Isothermal oxidation behavior and mechanism of a nickel-based superalloy at 1000℃

The oxidation behavior of a nickel-based superalloy at 1000℃ in air was investigated through X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy analysis. A series of oxides, including external oxide scales (Cr₂O₃, (TiO₂ + MnCr₂O₄)) and internal oxides (Al₂O₃,TiN), were formed on the surface or sub-surface of the substrate at 1000℃ in experimental still air. The oxidation resistance of the alloy was dependent on the stability of the surface oxide layer. The continuity and density of the protective Cr₂O₃ scale were affected by minor alloying elements such as Ti and Mn. The outermost oxide scale was composed of TiO₂ rutile and MnCr₂O₄ spinel, and the growth of TiO₂ particles was controlled by the outer diffusion of Ti ions through the pre-existing oxide layer. Severe internal oxidation occurred beneath the external oxide scale, consuming Al and Ti of the strength phase γ' (Ni₃(Al,Ti)) and thereby severely deteriorating the surface mechanical properties. The depth of the internal oxidation region was approximately 35 μm after exposure to experimental air at 1000℃ for 80 h.     

Low cycle fatigue properties and microstructure evolution at 760℃ of a single crystal superalloy

Low cycle fatigue(LCF) behavior of a single crystal superalloy was investigated at 760 ℃. Microstructure evolution and fracture mechanism were studied by scanning electron microscopy(SEM) and transmission electron microscopy(TEM), respectively. The results show that the fatigue data fluctuation was small and the fatigue parameters of the alloy had been determined. On increasing the cyclic number, the alloy initially showed slight cyclic softening at the early two or three cycles and slowly hardened to some extent afterwards, then kept stable for the most of the remaining fatigue life. The LCF of the alloy at 760 ℃ can be attributed to the main elastic damage in fatigue processing. The initiation site of fatigue crack was at or near the surface of the samples. Crack propagated perpendicularly to the loading direction at first and then along {111}octahedral slip planes. The fatigue fracture mechanism was quasi-cleavage fracture. The γ'phase morphology still maintained cubic shape after fracture. There were a number of slip bands shear the γ'precipitates and γ matrix near the fracture surface of the specimen. The inhomogeneous deformation microstructure was developed by dislocation motion of cross-slip and a limited γ'precipitate shearing by slip band, stacking faults or single dislocation was observed.     

Rare earth effects on high temperature oxidation of pure nickel at 1000 ℃

Isothermal and cyclic oxidation behaviors of pure and yttrium-implanted nickel were studied at 1000 ℃ in air.SEM and TEM were used to examine the oxide scales formed on nickel substrate.It was found that Y-implantation greatly improved the anti-oxidation ability of nickel both in isothermal and cyclic oxidizing experiments.Acoustic emission(AE)technique was used to study the size and number distribution of defects at the oxide/metal interface.Laser Raman microscopy was also used to study the stress status of oxide scales formed on nickel with and without yttrium.The main reason for the improvement in anti-oxidation and adhesion of oxide scale was that Yimplantation greatly reduced the grain size of NiO and lowered the compressive stress within the scale.In the meantime,Y-implantation inhibited ion diffusion rate in the oxide scale and reduced the size and number of interfacial defects,hence remarkably enhanced the adhesion of protective NiO oxide scale formed on nickel substrate.     

Isothermal section of ternary Sn-Zn-Ni phase equilibria at 250℃

Ternary Sn-Zn-Ni alloys were prepared and equilibrated at 250℃for 4-15 weeks.The phases formed in these equilibrated alloys were determined experimentally.The isothermal section of Sn-Zn-Ni system was constructed,based on the phase diagrams of the three constituent binary systems and the ternary phase equilibria data,determined in this study and referenced in literatures.12 single-phase regions were identified in the Sn-Zn-Ni ternary system at 250℃,including the three ternary compounds,δ,τ_1 andτ_2.There...     

