Oxidation behavior of Ni（Co）CrAlYHf（Si） coatings on DS superalloy at 1150℃

Two Ni（Co）CrAlY coatings were deposited by EB-PVD method on a DS superalloy of Ni-Al-Cr-Co-W-Mo-Ta-Hf system. SEM, XEDS and XRD were used to study the oxidation behavior of the coatings. The two coatings show a good protection for the DS superalloy. The results of the isothermal oxidation test at 1150 ℃ for 100 h show that the oxidation tendency obeys the parabolic law, and the oxidation rate constant Kp of the coated specimens decreases to about 1/3 of that for the bare superalloy. After oxidation, a continuous alumina-based scale is formed at the surfaces of the coated samples. Y2O3, NiO and SiO2 are also detectable in the oxide scale. A large number of Al in the coating is consumed due to high-temperature diffusion and oxidation reactions, and the NiAl phases in the coating are almost completely transformed to Ni3Al phases. For the Hf-bearing coating, some HfO2 particles exist at the interface between the coating and the substrate. Although internal oxidation occurs, the coating still shows a good adhesion with the superalloy substrate even after oxidation for 100 h. For the bare DS superalloy, after 100 h oxidation at 1 150℃, only discontinuous alumina-based oxide particles exist on the surface. Oxide spallation occurs for the bare alloy.     

Isothermal oxidation behavior of cast Ni-base superalloy K44

The oxidation behavior of a cast Ni-base superalloy K44 in air at 850-1 000℃ for 100 h was studied. The scales on the surface were determined by SEM and EPMA equipped with an EDXS. The results show that oxidation kinetics obey the parabolic law fi＇om which the values of activation energy Qp1=221.1 kJ/mol and Qp2=247.6 kJ/mol are estimated. The oxidation scales are composed of loose outer layer of TiO2/TiO-Cr2O3 and a small amount of NiCr204 and NiAl2O4, compact intermediate layer Cr2O3, and precipitate of internal oxides Al2O3.     

Effect of Nb on oxidation behavior of NiTiNb alloys

The oxidation behavior of NiTi and NiTiNb alloys containing different amounts of Nb （7%, 9%, mole fraction） were studied at 800℃ in air. It is found that the oxidation resistance of NiTi alloy can be effectively increased by the Nb addition. Under the same oxidation condition, the mass gain of NiTi is about 7 mg/cm^2, while the inass gains are only 3 mg/cm^2 for Ni47Ti44Nb9 alloy and 2.4 mg/cm^2 for Ni52Ti41NbT. Moreover the oxidation resistance of single phase NiTiNb alloy is better than that of the dual-phase alloy with large amount of Nb precipitates. On the basis of thermodynamics and kinetics of oxidation, the effect of Nb alloying element on the oxidation behavior of NiTi-based alloys was discussed.     

Effect of 5%Cr on oxidation of Ni- 10Al in 0.1MPaO2 at 900 ℃ and 1 000 ℃

The effect of 5%Cr（mole fraction） on the scaling behavior of Ni-10Al was investigated at 900 ℃ and 1 000 ℃ with the aim to discuss the third element effect（TEE）. The oxidation rate of the ternary alloy containing Cr is much slower than the corresponding binary alloys at both temperatures. A dense external scale of NiO overlying a zone of internal oxide precipitates forms on Ni-10Al, while Ni-5Cr-10Al develops a protective layer. So the addition of Cr promotes the formation of alumina scale. The possible mechanism of Cr on the oxidation of Ni-Al alloy is discussed.     

Oxidation behavior of temary Fe- 15Cu-5Al and Fe-85Cu-5Al alloys in pure oxygen at 900℃

The oxidation of two two-phase ternary Fe-Cu-Al alloys containing about 5%（mole fraction） aluminium, one Fe-rich and one Cu-rich, has been studied at 900 ℃ in 1~105 Pa pure oxygen. The Fe-rich Fe-15Cu-5Al alloy presents two quasi-parabolic stages, with a large decrease of the parabolic rate constant after about 50min. The presence of 5% Al greatly reduces the oxidation rate of this alloy with respect to a binary Fe-Cu alloy of similar composition by forming an external protective Al203 layer, which is present near the scale/alloy interface. Due to the high stress-growth effect, this thin Al203 layer cannot completely prevent further oxidation of the alloy underneath. On the contrary, the Cu-rich Fe-85Cu-5Al alloy presents a single parabolic stage and forms a thick and porous external scale, coupled to the internal oxidation of Fe and Al. As a result, the oxidation of Cu-rich alloy at 900 ℃ is much faster than that of the Fe-rich alloy.     

