TEM studies of oxidized NiAl and Ni3Al cross sections

Cross sections of oxide scale/(Ni-Al) intermetallics were prepared by a new method and studied using primarily transmission electron microscopy (TEM). The cross sections were prepared by encasing an oxidized metal specimen sandwich in a low-melting-temperature zinc alloy. Observations of oxidized zirconium-doped -NiAl cross sections revealed crystallographic voids beneath an adherent Al₂O₃ scale. The oxide-metal interface was incoherent, but a high dislocation density in the metal near the interface suggested that a large tensile stress was induced by the attached oxide scale. A duplex Al₂O₃-NiAl₂O₄ scale formed on zirconium-doped and zirconium/boron-doped -Ni₃Al alloys. Additional results are presented involving oxidation mechanisms and oxide-metal interface structures.     

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。     

Sulfidation behavior of Fe-25Cr-20Ni-1.5Al2O3 in simulated coal combustion/gasification environments

The effect of minor addition of -Al₂O₃ dispersoids on the sulfidation behavior of Fe-25Cr-20Ni was investigated over a range of pO₂, 1.13×10^–20 to 1.18×10 ****^–22 atm. at constant pS₂=1.22×10^–8 atm. Fe-25Cr-20Ni and Fe-25Cr-20Ni 1.5 Al₂O₃ with and without preformed oxide scales were exposed to bioxidant gas mixtures H₂/H₂O/H₂S/Ar at 700° C. Both isothermal and cyclic exposures were included. Scales were characterized by a combination of several surface analytical tools. A remarkable improvement in sulfidation resistance is observed in Fe-25Cr-20Ni-1.5Al₂O₃ under the conditions investigated here. This is attributed to the ability of the alloy to form and maintain a predominantly Cr₂O₃ scale with reduced Fe-diffusion and content. Possible scientific reasons for such improvement are discussed. The base alloy, Fe-25Cr-20Ni, fails to develop and retain such a Cr₂O₃ scale and undergoes sulfidation within a few minutes of exposure. The scale breakdown process by sulfidation is explained qualitatively. Experimental evidence suggests that sulfur in the environment enhances Fe-diffusion and content in the scale.     

High-temperature oxidation of Fe-2 1 4 Cr-1Mo in oxygen

The oxidation of a 2 ₁ ⁴ Cr-1Mo steel in dry flowing oxygen has been studied in the temperature range 550–700°C for periods of up to 100 hr. A detailed low-resolution microstructural investigation revealed a layered oxide consisting of a very fine-grained and finely pored innermost layer of doped spinel, a central columnar-grained relatively coarsely pored layer of magnetite, and an outer fine-grained hematite layer with fine pores and covered with whiskers of -Fe₂O₃. This structure is compared with previous results on Fe and model Fe-Cr alloys, as are the kinetics of the oxidation reaction.     

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.     

High-Temperature Oxidation of Ni Coated with La2O3 by Atomic-Layer Chemical-Vapor Deposition (ALCVD)

The effects of superficial (30–100 nm) La₂O₃ surface coatings on the oxidation kinetics of Ni from 700 to 1100°C in air and the oxide morphology of the NiO scales have been investigated. The parabolic rate constant is lower than for uncoated Ni by a factor of 5 to 10. The oxide morphology changes with the La₂O₃ coatings: The oxide scale consists of an outer fine-grain layer with an inner region of coarser, but still equiaxed, grains. SIMS shows that the majority of the La remains at the surface where a highly oxygen-defective spinel, La₂Ni₄O₇, was found by TEM. Two-stage oxidation followed by SIMS profiling reveals that the oxide growth occurs inside the scales.     

Reactive-element effect of electrodeposited Y2O3 oxide films on the oxidation of Fe−25Cr and Fe−25Cr−10Al alloys

Thin Y₂O₃ films were deposited by the electrochemical deposition-pyrolysis process on Fe–25Cr and Fe–25Cr–10Al alloys. The influence of the films on the oxidation behavior of the alloys was studied at 850°C and 1000°C. The results showed that Y₂O₃ films remarkably decreased the oxidation rate of Cr₂O₃-forming alloys and spallation of the scales, but they did not decrease the oxidation rate of the Al₂O₃-forming alloys, although they do reduce the spallation of Al₂O₃ scales. Y₂O₃ films remarkably change the morphology of the scales on both alloys, depending on the oxidation temperatures. These results show that the reactive-element effects of Y₂O₃ films on the Cr₂O₃ formers and Al₂O₃ formers are different.     

Synthesis of lanthanum carbonate nanoparticles via sonochemical method for preparation of lanthanum hydroxide and lanthanum oxide nanoparticles

Lanthanum carbonate nanoparticles were synthesized from the reaction of lanthanum acetate and Na₂CO₃ under sonication via sonochemical method. Lanthanum hydroxide nanoparticles were prepared by facial hydrothermal processing from the resulted product at 110 °C for 24 h. The role of surfactant, calcination temperature and sonication time were investigated on the morphology and particle size of the products. Products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectrum (XPS), and Fourier transform infrared (FT-IR) spectra. La₂O₃ nanoparticles were obtained by calcinations of the nanoparticles of lanthanum carbonate at 600 °C.     

