Failure Behavior of an Aluminide Coating on a Co-Base Superalloy during High-Temperature Oxidation in Air

A surface-morphology change has been observed for an aluminide coating on the Co-base superalloy DZ40M during oxidation at 1100°C in air. The oxide formed on the coating remained an even grayish-white color during the stable oxidation stage, whereas, dark-colored corrosion nodules were observed when oxidation of the coating accelerated. The number of nodules increased with time, along with growth of the nodules, eventually covering the entire surface which signified failure.     

The Main Degradation Modes of an Aluminide Coating on a Co-Base Superalloy During High-Temperature Oxidation in Air

Degradation of aluminide coatings occurs by two ways, one by coating oxidation, and the other by interdiffusion of Al. In this paper, the structure variation and phase transformation are analyzed for the aluminide coating on a newly developed Co-base superalloy (DZ40M alloy) after oxidation at 900 and 1000°C in air. The results show that degradation of this coating was mainly by oxidation at 900°C, but principally by interdiffusion at 1000°C. The main degradation mode of the coating is primarily dependent on the oxidation temperature and the specific structure of the coating itself.     

High-Temperature-Oxidation Behavior of Iron–Aluminide Diffusion Coatings

Aluminide diffusion coatings were oxidized in air under atmospheric pressure under isothermal and cyclic conditions. The high-temperature efficiency of the pack-aluminized alloys was tested by comparing their oxidation behavior in the temperature range 800–1080°C. The k _p values deduced from the parabolic plots of weight-gain curves showed that α-Al₂O₃ composed the major phase of the oxide scale on samples oxidized at T > 1000°C. For lower temperatures, transient-alumina phases were observed. The aluminide materials also exhibited excellent resistance to cyclic oxidation at 1000°C. The second aim of this study was to dope the aluminide compounds obtained by a pack-cementation process with yttria, which was introduced by metal-organic chemical-vapor deposition (MOCVD). The beneficial effect of the reactive-element-oxide coating is strongly dependent on its mode of introduction, since the oxidation resistance is drastically increased when the Y₂O₃ coating was applied prior to the aluminization process. When applied after the aluminization, the reactive element gave negative effects on the high-temperature oxidation behavior of the iron aluminides. The oxide morphologies, X-ray diffraction patterns and two-stage experiments helped to understand the oxide-scale-growth mechanisms.     

High-Temperature Oxidation Behavior of the Co-Base Superalloy DZ40M in Air

The oxidation behavior of the Co-base superalloy DZ40M was studied in air at900–1100°C for times of up to 2000 hr. The results indicated thatthis alloy can grow a protective oxide scale at 900 and 1000°C duringisothermal oxidation, but not at 1100°C because of serious cracking andspalling of the oxide scales. Moreover, an internal-precipitate zone formedin the subsurface region of the alloy at all temperatures and times. Theprecipitates were rich in Cr in the vicinity of the alloy–scaleinterface and rich in Al deep in the alloy. The internal-precipitatemorphology changed from a granular to needlelike shape with increasingoxidation temperature.     

Effect of Hf Additions on the Isothermal-Oxidation Behavior of TiAl at High Temperatures

The isothermal-oxidation behavior of TiAlcoupons containing Hf of up to 5.2 mass % has beenstudied in the temperature range 1100-1400 K in a flowof purified oxygen under atmospheric pressure. Theaddition of 0.2% Hf is very effective to decrease theoxidation rate at 1200 and 1300 K. Metallographicexamination using conventional methods revealed that theinitially-formed Al₂O₃ scale ismaintained very sound by the addition. However, further additions ofHf result in a slight enhancement of oxidation at 1200K and a gradual decrease of the effect at 1300 K.Finally, there is almost no effect by the addition of 5.2% Hf at 1300 K. Excess amounts of Hf leadto the formation of oxide mounds on theAl₂O₃ scale. They grow in size andnumber during subsequent oxidation until the wholespecimen surface is covered with a thick scale. Such a scale is notprotective having a structure often reported in theliterature. The effect of the addition of 0.2% Hfbecomes small at 1350 K and at 1400 K it is inverted.Possible mechanisms for the improvement attained by thesuitable addition are discussed.     

