Oxidation Behavior of the Single-Crystal Ni-base Superalloy DD32 in Air at 900, 1000, and 1100°C

The isothermal-oxidation behavior of the single-crystal Ni-base superalloy DD32 was studied over the temperature range from 900–1100 °C and analyzed by OM, TGA, XRD, EDX, SED, and EPMA. The alloy DD32 obeyed a subparabolic rate law during oxidation at 900 and 1000°C, while the alloy showed a rather high oxidation rate at 1100°C. The severe composition segregation, which resulted from the solidification process, led to the formation of different scale on the dendritic and interdendritic regions. Internal nitride (AlN) was observed in the subsurface zone of dendritic areas, but absent in interdendritic areas. The oxidation mechanism of the alloy DD32 is discussed by comparing with other alloys.     

Oxidation Behavior of a Single-Crystal Ni-base Superalloy in Air at 900, 1000 and 1100°C

The isothermal-oxidation behavior of a single-crystal (SC) Ni-base superalloy was studied over the temperature range from 900 to 1100°C and analyzed by OM, TGA, XRD, EDX, SEM and EPMA. The alloy obeyed a subparabolic rate law during oxidation at 900 and 1000°C, whereas the alloy showed parabolic behavior at 1100°C exposure. The variations in the chemical composition due to segregation, which resulted from the solidification process, led to the formation of different kinds of oxide scale on the dendritic and interdendritic areas during oxidation at 900 and 1000°C, while the alloy exhibited relatively uniform oxidation at 1100°C. The growth mechanism of the scale is discussed.     

AlCoCrFeNiTi_(0.5)高熵合金的高温氧化行为

采用水冷铜坩埚真空感应悬浮熔炼法制备AlCoCrFeNiTi_(0.5)多主元高熵合金,研究合金在800、900、1000和1100℃下的高温氧化行为,采用XRD,SEM及EDS对氧化膜的成分及形貌进行了分析,探索了合金的氧化机制。结果表明,合金的氧化动力学曲线在800和900℃时近似遵循六次方抛物线规律,在1000和1100℃时近似遵循四次方抛物线规律。合金具有优异的抗氧化性,在800、900和1100℃下为抗氧化级别,而在1000℃下为完全抗氧化级别。合金的氧化主要发生在枝晶间和共晶区,呈岛状团聚堆叠生长,1100℃氧化时该区域的氧化物发生明显剥落,氧化产物主要是TiO_2、Fe_2TiO_5和FeCr_2O_4等;而枝晶相的氧化产物较单一,1000℃及以下温度氧化时为弥散分布的Al_2O_3颗粒,1100℃氧化时为致密的Al_2O_3氧化层。高温氧化后,合金基体相结构稳定,未出现软化现象。     

Oxidation Behavior of the Directionally Solidified Ni-base Superalloy DS951 in Air

The isothermal oxidation behavior at 1000, 1050 and 1100°C and the cyclic oxidation behavior at 1000°C of the directionally solidified Ni-base superalloy DS951 were investigated. The oxidized samples were characterized by SEM, EDAX, XRD. The alloy DS951 obeyed a two-stage parabolic rate law during isothermal oxidation at 1000–1100°C. In cyclic conditions, the alloy showed no weight loss even after 1100 cycles. Cross-sectional examination revealed the development of faceted and needle-shaped AlN precipitates in the alloy subsurface region during cyclic oxidation, while no internal corrosion products were found in isothermal tests. Furthermore, a selective oxidation of MC carbides (M was predominately niobium with some chromium and tungsten) was observed along alloy grain boundaries and in interdendritic areas.     