Mechanical properties and microstructure changes after long-term aging at 700℃ for a nickel-base superalloy

Mechanical properties and microstructure changes have been investigated on a new nickel-base superalloy after long-term aging at 700℃. It is found that the major precipitates of the tested alloy are MC, M₂₃C₆, M₆C and γ' in the course of long-term aging at 700℃. The carbides maintain good thermal stability with the aging time up to 5008 h. The growth rate of gamma prime precipitates is relatively high in the early aging period and then slows down. The coarsening behavior of gamma prime follows a diffusion-controlled growth procedure. The room temperature Rockwell hardness of the alloy aged at 700℃ increases slightly at the initial stage of aging, but it decreases with the prolonged time. It mainly depends on the size of gamma prime. In comparison with Nimonic lloy 263, the new alloy characterizes with higher tensile and stress-rupture strengths at high temperatures. The new nickel-base superalloy offers a combination of microstructure stability, strength, ductility and toughness at 700℃.     

Creep behavior and dislocation mechanism of Ni3Al based single crystal alloy IC6SX at 760℃

The creep behavior and dislocation mechanism of Ni₃Al-based single crystal alloys IC6 SX with crystal orientation[001] which was prepared by seed crystal method under the testing conditions of 760℃/500 MPa,760℃/540 MPa and 760℃/580 MPa were investigated. The experimental results showed that the creep properties,dislocation morphology and mechanism of this alloy were different under different stress conditions. With the stress increasing from 500 MPa to 580 MPa, the creep life decrease...     

Influence of γ' precipitate on deformation and fracture during creep in PM nickel-based superalloy

FGH96 is a powder metallurgy nickel based superally used for turbine disk of aero-engines. In the present study FGH96 alloy with four different γ' precipitate microstructures were produced via solution heat-treatment with different cooling rates, and the maximum cooling rate reached 400°C/min which was a super cooling rate for Nickel-based superalloy. The creep tests were conducted for PM FGH96 alloy under the testing condition of 704°C and 690 MPa. The relationship between the creep properties and the distribution of γ' precipitate was established. The creep mechanism was analyzed by using TEM and ACTEM, and the dislocation movement was studied at the atomic scale. The creep strain rate was calculated through a physically based crystal slip model established based on crystal plasticity. The calculated results were consistent with the test ones, illustrating the validity of the model. The fracture mechanism was also investigated, and the results showed that the creep cracks generated on the surface due to the oxidation. It was observed that the cracks propagated in different ways depending on the different average diameters of γ' precipitate. With the decrease of the average γ' precipitate size,the critical shearing stress increased and the resistance of the dislocation slipping increased. The fracture mechanism for the primary stage transformed from intragranular to intergranular due to the change of dislocation slipping.     

Dissolution behavior and dissolution mechanism of palygorskite in HCl solution

The 1 mol/L, 3 mol/L and 5 mol/L HCl solutions were employed to leach two palygorskites with different trioctahedral contents in their crystal structure for different period of time. The results of the dissolution experiments show that the dissolution process could be divided into three steps, and that this dissolution behavior can be attributed to its higher Mg2+ content, and is restricted by the extraction behavior of Si4+. The preferential extraction for Mg2+ promotes the extraction behavior of Si4+ from Si-O framework. Because the Si4+ in the form of amorphous SiO2 is adsorbed onto the surface of palygorskite fibre, the reaction between palygorskite and acid is obstructed. With the elapsing of time, or the increasing of the acid concentration, the amorphous SiO2 flocculates, and then the channels of chemical reaction are reopened. The ratio value of Mg2+/(Fe3++Al3+) in leaching solution tends to a fixed value, showing that the acid attacks not only the surface but also the structural channels. There are no obvious three steps observed during the acid attack on the palygorskite with a lower trioctahedral content. The differential behavior for two palygorskites is discussed.     

