Surface oxidation behavior of Mg-Y-Ce alloys at high temperature

After the Mg-Y-Ce magnesium alloy was exposed to air at a temperature up to 1173 K for 0.5 hours, the dense and compact oxide film formed on the surface. Accordingly, oxidation and ignition of magnesium alloys at elevated temperature was successfully retarded by the Y and Ce additions. Thermogravimetric measurements in air revealed that the oxidation dynamics curves measured at 673 and 773 K followed the parabolic-line law and the curve investigated at 873 K followed the complicate quartic law. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis indicated that the oxide film on the surface of Mg-Y-Ce alloys exhibited a duplex structure, which agreed with the results of thermodynamic analysis. The oxidation film included two layers: the outer layer was a multiple structure of Ce_0.202Y_0.798O_1.601 and Y₂O₃, and the inner layer mainly consisted of metal Mg and MgO.     

Rare earth application for heat-resisting alloys

High-temperature oxidation resistance of Al₂O₃- and Cr₂O₃-forming heat-resisting alloys with rare earths (yttrium-implanted FeCrAl, -added FeCrAl, -added FeCrAlPt alloys, Y₂O₃- or CeO₂-coated NiCrSi, yttrium- or lutetium-added NiCr and NiCrSi) was studied in oxygen at high temperatures, by mass gain measurements, mass change measurements, amount of spalled oxide, observation of surface appearance, X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe X-ray microanalysis (EPMA) and transmission electron microscopy (TEM). After oxidation at 1573 K for 18 ks in oxygen, oxide scale on FeCrAl alloy spalled from the entire surface, however, yttrium-implanted FeCrAl alloys showed good oxide adherence. After oxidation at 1473 K for 18 ks in oxygen, mass gain of FeCrAlY alloys decreased with increasing yttrium of up to 0.1 wt.% follwed by an increase with the yttrium content, and the mass gain of FeCrAl0.005Pt0.05Y alloy with appropriate additions of platinum and yttrium was lower than that of FeCrAl0.1Y alloy. Yttrium-added FeCrAl alloys showed good oxide adherence. TEM analysis revealed that the alumina/alloy interface of FeCrAl0.005Pt0.05Y alloy showed good coherency. The scale surface of FeCrAl alloy was rough, however, those of FeCrAlY and FeCrAlPtY alloys were smooth. Cyclic oxidation of NiCrSi, Y₂O₃- or CeO₂-coated NiCrSi alloys was studied up to 10 cycles (1 cycle: 300 s) at 1523 K in oxygen. Mass change of NiCrSi alloy increased up to 3 cycles and then decreased up to 10 cycles because of oxide spallation during cooling. On the other hand, mass change of Y₂O₃- or CeO₂-coated NiCrSi alloy increased up to 10 cycles, and these alloys showed good oxide adherence. Granular Cr₂O₃ particles on Y₂O₃-coated NiCrSi alloy were in size smaller than these on CeO₂-coated NiCrSi alloy. This result suggested that oxidation rate of Y₂O₃-coated NiCrSi alloy was lower than that of CeO₂-coated NiCrSi alloy. After oxidation at 1473 and 1573 K for 18 ks in oxygen, mass gain of yttrium- or lutetium-added NiCr and NiCrSi alloys decreased. Oxide scales on NiCrSi alloy markedly spalled along with alloy grain boundaries during cooling. On the other hand, yttrium- or lutetium-added NiCrSi alloys showed good oxide adherence. Granular Cr₂O₃ particles on yttrium- or lutetium-added NiCr and NiCrSi alloys decreased in size with increasing yttrium or lutetium, and increased with increasing oxidation temperature.     

