The influence of grain-boundary segregation of Y in Cr2O3 on the oxidation of Cr metal

The oxidation behavior at 900°C of pure Cr and Cr implanted with 2×10¹⁶ Y ions/cm² was studied. The kinetics of oxidation were measured thermogravimetrically and manometrically. The mechanisms of oxide growth were studied using¹⁸O-tracer oxidation experiments, and the composition and microstructure of the oxide scales were characterized by TEM and STEM. Segregation of Y cations at Cr₂O₃ grain boundaries was found to be the critical factor governing changes in the oxidation behavior of Cr upon the addition of Y. In the absence of Y, pure Cr oxidized by the outward diffusion of cations via grain boundaries in the Cr₂O₃ scale. When Y was present at high concentration in the scale, as when Cr implanted with 2×10¹⁰ Y ions/cm² was oxidized, anion diffusion predominated. It is concluded that strain-induced segregation of Y at grain boundaries in the oxide reduced the cation flux along the grain boundaries. The rate of oxidation was reduced because the grain-boundary diffusivity of cations became lower than the grain-boundary diffusivity of the anions, which then controlled the rate of oxidation. Changes in the relative rates of Cr³⁺ and O^2– transport, as well as a solute-drag effect exerted by Y on the oxide grain boundaries, resulted in changes in the microstructure of the oxide.     

The influence of reactive-element coatings on the high-temperature oxidation of pure-Cr and high-Cr-content alloys

The influence of various reactive-element (RE) oxide coatings (Y₂O₃, CeO₂, La₂O₃, CaO, HfO₂, and Sc₂O₃) on the oxidation behavior of pure Cr, Fe–26Cr, Fe–16Cr and Ni–25Cr at 900°C in O₂ at 5×10^–3 torr has been investigated using the¹⁸O/SIMS technique. Polished samples were reactively sputtercoated with 4 nm of the RE oxide and oxidized sequentially first in¹⁶O₂ and then in¹⁸O₂. The effectiveness of each RE on the extent of oxidation-rate reduction varied with the element used. Y₂O₃ and CeO₂ coatings were found to be the most beneficial, whereas Sc₂O₃ proved to be ineffective, for example, for the oxidation of Cr. SIMS sputter profiles showed that the maximum in the RE profile moved away from the substrate-oxide interface during the early stages of oxidation. After a certain time the RE maximum remained fixed in position with respect to this interface, its final relative position being dependent on the particular RE. The position of the RE maximum within the oxide layer also varied with the substrate composition. For all coatings¹⁸O was found to have diffused through the oxide to the substrate-oxide interface during oxidation, the amount of oxide at this interface increasing with increasing time. The SIMS data confirm that for coated substrates there has been a change in oxidegrowth mechanism to predominantly anion diffusion. The RE most probably concentrates at the oxide grain boundaries, generally as the binary oxide (RE) CrO₃. Cr³⁺ diffusion is impeded, while oxygen diffusion remains unaffected.     

Effect of a marker material on the oxidation of nickel in sulfur dioxide

A marker study of nickel oxidation in SO₂ at 600°C has proved that after the reaction metallic markers (Au, Pt, W) are covered with a sulfide rim. This effect is not observed on quartz markers. The metallic markers make the intermediate NiO layer adjacent to them increase, whereas the quartz marker makes this layer disappear.     

The influence of grain-boundary segregation of Y in Cr2O3 on the oxidation of Cr metal. II. Effects of temperature and dopant concentration

The oxidation behavior of pure Cr and Cr implanted with Y was studied as a function of temperature (900 and 1025°C) and ion-implantation dose (1×10¹⁵ and 2×10¹⁶ Y ions/cm²). The microstructures of the Cr₂O₃ scales were affected by both of the variables studied. Yttrium ions segregated at the grain boundaries in the Cr₂O₃ scales formed on the implanted alloys and the concentration of Y at the grain boundaries decreased with a decrease in the dose of implanted Y. The mechanism of growth of the Cr₂O₃ scales was altered by the presence of the Y ions at the Cr₂O₃ grain boundaries only when a critical concentration of Y at the grain boundaries was exceeded.     