Changes of an Outer β-NiAl and Inner α-Cr Coating on Ni–40 at% Cr Alloy during Oxidation at 1373K in Air

Formation mechanisms of a coating with a duplex layer, outer β-NiAl(Cr) and inner α-Cr(Ni) layer structure on a Ni–40.2 at% Cr alloy were proposed and change in the coating structure was investigated during high temperature oxidation. The Ni–40.2 at% Cr alloy was electro-plated with about 12μm Ni followed by a high Al activity pack cementation at 1073K to form a coated layer with an outer δ-Ni₂Al₃ and an inner layer containing Al more than 70at% which grew with an inward diffusion of Al. The coated Ni–40.2at% Cr alloy was oxidized at 1373K in air for up to 2592ks. It was found that at the initial stage of oxidation the as-coated layer structure changed to a two-layer, outer β-NiAl(Cr) and inner α-Cr(Ni), structure. Al contents in the α-Cr(Ni) layer was less than 0.3at%. With long term oxidation an intermediate γ-Ni(Cr, Al) layer formed between the outer and inner layers, whereas the inner α-Cr(Ni) layer became thinner and then disappeared after the 2592ks oxidation at 1373K. Coating processes and changes in the coating structure during high temperature oxidation were discussed based on diffusion and composition paths plotted on a Ni–Cr–Al phase diagram     

Effect of Micro Arc Oxidation Coatings on Corrosion Resistance of 6061-Al Alloy

In the present study, the corrosion behavior of micro arc oxidation (MAO) coatings deposited at two current densities on 6061-Al alloy has been investigated. Corrosion in particular, simple immersion, and potentiodynamic polarization tests have been carried out in 3.5% NaCl in order to evaluate the corrosion resistance of MAO coatings. The long duration (up to 600 h) immersion tests of coated samples illustrated negligible change in weight as compared to uncoated alloy. The anodic polarization curves were found to exhibit substantially lower corrosion current and more positive corrosion potential for MAO-coated specimens as compared to the uncoated alloy. The electrochemical response was also compared with SS-316 and the hard anodized coatings. The results indicate that the overall corrosion resistance of the MAO coatings is significantly superior as compared to SS316 and comparable to hard anodized coating deposited on 6061 Al alloy.     

High-Temperature Oxidation of an Aluminized NiCr Alloy formed by a Magnetron-Sputtered Al Diffusion Coating

Aluminium diffusion coatings were obtained on Ni–20Cr substrate by sputtering an aluminium film, followed by a two stage diffusion treatment in an argon inert gas atmosphere (first stage at 600°C, second at 900 or 1100°C). Aluminides obtained at 900°C and 1100°C are close to those obtained by pack cementation process with high aluminium activity. These diffusion coatings are able to develop alumina scales during isothermal oxidation at high temperatures, whereas the untreated substrate had a chromia-forming behaviour. The weight gain recorded at 1100°C on coated sample is then smaller than the one of uncoated NiCr at 950°C. Presence of chromium was detected in the diffusion coating and Cr-rich precipitates were observed at the diffusion coating/substrate interface. After oxidation at 900°C and 1100°C, only α-Al₂O₃ was revealed by XRD. An intermediate scale with a “whiskered” morphology could however be observed after 48 hr oxidation at 900°C. After 100 hr of oxidation at 1100°C, the Ni_xAl_y diffusion phases were no longer detectable and the upper part of the oxide scale spalled away during cooling. Large cavities appeared at the initial location of the diffusion coating/substrate interface.     

Effect of Palladium Incorporation on Isothermal Oxidation Behavior of Aluminide Coatings

A (Ni, Pd)Al coating has been prepared by low-pressure pack cementation on the nickel-base superalloy IN738 with a preplating of Pd–20 wt.% Ni alloy. The coating consists mainly of a single phase of -Pd(Ni)Al in the outer part and -Ni(Pd)Al in the inner part. The oxidation behavior of the (Ni,Pd)Al coating at 900–1100°C was studied by TGA, XRD, and SEM/EDS. Results show that the transformation of - to -Al₂O₃ is more rapid on the (Ni,Pd)Al coating than that on a simple NiAl coating. The addition of Pd to the aluminide coating facilitates the transformation of - to -Al₂O₃e oxide scale formed and accelerates the diffusion of Ti from the substrate to the coating surface simultaneously.     

Oxidation resistance of refractory γ-TiAlW coatings

The resistance to oxidation of the W-alloyed magnetron sputtered γ-TiAl fine-crystalline 4 μm thick coatings have were investigated in this work. The oxidation tests were performed in an atmosphere of pure oxygen or in the air at a temperature of 1173 K for 120 h. The resistance to high-temperature oxidation was investigated by means of micro-thermogravimetric analysis with continuous or stepwise control of the substrate weight. Before and after the oxidation the morphology of the coatings as well as their chemical and phase composition were investigated by SEM, EDS and EBSD, respectively. The results have been compared with those obtained for the uncoated γ-TiAl substrate.It was found that: (1) the W-alloyed γ-TiAl coatings have a considerably higher oxidation resistance (about four orders of magnitude) than the uncoated γ-TiAl substrates and that their resistance to oxidation increases with the concentration of alloying element in the range of the concentrations investigated in the work; (2) the high resistance to oxidation of the coatings is a result of the thin α-Al₂O₃ layer formation on the surface of the substrate during oxidation; (3) the formation of dense and uniform α-Al₂O₃ layer on the coating surface is due to a fine-crystalline structure of the magnetron deposited γ-TiAlW coating.     

