Zirconium oxide film formation on zircaloy by water corrosion

The objective of this work is to study the morphological, micro-chemical and micro-mechanical characteristics of the thick zirconium oxide layer formed on a zircaloy-4 clad after a standard water corrosion test (ASTM G2-67). The oxidized samples were analyzed by Raman and Auger spectroscopy, SEM–EDS, XRD and the mechanical properties of the oxide film were evaluated using the Vickers microindentation method. The results reveal the formation of a textured oxide layer constituted of m-ZrO₂ crystals oriented primarily in the direction perpendicular to the (002) crystallographic plane. In addition, a not previously reported thin zirconium carbide film on the top surface of the oxide layer was detected by SEM–EDS, XRD and Auger spectroscopy.     

Evaluation of the corrosion and oxide microstructural characteristics of pure Zr and the Zr−1.5Nb alloy

The corrosion behavior and the oxide microstructural characteristics of pure Zr and the Zr-1.5Nb alloy were investigated. The crystalline structure and morphology of zirconium oxide formed in a water at 360°C and 18.9 MPa was analyzed using a High Voltage Electron Microscope (HVEM). The corrosion rate increased more in pure Zr than in the Zr-1.5Nb alloy after the transition of the corrosion rate. From a HVEM oxide analysis and an imaging process using the DigitalMicrograph^™ program for both oxides formed on pure Zr and the Zr−1.5Nb alloy, it was found that most of the oxide amorphology consisted of both equiaxed and columnar grains. They formed in the outer and middle region of the oxide layer and were identified as a monoclinic phase. However, a complex oxide structure formed near the metal/oxide interface in the Zr−1.5Nb alloy, which showed good corrosion resistance. Thus, this implies that the Nb affected the oxide characteristics of the crystal structure and changed the corrosion behavior.     

Microstructural Evolution of Creep-Induced Cavities and Casting Porosities for a Damaged Ni-based Superalloy Under Various Hot Isostatic Pressing Conditions

The microstructural evolution of casting porosities and creep-induced cavities for a damaged nickel-based superalloy under different hot isostatic pressing(HIP) conditions was investigated in order to understand the effects of HIP parameters on the healing behavior of micropores. A number of small-sized creep cavities formed during long-term service and large-sized porosities formed during the casting process were observed. These microdefects were partially healed after treated at high temperature of 1100 °C combined with 150 MPa pressure for 2 h, together with the formation of the socalled concentrically oriented c0 rafting structure. When HIP temperature was increased to 1150 and 1175 °C, both the amount and the size of the microdefects were decreased. The concentrically oriented c0 rafting around creep cavities became more remarkable, and the primary c0 denuded zone was also formed between the raft structure and the cavity.Energy-dispersive X-ray spectroscopy analysis revealed that the c matrix solute atoms diffused toward the cavity under the concentration gradient, whereas the c0-forming elements diffused in a negative direction. When increasing HIP temperature up to 1200 °C, the micropores were hardly observed, indicating that both casting porosities and creep-induced cavities had almost been healed. Meanwhile, the c0 rafting structure disappeared since HIP temperature was beyond the c0 solvus temperature. It is revealed by the experimental results that the atomic diffusion could mainly dominate the healing process of micropores.     

The effect of titanium support on the morphological properties of growth of titanium-oxide nanotubes and platinum deposit

It is shown that the thickness and structure of the layer of titanium-oxide nanotubes obtained by anodization of titanium foil in fluoride-containing solutions are determined by crystallographic orientation of surface metallic grains. In addition, this orientation appears when platinum deposits are applied onto nanooxide. Optical microimages in polarized light can be used for estimation of the quality of the crystallographic orientation of surface metallic grains, as well as for control of the thickness of oxide nanotubular layers. Shiny grains are characteristic for a nonuniformly (steplike) etched support with formation of a “humpy” light-scattering grain structure. Dull grains usually have the flat (0001) orientation; the process of nanotube growth occurs uniformly over the surface and is considerably hindered, which is probably related to the higher atomic density of the (0001) plane and protective properties of the barrier oxide. It is shown that TiO₂ nanotubes are formed at a growing rate on bright grains and grains with the crystallographic orientation allowing formation of a thick oxide layer.     