The effect of particle size of alumina dispersions on the oxidation resistance of Ni-Cr alloys

The oxidation behavior of Ni-Cr alloys with various chromium concentrations and particle sizes of a dispersion of 10 vol.% Al₂O₃ was observed in 1 atm of oxygen at 1000°C. This study was intended to determine the critical chromium concentration to form a protective Cr₂O₃ oxide layer for different Al₂O₃ particle sizes. The oxidation rate of Ni-Cr alloys containing 10 vol.% Al₂O₃ followed a parabolic rate law and a Cr₂O₃ protective layer continuously formed when the oxidation rate decreased rapidly. Times to form a continuous and protective Cr₂O₃ layer during the initial oxidation shortened as the size of the dispersion decreased. The critical chromium concentration to form a protective Cr₂O₃ layer in the oxide scale was 69 wt.% and was related strongly to the particle size of the Al₂O₃ dispersion.     

Characterisation of tin–copper intermetallic growth in electrodeposited tin coatings using electrochemical oxidation techniques

In the present investigations electrochemical oxidation has allowed for the removal of tin from coated copper substrates permitting scanning electron microscopy to be employed to assess intermetallic compound distribution and morphology. It was shown that the initial growth of intermetallic compound was not uniform, with its distribution dependent upon the grain structure of the copper substrate, and the grain orientation and distribution within the tin coating. After a period of further nucleation and coarsening, the intermetallic compound forms an almost complete layer along the interface. It is thought this uniform layer of intermetallic compound, formed soon after deposition, may be responsible for this specific electrodeposited tin system having a low whiskering propensity.     

ZrB2-玻璃陶瓷复合材料氧化分析

对ZrB₂-玻璃陶瓷复合材料氧化行为进行热力学分析,对氧化形成的氧化层进行物相分析和显微结构分析。结果表明：在1000°C-1400°C的反应温度范围内,ZrB₂氧化生成ZrO₂,B₂O₃玻璃相,氧化产物ZrO₂与SiO₂反应生成ZrSiO₄,当温度低于1177°C(1450K)时,氧化层主要包括ZrO₂,B₂O₃玻璃相,ZrSiO₄。当氧化温度超过1177°C(1450K)时,B₂O₃玻璃相蒸发,此时SiO₂玻璃相具有良好的流动性,氧化层主要包括ZrO₂,SiO₂玻璃相,ZrSiO₄。氧化过程中的反应产物B₂O₃玻璃相,ZrSiO₄和流动性良好的SiO₂玻璃相,均对氧气向基体的扩散均起到了良好的阻碍作用。     

The effect of fe on high temperature oxidation of NiAl

A study was conducted to observe the oxidation of NiAl+3.5at.%Fe alloy inβ-NiAl phase field at air temperatures of 1000, 1200 and 1400°C, and these results were compared with those of pure NiAl alloys. The primary effects of the Fe-addition in NiAl were found to be: 1) decrease in oxidation resistance and adherence of scales during both isothermal and cyclic oxidation tests, 2) enhancement of phase transformation rate from θ toα-AL₂O₃, 3) more rapid formation of characteristically ridgedα-A1₂O₃ scales during initial oxidation stages, and 4) partial sealing of voids formed at the scale-substrate interface and dissolution of Fe inside the alumina scale by the outward diffusion of Fe from the substrate alloy.     

均匀化退火处理对AZ91镁合金轧制成形行为的影响

为改善铸态AZ91镁合金组织不均匀性,提高其轧制成形能力,本文研究了均匀化退火处理对AZ91镁合金轧制变形前后微观组织及力学性能的影响。实验结果表明：均匀化退火处理可以有效改善合金组织中第二相分布;经400℃、多道次轧制后,沿晶界附近分布的细长条状Mg₁₇Al₁₂相数量显著减少,部分脆性Mg₁₇Al₁₂相发生断裂,以小颗粒状弥散分布于晶界附近和基体内部。均匀化后轧制组织比原始轧制组织强度略有提高,而伸长率提高达50%。轧制后的拉伸断口形貌也显示合金塑性得到明显改善。这为后续进一步研究AZ91镁合金在不同工艺参数条件下的轧制成形奠定基础。     