Morphological evolution of γ＇ phase in K465 superalloy during thermal fatigue

The alternative heating/cooling cycles （thermal fatigue ） of K465 superalloy were carried out. The specimens were held at 1 050 ℃ for 300 s, then quenched into 20℃ recycling water for 10 s as a cycle. During thermal fatigue, γ＇ precipitates changed typically from cubical to irregular shape aider 10 cycles, to complex configuration aider 20 cycles and raft-like shape aider 30 cycles. The very fine γ＇ particles precipitated inter the original γ＇ particles. The elastic energy dominated morphological evolution of large γ＇ precipitates, and the thermal stress induced the directional growth of precipitates that minimized the total energy of the system, and the nucleation theory controlled the formation of fine γ＇ precipitate. The results show that the volume fraction of γ＇ precipitates is increased with the increase of heating/cooling cycles, which improves the mechanical property of this alloy.     

Influence of high-intensity pulsed ion beams irradiation on oxidation behavior of 316L stainless steel at 700 ℃

316L stainless steel samples, as a widespread used material, were irradiated with HIPIB at the beam parameters of ion energy 300 keV, current density 100, 200 and 300 A/cm^2, shot number 10 and pulse duration 75 ns. The surface morphology and the phase structure in the near surface region of original and treated samples were analyzed with scanning electron microscopy （SEM） and X-ray diffractometry （XRD）. It is shown that the HIPIB irradiation can smooth the surface of the samples, and the preferred orientation is present in the surface layer of irradiated coupons. The influence of HIPIB irradiation on the oxidation behavior of 316L stainless steel at 700 ℃ for up to 100 h was investigated. Electron probe microanalysis （EPMA） was used to study the distribution of elements in the oxidation products. It is found that the oxidation behavior of the irradiated coupons depends greatly on the ion current density of HIPIB. HIPIB irradiation with ion current density of 100 A/cm^2 slightly reduces the oxidation rate with respect to the unirradiated coupon. The improvement of the oxidation resistance can be attributed to more oxide of Cr that forms on the surface of the irradiated coupons. In contrast, HIPIB irradiation with ion current density of 200 or 300 A/cm^2 is proved to be detrimental, causing a higher oxidation rate.     

Transient oxidation behavior of nanocrystalline CoCrAlY coating at 1050℃

Nanocrystalline CoCrA1Y overlay coating was prepared on M38G superalloy by magnetron sputtering deposition. To investigate the oxidation behavior and phase transformation of alumina during oxidation, the oxidation experiments were conducted at 1 050 ℃ for various time in the range of 5-180 min. The phase compositions of the oxide scales were investigated by using glancing angle X-ray diffraction（XRD）. The microstructure analysis of oxide scales was carried out by means of scanning electron microscopy（SEM）. The growth process of metastable alumina at the grain boundaries and transformation to stable alumina were discussed. The results show that at the initial oxidation stage the mixture of δ-Al2O3, γ-Al2O3 and α-Al2O3 is formed on the sample surface rapidly. Especially, δ-Al2O3 and γ-Al2O3 prefer growing at the grain boundaries of CoCrA1Y coating. With increasing oxidation time, δ-Al2O3 and γ-Al2O3 transform to θ-Al2O3, afterwards θ-Al2O3 transforms to α-Al2O3 gradually. After 180 min oxidation, θ-Al2O3 transforms into α-Al2O3 completely.     

Corrosion behavior of NiCr alloys in HCl-containing oxidation atmosphere at 700-800℃

Corrosion behaviors of pure Ni and three NiCr alloys were investigated in an HCl-containing oxidizing at-mosphere at 700℃ and 800℃. All materials suffer from accelerated corrosion at both temperatures. NiCr alloys show an initial mass loss due to the formation of volatile CrCl3 and CrO2Cl2. Some chlorides are detected at the scale/substrate interface and many voids are also found there. NiCr alloys with higher chromium content have better corrosion resistance. However, Ni50Cr is inferior to Ni25Cr due to its two-phase structure, which makes it easy for chlorine to diffuse along grain boundary and to occur inner oxidation. The relevant corrosion mechanism was also discussed.     