Deformation behaviour and oxidation resistance of single-phase and two-phase L21-ordered Fe–Al–Ti alloys

Single-phase Fe–Al–Ti alloys with the Heusler-type L2₁ structure and two-phase L2₁ Fe–Al–Ti alloys with MgZn₂-type Laves phase or Mn₂₃Th₆-type τ₂ phase precipitates were studied with respect to hardness at room temperature, compressive 0.2% yield stress at 20–1100 °C, brittle-to-ductile transition temperature (BDTT), creep resistance at 800 and 1000 °C and oxidation resistance at 20–1000 °C. At high temperatures the L2₁ Fe–Al–Ti alloys show considerable strength and creep resistance which are superior to other iron aluminide alloys. Alloys with not too high Ti and Al contents exhibit a yield stress anomaly with a maximum at temperatures as high as 750 °C. BDTT ranges between 675 and 900 °C. Oxidation at 900 °C is controlled by parabolic scale growth.     

High temperature oxidation behavior of Ti3SiC2-based material in air

The oxidation behavior of Ti₃SiC₂-based material in air has been studied from 900°C to 1200°C. The present work showed that the growth of the oxide scale on Ti₃SiC₂-based material obeyed a parabolic law from 900°C to 1100°C, while at 1200°C it followed a linear rule. The oxide scale was generally composed of an outer layer of coarse-grained TiO₂ (rutile) and an inner layer of fine-grained TiO₂ and SiO₂ (tridymite) above 1000°C. A discontinuous coarse-grained SiO₂ layer was observed within the outer coarse-grained TiO₂ layer on the samples oxidized at 1100°C and 1200°C. Marker experiments showed that the oxidation process was controlled by the inward diffusion of oxygen, outward diffusion of titanium and CO or SiO, and that internal oxidation predominated. The TiC content in Ti₃SiC₂ was deleterious to the oxidation resistance of Ti₃SiC₂.     

Improved Spallation Resistance of the Oxide Scale by Hf/Y Co-doping in Ni-Based Superalloy at High Temperature

Ni-based superalloys added with comparably higher concentrations of single-doped Hf and co-doped Hf/Y are prepared by vacuum induction melting (VIM). The oxidation properties up to 300 h at 900 °C, 1000 °C, and 1100 °C are investigated. The undoped alloy exhibited a minimum oxidation rate at 900 °C and 1000 °C. The co-doped alloy showed a higher oxidation rate; however, it possesses better scale adhesion, and no spallation occurs. Hf-doped alloy showed a lower oxidation rate and better scale adhesion at 900 °C and 1000 °C, but exhibited a shorter lifetime at 1100 °C. The spallation of the Hf-doped alloy is attributed to the precipitation of the HfO₂ in and beneath the oxide scale. The spallation in the undoped alloy is accredited to the thermal expansion mismatch between the growing oxide scale and superalloy substrate. Incorporating two reactive elements (REs) in alloy minimized the precipitation of RE oxide in the oxide scale, diminished internal oxidation in the alloy, and decreased oxide scale spallation.     

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.     

Beneficial Effects of Rhenium Additions on the Cyclic-Oxidation Resistance of β-NiAl + α-Cr Alloys

This study reports the effects of up to 4 at.%rhenium addition on the cyclicoxidation behavior of-NiAl + -Cr alloys having a basecomposition (in at.%) Ni-40Al-17Cr. Tests were conductedin still air at 1100°C for up to 250 1-hr cycles.The ternary alloy (without rhenium addition) exhibitedpoor cyclic-oxidation resistance, undergoing extensivescale spallation and internal oxidation. Additions of rhenium considerably improved the oxidationbehavior, reducing the extent of both scale spallationand internal oxidation. These beneficial effectsincreased with increasing rhenium content. Rhenium additions improved cyclic-oxidation resistanceby both decreasing the solubility of chromium in the phase and causing the interdendritic -Crprecipitates in the alloy microstructure to become more spheroidized and disconnected. Theseeffects aided in preventing both interdendritic attackand the dissolution of the -Cr precipitates fromthe subsurface region of the alloy. The maintenance of -Cr precipitates at the alloy-scaleinterface decreased the extent of scale spallation byproviding a lower coefficient of thermal-expansion (CTE)mismatch between the alloy and theAl₂O₃-rich scale.     

