9Cr低活化马氏体钢在超临界水中的腐蚀行为

利用扫描电子显微镜(SEM)、电子能谱(EDS)以及X射线衍射(XRD)技术研究了9Cr低活化马氏体钢在650℃/25MPa超临界水中的腐蚀行为。结果表明,9Cr低活化马氏体钢腐蚀产物的晶粒随腐蚀时间的延长而长大,晶粒尺寸从200h的5.7μm长大到1000h的10.1μm。表面形成的氧化膜为双层结构,外层为Fe3O4,内层由Fe3O4和FeCr2O4共同组成。由氧化增重结果获得了9Cr低活化马氏体钢在超临界水中腐蚀的氧化动力学表达式,同时其腐蚀机理表现为吸氧腐蚀。     

The effect of thermal explosion in a supercritical water

The conversion of hydrocarbons (eicosane, naphthalene, and synthetic bitumen) dissolved in super-critical water (SCW) was studied in a batch reactor at a pressure of P=30 MPa and a range of temperatures from 450 to 75°C. It was established that water participates in the conversion process on a chemical level: in particular, oxygen from water molecules is involved in the formation of carbon oxides. Even in the absence of added molecular oxygen, the process of naphthalene and bitumen conversion in a certain temperature interval exhibited an exothermal character. Upon adding O₂ into SCW, the oxidation reaction may proceed in a burning regime with self-heating of the mixture. Under certain conditions, the self-heating process may lead to the thermal explosion effect accompanied by ejection of the substance from the reactor, which is explained by the high rate of hydrocarbon burning in SCW.     

Synthesis of ZrO<Subscript>2</Subscript> nanoparticles during zirconium oxidation by supercritical water

We have discovered that massive samples of solid zirconium (Zr)_s are completely oxidized by supercritical water (SCW, T > 647 K, P > 22.1 MPa) with the formation of zirconium oxide nanoparticles (ZrO₂) _n . The particle size distribution, morphology, and features of the nanostructure formation depend on the process conditions. The kinetics of H₂ production and zirconium oxidation has been determined using the method of SCW injection into a reactor with (Zr)_s at various temperatures. The dependence of the oxidation induction time on the SCW parameters has been studied.     

Creep Behavior in Interlaminar Shear of a SiC/SiC Ceramic Composite with a Self-healing Matrix

Creep behavior in interlaminar shear of a non-oxide ceramic composite with a multilayered matrix was investigated at 1,200 °C in laboratory air and in steam environment. The composite was produced via chemical vapor infiltration (CVI). The composite had an oxidation inhibited matrix, which consisted of alternating layers of silicon carbide and boron carbide and was reinforced with laminated Hi-Nicalon? fibers woven in a five-harness-satin weave. Fiber preforms had pyrolytic carbon fiber coating with boron carbide overlay applied. The interlaminar shear properties were measured. The creep behavior was examined for interlaminar shear stresses in the 16–22 MPa range. Primary and secondary creep regimes were observed in all tests conducted in air and in steam. In air and in steam, creep run-out defined as 100 h at creep stress was achieved at 16 MPa. Larger creep strains were accumulated in steam. However, creep strain rates and creep lifetimes were only moderately affected by the presence of steam. The retained properties of all specimens that achieved run-out were characterized. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Notch Sensitivity of Fatigue Behavior of a Hi-Nicalon™/SiC-B4C Composite at 1,200 °C in Air and in Steam

The effect of holes on the fatigue life of a non-oxide ceramic composite processed via chemical vapor infiltration (CVI) was examined at 1,200 °C in laboratory air and in steam. The effect of holes on tensile strength at 1,200 °C was also evaluated. The composite comprised laminated woven Hi-Nicalon? fibers in an oxidation inhibited matrix, which consisted of alternating layers of silicon carbide and boron carbide. Fiber preforms had pyrolytic carbon fiber coating with boron carbon overlay applied. Unnotched specimens and specimens with a center hole having a radius to width ratio of 0.24 were tested in tension-tension fatigue at 0.1 Hz and at 1.0 Hz. The fatigue stresses ranged from 100 to 140 MPa in air and in steam. Fatigue run-out was defined as 10⁵ cycles at 0.1 Hz and as 2?×?10⁵ cycles at 1.0 Hz. The net-section strength was less than the unnotched ultimate tensile strength. Comparison of notched and unnotched data also revealed that the fatigue performance was notch insensitive in both air and steam environments. Composite microstructure, as well as damage and failure mechanisms were investigated.     