La2O3改性ZrB2-SiC涂层C/C复合材料全温域抗氧化行为研究

采用高温反应熔渗工艺制备了ZrB2-SiC和La2O3改性ZrB2-SiC涂层C/C复合材料,对比了2种涂层试样在中温(7001100℃)、高温(12001500℃)和超高温(2000℃以上)3个温域范围内的抗氧化性能。结果表明:7001100℃范围内,随着温度的升高,La2O3改性涂层试样的抗氧化性能提升幅度在逐渐提高。1200℃1500℃范围内,涂层均表现出良好的长时抗氧化性能,La2O3改性ZrB2-SiC在1200℃下恒温氧化250 h后,仍保持微量的增重;涂层复合材料良好的高温抗氧化性能主要其在是由于氧化过程中涂层表面形成的La-Si-O复合玻璃层和钉扎相ZrSiO4的协同作用提升了氧化膜的高温稳定性。在2000℃以上的氧乙炔火焰烧蚀环境下,La2O3的添加使得ZrB2-SiC涂层的质量烧蚀率和线烧蚀率均降低了近50%,其主要归因于表层La-Si-O和ZrO2玻璃层对烧蚀缺陷的愈合作用。     

A TEM study of the oxide scale development in Ni-Cr-Al alloys

In the present study the isothermal oxidation behaviours of Ni-10Cr-5Al, Ni-20Cr-5Al and Ni-30Cr-5Al alloys were investigated. The alloys were oxidised in air for 50 h at 1000 °C. Analytical transmission electron microscopy was used to characterize the morphology, structure and composition of the oxide scale. The oxide formed adjacent to the alloy was α-Al₂O₃ such that the higher was the Cr content of the alloy the easier was its formation. The Ni-30Cr-5Al alloy formed a complete layer of α-Al₂O₃ in the initial stages of oxidation through ‘oxygen gettering’ by Cr. A decrease in scale thickness and an increase in scale adherence were observed with an increase in Cr content from 10 to 30 wt.%.     

Al含量对Fe-22Cr-25Ni含铝奥氏体耐热钢高温抗氧化性能的影响

以HR3C合金成分为基础,通过调控Cr、Ni含量和添加1.5%,2.5%和3.5% (质量分数) 的Al制备了Fe-22Cr-25Ni型含铝奥氏体耐热钢,并研究了合金的高温抗氧化性能。利用SEM、EDS和XRD对含铝奥氏体钢700、800和900 ℃氧化后的氧化膜组成、结构进行了表征。结果表明：22Cr-25Ni-2.5Al和22Cr-25Ni-3.5Al含铝奥氏体耐热钢在700和800 ℃下具有优异的抗高温氧化性能。氧化后表层形成了连续致密的Al₂O₃保护膜,提高了其高温抗氧化性能。3种耐热钢经900 ℃氧化时形成外层为Cr₂O₃和MnCr₂O₄的复合氧化层,且氧化层下存在Al₂O₃内氧化物和AlN析出相,不能对基体起到有效保护作用。     

Rutherford-backscattering study of high-temperature oxidation of Y-implanted Fe-24Cr

We have investigated the effect of Y ion implantation on the high-Temperature oxidation of Fe-24Cr using Rutherford-backscattering spectroscopy, secondary ion mass spectroscopy, and analytical electron microscopy. Results indicate that implantation of Y has a very large effect on the growth rate of the oxide compared to metals alloyed with Y. Analytical tools have been applied to determine the spatial distribution of Y, the microstructure of the oxide, and contribution of oxygen transport to the oxidation process. Results are compared with those of recent cation-diffusion measurements in Cr₂O₃ and Cr₂O₃ doped with Y₂O₃.     

La2O3含量对Al2O3/Cu基复合材料组织与性能的影响

以La₂O₃粉、Al粉、CuO粉为反应物原料、纯铜为基体,采用原位合成技术和近熔点铸造法制备颗粒增强Cu基复合材料,研究La₂O₃对Al-CuO体系制备的Cu基复合材料组织及性能的影响。结果表明：添加La₂O₃可获得纳米Al₂O₃颗粒,且弥散分布于Cu基体中,制备的材料组织更加细小、均匀,其材料的电导率及摩擦磨损性能明显提高。当添加0.6%wtLa₂O₃,复合材料的电导率达到90.2%IACS,磨损量达到最小,相比未添加La₂O₃,其导电率提高10.1%,磨损量减小36.6%。     

Biomimetic deposition of apatite coatings on micro-arc oxidation treated biomedical NiTi alloy

The biomedical NiTi alloy was treated by micro-arc oxidation in an electrolytes containing sodium aluminate and sodium hypophosphite at 400 V constant voltages for 30 min. The MAO-treated NiTi has a porous microstructure on its surface and coatings consisting only of the γ-Al₂O₃ phase. The ceramic coating prepared by micro-arc oxidation is composed of Al, Ti, Ni, O, and P with the atomic concentration of 26.98%, 3.67%, 3.33%, 65.30% and 0.72%, respectively. The MAO-treated NiTi was soaked in a simulated body fluid (1.0SBF) to investigate the biomimetic deposition of apatite on the surface of Al₂O₃ coated NiTi alloy. It was found that Al₂O₃ coated NiTi alloy shows an excellent apatite-forming ability after soaking in a simulated body fluid (1.0SBF) for 14 days, while no apatite-forming ability was observed on bared NiTi alloy even though soaking time is up to 28 days.     