A Study on the Oxidation Behavior of Nb Alloy (Nb-1 pct Zr-0.1 pct C) and Silicide-Coated Nb Alloys

In the current work, silicide coatings were produced on the Nb alloy (Nb-1 pct Zr-0.1 pct C) using the halide activated pack cementation (HAPC) technique. Coating parameters (temperature and time) were optimized to produce a two-layer (Nb₅Si₃ and NbSi₂) coating on the Nb alloy. Subsequently, the oxidation behavior of the Nb alloy (Nb-1 pct Zr-0.1 pct C) and silicide-coated Nb alloy was studied using thermogravimetric analysis (TGA) and isothermal weight gain oxidation experiments. Phase identification and morphological examinations were carried out using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. TGA showed that the Nb alloy started undergoing accelerated oxidation at and above 773 K (500 °C). Isothermal weight gain experiments carried out on the Nb alloy under air environment at 873 K (600 °C) up to a time period of 16 hours exhibited a linear growth rate law of oxidation. In the case of silicide-based coatings, TGA showed that oxidation resistance of silicide coatings was retained up to 1473 K (1200 °C). Isothermal weight gain experiments on the silicide coatings carried out at 1273 K (1000 °C) in air showed that initially up to 8 hours, the weight of the sample increased, and beyond 8 hours the weight of the sample remained constant. The oxide phases formed on the bare samples and on the coated samples during oxidation were found to be Nb₂O₅ and a mixture of SiO₂ and Nb₂O₅ phases, respectively. SEM showed the formation of nonprotective oxide layer on the bare Nb alloy and a protective (adherent, nonporous) oxide layer on silicide-coated samples. The formation of protective SiO₂ layer on the silicide-coated samples greatly improved the oxidation resistance at higher temperatures.     

阻燃镁合金ZM5-0.1RE阻燃机理的研究

在ZM5镁合金中添加0.1％混合稀土时,阻燃效果最佳,镁合金起燃温度提高到800℃以上。X射线衍射分析（XRD）和扫描电镜分析（SEM＆EDS）表明其阻燃机理是在合金液表面形成致密的RE2O3和MgO复合氧化膜,阻止了镁合金的进一步氧化燃烧。高温氧化动力学研究显示：该合金在400℃和700℃氧化符合抛物线规律,而在600℃氧化遵循立方规律。     

Structural Studies of Dispersoids in Fe–15 wt% Y<Subscript>2</Subscript>O<Subscript>3</Subscript>–5 wt% Ti Model ODS Alloys During Milling and Subsequent Annealing

Oxide dispersion strengthened (ODS) steels have very high thermal stability and creep resistance due to reinforcement of hard and stable nano-sized ceramic dispersoids in metallic matrix which act as barriers to dislocation motion. This study established the role of Ti in the structural evolution of yttria during mechanical milling and subsequent annealing in a Fe–15 wt% Y₂O₃–5 wt% Ti model ODS alloy, using electron microscopy and XRD techniques. The alloy was synthesized in a high energy planetary ball mill in Ar atmosphere by varying the milling durations in the range of 0 (un-milled) to 60 h. The XRD result revealed amorphisation of Y₂O₃/Ti during milling and evolution of YTiO₃ complex oxide upon annealing at 1273 K for 1 h. The electron microscopy studies revealed the refinement of alloy powders from ~50  μm to few nanometers during milling. Electron diffraction analysis and high resolution transmission electron microscopy of 60 h milled as well as and annealed powder showed formation of different types of Y–Ti–O complex oxides such as Y₂Ti₂O₇, Y₂TiO₅ and YTiO₃.     

Diffusion of oxygen in the Al2O3 oxidation product of TiAl3

The oxidation behavior of TiAl₃ was investigated. The studies were carried out using thermogravimetric analysis (TGA) in the temperature range from 1123 to 1273 K in a 1 atm pure oxygen environment. Samples were analyzed using the X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersion X-ray analysis techniques. The oxidation product was determined to be Al₂O₃ over the temperature range of the investigation. The parabolic-rate constant for TiAl₃ was deduced and compared with those for Ti, TiAl, and Ti₃Al. The electronprobe microanalysis technique was used to obtain concentration profiles for O, Al, and Ti in the oxide layer and the matrix alloy. The parabolic-rate data were used to calculate the diffusivities of oxygen at various temperatures. The activation energy for diffusion was determined to be 337.66 kJ/mol, while the frequency factor (D ₀) was 167.2 × 10⁻⁴ m²/s.     