Substrate Effect on the High Temperature Oxidation Behavior of a Pt-modified Aluminide Coating. Part II: Long-term Cyclic-oxidation Tests at 1,050 °C

This second part of a two-part study is devoted to the effect of the substrate on the long-term, cyclic-oxidation behavior at 1,050 °C of RT22 industrial coating deposited on three Ni-base superalloys (CMSX-4, SCB, and IN792). Cyclic-oxidation tests at 1,050 °C were performed for up to 58 cycles of 300 h (i.e., 17,400 h of heating at 1,050 °C). For such test conditions, interdiffusion between the coating and its substrate plays a larger role in the damage process of the system than during isothermal tests at 900, 1,050, and 1,150 °C for 100 h and cyclic-oxidation tests at 900 °C which were reported in part I [N. Vialas and D. Monceau, Oxidation of Metals 66, 155 (2006)]. The results reported in the present paper show that interdiffusion has an important effect on long-term, cyclic-oxidation resistance, so that clear differences can be observed between different superalloys protected with the same aluminide coating. Net-mass-change (NMC) curves show the better cyclic-oxidation behavior of the RT22/IN792 system whereas uncoated CMSX-4 has the best cyclic-oxidation resistance among the three superalloys studied. The importance of the interactions between the superalloy substrate and its coating is then demonstrated. The effect of the substrate on cyclic-oxidation behavior is related to the extent of oxide scale spalling and to the evolution of microstructural features of the coatings tested. SEM examinations of coating surfaces and cross sections show that spalling on RT22/CMSX-4 and RT22/SCB was favored by the presence of deep voids localized at the coating/oxide interface. Some of these voids can act as nucleation sites for scale spallation. The formation of such interfacial voids was always observed when the β to γ′ transformation leads to the formation of a two-phase β/γ′ layer in contact with the alumina scale. On the contrary, no voids were observed in RT22/IN792, since this β to γ′ transformation occurs gradually by an inward transformation of β leading to the formation of a continuous layer of γ′ phase, parallel to the metal/scale interface.     

铌合金表面高温抗氧化涂层

介绍了铌合金表面高温抗氧化涂层的4大体系——耐热合金涂层、铝化物涂层、硅化物涂层和贵金属涂层的组成、特点及制备条件。我国研究人员围绕飞机发动机涡轮叶片和火箭发动机燃烧室及尾气喷管用铌合金的防护进行了大量研究工作，研制的高温抗氧化涂层已经用于49kN推力发动机铌合围裙和姿态控制铌合金喷管。通过研究认为，PVD和传统熔烧工艺相结合的新工艺及纳米涂层技术是今后铌合金表面高温涂层制备的研究方向。     

High-Temperature Oxidation Behavior of Ti2AlC in Air

The isothermal oxidation behavior of bulk Ti₂AlC in air has been investigated in temperature range 1000–1300°C for exposure time up to 20 hr by TGA, XRD, and SEM/EDS. The results demonstrated that Ti₂AlC had excellent oxidation resistance. The oxidation of Ti₂AlC obeyed a cubic law with cubic rate constants, k_c, increasing from 2.38×10^-12 to 2.13×10^-10 kg³/m⁶/sec as the temperature increased from 1000 to 1300°C. As revealed by X-ray diffraction (XRD) and SEM/EDS results, scales consisting of a continuous inner -Al₂O₃ layer and a discontinuous outer TiO₂ (rutile) layer formed on the Ti₂AlC substrate. A possible mechanism for the selective oxidation of Al to form protective alumina is proposed in comparison with the oxidation of Ti–Al alloys. In addition, the scales had good adhesion to the Ti₂AlC substrate during thermal cycling.     

Competition Between Stress Generation and Relaxation During Oxidation of an Fe-Cr-Al-Y Alloy

The residual biaxial stress inAl₂O₃ scales formed duringoxidation of Fe-22.0%Cr-4.8%Al-0.3%Y in the temperaturerange 1000-1300°C have been measured usingCr³⁺ luminescence piezospectroscopy. Thestress measurements together with substantialdimensional changes of the specimens show that oxidationproduces high compressive growth stresses in theAl₂O₃ scale in addition tothermal-mismatch stresses that arise during cooling from the oxidationtemperature. The magnitude of the growth stress isdetermined by two opposing processes: stress generation,associated with lateral growth of the scale, and stress relaxation in both the metal and oxide. Thescale lateralgrowth strain continuously increasesconcurrently with the scale thickening. Creep in theoxide is a major relaxation process when the scale isthin, while metal deformation becomes significantduring the later stages of oxidation at highertemperatures.     