Some aspects of the mechanism of high-temperature oxidation of nickel in SO2

A number of investigations on the mechanism of reaction of nickel with SO₂ has been summarized. The calculation results of the equilibrium gas composition in homogeneous SO₂+O₂ mixtures are described over wide ranges of temperatures (500–1100°C) and initial gas compositions. The Ni–O–S phase diagram at 540°C has been compared with data on the stability of interaction products under conditions close to equilibrium. The catalytic activity of NiO has been verified to accelerate the attainment of thermodynamic equilibrium in the SO₂–O₂–SO₃ system. The most effective catalytic activity of NiO occurred at 650–800°C. A monolayer (6 Å) of NiSO₄ was detected on the scale surface by ESCA. This surface phase is assumed to be formed either as an activated complex on the NiO catalyst or as the locally stable NiSO₄ phase. Both assumptions lead to a possible recognition of the sulfate intermediate mechanism.     

The high-temperature oxidation behavior of vanadium-aluminum alloys

The oxidation behavior in air of pure vanadium, V-30Al, V-30Al-10Cr, and V-30Al-10Ti (weight percent) was investigated over the temperature range of 700–1000° C. The oxidation of pure vanadium was characterized by linear kinetics due to the formation of liquid V₂O₅ which dripped from the sample. The oxidation behavior of the alloys was characterized by linear and parabolic kinetics which combined to give an overall time dependence of 0.6–0.8. An empirical relationship of the form: W/A=Bt + Ct^1/2 + D was found to fit the data well, with the linear contribution suspected to be from V₂O₅ formation for V-30Al and V-30Al-10Cr, and a semi-liquid mixture of V₂O₅ and Al₂O₃ for V-30Al-10Ti. The parabolic term is presumed related to the formation of a solid mixture of V₂O₅ and Al₂O₃ for V-30Al and V-30Al-10Cr, and TiO₂ for V-30Al-10TiThe addition of aluminum was found to reduce the oxidation rate of vanadium, but not to the extent predicted by the theory of competing oxide phases proposed by Wang, Gleeson, and Douglass. This was attributed to the formation of a liquid-oxide phase in the initial stages of exposure from which the alloys could not recover. Ternary additions of chromium and titanium were found to decrease the oxidation rate further, with chromium being the most effective. The oxide scales of the alloys were found to be highly porous at 900° C and 1000° C, due to the high vapor pressure of V₂O₅ above 800° C.     

Sulfide-forming features during oxidation of predeformed nickel in SO2

Loaded parts are exposed to hot corrosion to a greater extent than unloaded components. Pure nickel predeformed to various degrees by compression (up to 27%) has been oxidized in SO₂ at 600°C for different periods (22 to 95 hr). It has been shown that transport properties of the scale, formed on the initial metal surface containing physical defects, depend on their surface density. A general behavior was established for the same exposure (>70 hr): the higher the preliminary strain, the greater the amount of Ni₃S₂ in the scale. Nickel predeformed 21% and 27%, oxidized in SO₂ over 70 hr, formed scales consisting mainly of a single-phase Ni₃S₂ layer. An increase of the scale defectiveness accelerated attainment of heterogeneous equilibrium in a gas-scale system and intensified the formation of Ni₃S₂—the stable phase for the conditions used.     

The effect of aluminum as an alloying addition or as an implant on the high-temperature oxidation of Ni-25Cr

The oxidation behavior of aluminum-implanted Ni-25Cr and Ni-25Cr containing 1 wt.% Al has been studied at 1000°C and 1100°C in oxygen. As did Y alloying addition or Y-implantation, 1 wt.% Al added to Ni-25Cr prevented nodular formation of Ni-containing oxides, improved spalling resistance of the scale upon cooling to a similar degree, and eliminated the formation of large voids between the alloy and the scale at the oxidation temperature. However, the Al addition did not alter the rate of growth of the Cr₂O₃ scale, nor did it change the growth direction. Al-implantation produced no effect even when the maximum concentration and depth of penetration were adjusted to be identical with those of the yttrium in the Y-implanted alloy. The implications of these results concerning the reactive element effect are discussed.     