High temperature oxidation behavior of ferritic steel in supercritical water at 550–700°C

Oxidation tests were conducted on ferritic steel T22 exposed to deaerated supercritical water at 550–700°C and 25 MPa. Oxide films formed on T22 had a double-layered structure with the outer layer consisting of iron oxide and the inner layer consisting of spinel oxide. Pores formed on the surface of samples initially but healed at longer exposure time and higher temperature. Cracks occurred along the grain boundaries in the oxide scale at 600–700°C for 200 and 400 h. The oxidation kinetics obeyed a near-parabolic law in all cases. The data of activation energy of T22 indicated that the likely oxidation rate-controlling step may be the outward diffusion of iron along the magnetite bulk.     

Impacts of temperature and surface finish upon steam oxidation of austenitic steel TP347HFG

Abstract

The effects of the steam oxidation process on an austenitic steel (TP347HFG) exposed under isothermal conditions between 600 and 800 °C for up to 2500 h have been investigated. Samples with both as-received and ground surfaces have been exposed and the impact of surface finish on the oxidation process analysed using scanning electron microscopy with energy dispersive X-ray analysis. Exfoliated oxide flakes have also been examined to characterise their microstructures on fractured sections as well as external and spalled surfaces. Microscopic analyses demonstrated that ground surfaces possess better steam oxidation resistance than as-received surfaces due to their ability to form a more protective chromium-rich layer. The formation of regions of thicker multi-layered oxides was noted on both types of surface finish, covering large areas on as-received surfaces and only nodules on ground surfaces (spreading with increasing exposure temperature and time).     

Tension-Compression Fatigue of a Nextel™720/alumina Composite at 1200 °C in Air and in Steam

Tension-compression fatigue behavior of an oxide-oxide ceramic-matrix composite was investigated at 1200 °C in air and in steam. The composite is comprised of an alumina matrix reinforced with Nextel?720 alumina-mullite fibers woven in an eight harness satin weave (8HSW). The composite has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. Tension-compression fatigue behavior was studied for cyclical stresses ranging from 60 to 120 MPa at a frequency of 1.0 Hz. The R ratio (minimum stress to maximum stress) was ?1.0. Fatigue run-out was defined as 10⁵ cycles and was achieved at 80 MPa in air and at 70 MPa in steam. Steam reduced cyclic lives by an order of magnitude. Specimens that achieved fatigue run-out were subjected to tensile tests to failure to characterize the retained tensile properties. Specimens subjected to prior cyclic loading in air retained 100 % of their tensile strength. The steam environment severely degraded tensile properties. Tension-compression cyclic loading was considerably more damaging than tension-tension cyclic loading. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Oxidation and chromia evaporation of austenitic steel TP347HFG in supercritical water

The evaporation of protective chromia scales is expected to be a primary oxidation mechanism. An oxidation test of austenitic steel TP347HFG in supercritical water was carried out at 550 and 600°C under 25 MPa with a dissolved oxygen content of 2000 ppb. The results show that the protective chromia scale forms at the earlier stage and then disappears gradually with the increasing exposure time due to chromium evaporation in the presence of both dissolved oxygen and supercritical water. The effect of chromia evaporation on oxidation resistance of austenitic steels in supercritical water is discussed.     

