Al对Fe-20Cr-35Ni-0.6Nb合金显微组织和抗氧化性的影响

采用增量法研究了不同Al含量(0.5%、1.5%、2.5%,质量分数)的Fe-20Cr-35Ni-0.6Nb含Nb合金在1000 ℃空气条件下的抗氧化。采用SEM、EDS、TEM、拉曼光谱等手段研究了合金的显微组织和氧化膜特性。结果表明,3种含Nb合金组织为单相奥氏体,基体中存在少量弥散分布的NbC沉淀相,氧化前后沉淀相含量和晶粒大小保持不变。添加0.5%和1.5%的Al后,含Nb合金的表面形成多层结构的氧化膜,最外层和第三层为Cr₂O₃,次表层主要为NiCr₂O₄、NiFe₂O₄和Fe₂O₃,最内层为Al₂O₃内氧化层。基体中的NbC析出相和氧化膜中少量Nb的氧化物(Nb₂O₅)加剧了氧化膜的疏松。当Al含量增加到2.5%时,含Nb合金表面形成连续致密的Al₂O₃氧化膜,降低了Fe-20Cr-35Ni-0.6Nb合金的氧化速率,提高了抗氧化性。     

Influence of Zr or Hf additions to Fe-23Cr-5Al on the protectiveness of preformed Al2O3 scales against sulfidation

An Fe-23Cr-5Al alloy and those containing 0.17 w/o Zr or 0.12 w/o Hf were oxidized to form -Al₂O₃ scales in a flow of pure O₂ at 1300 K for specified periods up to 400 ks, and subsequently sulfidized at 1200 K in an H₂ –10% H₂S atmosphere without intermittent cooling. The protectiveness of the preformed scale was evaluated by the protection time after which a remarkable mass gain takes place owing to the rapid growth of sulfides. In general, the protection time increases as the scale thickens. Both additives increase the protection time to some degree by forming more structurally perfect scales. However, ZrO₂ particles on or near the outer surface of the scale on the Zr-containing alloy provide sites for sulfide formation. The scales formed on the grain boundaries of the Hf-containing alloy are ridged. The tops of the ridges are associated with cracks, which provide preferential sites for sulfide growth.     

The effect of preoxidation on the sulfidation of Ni-20Cr (2–5)Al Alloys

Ni-20Cr alloys with 2, 3.5, and 5 wt.% Al have been preoxidized up to 100 hr at 1000°C in dry H₂, in H₂/23% H₂O and in air and subsequently exposed to an H₂/5% H₂S atmosphere at 750° C. During the preoxidation treatment different types of oxide scales were formed which affect the sulfidation protection in different ways. Optimum results were obtained for alloys with 3.5 and 5 wt.% Al after 20 hr exposure to dry H₂ at 1000°C. A thin Al₂O₃ scale is formed which decreases the sulfur attack by more than one order of magnitude. Preoxidation conditions for Ni-20Cr-2Al alloys in H₂ and for Ni-20Cr-2Al and Ni-20Cr-3.5Al in H₂/H₂O were observed to be less effective. No improvement was found for preoxidation in air or for Ni-20Cr-5Al alloys preoxidized in H₂/H₂O.     

Effect of Y2O3 dispersoids in 80Ni-20Cr alloy on the early stages of oxidation at low-oxygen potential

Specimens of a 80Ni-20Cr type alloy, with and without Y₂O₃ dispersoid particles, were oxidized at 1000°C in H₂/H₂O mixtures where the partial pressure of oxygen (P _{O 2}) was varied between 10₃ and 10₂₄ atm. Oxide particles nucleated homogeneously on both alloys, and preferential nucleation on dispersoid particles at the surface was not observed. Continuous Cr₂O₃ films formed slightly faster at aP _{O 2} of 10^–21 atm on the alloy containing the dispersoid, but the difference was negligible at higher pressures. Oxidation atP _{O 2}=10^-19 and 10^–21 atm involved both the formation of Cr₂O₃ and the evaporation of chromium. Thin films of -Al₂O₃ were observed on both alloys after oxidation atP _{O 2}.     

The high-temperature corrosion behavior of Fe-Mo-Al alloys in H2/H2O/H2S mixed-gas environments