Effects of secondary precipitation on recrystallization in Co-base superalloy DZ40M

A series of experimental studies were conducted on the recrystallization of directionally solidified cobalt-base superalloy DZAOM. It is found that the secondary M23C6 precipitation influences the size and shape of the recrystal grains. When the annealing temperature is below 1 473 K, a large amount of the fine secondary M23C6 precipitations are distributed around the primary carbides, and such carbides impede the movement of grain boundary because the effect, the size and shape of recrystal grains become irregularly. When the temperature exceeds 1 473 K, the recrystal grains grow rapidly due to the dissolved secondary M23C6 precipitation.     

Creep behavior and microstructure evolution of directionally solidified superalloy IC 10 at high temperature

The creep behavior of directionally solidified （DS） superalloy IC 10 was investigated under 192 MPa and 218 MPa at 980 ℃. Under the testing conditions, the marked creep characteristic of the superalloy is that the creep curve has a short primary and secondary stage and a long tertiary stage. Another creep characteristic is that the superalloy has excellent plasticity at high temperature. To study the creep behavior, the microstructure was observed by SEM and TEM. Different from other microstructure of Ni-base superalloys, superalloy IC10 forms incompletely rafted γ＇ phase during the creep processes. To understand the creep deformation mechanism of superalloy IC10, the movement of dislocations was analyzed. The results show that the dislocations moving in the γ matrix and climbing over the γ＇ precipitates is the main deformation mechanism under the experimental conditions.     

Static oxidation behaviour of silicide coating on niobium alloy at high temperature

A silicide coating was prepared on the surface of the Nb521 alloy by the complex pack cemented method. The oxidation resistance properties of the present coating were exeamed by the static oxidation tests at 1 700 ℃in air. The compositions and the microstructures of the coating before and after test were characterised and analysed through scanning electron microscopy （SEM）, X-ray diffraction analysis （XRD）, energy dispersive X-ray spectrometry （EDS） and electron probe microanalysis （EPMA）, respectively. The present silicide coating can provide an effective protection for the Nb alloy for 25 h at 1 700 ℃ in air. The results show that the oxidation kinetics of the present silicide coating is parabolic. The diffusion of Si leads to the phase transformation and evolution during the oxidation.     

Isothermal oxidation behavior of a new Re-free nickel-based single-crystal superalloy at 950 ℃

The isothermal oxidation behavior of a new Refree nickel-based single-crystal superalloy in air at 950 ℃ for 200 h was studied by scanning electron microscopy(SEM)with energy-dispersive spectroscopy(EDS)and X-ray diffraction(XRD).The results indicate that oxidation kinetics obeys parabolic law approximately,and the mass gain increases rapidly during initial oxidation stage and then gradually slows down.The oxidation scales are composed of three layers:the outer layer mainly consists of NiO with a small amount of CoO;the intermediate layer is mainly composed of Cr_2O_3 with a small amount of spinel compounds such as CrTaO_4,NiCr_2O_4,CoCrAl_2O_4,CoAl_2O_4,and NiAl_2O_4;and the inner layer is composed of Al_2O_3.Inner Al_2O_3 layer suppresses the diffusion of elements between oxygen and alloy elements,slows down the alloy oxidation speed,and also suppresses the growth of the oxide scale and reduces the oxidation rate,which is agreeable with the oxidation kinetics.     

The Sulfidation/Oxidation Resistance of Two Ni-Cr-Al-Y Alloys at 700℃

The high temperature corrosion resistance of Ni-25.9Cr-13.5Al-1.2Y-0.6Si and Ni-10.2Co-12.4Cr-16.0Al-0.5Y-0.2Hf alloys was assessed in sulfidation/oxidation environments.In the environment with a sulfur partial pressure of 1Pa. and an oxygen partial pressure of 10^-19Pα,both these alloys exhibited three distinct stages in the weight gain-time curve when tested at 700℃.In the initial stage, selective sulfidation of Cr suppressed the formation of the other metal sulfides,resulting in lower weight gains.In the transient stage, breakdown and cracking of Cr sulfides and insufficient concentration of Cr at the outer zone led to the rapid formation of Ni sulfides and a rapid increase in weight.In the steady-state stage, corrosion was controlled by the diffusion of anions and/or cations, which led to a parabolic rate law.     

Influence of surface modification on isothermal oxidation behavior of EB-PVD NiA1 coating

The isothermal oxidation behaviors of the as-deposited NiAl coating on the nickel-based superalloy by electron beam physical vapour deposition（EB-PVD） and the NiAl coating after surface modifications of grinding and polishing were investigated. The as-deposited coating shows the least mass gain, the initially formed θ-Al2O3 scale spalls after only 1h, and the succeeding scale formed is coarse and discontinuous and thus can not be used as protective coatings. Among the two surface-modified coatings, the ground coating results in the highest oxide growth rate, which is consistent with the SEM results where the scale spalls heavily and many voids appear between the scale and the NiAl coating. The scale spallation and void formation mechanisms during isothermal oxidation test ofEB-PVD NiAl coating were also discussed.     