An Evaluation of Quality of Joints of Two Dissimilar Metals by Diffusion Bonding using Ultrasonic C Scan

Diffusion bonding of commercially available pure aluminum/copper was carried out between the temperatures of 400°C and 500°C for 60 min under the pressure of 5–15 MPa in vacuum. The effects of temperature and pressure on the microstructure of aluminum/copper diffusion bonded joints were analyzed. The interface micrographs of the bonded samples were observed in optical and scanning electron microscope (SEM) images. The soundness of the bond was evaluated by destructive and nondestructive (ultrasonic C scan) testing methods. The quality of the bonded joints was evaluated by the intensity of the echo and its images of ultrasonic testing and was correlated with destructive parameters such as the strength ratio. Chemical compositions of the interface and the fractured surface of the bonded samples were characterized by energy dispersive spectroscopy (EDS). EDS patterns were confirmed by the formation of the different compositions at the interface of the bonded samples. Better bonding characteristics were observed by diffusion bonding optimum parameters at 450°C with an applied pressure of 15 MPa for 60 min.     

The effect of thermal cycling on steam oxidation behaviour of TP347H FG at 650 °C

Abstract

The cyclic oxidation behaviour of fine-grained Type 347 stainless steel (TP347H FG) at 650 °C in air saturated steam and deoxygenated steam environments for 100–1000 h has been investigated. Electron microscopy, Energy Dispersive and Wavelength Dispersive X-ray Spectroscopy (EDS and WDS, respectively) have been used to characterise the samples. Short term oxidation tests have shown only haematite spallation occurs whereas longer term tests have shown magnetite also spalls on cooling to room temperature. In all cases cyclic oxidation showed spallation does not occur after long term tests and is only visible in small amounts after short term tests subsequent to the initial spallation event.     

On the influence of alloy composition on the oxidation performance and oxygen-induced phase transformations in Ti–(0–8) wt%Al alloys

Adopting a high-throughput combinatorial approach, a compositionally graded Ti–xAl (0 ≤ x ≤ 8 wt%) specimen was prepared to conduct a rapid systematic investigation of the influence of composition and exposure time on the oxidation performance of the titanium-rich section of the binary Ti–Al system. The compositionally graded specimen was solution heat treated and subjected to oxidation tests at 650 °C for different exposure times. The morphology, structure, and composition of the oxide scale as well as the microstructural changes in the base material were studied across the entire composition range, using a suite of characterization techniques. The observations revealed the presence of Al₂O₃ in the topmost layer of the oxide scale in addition to TiO₂, indicating its early formation during oxidation. An increase in Al concentration improves the scaling rate of Ti; however, this is observed only for extended exposure times (i.e., 50 and 100 h), and a parabolic oxidation law is obeyed in the composition-time domain. The formation of the α₂ phase (Ti₃Al) also takes place for relatively higher Al contents (i.e., 8 wt%).     

Characterisation of oxide scales developed on high temperature resistant alloys in pure steam environments

Abstract

Steam oxidation of heat exchanger tubes and pipe work is of growing interest as research into the improvement of power plant efficiencies shows the need for much higher steam temperatures and pressures. This paper reports on the characterisation of the oxide scales grown during the steam oxidation of four alloys (T23, T92, TP347HFG and Inconel 740) in atmospheric pressure steam at four temperatures (600, 650, 700 and 750°C) for periods of 250, 500 and 1000 h. Three methods have been employed in analysing these scales: reflected light optical microscopy, scanning electron microscopy with energy dispersive X-ray analysis and X-ray diffraction.

The thickness, composition, morphology and spalling behaviour of the oxides differed with alloy composition, exposure times/temperatures and sample shapes. The ferritic steels exhibited the most severe oxidation, with the scales formed on these typically being triple-layered: an inner layer of Fe – Cr spinel, central layer of magnetite and outermost layer of haematite. However, the amount of haematite formed changed with the exposure time/temperature, alloy and sample orientation. In comparison TP347HFG and Inconel 740 showed significantly slower oxidation, with generally thin oxide scales (<5 µm) developing even at the highest exposure temperatures, though TP347HFG started to form some nodular growths after 1000 h exposure at the two higher temperatures.     