微弧氧化Mg-3Al-1Zn镁合金应力腐蚀和腐蚀疲劳行为研究

本文主要对微弧氧化Mg-3Al-1Zn镁合金在空气和3.5 wt.%硫酸钠溶液两种环境下的应力腐蚀和腐蚀疲劳行为进行研究,并讨论其相互关系。微弧氧化处理后,Mg-3Al-1Zn镁合金的耐蚀性能得到明显改善。与Mg-3Al-1Zn镁合金基体相比,在空气中,微弧氧化后合金的应力腐蚀和腐蚀疲劳强度均下降了大约10 MPa。在3.5 wt.%硫酸钠溶液环境中,微弧氧化后合金的腐蚀疲劳性能仍然是恶化的,但是应力腐蚀强度却得到了显著改善,从58.24 MPa提高至202.08 MPa。这表明材料的力学性能（应力腐蚀和腐蚀疲劳）并不是与其腐蚀性能完全保持线性关系的。微弧氧化处理后镁合金在空气中的应力腐蚀断口为韧性断裂,在硫酸钠溶液环境中为解理断裂。而其腐蚀疲劳断口不论是在空气中还是在腐蚀环境中均为解理断裂。这主要是由于腐蚀环境和变换循环载荷的影响,两者共同作用将会加速裂纹扩展。这表明,周围环境和加载类型对材料断裂机理有重要影响。     

Effect of Hf and Y alloy additions on aluminide coating performance

Iron- and Ni-base alloys, with and without Hf or Hf and Y alloy additions, were aluminized by chemical vapor deposition to study the potential for minor alloy additions to improve oxidation resistance of coated alloys. Compared to uncoated specimens, the coated specimens showed improved cyclic oxidation resistance at 1100° and 1150 °C. However, alumina scale spallation was observed at relatively short times and, particularly for the Ni-base alloy X, the aluminized lab-cast alloy with Hf tended to have poor coating performance compared to the commercial alloy without Hf. Internal oxidation of Hf at 1150 °C and rapid Al depletion in the relatively thin aluminide coatings contributed to the observed detrimental Hf effect. For the Ni-base alloys, the increased scale spallation could be attributed to much higher S contents (10-50 ppma) in the laboratory-cast alloys. Oxide scale spallation from the coating surface was minimized when Hf and Y were added to a casting and the [Y]/[S] content ratio was ∼ 1.     

Chromium transport through Cr2O3 scales. II. Changes in scale morphology during high vacuum treatment of oxidized chromium specimens

Chromium specimens oxidized at 1200 and 1300° O to give Cr₂O₃ scales with varying thicknesses have been high vacuum annealed for extended periods at temperature. During the high vacuum anneal chromium is transported through the scale and evaporates from the scale surface. Initially the rate of chromium evaporation decreases with time as a result of recrystallization and densification of the scale. On extended high vacuum treatment the rate of chromium evaporation again increases and major changes in scale morphology takes place. The outer scale surfaces develop hollows in the oxide grains while the grains protrude from the scale at the inner surfaces. The morphological changes are interpreted in terms of differences in diffusion rates along grain boundaries and through the lattice and resultant variations in surface energy along the surfaces.     

Oxidation properties of Fe-5Cr-4Al (wt.%) alloys in oxygen at temperatures 1000°C–1320°C

Oxidation kinetics of a parent Fe-5Cr-4Al alloy subjected to two types of anneals were investigated at temperatures ranging from 1000°C to 1320°C. The alloy annealed at 850°C exhibited a rapid transient oxidation stage associated with growth of nodules containing iron oxides and internal precipitation of -Al₂O₃ in the alloy beneath these nodules. The nodules nucleated and grew from sites located in the regions of the alloy grain boundaries during the period of rapid alloy grain growth. Nodular growth virtually ceased when a continuous -Al₂O₃ film formed at the nodule-alloy interface. The alloy subjected to anneal at 1000°C and at the reaction temperature to stabilize the alloy grain size tended upon oxidation to form a protective -Al₂O₃, layer by parabolic kinetics at temperatures to 1250°C. If this alloy was oxidized in stages at 1000°C, a protective -Al₂O₃ scale was formed up to 1320°C. The temperature coefficient of the parabolic oxidation kinetics was consistent with diffusion processes at boundaries of the -Al₂O₃ grains playing an essential role during growth of this protective oxide layer.     

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 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.