Effect of temperature on oxidation behavior of ternary Fe-15Cu-5Al alloy in pure oxygen

The effect of temperature on oxidation behavior of ternary Fe-15Cu-5Al （mole fraction, %） alloy in pure oxygen was studied. Fe-15Cu-5Al alloy presents an irregular high-temperature oxidation behavior between 700-1 000 ℃, though the kinetic curve at each temperature can be approximately considered as being composed of two quasi-parabolic stages. At 700 ℃ the alloy forms bulky stratified scales. On the contrary, at 800℃ the alloy forms an external protective Al2O3 layer. The trend of decrease of oxidation rate does not continue at elevated temperatures. Due to the high stress-growth effect at 900℃, the thin Al2O3 layer cannot completely prevent further oxidation of the alloy underneath. When the temperature rises to 1 000℃, the high stress-growth effect is more obvious and the alloy forms overgrown large nodules. Compared with the Fe-10Al binary alloy, the presence of 15 % Cu tends to greatly increase the flaw of the formed Al2O3 layer at elevated temperatures, resulting in a peculiar oxidation pattern of Fe- 15Cu-5Al alloy.     

Oxidation and hot corrosion behavior of Ti2AlNb-based alloy with and without enamel coating at 800℃

The oxidation and hot corrosion behavior of Ti2AlNb-based alloy with and without enamel coating at 800 ℃ was investigated. The results indicated that Ti-22Al-25Nb alloy exhibited poor oxidation resistance at 800 ℃. The constitution of oxide scale had the effect on its oxidation rate. Because of the S and Cl accelerating the corrosion process, Ti-22Al-25Nb alloy suffered severe hot corrosion and exhibited very poor hot corrosion resistance, Enamel coating could remarkably improve the high temperature oxidation resistance of Ti-22Al-25Nb alloy because it had good chemical stability and matched thermal expansion coefficient with the substrate. In （Na,K）2SOa＋NaCl molten salts at 800 ℃, chemical reactions between molten salts and enamel coating occurred and complicated products formed on the surface of the enamel coating; Cl in the molten salts could penetrate through the coating and induced the substrate corrosion, but enamel coating still had good hot corrosion resistance.     

Isothermal oxidation behavior of Ti3Al-based alloy at 700-1000℃ in air

The isothermal oxidation behavior of a Ti3Al-based alloy (Ti-24Al-14Nb-3V-0.5Mo-0.3Si, molar fraction, %) at 700- 1 000 ℃ in air was investigated. The oxidation kinetics of tested alloy approximately obeys the parabolic law, which shows that the oxidation process is dominated by the diffusion of ions. The oxidation diffusion activity energy is 241.32 kJ/mol. The tested alloy exhibits good oxidation resistance at 700 ℃. However, when the temperature is higher than 900 ℃, the oxidation resistance becomes poor. The XRD results reveal that the oxide product consists of a mixture of TiO2 and Al2O3. Serious crack and spallation of oxide scale occur during cooling procedure after being exposed at 1 000 ℃ in air for 16 h. According to the analysis of SEM/EDS and XRD, it is concluded that the Al2O3 oxide forms at the initially transient oxidation stage and most of it keeps in the outer oxide layer during the subsequent oxidation procedure.     

Oxidation behavior of three-phase Cu-25Ni-30Cr alloy at 700-800℃ under high oxygen pressures

The thermogravimetric analysis of a ternary Cu-25Ni-30Cr alloy prepared by conventional casting was performed in 0. 1 MPa pure O2 at 700-800℃. The results show that the alloy is composed of three phases,where the phase with the largest copper and lowest chromium content forms the matrix, while the other two,much richer in chromium, form a dispersion of isolated particles. At variance with another three-phase Cu-20Ni-20Cr alloy, which forms complex scales containing the oxides of the various components and double oxides plus an irregular region composed of alloy and oxides, the present alloy can form a very irregular but continuous chromia layer at the base of the mixed internal region, producing a gradual decrease of the oxidation rate down to very low values. A larger chromium content needed to form chromia layer for a ternary three-phase alloy is attributed to the limitations to the diffusion of the alloy components in the metal substrate imposed by their multiphase nature.