Effect of Ca and Y additions on oxidation behavior of magnesium alloys at high temperatures

Oxidation and ignition of magnesium alloys at elevated temperature were successfully retarded by additions of Y and Ca,which could be melted at 1173 K in air without any protection.Thermogravimetric measurements in dry air revealed that the oxidation dynamics curves of Mg-2.5Ca alloy and Mg-3.5Y-0.79Ca alloy at high temperatures followed the parabolic-line law or the cubic-line law.X-ray diffraction(XRD) and scanning electron microscopy(SEM) analysis indicated that the oxide film on the surface of Mg-3.5Y-0.79Ca and Mg-2.5Ca alloys exhibited a duplex structure,which agreed with the results of thermodynamic analysis.By comparison,the ignition-proof effect of the combination addition of Y and Ca was better than that of the single addition of Ca.     

Corrosion Behavior of Alloy 625 in PbSO4-Pb3O4-PbCl2-ZnO-10?Wt?Pct CdO Molten Salt Medium

Corrosion behavior and degradation mechanisms of alloy 625 under a 47.288 PbSO₄-12.776 Pb₃O₄-6.844PbCl₂-23.108ZnO-10CdO (wt pct) molten salt mixture under air atmosphere were studied at 873?K, 973?K, and 1073?K (600?°C, 700?°C, and 800?°C). Electrochemical impedance spectroscopy (EIS), open circuit potential (OCP) measurements, and potentiodynamic polarization techniques were used to evaluate the degradation mechanisms and characterize the corrosion behavior of the alloy. Morphology, chemical composition, and phase structure of the corrosion products and surface layers of the corroded specimens were studied by scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) and X-ray map analyses. Results confirmed that during the exposure of alloy 625 to the molten salt, chromium was mainly dissolved through an active oxidation process as CrO₃, Cr₂O₃, and CrNbO₄, while nickel dissolved only as NiO in the system. Formation of a porous and nonprotective oxide layer with low resistance is responsible for the weak protective properties of the barrier layer at high temperatures of 973?K and 1073?K (700?°C and 800?°C). There were two kinds of attack for INCONEL 625, including general surface corrosion and pitting. Pitting corrosion occurred due to the breakdown of the initial oxide layer by molten salt dissolution of the oxide or oxide cracking.     

Effect of the method of introduction of Y2O3 into NiAl-based powder alloys on their structure: I. Agitation in a ball mill

The effect of the sintering temperature (1100–1400°C) of NiAl alloy samples with oxide Y₂O₃ produced by hydrostatic pressing on their structure and phase composition and the distribution of oxide particles in a NiAl-based intermetallic matrix alloyed with ～0.5 at % Fe is considered. It is found that dispersed oxide particles in the compact material prepared from a mixture of oxide Y₂O₃ powder and a NiAl alloy (produced by calcium hydride reduction of a mixture of nickel and aluminum oxides) powder in a standard ball mill are nonuniformly distributed in the volume. The morphology of oxides changes during sintering: sintered samples contain rounded particles, which differ strongly from the clearly faceted angular particles of oxide Y₂O₃ added to a mixture (they represent conglomerates of single crystals). In the sintered samples, large aggregates of oxides are revealed along grain boundaries. Mass transfer is possible at the NiAl/Y₂O₃ interface in the system: it leads to partial substitution of aluminum and/or iron atoms for yttrium atoms in the Y₂O₃ lattice and to the formation of submicroscopic particles of (Fe,Al)₅Y₃O₁₂-type oxides.     