The oxidation behavior of Cu–Zr–Ti–base bulk metallic glasses in air at 350–500 °C

The oxidation behavior of two Cu-base bulk metallic glasses (BMGs), having compositions Cu–30Zr–10Ti and Cu–20Zr–10Ti–10Hf (in at.%), was studied over the temperature range of 350–500 °C in dry air. In general, the oxidation kinetics of both BMGs followed the parabolic rate law, with the oxidation rates increasing with increasing temperature. The addition of Hf slightly reduced the oxidation rates at 350–400 °C, while the opposite results observed at higher temperatures. It was found that the oxidation rates of both BMGs were significantly higher than those of polycrystalline pure-Cu. The scales formed on both BMG alloys were strongly composition dependent, consisting of mostly CuO/Cu₂O and minor amounts of cubic-ZrO₂ and ZrTiO₄ for the ternary BMG, and of CuO, cubic-ZrO₂, and Zr₅Ti₇O₂₄ for the quaternary BMG. The formation of ternary oxides (ZrTiO₄ and Zr₅Ti₇O₂₄) was inferred to be responsible for the fast oxidation rates of the BMGs.     

High temperature oxidation of chromium nitrides

Chromium nitride powders oxidized to Cr₂O₃ noticeably above 400°C. Bulk chromium nitrides that were manufactured by sintering were oxidized between 900 and 1100°C in atmospheric air. The Cr₂O₃ layer that formed on bulk chromium nitrides having pores was relatively dense, and grew primarily via the inward transport of oxygen. The Cr₂O₃ layer to some degree deterred the nitrogen evolution from bulk chromium nitrides.     

The air oxidation of an austenitic Fe-Mn-Cr stainless steel for fusion-reactor applications

The oxidation in air of an austenitic Fe-Mn-Cr steel containing 17.8 Mn, 9.5 Cr, 1.0 Ni, 0.27 C, and 0.03 N was studied over the range 700–1000°C. Oxidation of surface-abraded samples at low temperatures, 700–750°C, resulted in only Mn ₂O₃ containing dissolved chromium, except at corners, where large nodules containing spinel and manganowustite formed. The Mn₂O₃ layer grew into the substrate forming a globular-type film. This growth mode was the result of slow interdiffusion in the alloy after the cold-worked surface layer had been recrystallized and/or consumed, as evidenced by the formation of a ferrite layer subjacent to the scale and by the instability of the planar interface. No internal oxidation was observed beneath the Mn₂O₃ film at either 700 or 750°C. Samples oxidized in thehigh-temperature region, 800–1000°C, exhibited vastly different behavior, forming thick stratified scales at long times (24 hr), the scales consisting of a very thin outer layer of Mn₂O₃ (with appreciable iron in solution), Fe-Mn spinel beneath the outer layer, and a thick inner layer of manganowustite and a chromium-containing spinel. No chromium was found in the outer two layers. A thin layer of nearly pure Fe₂O₃ formed between Mn₂O₃ and the outer spinel. Quasiparabolic kinetics were observed. The high-temperature rates were about 10³ to 10⁴ times greater than at low temperatures at the transition temperature. The rapid rates at high temperatures were attributed to manganowustite growth. However, oxidation of an electropolished sample at 750°C, from which the superficial cold-worked layer had been removed, formed scales similar to those observed at high temperatures at comparable rates. A difference by a factor of over 10⁴ existed between the oxidation rate of the electropolished sample and the surface-abraded sample at 750°C. The much slower oxidation rate of the latter is attributed to greatly enchanced manganese diffusion through the high dislocation-density, cold-worked layer. Short-time tests at 800°C revealed an incubation period during which a thin protective layer of Mn²O₃ formed. The incubation period corresponded to the recrystallization time of the cold-worked layer. Subsequently, nodular growth occurred which was associated with internal oxidation. The nodules, consisting of spinel and manganowustite, eventually linked up to form a thick, stratified scale. Comparison of the scale structures with calculated phase diagrams of composition versus oxygen activity (at constant temperature), showed that the protective films formed at low temperatures were due to kinetics factors, involving enhanced manganese diffusion through the cold-worked layer, rather than to thermodynamics. A model for the breakdown of protective films is proposed which involves internal oxidation.     