The complementary nature of x-ray photoelectron spectroscopy and angle-resolved x-ray diffraction part II: Analysis of oxides on dental alloys

X-ray photoelectron spectroscopy (XPS) and angle-resolved x-ray diffraction (ARXRD) were used to analyze the oxide layer on three palladium-gallium-based dental casting alloys. The oxide layers were approximately 10 Μm thick. The use of the techniques helped to determine which mechanism was responsible for oxide formation—either (a) oxide layer growth via diffusion of oxygen through the scale to the metal, causing the scale to grow at the metal-oxide interface, or (b) an oxide layer formed by metal ions diffusing through the scale to the surface and reacting with oxygen, causing the scale to grow at the oxide-air interface. The oxide growth mechanisms were correlated to previous layer adhesion results determined with biaxial flexure testing.     

热电厂15CrMo钢管高温蒸汽氧化腐蚀机理研究

研究某热电厂锅炉过热器15CrMo钢管在540℃/3.9 MPa条件下运行3000 h发生高温蒸汽氧化失效行为.利用X衍射、扫描电镜和电子探针等对15CrMo锅炉氧化层形貌和氧化膜成分进行组织结构观察和分析,结合高温水蒸气存在的情况下氧化膜生长理论,分析了这一特定条件下高温蒸汽氧化层的形成机理及其非正常失效原因.15CrMo钢在较短的服役过程中珠光体组织球化严重,达到4～5级,组织结构的退化导致力学性能严重下降同时炉管内壁发生严重的内氧化,从而降低管壁的有效厚度造成承压能力下降.晶界优先腐蚀,靠近晶界的基体组织内部形成大量孔洞,孔洞的形成是因离子扩散优先形成氧化点引起.氧化腐蚀产物分为内外两层,内氧化层结构疏松,方便了气体分子和金属离子的扩散,加剧了高温氧化的进行.     

The Influence of Oxidizing Medium on Structure,Mechanical Properties,and Breakage Pattern of Oxide Films of Zr—1% Nb Alloy under Conditions Simulating Oxidizing Media of WWER and PWR Reactors

This article investigates into the structure, phase composition, mechanical properties, and breakage pattern of oxide films formed on the surface of Zr—1% Nb alloy after corrosion tests in autoclave in water, steam, and water with lithium. Oxidation in water with lithium promotes formation of oxide films with a thickness of more than 100 μm, and, after oxidation in water or steam, the thickness is 5–15 μm. Upon oxidation in water, oxide films are generated with laminar structure: in the substrate, the grains are extended with the thickness of ~80 nm, near film surface the grains are equiaxial with the diameter of ~30 nm. After holding of the specimens in steam the oxide films are comprised mainly extended grains with the thickness of ~95 nm with a minor amount of equiaxial grains, 10%, with a diameter of ~30 nm, after oxidation in water with lithium the mixture of equiaxial and weakly extended nanograins is observed in the film structure. Upon loading of specimens with the structure of equiaxial and weakly extended grains the oxide films are broken due to generation of transversal cracks propagating to basic metal. The oxide films with laminar structure oxidized in water are broken due to exfoliation along the interface between the layers of extended and equiaxial grains. Breakage of films with the structure of equiaxial and weakly extended grains starts at the stage of elastic deformation at a stress of 300 MPa. The highest breakage stresses of 1150 MPa are characteristic for alloy specimens oxidized in steam. Breakage of films with laminar structure after oxidation in water occurs at average stresses of 798 MPa. The cohesive/adhesive strengths of films oxidized in water and steam are nearly the same. In thick alloys, oxide films oxidized in water with lithium, substrate does not open even at loading of 100 N. According to data of Raman spectroscopy, oxide film is mainly comprised of monoclinic phase of zirconium oxide; however, after oxidation in steam at the metal—film interface, a barrier layer of tetragonal phase of zirconium dioxide has been detected that prevents accelerated oxidation.     