Oxide Scales Formed on NiTi and NiPtTi Shape Memory Alloys

Ni-49Ti and Ni-30Pt-50Ti (nominal at. pct) shape memory alloys (SMAs) were isothermally oxidized in air over the temperature range of 773?K to 1173?K (500?°C to 900?°C) for 100?hours. The oxidation kinetics, presented in detail in a companion study, show ~4 times reduction in oxidation rate due to Pt.[1] The microstructure, composition, and phase content of the scales and depletion zones were determined by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). A relatively pure TiO₂ rutile structure was identified as the predominant scale surface feature, typified by a distinct highly striated and faceted crystal morphology, with crystal size proportional to oxidation temperature. The complex layered structure beneath these crystals was characterized by semiquantitative XRD of serial/taper polished sections and SEM/EDS of cross sections for samples oxidized at 973?K (700?°C). In general, graded mixtures of TiO₂, NiTiO₃, NiO, Ni(Ti), or Pt(Ni) metallic dispersoids, and continuous Ni₃Ti or Pt-rich metal depletion zones, were observed from the gas surface to the substrate interior. Overall, substantial depletion of Ti occurred due to the formation of predominantly TiO₂ scales. It is proposed that the Ni-30Pt-50Ti alloy oxidized more slowly than the binary Ni-49Ti alloy by decreasing oxygen and titanium diffusion through the thin Pt-rich layer.     

The oxidation of llmenite and its relationship to the FeO-Fe2O3-TiO2 phase diagram at 1073 and 1140 K

The oxidation of synthetic ilmenite was investigated in the temperature range of 1048 to 1273 K using a continuous flow electrobalance, X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The phases observed and their relative distribution in partially oxidized samples were related to the FeO-Fe₂O₃-TiO₂ phase diagrams at 1073 and 1140 K. Initial stages of oxidation were characterized by the formation of rutile and ilmenite-hematite solutions containing up to 10 mole pct hematite at 1073 K and 7 mole pct at 1140 K. Final stages of oxidation included the formation of ferropseudobrookite-pseudobrookite solutions containing up to 56 mole pct ferropseudobrookite at 1073 K and up to 66 mole pct ferropseudobrookite at 1140 K. Although not all of the predicted phase fields were observed during oxidation, generally the expected oxidation sequences were followed.     

Oxidation Behavior of NiAl-30.75Cr-3Mo-0.25Ho Alloy at High Temperatures

The oxidation behavior of NiAl-30.75Cr-3Mo-0.25Ho alloy from 1300 to 1500 K in air atmosphere was investigated. The results reveal that oxidation resistance of the alloy is improved by the addition of Ho. At 1500 K, the oxidation kinetic curve obeys the parabolic law (n≈0.5), whereas the oxidation kinetic curve of the tested alloy follows the cubic relations (n≈0.3～0.4) from 1300 to 1450 K. An activation energy of about 261 kJ·mol-1 was determined for the tested alloy. It is found that a continuous and compact Al2O3 layer has formed on the surface of NiAl-30.75Cr-3Mo-0.25Ho alloy after oxidation 100 h at various tested temperatures. A rich-Ho solution formed on the boundaries of Cr(Mo) phase. Doped little amount of Ho in NiAl-31Cr-3Mo alloy promotes the transformation from θ-Al2O3 phase to α-Al2O3 phase, and decreases the size of Al2O3 and the crack forming in the oxidation scale prolongs the spalling time of the film at high temperature. The volatilizing oxides of Cr, Mo and the reactive element effects (REEs) make the mass gain lower than that of pure NiAl.     