The combined effect of refractory coatings containing reactive elements on high temperature oxidation behavior of chromia-forming alloys

The high temperature oxidation behaviors of chromia-forming alloys (F17Ti and Fe-30Cr alloys) have been studied at 1273 K under isothermal conditions and at 1223 K under cyclic conditions, in air under the atmospheric pressure. To extend the oxidation lifetime, coatings have been applied onto the alloy surfaces. Al₂O₃ and Cr₂O₃ films doped with Sm₂O₃ or Nd₂O₃ were prepared via the metal-organic chemical vapor deposition technique. Single Cr₂O₃, Al₂O₃, Nd₂O₃ and codeposited Cr₂O₃-Nd₂O₃, Al₂O₃-Nd₂O₃, Al₂O₃-Sm₂O₃ coatings drastically improved the chromia-forming alloy high temperature oxidation behavior, since they decreased the oxidation rate and enhanced the oxide scale adhesion. Results showed that a critical amount of reactive element (Nd or Sm) in chromia or alumina coatings (11-18 at.%) was needed to observe the most effective effect. The fast precipitation of NdCrO₃ or NdTi₂₁O₃₈ and the segregation of reactive elements at the chromia grain boundaries slowing down outward cation transport and consequently blocking the chromia grain growth, was supposed to be the main reasons of the beneficial effect ascribed to the reactive elements in chromia scales.     

Effect of internal oxidation pretreatments and Si contamination on oxide-scale growth and spalling

Internal oxidation pretreatments carried out in quartz capsule with a Rhines pack were found to have a profound effect on the subsequent oxidation behavior of alloys. Specimens of Co-15 wt.% Cr, Co-25 wt.% Cr, Ni-25 wt.% Cr, and Ni-25 wt.% Cr-1 wt.% Al were tested at 1100°C after pre-oxidation treatments. Even without the development of internal oxide particles, pretreated binary CoCr and NiCr alloys oxidized with significantly lower rates. Selective oxidation of chromium was observed on the non-Cr₂O₃-forming Co-base alloys, whereas on the Cr₂O₃-forming Ni-base alloys, elimination of base-metal oxide, reduction in the Cr₂O₃ growth rate, and better scale adhesion were found. These effects were more apparent with pre-oxidation temperatures greater than 1000°C and with longer pretreatment times. Contaimination of Si from the quartz is believed to be the cause.     

The low-temperature oxidation of calcium by water vapor

The oxidation kinetics of calcium in water vapor have been studied over the temperature range 25–300°C. There is a change in the form of the oxidation kinetics with temperature, from essentially linear at temperatures below 150°C to logarithmic at 300°C. This is coupled with a change in the manner of growth of the oxide layer as well as the chemical composition of the reaction product. In addition, the oxidation rate decreases with temperature, reaching a minimum at about 150°C. At temperatures below 150°C oxidation appears to be a result of the formation of cracks or fissures in the oxide film. Above 150°C no single oxidation mechanism can be deduced.     

Effects of sputtering pressure and post-metallization annealing on the physical properties of rf-sputtered Y2O3 films

Y₂O₃ thin films were prepared by rf-sputtering under various sputtering pressures at room temperature. Spectroscopic ellipsometer, X-ray diffraction and semiconductor parameter analyzer were used to characterize the studied films. The results show the crystallinity and leakage current density of the films improved with decreasing sputtering pressure. The effects of post-metallization annealing (PMA) on optical, structural and electrical properties of the films were also evaluated. It is found that PMA can significantly enhance the electrical performance of Y₂O₃ film, and the lowest leakage current is found to be 1.54 × 10⁻⁸ A/cm² at 1 MV/cm for the samples treated at 350 °C for 30 min. The leakage current mechanisms were discussed as well, which reveals that space charge limited current dominates the as-deposited films while Schottky mechanism describes the PMA treated ones well.