behavior of a Ti3Al–Nb Alloy with Surface Thin Oxide Films

Thin films of aluminum, cerium, and yttrium oxides were applied onto the surfaces of Ti₃Al–11Nb samples using an electrodeposition technique. The oxidation behaviors of the Ti₃Al–Nb alloy, with and without these surface-applied films, were studied in air at 800–1000°C. The results showed that the oxidation rate of the alloy can be reduced by Ce oxide and Y oxide films, and the greatest improvement comes from oxidation of the Y oxide-coated specimens at 800°C. With increasing oxidation temperature, the difference between the Co-oxide and Y-oxide films becomes smaller. The results also indicated that the Ce-oxide and Y-oxide films can significantly improve the oxide scale-spallation resistance. On the other hand, Al-oxide films result in detrimental effects on the oxidation and scale-spallation resistance of the Ti₃Al–Nb alloy. Based on the experimental results, the effects of the different surface films on the oxidation mechanism are discussed.     

The corrosion of Ni3Al in a combustion gas with and without Na2SO4-NaCl deposits at 600–800°C

The corrosion behavior of Ni₃Al containing small additions of Ti, Zr, and B in combustion gases both with and without Na₂SO₄–NaCl deposits at 600–800°C has been studied for times up to four days. The corrosion of the saltfree Ni₃Al leads to the formation of very thin alumina scales at 600°C but of mixed NiO–Al₂O₃ scales containing also some sulfur compounds at higher temperatures, while the rate increases with temperature up to 800°C. The presence of the salt deposits considerably accelerates the corrosion rate, especially at 600 and 800°C. The duplex scales formed at 600°C are composed mostly of a mixture of NiO and unreacted salt in the outer layer and of alumina and aluminum sulfide with some nickel compounds in the inner layer. The scales grown at 700°C contain only one layer of complex composition, while those grown at 800°C are similar but have an additional outer layer containing similar amounts of nickel and aluminum. At 600 and 700°C NiSO₄ can be detected also in the salt layer. The samples corroded at 700°C and 800°C also show an Al-depleted zone containing titanium sulfide precipitates at the surface of the alloy. The hot corrosion of Ni₃Al involves a combination of various mechanisms, including fluxing of the oxide scale as well as mixed oxidation-sulfidation attack. At all temperatures Ni₃Al shows poor resistance to hotcorrosion attack as a result of the formation of large amounts of Ni compounds in the scales.     

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.     

Influence of niobium additions on high-temperature-oxidation behavior of Ti3Al alloys and coatings

In this study, the oxidation properties of Ti₃Al+Nb bulk alloys, as well as IMI 829 alloy, coated with a Ti₃Al+Nb layer, have been considered. Model alloys have been prepared, with 5–25 at.% niobium contents; 50-m-thick Ti₃Al+10 at.% Nb coatings have also been deposited on IMI 829 by triode sputtering. Bulk alloys and coated substrates have been exposed to cyclic and isothermal oxidation in air between 700 and 800°C. Niobium additions generally caused the oxidation rate of Ti₃Al to decrease significantly. In all cases rutile is the main oxide formed. It is believed that the ability of niobium to dissolve in the rutile lattice, and therefore to lower the oxygen diffusion rate through the oxide layer, is a contribution to the observed oxidation resistance enhancement. The formation of niobium oxide has also been envisaged for this matter.     

一种钴基耐磨材料的高温氧化行为研究

研究了NbC颗粒增强钴基耐磨材料在950～1050℃空气中的氧化行为。合金在950℃下属于抗氧化级，1000℃下属于次抗氧化级，1050℃下属于弱抗氧化级，氧化动力学曲线基本符合抛物线规律。合金表面形成由CoCr₂O₄、CoNb₂O₆、Cr₂O₃和Al₂O₃组成的混合氧化物层，基体中块状NbC的优先原位氧化行为造成氧化层厚度不均匀且疏松多孔。经过1050℃，100 h氧化后，氧化层下方基体中形成贫Cr层，导致氧化皮剥落后无法重新形成具有保护作用的连续氧化膜。     

Pt改性Ni3Al基合金短时高温氧化行为研究

用冷坩埚悬浮熔炼制备了Ni-xPt-25Al(x=0,10,20,30,at%)系列合金,利用X射线衍射仪、同步热分析仪、扫描电镜和光电子能谱仪(XPS)分析了Pt的添加对晶体结构的影响,研究了Ni-xPt-25Al合金的氧化动力学曲线、氧化物形貌以及抗氧化性能。结果表明:随着Pt含量的升高,Pt改性Ni_3Al基合金仍保持γ′相。升温过程和短时等温阶段合金的氧化动力学特征分别符合线性和抛物线氧化物生长动力学规律。Pt有利于氧化物的快速形成,且随Pt含量的增加,氧化膜的完整性和致密性均有所改善。     

Degradation behavior of Ni<Subscript>3</Subscript>Al plasma-sprayed boiler tube steels in an energy generation system

Boiler steels, namely, low-C steel, ASTM-SA210-Grade A1 (GrA1), 1Cr-0.5Mo steel, ASTM-SA213-T-11 (T11) and 2.25Cr-1Mo steel, ASTM-SA213-T-22 (T22) were plasma sprayed with Ni₃Al. The alloy powder was prepared by mixing Ni and Al in the stoichiometric ratio of 3 to 1. The Ni-22Cr-10Al-1Y alloy powder was used as a bond coat, with a 150 μm thick layer sprayed onto the surface before applying the 200 μm coating of Ni₃Al. Exposure studies have been performed in the platen superheater zone of a coal-fired boiler at around 755 °C for 10 cycles, each of 100 h duration. The protection to the base steel was minimal for the three steels. Scale spallation and the formation of a porous and nonadherent NiO scale were probably the main reasons for the lack of protection. In the case of T22-coated steel, cracks in the coatings have been observed after the first 100 h exposure cycle.