Oxidation of an Fe-Cr-Al-Y-Zr alloy forming an alumina scale in a Ga<Subscript>2</Subscript>O atmosphere at 1473 K

The oxidation of an Fe-Cr-Al-Y-Zr alloy exposed to a Ga atmosphere at 1473 K has been investigated by an encapsulation method. A Ga/Ga₂O₃ mixture was placed within the quartz ampoules to establish the Ga₂O and the O₂ partial pressures enveloping a Fe-Cr-Al-Y-Zr alloy (specifically, 72.8Fe-22Cr-5Al-0.1Y-0.1Zr by mass pct), which contains minor additions of yttrium and zirconium to improve scale adherence. A deoxidized Ar atmosphere surrounded the ampoules to control oxygen permeation through the silica wall. The Fe-Cr-Al-Y-Zr alloy developed an Al₂O₃ scale growing parabolically and protecting the substrate from gallium attack for 5, 10, 20, 40, and 50 days, as examined with a scanning electron microscope (SEM) and energy dispersive X-ray (EDX) analysis. The Ga₂O atmosphere decomposes to liquid Ga and Ga₂O₃ on the alloy surface, but gallium species do not penetrate the oxide layer according to the limits of detection for EDX analysis. Defect equilibria incorporating Ga₂O are presented with neutral interstitial oxygen to explain the oxidation of the Fe-Cr-Al-Y-Zr alloy.     

Isothermal Grain Growth Studies on Nanostructured 9Cr-1Mo and 9Cr-1W Ferritic Steels Containing Nano-sized Oxide Dispersoids

Analysis of isothermal grain growth kinetics of nanocrystalline Fe-9Cr-1Mo and Fe-9Cr-1W-based ferritic oxide dispersion strengthened alloys is reported. Fe-9Cr-1Mo-0.25Ti-0.5Y₂O₃ alloy exhibited ~900 and ~250 pct enhancement in grain-coarsening resistance at 1073 K (800 °C) in comparison with Fe-9Cr-1Mo-0.5Y₂O₃ alloy and Fe-9Cr-1W-0.5Y₂O₃ alloy, respectively. Comparison of grain growth time exponents also revealed that addition of Ti and Y₂O₃ to nanocrystalline Fe-9Cr alloy has significantly enhanced the grain growth resistance. This is attributed to the possible presence of Y-Ti-O-based nanoclusters (<5 nm).     

Effect of yttrium,calcium and zirconium on ignition-proof principle and mechanical properties of magnesium alloys

Good ignition-proof principle and mechanical properties were realized in Mg-Y-Ca-Zr alloy system.By adding Y and Ca elements,the ignition point of Mg-3.5Y-0.8Ca alloy was improved to over 1173 K,and the alloy could be melted in air without any protections.The ef-fect of Zr addition on the microstructures and mechanical properties of Mg-3.5Y-0.8Ca alloys were investigated,and Mg-3.5%Y-0.8%Ca-0.4%Zr alloy had good comprehensive properties with tensile strength of 190 MPa and elongation of 11%.Auger electron spectros-copy(AES) and X-ray diffraction(XRD) analysis revealed that the oxide film formed on the surface of Mg-3.5Y-0.8Ca alloy was mainly composed of Y2O3.Thermogravimetric measurements in dry air indicated that the oxidation dynamics curves measured at 773,873 and 973 K followed the cubic law.Moreover,the semiconductor characteristic of Y2O3 film and its effect on ignition-proof properties of Magnesium al-loys were discussed from the viewpoint of electrochemistry.     

The effect of dispersed reactive metal oxides on the oxidation resistance of nickel-20 Wt pct chromium alloys

Nickel-20 wt pet chromium alloys containing ThO₂, Y₂O₃, La₂O₃, Al₂O₃ and Li₂O, as prepared by the mechanical alloying technique, were examined for isothermal and cyclic oxidation resistance in dry air at 1000, 1100 and 1200°C. TDNiCr, a commercial electrical heating element alloy (Com Ni-20Cr) and a laboratory melted alloy⁹Lab Ni-20Cr) were also tested. It was found that Y₂O₃, La₂O₃, Al₂O₃ and ThO₂ dispersoids markedly increased both isothermal and cyclic oxidation resistance compared to Lab Ni-20Cr at all temperatures; in contrast Li₂O additions gave no improvement in protection. Com Ni-20Cr was in between Lab Ni-20Cr and the Y₂O₃, A1₂O₃ and ThO₂ containing alloys in both cyclic and isothermal oxidation performance. A mechanism based on alterations in the defect structure of Cr₂O₃ is proposed to explain these dispersed oxide effects on isothermal oxidation behavior. It is based on a reduction in cation transport rates which in turn alter the rate of oxide growth. ThO₂-containing alloys fabricated by the mechanical alloying technique were found to have oxidation resistance fully equal to commercial TDNiCr. Com Ni-20Cr performed better than Lab Ni-20Cr, but not as well as TDNiCr.     