Oxidation behavior of a new Fe–Ni–Cr-based alloy in pure steam at 750 °C

The isothermal oxidation of a new Fe–Ni–Cr-based alloy has been investigated in pure steam at 750 °C for exposure time up to 500 h using secondary electron microscope (SEM)/ X-ray energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD). Results showed that the alloy was oxidized approximately following a parabolic law with a parabolic rate constant k_p of 2.36 × 10^?13 g²/m⁴/s. As revealed by SEM/EDS and XRD results, a duplex-layered external oxide scale was formed, consisting of a thin outer layer of Ni(Fe, Al)₂O₄ and a thicker inner layer of (Cr, Mn)₂O₃. Underneath the external oxide scale, the internal oxidation of Ti to be TiO₂ occurred particularly along the grain boundaries of the matrix alloy. Internal oxide of Al₂O₃ was also observed but at a deeper depth. Based on the detailed compositional and microstructural characterization of the oxidized zone, the mechanism of the external and internal oxidation in steam is presented.     

Injection Parameters Optimization and Artificial Aging of Automotive Die Cast Aluminum Alloy

High-pressure die castings are expected to be used in the near future as high-duty structural components in the automotive industry. The effects of die casting parameters and aging on the tensile properties of high-performance die cast aluminum alloy are therefore investigated in this work. Our results indicate that HPDC AlMg5Si2Mn specimens (formed under an injection pressure of 100 MPa, high-level fast-shot velocity, and speed transition point location 220 mm) possess good internal quality and superb tensile properties (351.1 MPa, 200.7 MPa, 13.77%). Sample density decreased along the die filling direction due to pressure loss. After 3 h aging at 250 °C, tensile strength and yield strength were significantly increased from 351.1 and 200.7 MPa to 380.5 and 246.9 MPa, respectively. Elongation decreased initially from 13.77 to 5.5% after 1 h aging and then recovered to 11.48%. In addition, the effect of cooling methods on mechanical properties was found to be insignificant.     

The effect of constant electric field on the oxidation rate of massive zinc in supercritical water and the formation of zinc oxide nanocrystals

It is established that the rate of oxidation of a massive zinc plate by supercritical water (SCW) at 673 K and 23 MPa increases and the morphology of obtained ZnO nanocrystals changes when strength E of the constant electric field applied perpendicular to the plate increases from 0 to 286 kV/m. A decrease in the SCW density at the same temperature results in the formation of a more compact nanostructured ZnO layer, while the increase in E leads to loosening of structure in the inner part of the ZnO layer.     

Rate-Dependent Elasto-Viscoplastic Constitutive Model for Industrial Powders. Part 1: Parameter Quantification

ABSTRACT

The rate-dependent mechanical behavior of a dry industrial powder (MZF powder) was studied using a cubical triaxial tester (CTT) within the context of a new elasto-viscoplastic model (PSU-EVP model). The compression and shear properties of the powder were quantified at compression rates of 0.62, 6.21, and 20.7 MPa/minute with pressures up to 11 MPa. Test results demonstrated that the compression and shear responses of the powder were nonlinear, consistent, and reproducible (coefficient of variation or COV ≤ 15%). Also, MZF powder exhibited varying elastic and plastic deformation at different pressure levels that were quantified using statistical correlations (R² > 0.90). For example, the average bulk modulus and shear modulus values for MZF powder increased linearly with pressure (R² > 0.90) at all compression rates. The failure stress values also increased with the increase in mean pressure. For instance, at a compression rate of 0.62 MPa/minute, failure stress increased from 5.0 to 13.3 MPa as the confining pressure increased from 2.2 to 8.5 MPa. Similar effects were noted at compression rates of 6.21 and 20.7 MPa/minute. Overall, failure stress decreased with increasing compression rate. From the data collected, it was demonstrated that compression rate does have substantial effect on the compressibility and shear behavior of powders that can be quantified using the CTT and is suitable for use in the PSU-EVP model.     

Characterization of alloy 3033 after exposure to superheated steam at 800°C

Alloy 3033 was evaluated in superheated steam (SHS) at 800°C for a duration of 3000 hours. The SHS was able to simulate the supercritical water (SCW) condition at higher temperatures which no commercial SCW rig is currently capable of reaching. After exposure to the SHS, the weight change and surface oxide formation of Alloy 3033 were analyzed. Alloy 3033 had an initial weight gain after 1000 hours; however, the net weight gain reduced after 2000 and 3000 hours of exposure, suggesting oxide spallation. Formation of both Cr₂O₃ and MnCr₂O₄ was observed on the surface after 2000 and 3000 hours of SHS exposure. However, as exposure progressed, the XRD peak intensity ratio of MnCr₂O₄ to Cr₂O₃ decreased, in addition to the observation of more exposed Cr₂O₃. Based on this preliminary investigation, Alloy 3033 may not be suitable for extended use in SHS due to weight loss associated with oxide spallation.     