Oxidation behavior of atomized Fe40Al intermetallics doped with boron and reinforced with alumina fibers

Isothermal oxidation resistance of Fe40 (at.%) Al-based atomized and deposited intermetallic alloys has been evaluated. The alloys included Fe40Al, Fe40Al + 0.1B, and Fe40Al + 0.1B + 10Al₂O₃ at 800, 900, 1000, and 1100 °C. The tests lasted approximately 100 h, although in most cases there was scale spalling. At 800 and 900 °C, the Fe40Al + 0.1B alloy had the lowest weight gain, whereas the Fe40Al alloy had the highest weight gain at 800 °C (0.10 mg/cm²) and the Fe40Al + 0.1B + 10Al₂O₃ alloy was the least oxidation resistant at 900 °C with 0.20 mg/cm². At 1000 °C, the Fe40Al + 0.1B alloy showed the highest weight gain with 0.12 mg/cm² and the Fe40Al alloy the lowest. At 1100 °C, again, as at 900 °C, the Fe40Al alloy was the least resistant, whereas the Fe40Al + 0.1B alloy performed the best, but the three alloys exhibited a paralinear bahavior on the weight-gain curves, indicating the spalling, breaking down, and rehealing of the oxides. This spalling was related to voids formed at the metal-oxide interface.     

Comparative Study of the Alumina-Scale Integrity on MA 956 and PM 2000 Alloys

The present work analyzes the oxidation kinetics of MA 956 and PM 2000 alloys at 900 and 1100°C for exposure times up to 1000 hr. Special emphasis was placed on a comparison of the alumina-scale integrity formed at 1100°C by means of electrochemical tests at room temperature, which have been shown to be very reliable methods to detect the presence of microdefects within oxide scales. To check whether a preoxidation treatment makes these materials corrosion resistant against aggressive fluids, an electrolyte containing chloride ions was chosen. The mass gain of MA 956 was found to be slightly lower than that of PM 2000 up to 200 hr exposure at 1100°C and for the whole exposure range at 900°C. A subparabolic time dependence (n=0.3) of the oxide growth rate was determined for both alloys at both temperatures. On the other hand, the electrochemical-impedance spectroscopy (EIS) and anodic-polarization tests performed on preoxidized alloys (1100°C/100 hr) revealed good room-temperature corrosion behavior for both alloys, the corrosion resistance and polarization values being somewhat higher for preoxidized PM 2000. Consideration of these results and those of both surface and cross-section examinations of the scale, the better room-temperature corrosion behavior of preoxidized PM 2000 denotes the formation of a denser and mechanically more stable alumina scale containing a lower number of microdefects. This could result from the higher aluminum content of this alloy and the lower density of chemical heterogeneities within the scale. The higher mass gain of PM 2000 could be related to the higher concentration of oxide nodules on top of the alumina scale, as deduced from SEM examination.     

The oxidation behavior and mechanical performance of Ti60 alloy with enamel coating

In the present paper, the influence of enamel coating, compared to the traditional TiAlCr coating, on oxidation behavior and oxygen contamination of Ti60 alloy at 700 and 800 °C was studied. A continuous protective alumina scale formed on TiAlCr coating during oxidation at the two temperatures; but a rather heavy interdiffusion layer appeared at the interface of TiAlCr/Ti60 during oxidation at 800 °C. The uniform and dense enamel coating could provide excellent protectiveness to Ti60 due to its thermal chemical stability and good compatibility in terms of thermal expansion coefficient to the substrate Ti60 alloy. According to the microhardness measurement results, there exists a layer of contamination of about 30 μm into the alloy after the enamel was vitrified for 30 min at 900 °C in air; but the depth of oxygen contamination into the alloy changed little after oxidation for 1000 h at 600 °C. The strength and the elongation at ambient temperature of Ti60 alloy with enamel coating decreased about 7.4% and 3.4% in comparison to the original bare alloy, respectively. From the results, the enamel coating could protect Ti60 alloy from oxidation and oxygen embrittlement.     