Oxidation behaviour of selected Ni- and Fe- based superalloys in air at 900°C under cyclic conditions

Oxidation behavior of Ni- and Fe- based superalloys has been studied in air at 900°C under cyclic conditions. The mass change measurement with time was used to establish the oxidation kinetics. The oxide scales formed on the surface of the superalloys were characterised by various techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDAX) analyses. The oxide scales formed on the superalloys contained NiO, Cr₂O₃ and Fe₂O₃. The scales were adherent and only microspalling was observed. Superfer 800 has provided best resistance to oxidation among the alloys studied.     

A New Oxide Morphology Map: Initial Oxidation Behavior of Ni-Base Single-Crystal Superalloys

Predictions for oxidation behavior of Ni-base superalloys become more difficult than before because of the complex alloy composition. In this study, we focus on the initial oxidation behavior of Ni-base superalloys, and we suggest a new diagram to predict the initial oxide morphology of Ni-base superalloys with 63 binary, ternary, and multicomponent Ni-base single-crystal superalloys at 1373 K (1100 °C). As a comparison of observed and calculated weight changes after one cycle at 1373 K (1100 °C) obtained by a regression analysis, 63 alloys demonstrated two distinct behaviors, which are divided heretofore into group A and group B. Microstructural observation revealed that an oxide layer in the group A alloys consists of Al₂O₃ and/or spinel or complex oxide, whereas an oxide layer in the group B alloys consists of a thick NiO layer with an Al₂O₃ internal subscale. Thermodynamic properties can reflect more effects of alloy elements in Ni-base superalloys, and Al and Cr activities, calculated by Thermo-Calc, were used as factors to predict initial oxidation morphology. Groups A and B alloys can clearly be divided according to Al and Cr activities. This was suggested as a new diagram to predict the initial oxide morphology of Ni-base superalloys, and possibly it can apply for any generation of Ni-base superalloys.     

Oxidation Studies of SiAlON/MgAlON Ceramics with Fe2O3 and CaO Impurities, Part II: Phase Evolution

The oxidation behavior of composite SiAlON/MgAlON phases, synthesized from the leaching residue after the aqueous treatment of salt cake from aluminum remelting, is compared with the oxidation of corresponding synthetic samples. The samples were subjected to oxidation under air as the oxidant atmosphere in the temperature range of 1373 K to 1773 K (1100 °C to 1500 °C). The phases present were analyzed by scanning electron microscopy (SEM)-electron-dispersive spectroscopy (EDS) and X-ray diffraction (XRD) to arrive at the evolution of the various phases formed during oxidation. From the experimental results, especially by the characterization of the oxidation products, the mechanism of the oxidation reaction was deduced as follows: With the progress of oxidation, the composition of the material being oxidized moved toward the Al₂O₃-rich corner of MgO-Al₂O₃-SiO₂ and CaO-MgO-Al₂O₃-SiO₂ phase diagrams relevant to the SiAlON/MgAlON composite. At lower temperatures, the addition of Fe₂O₃ and CaO facilitated the formation of cordierite and anorthite, respectively. With increasing temperature, islands of silicate melt were formed dissolving these oxides, with the liquidus temperature getting lowered as a consequence. The liquid phase formed engulfed the adjacent solid phases providing strong mobility for the cations and enabling the crystal growth. As a result, intermediate products, i.e., cordierite, anorthite, and spinel, which were formed earlier during oxidation, are found to get dissolved in the liquid phase.