Oxidation behaviour of an intermetallic Ti–Al–Cr–Zr bond coat on a γ–TiAl based TNB alloy with 7YSZ thermal barrier coating

Abstract

Intermetallic titanium aluminide alloys are attractive light-weight materials for high temperature applications in automotive and aero engines. The development of γ-TiAl alloys over the past decades has led to their successful commercial application as low pressure turbine blades. The operating temperatures of γ-TiAl based alloys are limited by deterioration in strength and creep resistance at elevated temperatures as well as poor oxidation behaviour above 800 °C. Since improvement in oxidation behaviour of γ-TiAl based alloys without impairing their mechanical properties represents a major challenge, intermetallic protective coatings have aroused increasing interest in the last years.

In this work, a 10 μm thick intermetallic Ti–46Al–36Cr–4Zr (in at.-%) coating was applied on a TNB alloy using magnetron sputtering. This layer provided excellent oxidation protection up to 1000 °C. Microstructural changes in this coating during the high temperature exposure were extensively investigated using scanning and transmission electron microscopy. The coating developed a three-phase microstructure consisting of the hexagonal Laves-phase Ti(Cr,Al)₂, the tetragonal Cr₂Al phase and the cubic τ-TiAl₃ phase. After long-term exposure the three-phase microstructure changed to a two-phase microstructure of the hexagonal α₂-Ti₃Al phase and an orthorhombic body-centred phase, whose crystal structure has not yet been definitely identified. On the coating, a thin protective alumina scale formed. Applying this intermetallic layer as bond coat, thermal barrier coatings (TBCs) of yttria partially stabilized zirconia were deposited on γ-TiAl based TNB samples using electron-beam physical vapour deposition. The results of cyclic oxidation testing (1 h at elevated temperature, 10 min. cooling at ambient temperature) revealed a TBC lifetime of more than 1000 h of cyclic exposure to air at 1000 °C. The ceramic topcoat exhibited an excellent adhesion to the thermally grown alumina scale which contained fine ZrO₂ precipitates.     

Characterization of the viscoelastic behavior of bismaleimide resin before and after exposure to high temperatures

Creep and high strain rate mechanical properties, shrinkage strain, and thermal properties of a bismaleimide neat resin after exposure to a high temperature in air were evaluated and compared with the corresponding properties for a pristine resin. Under tension at a strain rate of 6×10^?4 s^?1, the Young’s modulus decreases and Poisson’s ratio increases with temperature, measured up to 310 °C. The tensile creep behavior was determined at stress levels of 12, 24, and 33 MPa at elevated temperatures. At each stress level, the creep compliance curves at different temperatures were shifted horizontally to form a master curve. These creep compliance master curves are nearly identical, indicating a linearly viscoelastic behavior up to 33 MPa. The bismaleimide resin was also exposed to air at other temperatures of 245, 260, and 280 °C for 1500 hours. After exposure to a high temperature, three regimes were observed in the resin through optical micrographs: an outer layer showing darker color, an interior that nearly maintained its original color, and a transition (or reacting) region in between. The average shrinkage on surface was determined as 3.4 % strain after 1500 hours of exposure to 260 °C in air. Compression at a high strain rate using a long split Hopkinson pressure bar shows that the bulk bismaleimide resin is rather insensitive to the exposure to a high temperature, exhibiting only a slight reduction in mechanical properties after 1500 hours of exposure to 245 °C. The uniaxial creep compliance of the neat resin was converted into the Young’s relaxation modulus, which was then used to calculate the Young’s modulus under tension at the strain rate and temperatures involved, and a good agreement was achieved between the calculated results and the experimental data, indicating that the rate-dependent Young’s modulus is the representation of viscoelastic properties.