Creep behavior and effect factors of a TiAl–Nb alloy at high temperature

The effect of solution treatment on the microstructure and creep properties of forged TiAl-Nb alloys was investigated.The results showed that the microstructure of forged alloy mainly consisted of γ/α₂ lamellar colonies and fine equiaxed recrystallized γ/α₂ grains.During the solution treatment the microstructure of the alloy transformed into a fully lamellar structure due to the lamellar colonies growth by consuming equiaxed grains.Compared with the forged alloy the creep l...     

Effect of niobium alloying level on the oxidation behavior of titanium aluminides at 850°C

This work addresses the alloying of titanium aluminides used in aircraft engine applications and automobiles. The oxidation resistance behavior of two titanium aluminides of α₂ + γ(Ti₃Al + TiAl) and orthorhombic Ti₂NbAl, recognized as candidates for high-temperature applications, was investigated by exposure of the alloys for 100 h in air. Thus, oxidation resistance was expressed as the mass gain rate, whereas surface aspects were analyzed using scanning electron microscopy in conjunction with energy-dispersive X-ray spectroscopy, and the type of oxidation products was analyzed by X-ray diffraction and Raman spectroscopy. The orthorhombic Ti₂NbAl alloy was embrittled, and pores and microcracks were formed as a result of oxygen diffusion through the external oxide layer formed during thermal oxidation for 100 h.     

The radiolytic behavior and mechanism of calixarene crown ether under γ-irradiation

Bis(2-propyloxy)calix[4]crown-6 (BPC6) is an effective separation agent for cesium removing from spent nuclear fuel. The study on radiolytic behavior of BPC6 itself under γ-irradiation was required for evaluating its feasibility of practical application. It was found that BPC6 exhibited excellent radiation stability at dose less than 100 kGy. However, the isopropyl groups in BPC6 under- went oxidation and dealkylation to form carbonyl groups and phenolic hydroxyl groups at dose above 300 kGy, respectively. When the dose was more than 1000 kGy, the ring opening of crown ether structure of BPC6 resulted in the formation of phenolic hydroxyl groups as well. The formation of radiolytic products containing phenolic hydroxyl groups during the irradiation of BPC was confirmed by using FeCl 3 as a probe. Combining with the radiolytic behavior of two model chemicals (1-isopropoxybenzene and benzo-18-crown-6 ether), the radiolytic behavior and mechanism of BPC6 under -irradiation was testified further.     

Effect of Zr and Ti on isothermal oxidation behavior of Nb–Si based alloys

The isothermal oxidation behavior of 56Nb-16Si-(20-x)Ti–3Cr–3Al-2Hf-xZr (x ?= ?0, 2, 5, 10 ?at. %) alloys was investigated at 800 ?°C and 1250 ?°C, respectively. The results show that increasing the Zr content evidently increased the oxidation rates at 800 ?°C, accompanied by the obvious occurrence of pesting oxidation. The alloys showed alike linear oxidation kinetics at 1250 ?°C. With the increase of Zr content, the adherence and integrity of oxide scales were improved, but the overall oxidation resistance was slightly deteriorated. The observed oxidation behavior may be attributed to the composition variation of Zr and Ti in the alloys. The oxidation mechanism associated with the composition variation is discussed in this study.     

Coarsening behavior of γ′ and γ″ phases in GH4169 superalloy by electric field treatment

The coarsening behaviors of γ′ and γ″ phases in GH4169 alloy aged at 1023 and 1073 K with electric field treatment (EFT) were investigated by transmission electron microscopy (TEM) and positron annihilation lifetime spectroscopy (PALS). It is demonstrated that precipitation coarsening occurs, and the growth activation energies of γ′ and γ″ phases can be decreased to 115.6 and 198.1 kJ·mol?1, respectively, by applying the electric field. The formation of a large number of vacancies in the matrix is induced by EFT. Due to the occurrence of vacancy migration, the diffusion coefficients of Al and Nb atoms are increased to be 1.6–5.0 times larger than those without EFT at 1023 or 1073 K. Furthermore, the formation of vacancy clusters is promoted by EFT, and the increase in strain energy for the coarsening of γ′ and γ″ phases can be counterbalanced by the formation of vacancy clusters.     