High temperature scaling of Ni-xSi-10 at.% Al alloys in 1 atm of pure O2

The oxidation of Ni-xSi-10Al alloys (with x = 0, 2, 4 and 6 at.%), has been studied at 900 and 1000 °C in 1 atm of pure O₂ to examine the effect of different silicon additions on the behavior of ternary Ni-Si-10Al alloys. The kinetic curves of Ni-10Al are approximately parabolic at both 900 and 1000 °C. Conversely, the kinetics of the ternary alloys at both temperatures correspond generally to a rate decrease faster than predicted by the parabolic rate law, except for the oxidation of Ni-6Si-10Al at 1000 °C, which exhibits a single nearly-parabolic stage. Oxidation of the binary alloy formed at both temperatures an internal oxidation zone beneath a layer of NiO. Oxidation of Ni-2Si-10Al at both temperatures and of the other two alloys at 900 °C formed initially a zone of internal oxidation of Al + Si. However, a layer of alumina forming at the front of internal oxidation after some time blocked the internal oxidation and produced a gradual conversion of the metal matrix of this region into NiO, with a simultaneous decrease of the oxidation rate. Conversely, the oxidation of Ni-4Si-10Al and Ni-6Si-10Al at 1000 °C did not produce an internal oxidation, but formed an alumina layer directly on the alloy surface after an initial stage when also Ni was oxidized. Therefore, silicon exerts the third-element effect by reducing the critical Al content needed for the transition from its internal to its external oxidation with respect to the corresponding Ni-Al alloy. This result is interpreted by means of an extension to ternary alloys of Wagner’s criterion for the same transition in binary alloys based on the attainment of a critical volume fraction of internal oxide.     

Oxidation behavior of FeAl+Hf,Zr, B

The oxidation behavior of Fe-40Al-1Hf, Fe-40Al-1Hf-0.4B, and Fe-40Al-0.1Zr-0.4B (at.%) alloys was characterized after 900°, 1000°, and 1100°C exposures. Isothermal tests revealed parabolic kinetics after a period of transitional -alumina scale growth. The parabolic growth rates for the subsequent -alumina scales were about five times higher than those for NiAl+O.1Zr alloys. The isothermally grown scales showed a propensity toward massive scale spallation due to both extensive rumpling from growth stresses and to an inner layer of HfO₂. Cyclic oxidation for 200 1-hr cycles produced little degradation at 900 or 1000°C, but caused significant spaliation at 1100°C in the form of small segments of the outer scale. The major difference in the cyclic oxidation of the three FeAl alloys was increased initial spallation for FeAl+Zr, B. Although these FeAl alloys showed many similarities to NiAl alloys, they were generally less oxidation-resistant. It is believed that this resulted from nonoptimal levels of dopants and larger thermal-expansion mismatch stresses.     

Oxidation resistance of aluminized coatings on a directionally solidified Ni-Al-Cr3C2 eutectic alloy

Although a directionally solidified Ni-Al-Cr₃C₂ eutectic alloy has good high-temperature mechanical properties, it does not have adequate oxidation resistance for prolonged exposure to high surface temperatures. Thus the oxidation behavior of several aluminized coating systems on this alloy in flowing air at temperatures of 900 to 1100°C under isothermal and thermal cycling conditions has been investigated. Attempts to produce an oxidation-resistant system by direct aluminizing have not been successful since removal of carbide fibers results in a porous coating which gives little protection to the alloy. The deposition of a layer of nickel or a Ni-20%Co-10%Cr-4%Al alloy on the eutectic prior to aluminizing gives improved isothermal oxidation resistance for prolonged exposure to high surface temperatures. Thus the eutectic alloy substrate occur during thermal cycling. A more successful system has been produced by depositing a thin layer of platinum on the eutectic alloy prior to aluminizing. Protective -Al₂O₃ scales are formed and maintained during isothermal and thermal cycling oxidation at 900 and 1000°C. Similar scales are developed at 1100° C although these do break down during thermal cycling. However, surface -Al₂O₃ scales are able to re-form rapidly, thereby preventing excessive oxidation of the coating.     