Influence of Selective Laser Melting Process Parameters on Microstructure and Properties of a Typical Ni-Based Superalloy

The influence of selective laser melting (SLM) process parameters on the microstructure and mechanical properties of a typical Ni-based superalloy was researched. The optimum parameters of P = 170 W, V = 0.8 m/s were determined, under which the SLMed samples exhibited both the largest relative density of 99.57% and the best mechanical properties, including the microhardness (329.3 ± 3.8 HV), yield strength (726 ± 8.1 MPa), ultimate tensile strength (900 ± 5.9 MPa) and elongation ((31.9 ± 0.24)%). The average grain size ranges of SLMed samples are from 15.2 to 17.4 μm, with a typical mixed grain structure. Owing to the high cooling rate and remelting during SLM process, a large number of low-angle grain boundaries (LAGBs), dislocations and sub-grains were formed, and the fraction of LAGBs reached above 65%. At the same time, the content of low-Σ coincidence site lattice (CSL) boundaries was mostly less than 1%, while there was almost no γ′ phase precipitated in the matrix. The texture of SLMed samples was weak, and there was no obvious preferred growth direction. Combining with the microstructure characterization, both grain refinement strengthening and dislocation strengthening were considered as the main strengthening mechanisms. Moreover, the fracture mechanism of the optimum sample belonged to ductile fracture.     

Effects of Ribbon Thickness on Structure and Soft Magnetic Properties of a High-Cu-Content FeBCuNb Nanocrystalline Alloy

The effects of ribbon thickness (t) on the structure and magnetic properties of a Fe_82.3B₁₃Cu_1.7Nb₃ alloy in melt-spun and annealed states have been investigated. Increasing the t from 15 to 23 μm changes the structure of the melt-spun ribbons from a single amorphous phase to a composite with dense α-Fe nanograins embedded in the amorphous matrix. The grain size (D_α-Fe) of the α-Fe near the free surface of the ribbon is about 6.7 nm, and it gradually decreases along the cross section toward the wheel-contacted surface. Further increasing the t to 32 μm coarsens the D_α-Fe near the free surface to 15.2 nm and aggravates the D_α-Fe ramp along the cross section. After annealing, the ribbon with t = 15 μm has relatively large α-Fe grains with D_α-Fe > 30 nm, while the thicker ribbons possessing the pre-existing nanograins form a finer nanostructure with D_α-Fe < 16 nm. The structural uniformity of the ribbon with t = 23 μm is better than that of the ribbon with t = 32 μm. The annealed ribbons with t = 23 and 32 μm possess superior soft magnetic properties to the ribbon with t = 15 μm. The ribbon with t = 23 μm exhibits a high saturation magnetic flux density of 1.68 T, low coercivity of 9.6 A/m, and high effective permeability at 1 kHz of 15,000. The ribbon with t = 32 μm has a slightly larger coercivity due to the lower structural uniformity. The formation mechanism of the fine nanostructure for the ribbons with suitable t has been discussed in terms of the competitive growth effect among the pre-existing α-Fe nanograins.     

Effect of Annealing on the Morphology,Structure and Photoelectric Properties of AZO/Pt/FTO Trilayer Films

Aluminum-doped zinc oxide/platinum/fluorine-doped tin oxide(AZO/Pt/FTO) trilayer films were prepared by sputtering 5-nm-thick Pt layers and 150-nm-thick AZO layers in sequence on commercial FTO glass.The effects of onestep annealing and layer-by-layer annealing on the morphology,structure and photoelectric properties of the AZO/Pt/FTO trilayer films were comparatively analyzed.It is found that the both annealing approaches increased the grain size and improved the crystallinity of the films,leading to enhancement in transmittance and conductivity.However,layer-by-layer annealing led to the formation of quasi-continuous or continuous AZO layers,different from the sparsely distributed AZO particles brought about by one-step annealing,resulting in excellent optical and electrical properties.Specifically,after layer-by-layer annealing at 400 ℃ for both Pt and AZO layers,the AZO/Pt/FTO trilayer film showed an increase in average transmittance from 71.3% to 85.3% and a decrease in sheet resistance from 7.5 to 5.6 Ω/□,leading to the highest figure of merit of 3.64 × 10~(-2) Ω~(-1).     

Influence of Rust Layers on the Corrosion Behavior of Ultra-High Strength Steel 300M Subjected to Wet–Dry cyclic Environment with Chloride and Low Humidity

The influence of rust layers on the corrosion behavior of ultra-high strength steel 300 M subjected to a simulated coastal atmosphere was investigated by corrosion weight loss, surface analysis techniques, and electrochemical methods.The results exhibit the presence of a large proportion of c-Fe OOH and a-Fe OOH and a small amount of Fe3O4 in the outer rust layer. During the wet–dry cyclic process, the bonding performance and the density of outer rust layer deteriorate with the thickness of outer rust. The inner rust layer plays a main role on protectiveness, which can be attributed to the formation of an ultra-dense and adherent rust film with major constituent of a-Fe OOH and a-Fe2O3 on the steel.     