Oxidation behavior of the Fe-36Al-0.09C-0.09B-0.04Zr alloy at 1250℃

<正>To explore and study the Fe-Al system alloy presenting exceptional oxidation resistance at high temperature,the Fe-36Al-0.09C-0.09B-0.04Zr alloy was designed and developed.The microstructure and hardness of the backing at 1250℃were analyzed and measured.Thermodynamics and kinetics of the oxidation behavior were also analyzed by X-ray diffraction,scanning electron microscopy, and energy-dispersive X-ray spectroscopy techniques.The results show that the microstructure of the Fe-36Al-0.09C-0.09B-0.04Zr alloy is FeAl phase at ambient temperature and is stable at 1250℃.It displays the excellent property of oxidation resistance because the oxide film has only the Al_2O_3 layer,and its oxidation kinetics curve obeys the parabolic law at 1250℃.The oxidation mechanism at 1250℃is presumed that in the early oxidation period,the alloy oxidizes to form a large number of Al_2O_3 and a little Fe_2O_3,then,the enrichment of Al caused by Fe oxidization combines with O to form Al_2O_3.     

Temperature characteristics of electrical behavior of W-Bi-Ti-O ceramics at low field

Bi2WTi3O12 ceramics are fabricated by the conventional solid-state reaction process. With increasing temperature the sample first has metallic behavior, then strong electrical fluctuations above 100℃, and finally exhibits stable nonlinear properties characterized by semiconductivity above 300℃ at low field (E≤100 V/mm). The Arrhenius law for electrical conductivity by thermal activation is not suitable to explain the anomalous results. XRD analysis reveals that Bi2WO6 is the main phase and Bi4Ti3O12 is the second phase. Based on the phase transition of tungsten trioxide from room temperature to about 300℃, the electrical properties of Bi2WTi3O12 ceramics can be explained.     

Characterization and adsorption behavior of a novel triolein-embedded activated carbon composite adsorbent

A novel triolein-embedded activated carbon composite adsorbent was developed. Experiments were carried out in areas such as the preparation method, the characterization of physicochemicai properties, and the adsorption behavior of the composite adsorbent in removing dieldrin from aqueous solution. Results suggested that the novel composite adsorbent was composed of the supporting activated carbon and the surrounding triolein-embedded cellulose acetate membrane. The adsorbent was stable in water, for no triolein leakage was detected after soaking the adsorbent for five weeks. The adsorbent had good adsorption capability to dieldrin, which was indicated by a residual dieldrin concentration of 0.204μg·L^-1. The removal efficiency of the composite adsorbent was higher than the traditional activated carbon adsorbent.     

Hot-corrosion behavior of Cr2O3-CNT-coated ASTM-SA213-T22 steel in a molten salt environment at 700℃

The present work investigates the hot-corrosion behavior of carbon nanotube (CNT)-reinforced chromium oxide coatings on boiler steel in a molten salt (Na₂SO₄-60wt%V₂O₅) environment at 700℃ under cyclic conditions. The coatings were deposited via the high-velocity oxygen fuel process. The uncoated and coated steel samples were subjected to hot corrosion in a silicon tube furnace at 700℃ for 50 cycles. The kinetics of the corrosion behavior was analyzed through mass-gain measurements after each cycle. The corrosion products were analyzed by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray analysis techniques. The results revealed that uncoated steel suffered spallation of scale because of the formation of nonprotective Fe₂O₃ scale. The coated steel samples exhibited lower mass gains with better adhesiveness of oxide scale with the steel alloy until the end of exposure. The CNT-reinforced coatings were concluded to provide better corrosion resistance in the hot-corrosion environment because of the uniform dispersion of CNTs in the coating matrix and the formation of protective chromium oxides in the scale.