铬含量对镍基合金涂层高温氧化行为的影响机理

采用增重法对铬质量分数分别为30%和40%～45%的两种镍基合金涂层在500℃、650℃和800℃下的高温氧化行为进行了研究.并使用配有能谱分析仪的扫描电镜以及X射线衍射仪等检测设备对涂层氧化产物的形貌、成分和相组成进行了分析.发现铬含量对镍基合金涂层的抗高温氧化性能和氧化机理有着重要的影响.铬含量为40%～45%的涂层,其表面生成了连续的Cr2O3保护膜,具有相对较低的氧化速度.     

Nb521表面改性Si-Cr-Ti涂层的高温氧化行为

利用料浆烧结法在Nb521合金表面制备了Si-Cr-Ti-W和Si-Cr-Ti-Al-Y2O3涂层,并对这两种涂层在1 400℃静态空气中的静态氧化行为进行了测试和研究。利用扫描电镜(SEM)、电子探针(EPMA)和波谱仪(WDS)对涂层在氧化过程中的微观组织形貌、元素分布和相组成进行了测试和分析,并建立了动态高温氧化模型。结果表明:两种改性涂层的氧化过程都遵循抛物线规律,添加铝、Y2O3和钨均能提高涂层的抗氧化性能。由于Al2O3和SiO2组成的复合氧化膜均匀致密且高温稳定性强,Si-Cr-Ti-Al-Y2O3涂层的抗氧化性能优于Si-Cr-Ti-W涂层的。     

Oxidation Resistance of a Cr0.50Al0.50N Coating Prepared by Magnetron Sputtering on Alloy K38G

A Cr_0.50Al_0.50N coating has been prepared by a reactive-magnetron-sputtering method on alloy K38G. The coating possesses mainly the B1 type with a small amount of B4-type crystal structure phase. Isothermal oxidation tests were performed at 900–1,100 °C for 20 h by thermogravimetric analysis (TGA) in air. The results reveal that the coated samples have much lower mass gain than that of the bare alloy. The parabolic rate constants of the coated samples decrease by 2 orders of magnitude compared with the bare alloy at 1,000 and 1,100 °C. During the oxidation of the coated samples below 1,000 °C, the main oxide is Cr₂O₃, but above 1,000 °C, the scale changes to α-Al₂O₃. The observed oxidation behaviors demonstrate that the Cr_0.50Al_0.50N coating can provide good protection against corrosion over a wide temperature range.     

Cyclic Oxidation of Sputter-Deposited Nanocrystalline Fe-Cr-Ni-Al Alloy Coatings

Nanocrystalline coatings of AISI 310S stainlesssteel with a small amount of Al were produced usingunbalanced magnetron-sputter deposition with a compositetarget. Cyclic oxidation tests were performed at 800 and 900°C in air for 100 hr. Theresults showed that Al₂O₃ scalesformed on the coatings containing ~3 wt.% Al. Theoxidation rate of the coated specimens was much lowerthan that of the uncoated specimens and the scale spallationresistance was also improved significantly. Studies withatomic force microscopy (AFM) of the transient-oxidationsurfaces suggested that grain-boundary andsubgrain-boundary diffusion in the coatings may have played animportant role in promoting the formation of theAl₂O₃ scale. Fast creep of thefine-grain coating and alumina scale was considered tobe the mechanism for the improvement in spallation resistance.     

Biomimetic deposition of apatite coatings on micro-arc oxidation treated biomedical NiTi alloy

The biomedical NiTi alloy was treated by micro-arc oxidation in an electrolytes containing sodium aluminate and sodium hypophosphite at 400 V constant voltages for 30 min. The MAO-treated NiTi has a porous microstructure on its surface and coatings consisting only of the γ-Al₂O₃ phase. The ceramic coating prepared by micro-arc oxidation is composed of Al, Ti, Ni, O, and P with the atomic concentration of 26.98%, 3.67%, 3.33%, 65.30% and 0.72%, respectively. The MAO-treated NiTi was soaked in a simulated body fluid (1.0SBF) to investigate the biomimetic deposition of apatite on the surface of Al₂O₃ coated NiTi alloy. It was found that Al₂O₃ coated NiTi alloy shows an excellent apatite-forming ability after soaking in a simulated body fluid (1.0SBF) for 14 days, while no apatite-forming ability was observed on bared NiTi alloy even though soaking time is up to 28 days.     

NiAl涂层对NiAl-Cr(Zr)共晶合金1100℃空气中循环氧化行为的影响

研究了溅射NiAl涂层对NiAl-Cr(Zr)共晶合金在1100 ℃空气中抗氧化性能的影响.结果表明,在1100 ℃高温下,NiAl-Cr(Zr)共晶合金由于形成Al2O3 + Cr2O3+ ZrO2的混合氧化物膜而表现出较差的抗循环氧化性能.在其表面溅射与其成分接近的NiAl 微晶涂层,高温下形成致密的、粘附性好的A...