The corrosion behavior of 11 Fe-Mo-Al ternary alloys was studied over the temperature range 700–980°C in H₂/H₂O/H₂S mixed-gas environments. With the exception of Fe-10Mo-7Al, for which breakaway kinetics were observed at higher temperatures, all alloys followed the parabolic rate law, despite two-stage kinetics which were observed in some cases. A kinetics inversion was observed for alloys containing 7 wt.% Al between 700–800°C. The corrosion rates of Fe-20Mo and Fe-30Mo were found to be reduced by five orders of magnitude at all temperatures by the addition of 9.1 or higher wt.% aluminum. The scales formed on low-Al alloys (5 wt.% Al) were duplex, consisting of an outer layer of iron sulfide (with some dissolved Al) and a complex inner of Al_0.55Mo₂S₄, FeMo₂S₄, Fe_1.25Mo₆S_7.7, FeS, and uncorroded FeAl and Fe₃Mo₂. Platinum markers were always located at the interface between the inner and outer scales for the low-Al alloys, indicating that outer-scale growth was due mainly to outward diffusion of cations (Fe and Al), while the inner scale was formed primarily by the inward flux of sulfur anions. Alloys having intermediate Al contents (7 wt.%) formed scales that consisted of FeS and Al₂O₃. The amount of Al₂O₃ increased with increasing reaction temperature. The high-Al-content alloys (9.1 and 10 wt.%) formed only Al₂O₃ which was responsible for the reduction of the corrosion rates.     

The effect of Al on the corrosion behavior of Ni-Mo in a H2/H2O/H2S gas mixture

The corrosion behavior of seven Ni-Mo-Al alloys was investigated over the temperature range of 600–950°C in a mixed-gas atmosphere of H ₂/H ₂O/H ₂ S. The parabolic law was followed at low temperatures, while linear kinetics were generally observed at higher temperatures. At a fixed Mo content, the transition from parabolic to linear kinetics shifted to higher temperature with increasing Al concentration. Double-layered scales generally formed on alloys having a low Al content, consisting of an outer layer of nickel sulfide and a complex inner scale. The thickness of the outer scale and the inner scale decreased as the Al content increased. The outer scale became porous and discontinuous with increasing Al content and temperature. Al ₂ O ₃ was detected in the scales of all alloys corroded at higher temperatures ( 800°C), even though the amount of Al ₂ O ₃ was very small in some cases. The decrease in corrosion rate with increasing Al content may be attributed to the formation of Al ₂ O ₃,Al _0.55 Mo ₂ S ₄,and Al ₂ S ₃ in the inner scale.     

The effect of Al2O3 addition on the milling behavior of NiAl powder

The milling behavior of nickel aluminide, NiAl, powder in the presence of a fine Al ₂ O ₃ powder was investigated in the present study. The milling was carried out in an attrition mill. The size and shape of NiAl particles were not changed after milling while only NiAl powder was milled. When fine Al ₂ O ₃ powder was added to the NiAl powder, the Al ₂ O ₃ particles attached to the surface of NiAl particles during milling. As a consequence, the size of NiAl particles was reduced after milling. The shape of NiAl particles also changed. The presence of fine Al ₂ O ₂ particles enhanced the milling efficiency. The Al ₂ O ₃ particles on the surface of NiAl powder can be removed by washing repeatedly in an ultrasonic bath.     

Influence of a mixed Al2O3+Cr2O3 superficial coating on the non-isothermal oxidation behavior of Fe and Fe-Cr alloys

The non-isothermal oxidation behavior of electrolytic-grade iron and Fe-Cr alloys in dry air has been studied using linear heating rates of 6 K/min, 10 K/ min, and 15 K/min up to a final temperature of 1273–1473 K. Some of the iron and iron-chromium alloy samples were given a surface treatment by dipping them in an aqueous solution containing both Cr and Al ions before their oxidation studies. This pretreatment has resulted in improved oxidation resistance and scale adherence as depicted by no scale rupture even after a second thermal cycle. Mass changes were recorded gravimetrically, and scales have been characterized by SEM, EPMA, and x-ray diffraction analyses.     

High-temperature corrosion of Co-15Mo alloys containing ternary additions of Al in mixed-gas environments

The corrosion behavior of Co-15 at.% Mo alloys containing up to 20at.% Al in gaseous H ₂ -H ₂ O-H ₂ S mixtures was studied over the temperature range of 600–900°C. The corrosion kinetics of all alloys followed the parabolic rate law over the temperature range of interest. Corrosion resistance increased with increasing aluminum content. Complex scales formed on the alloys, consisting of an outer layer of cobalt sulfide and a heterophasic inner layer. Al ₂ O ₃ formed only at high temperatures in alloys having aluminum additions of 15at.% or more. The absence of Al ₂ O ₃ in some cases is due to the small volume fraction of the intermetallic phase CoAl in the alloys and the nature of the slow growth rate of Al ₂ O ₃.Improvement in corrosion resistance is attributed to the presence of a ternary sulfide, Al _0.55 Mo ₂ S ₄,and Al ₂ O ₃ in the inner layer.     