Oxidation Behavior of Ni–Cr–Fe-Based Alloys: Effect of Alloy Microstructure and Silicon Content

The oxidation behavior of Ni–Cr–Fe-based alloys in a low oxygen partial pressure atmosphere (H₂–H₂O) was investigated in terms of the effect of alloy microstructure and their silicon content. It was found that the formation and growth kinetics of the oxide scale are rather sensitive to the alloy microstructure and their corresponding Si contents. Oxide ridges were found to form in areas with eutectic structure, while a thin and homogeneous oxide scale formed on austenite matrix. The thicknesses of the oxide ridges and the oxide layer on the austenite matrix were dependent of their corresponding Si contents. The austenite/carbide phase boundaries in eutectic structure can offer fast diffusion paths for metal outward diffusion, which leads to the formation of ridge-like oxide features. The continuous SiO₂ sub-layer formed at the oxide scale/metal interface on the austenitic matrix acted as an effective diffusion barrier to metal outward diffusion, resulting in rather thin and uniform oxide scales.     

Tetragonal phase stability in ZrO2 film formed on zirconium alloys and its effects on corrosion resistance

A thermodynamic model is developed to understand the origin of variation in the microstructure of ZrO₂ film formed on zirconium alloys and its effects on corrosion resistance. The correlation among the tetragonal phase fraction, the stress (macroscopic and internal one), the ZrO₂ grain size and the microstructural change of oxide film is formulized, and then analyzed. The results show that many complicated factors simultaneously govern the microstructure of oxide film. The tetragonal phase content near the oxide/metal interface, the macroscopic compressive stress near the interface, the decline gradient of macroscopic compressive stress and the internal stress induced by the transformation from the tetragonal to the monoclinic phase have very important influences on the transition from columnar grains to equiaxed grains, the crack formation and the degradation of oxidation resistance. The presence of intermetallic precipitates in oxide film may effectively relax the internal stress caused by transformation strain, stabilize the columnar-grain structure and reduce the probability of crack formation. How to reduce the transformation stress in the oxide film is a key to improve the corrosion resistance of zirconium alloys.     

High temperature oxidation of an oxide-dispersion strengthened NiAl

The oxidation behavior of an oxide-dispersion strengthened (ODS) NiAl has been studied between 900 and 1100°C in air. The dispersoids of mostly Al₂O₃ in fine-grained β-NiAl were incorporated by mechanical alloying (MA) in an argon atmosphere and hot pressing. It was found that excessive amounts of dispersoids and voids within the matrix had serious negative effects on the oxidation resistance of β-NiAl, by allowing for a more rapid formation of oxide scales and by providing fast diffusion paths for oxygen. Below the thin surface oxide scales consisted of -Al₂O₃, NiAl₂O₄ and Ni₂O₃, an internal oxidation zone was formed deep into the matrix. No metastable transient aluminas were formed during oxidation. The oxide ridge structure began to evolve after oxidation at 1100°C at the oxide–gas interface.     

Oxidation Performance and Interdiffusion Behaviour of two MCrAlY Coatings on a Fourth-Generation Single-Crystal Superalloy

The oxidation behaviour of a fourth-generation single-crystal superalloy without coating and with two types of MCrAlY coatings at 1140 °C was studied. The results showed that both coatings greatly improved the oxidation resistance of the superalloy, and the addition of Hf further improved the oxidation resistance by pinning the oxide layer into the coating. Before and after oxidation, obvious Cr and Al interdiffusion was detected. Inward Cr diffusion induces the precipitation of a topologically close-packed phase, while the diffusion of Al affects the structure of the γ/γ' phase, the solubility of refractory elements, and the formation of an interdiffusion zone.     

Nano-scale chemical analysis of rust on a 2% Si-bearing low alloy steel exposed in a coastal environment

Corrosion products of the rust layer formed on a 2% Si-bearing low alloy steel during atmospheric exposure at a coastal area in Japan for three years were characterized. The inner layer of the rust, i.e., near oxide/metal interface, has a layer structure and the average grain size of corrosion products is finer than that in the upper region that is few micro meter above the interface. A series of nano probe energy disperse X-ray spectroscopy (EDS) and selected area electron diffraction (SAED) analyses performed using a field emission gun (FEG) transmission electron microscope (TEM) revealed that the inner layer consists of an iron rich, coarse grained β-FeOOH matrix and the silicon enriched narrow band composed a mixture of α-FeOOH and nano-meter size silicon oxide. On the other hand, the α-FeOOH single phase area exists where the layer structure did not develop. The result of detailed microstructural analyses indicated that the silicon plays an important role in modifying the rust layer structure.     