Oxidation Behavior of ODS Fe–Cr Alloys

Four experimental oxide dispersion strengthened (ODS)Fe-(13–14 at. %)Cr ferritic alloys were exposed for up to 10,000 hr at 700–1100 °C in air and in air with 10vol.% water vapor. Their performance has been compared to other commercial ODS and stainless steel alloys. At 700–800°C, the reaction rates in air were very low for all of the ODS Fe–Cr alloys compared to stainless steels. At 900°C, a Y₂O₃ dispersion showed a distinct benefit in improving oxidation resistance compared to an Al₂O₃ dispersion or no addition in the stainless steels. However, for the Fe-13 %Cr alloy, breakaway oxidation occurred after 7,000 hr at 900°C in air. Exposures in 10 % water vapor at 800 and 900°C and in air at 1000 and 1100°C showed increased attack for this class of alloys. Because of the relatively low Cr reservoirs in these alloys, their maximum operating temperature in air will be below 900°C.     

Oxidation behaviour of Ag-containing TiAl-based intermetallics

The effect of alloy composition on the oxidation behaviour of γ-TiAl alloys with silver additions was studied during exposure at 800 °C in air. For this purpose, a number of Ti–Al–Ag alloys with systematic variation of the chemical composition was prepared by levitation induction melting. Subsequently, the alloys were isothermally and/or discontinuously oxidised at 800 °C in air. The results showed that suitable silver additions can promote and stabilise formation of an alumina scale on γ-TiAl up to the maximum test times of 12,000 h. An oxidation map was derived which allows defining which type of surface oxide is formed on various TiAl–Ag alloy compositions. This result can be applied in further work for development of oxidation resistant coatings for γ-TiAl or/and Ti-based alloys.     

High-Temperature Oxidation Behavior of ODS–Fe3Al

The high-temperature oxidation behavior of an oxide dispersion-strengthened (ODS) Fe₃Al alloy has been studied during isothermal and cyclic exposures in oxygen and air over the temperature range 1000 to 1300°C. Compared to commercially available ODS–FeCrAl alloys, it exhibited very similar short-term rates of oxidation at 1000 and 1100°C, but at higher temperatures the oxidation rate increased because of increased scale spallation. Over the entire temperature range, the oxide scale formed was -Al₂O₃, with the morphological features typical of reactive-element doping and was similar to those formed on the ODS–FeCrAl alloys. Although initially this scale appeared to be extremely adherent to the Fe₃Al substrate, an undulating metal–oxide interface formed with increasing time and temperature, which led to cracking of the scale in the vicinity of surface undulations accompanied by a loss of small fragments of the full-scale thickness. In some instances, the surface undulations appeared to have resulted from gross outward local extrusion of the alloy substrate. Similar features developd on the FeCrAl alloys, but they were typically much smaller after a given oxidation exposure. The ODS–Fe₃Al alloy has a significantly larger coefficient of thermal expansion (CTE) than typical FeCrAl alloys (approximately 1.5 times at 900°C) and this appears to be the major reason for the greater tendency for scale spallation. The stress generated by the CTE mismatch was apparently sufficient to lead to buckling and limited loss of scale at temperatures up to 1100°C, with an increasing amount of substrate deformation at 1200°C and above. This deformation led to increased scale spallation by producing an out-of-plane stress distribution, resulting in cracking or shearing of the oxide.     

Influence of grain size on the oxidation behaviour of MA 956

The influence of grain size of alloy MA 956 on the oxidation kinetics and scale morphology is investigated at 900 and 1100°C for up to 1000 h exposure. Specimens with grain sizes of about 3, 150, and 300 μm mean diameter were studied. Grain growth during oxidation was not observed. The results of this work at 1100°C show that grain size of the alloy does not significantly influence the oxidation kinetics. At 900°C, however, mass gain values for the small‐ and medium‐grain size material are somewhat lower than those for the large‐grain size material, especially for exposures below 100 h. This feature indicates that grain boundaries could have played a role for the supply of Al to the scale.