Interfacial segregation during oxidation in O2 at 1173 K of CeO2-coated Fe-20Cr alloys

The interfacial segregation of sulphur and carbon during the oxidation in 1 torr O₂ at 1173 K of Fe-20Cr alloy, which was either free of Ce, alloyed with 0.078 wt.% Ce, or sputter-coated with a 4 nm-thick CeO₂ layer, was studied using polyatomic SIMS. Oxidation periods were up to 19 hr. During oxidation, sulphur and carbon accumulated at the alloy-oxide interface region of both uncoated and coated alloys. The amount of segregated sulphur did not vary appreciably with time, whereas carbon increased with time. The total amount of segregants was similar for both uncoated and coated alloys, although the scales formed on the sputter-coated alloy maintained adhesion and were about 10 times thinner than those on the uncoated alloys.     

Nano/Micro-Laminated (ZrO2–Y2O3)/(Al2O3–Y2O3) Composite Coatings and Their Oxidation Resistance

Nano/micro-laminated (ZrO₂–Y₂O₃)/(A1₂O₃–Y₂O₃) composite coatings were deposited onto an Fe–25Cr–7Ni–N alloy substrate by using alternate electrochemical and sintering processes. The thickness of each layer was in the range of 80–500 nm. Experimental results indicated that the multi-laminated coatings were more effective in providing oxidation resistance than monolithic ZrO₂–Y₂O₃ or A1₂O₃–Y₂O₃ coatings, with the oxidation resistance of the former increasing with increasing number of laminated layers. The microstructural studies suggest that the laminated coatings possess the advantages of these two types of coatings and avoid the weakness of single ZrO₂–Y₂O₃ or A1₂O₃–Y₂O₃ coatings. Reactive elements Y and Zr also played a role in this nano-layered setting in improving the oxidation resistance of the coatings.     

Deposition of Al2O3-TiO2 Nanostructured Powders by Atmospheric Plasma Spraying

Al₂O₃-13%TiO₂ coatings were deposited on stainless steel substrates from conventional and nanostructured powders using atmospheric plasma spraying (APS). A complete characterization of the feedstock confirmed its nanostructured nature. Coating microstructures and phase compositions were characterized using SEM, TEM, and XRD techniques. The microstructure comprised two clearly differentiated regions. One region, completely fused, consisted mainly of nanometer-sized grains of γ-Al₂O₃ with dissolved Ti⁺⁴. The other region, partly fused, retained the microstructure of the starting powder and was principally made up of submicrometer-sized grains of α-Al₂O₃, as confirmed by TEM. Coating microhardness as well as tribological behavior were determined. Vickers microhardness values of conventional coatings were in average slightly lower than the values for nanostructured coating. The wear resistance of conventional coatings was shown to be lower than that of nanostructured coatings as a consequence of Ti segregation. A correlation between the final properties, the coating microstructure, and the feedstock characteristics is given.     

The spalling modes and degradation mechanism of ZrO2-8 wt.% Y2O3/CVD-Al2O3/Ni-22Cr-10Al-1Y thermal-barrier coatings

State-of-the-art conventional thermal-barrier coatings consist of a thermalinsulating, partially-stabilized ZrO₂ top coat and a bond coat. In this study, a continuous alumina-diffusion-barrier layer was deposited and interposed between the top coat and bond coat by chemical-vapor deposition (CVD). Both the conventional and the experimental TBC systems were cyclically tested at 1000°C, 1050°C, 1100°C, and 1150°C to evaluate and compare oxidation, performance, and fracture behavior. Introduction of the intermediate CVD-Al₂O₃ layer effectively suppressed the oxidation rate of the bond coat and sufficiently altered its oxidation behavior. The thermal-cyclic life of TBCs was improved by the new system. The failure of the ZrO₂-8 wt.% Y₂O₃/CVD-Al₂O₃/Ni-22Cr-10Al-1Y TBC specimens was observed to propagate mainly along the lamellar splats of the top coat, and secondarily along the top coat/CVD-Al₂O₃ interface.     

Strengthening and toughening forming of Al/Al2O3 composite in pseudo-semi-solid state

A new technology—thixo-die-forging of the composite in pseudo-semi-solid state was proposed based on the powder metallurgy technology combing with semi-solid metal process, and the cup shells with Al/Al₂O₃  composite was prepared successfully. The metallographic analysis and performance test show that the microstructure of parts is dense and mechanical properties are excellent with the volume fraction of Al is 37 %. The bend strength and fracture toughness of the composite are about 570-690 MPa and 8.5-16.8 MPa m^1/2, respectively. Comparing with reaction in situ and high temperature oxidation technologies the bending strength and fracture toughness are improved greatly. At the same time, it shows that the technology parameters have great influences on the properties. So it is feasible to prepare metal/ceramics composites by the proposed technology.     