Intergranular crack tip oxidation in a Ni-base superalloy

High-temperature intergranular crack tip oxidation under a single 600 s long sustained tensile load at 700 °C was studied for the Ni-base superalloy Allvac 718Plus. High-resolution analytical techniques showed oxidation to take place at and immediately ahead of the tip of an open crack, forming a closed but layered oxide structure about the prior (now oxidized) grain boundary. Near the prior grain boundary the oxide is Ni-rich, with a Co-enriched layer furthest away from the metal and a Fe-enriched region below this. A Cr-rich oxide is present below the outer Ni-rich oxides throughout the crack, also in the direction of crack growth. This is believed to have a hindering effect on oxidation ahead of the crack. Ni₃(Nb,Al) γ′ precipitates close to the grain boundaries were found to oxidize and form regions of near-stoichiometric NiO within the oxide layers. Remaining constituents of γ′ (e.g. Al and Nb) were found to be enriched in the surrounding oxidized matrix and also to produce thin oxide layers near the interface between the unoxidized metal and the Cr-rich oxide. The formation of the crack tip oxides is discussed with regard to thermodynamics, kinetics and the influence of applied mechanical load.     

A study of the stresses generated in zirconia films during the oxidation of zirconium alloys

The deformation kinetics of zirconium and Zircaloy-2 strips, resulting from oxidation at 500°C in dry oxygen, were measured as a function of time. The stress distribution in the oxidized specimens was analyzed using a stress model based on the theory of elasticity and was correlated with the oxidation kinetics. The magnitude of the stress generated during oxidation differed significantly between zirconium and Zircaloy-2; while a simple linear relationship appeared to exist between the stress and the oxide thickness on zirconium, the stresses in oxides on Zircaloy-2 increased very rapidly and discontinuously to a maximum at a film thickness near the transition in the kinetic curve. In order to characterize the stress distribution in the oxide film, vacuum annealing experiments were performed on preoxidized specimens. These indicated that the stress relaxation which occurs during vacuum annealing resulted from dissolution in the metal of the highly stressed inner layers of the oxide film. Although these measurements have provided valuable information on the stresses generated during the oxidation of zirconium and Zircaloy-2, they do not require a causal relationship between the stresses and any oxidation process (e.g., transition).     

Microstructure,Mechanical Properties and Die-Filling Behavior of High-Performance Die-Cast Al-Mg-Si-Mn Alloy

This work aims to reveal the relationships between the microstructure,mechanical properties and flow behavior of die-casting AlMg_5Si_2Mn alloy.Results indicated that the microstructure of the die-cast AlMg_5Si_2Mn consists of α_1-Al grains,fine-size α_2~Al grains and(Al +Mg_2Si) eutectic.The surface layer observed has the thickness in a range of120-135 μm,while an ellipse-like surface layer edge is observed in the corner of the plate-like sample.Tensile strength and elongation(5) of the specimens are slightly decreased along the die-filling direction due to the backflow of melt.Pure(Al + Mg_2Si) eutectic layer and ultra-fine-size α_2-Al grains observed are around the overflow channels.Mass feeding is predominantly responsible for the superior mechanical properties of the round bars as compared to those of plate-like samples.     

Untersuchung von Korrosionseffekten durch Natrium an Metallen der vierten und fünften Nebengruppe

A study of corrosion effects produced by sodium on metals of the 4th and 5th subgroups The action of liquid sodium on Ti, Zr, V and Nb involves exchange of oxygen from the liquid to te solid metal phase; because of the high energy of formation of the oxides of these metals Na₂O contents below 10 ppm are sufficient for initiating this reaction. The oxygen is taken up by te metal matrix directly, without formation of an oxide layer; oxygen take-up is always connected with embrittlement. In many cases oxygen becomes enriched in teh surface zone of the metal; in such cases several discrete layers are formed which have poor adhesion and different structure. Oxygen concentration can be assessed by microardness measurements.