Cu-Al合金薄板内氧化析出相Al_2O_3晶型及分布

Al含量为0.50%(质量分数)的Cu-Al合金薄板在900℃下内氧化25 h制备Cu-Al_2O_3薄板复合材料,并用富集萃取法提取Cu-Al_2O_3复合材料中的Al_2O_3相。利用TEM分析了Cu-Al_2O_3薄板中的Al_2O_3相的种类、分布、与Cu基体的界面关系,用X射线衍射和TEM研究了萃取粉末的组成。结果表明,Cu-Al薄板内氧化法所得的Cu-Al_2O_3复合材料的析出相主要为γ-Al_2O_3,有少量的α-Al_2O_3和θ-Al_2O_3相存在。析出相Al_2O_3颗粒弥散分布在Cu基体上,且析出相γ-Al_2O_3与Cu基体完全共格;Cu-Al_2O_3薄板复合材料从表层至深约0.5 mm处,Al_2O_3颗粒粒径逐渐减小,从14 nm减小到5 nm,颗粒间距逐渐增大,从10 nm增加到15 nm。     

The corrosion behavior of Ni-Al alloys and Ni-Nb-Al alloys in a H2/H2O/H2S gas mixture

The corrosion behavior of two Ni-Al alloys and four Ni-Nb-Al alloys was studied over the temperature range of 600° C to 1000° C in a mixed-gas of H₂/H₂O/H₂S. The parabolic law was generally followed, although linear kinetics were also observed. Multiple-stage kinetics were observed for the Ni-Al alloys. Generally, the scales formed on Ni-13.5Al and Ni-Nb-Al alloys were multilayered, with an outer layer of nickel sulfide with or without pure Ni particles and a complex inner scale. The outer scale became porous and discontinuous with increasing temperature. Very thin scales formed on Ni-31Al. The reduction in corrosion rate with increasing Al content is ascribed to the formation of Al₂O₃ and Al₂S₃ in the scale. Platinum markers were found at the interface between the outer and inner scales.     

Evaluating Bending Strength of Plasma Sprayed Al2O3 Coatings

The bending strength of plasma sprayed Al₂O₃ coatings was evaluated using three-point bending tests. The measured strength data sets were statistically analyzed by employing Weibull distribution. It was found that the bending strength values of coatings show obvious anisotropic behavior. Additionally, the bending strength values measured by loading parallel to the spraying direction are lower than those antiparallel to the spraying direction, and larger variability was realized for the former. This may be related mainly to the distribution of residual stress and microstructural defects within coatings.     

18O Tracer studies of Al2O3 scale formation on NiCrAl alloys

Diffusion processes in Al ₂ O ₃ scales formed on NiCrAl + Zr alloys were studied by the proton activation technique employing the ¹⁸ O isotope as a tracer. The ¹⁸ O profiles identified a zone of oxide penetration beneath the external scale. Both this subscale formation and the outer Al ₂ O ₃ scale thickness were shown by this technique to increase with Zr content in the alloy. Estimated k _p 's from scale thicknesses were in agreement with gravimetric measurements for various Zr levels. Alternate exposures in O and ¹⁸ O revealed that oxygen inward transport was the primary growth mechanism. A qualitative analysis of these ¹⁸ O profiles indicated that the oxygen transport was primarily via short-circuit paths, such as grain boundaries.     

Formation of Al2O3 Scales on Single-Phase RuAl Produced by Reactive Sintering

The oxidation behavior of single-phase RuAl produced by powder metallurgy combined with arc melting was investigated. Oxidation was conducted at 1000°C; oxide scale growth and phase formation were studied using scanning-electron microscopy (SEM) and x-ray diffraction (XRD). A dense protective scale with an Al–depleted sublayer was formed during oxidation. The oxide scale is the stable -Al₂O₃. The oxide-scale morphology shows the presence of whiskers, with a needle-like form, which suggests that the growth of the oxide scale is produced by outward diffusion of Al. At the beginning, oxidation follows a parabolic law, but, after 100 hr of oxidation; the growth rate is slower than expected from a parabolic law.     

Mechanical Properties and Thermal Shock Resistance of Al2O3-TiC-Co Composites

Ductile cobalt was introduced into Al₂O₃-TiC (AT) composites by using a chemical deposition method to improve toughness and resistance to thermal shock. The mixture of Co-coated Al₂O₃ and TiC powders was hot-pressed into an Al₂O₃-TiC-Co (ATC) composite. The flexure strength and fracture toughness of the ATC composites have been improved considerably, compared with AT and Al₂O₃. The fracture surface of ATC shows a large proportion of transgranular cracks with some intergranular type, unlike the intergranular fracture modes of AT and Al₂O₃. The thermal shock properties of the composites were evaluated by water quenching technique and compared with the traditional AT and Al₂O₃. The composites containing only 3.96 vol.% cobalt exhibited higher critical temperature difference and retained flexure strength. The SEM examination of the fracture surfaces of the ATC composites after single thermal cycle showed that voids increased in number and size, and most isolated voids coalesced with increasing temperature difference, which caused the density and strength to decrease. The ATC composite is less sensitive to repeated thermal shock than the AT composite.