High temperature tensile properties of oxide dispersion strengthened T91 and their correlation with microstructural evolution

Abstract

In the present work, high temperature deformation behavior of oxide dispersion strengthened T91 was investigated and linked to the corresponding microstructure. First, tensile properties are presented and discussed in terms of yield strength, tensile stress and total elongation as a function of temperature. The results are compared to the matrix material and other ODS alloys. Second, transmission electron microscopy was applied to as received and deformed tensile test specimens. It is shown that the Y₂O₃ particle diameter increases slightly upon deformation at elevated temperatures. Additionally, distinctive coarsening of M₂₃C₆ carbides occurs at prior austenite grain boundaries. At temperatures above 500°C, dislocations are straight and pile up at grain boundaries due to thermally activated climbing. Oxide dispersion strengthened T91 provides high strength due to strong particle/dislocation interactions and good toughness properties.     

Room temperature synthesis and characterization of different morphological TbF3 nano/microcrystals

At room temperature, a simple solution-based procedure employing NaBF₄ as the fluoride source has been developed to selectively prepare different morphological TbF₃ nano/microcrystals (disks, peanuts and spindles) with an orthorhombic structure. The crystal structure, morphology and photoluminescence properties of the as-prepared TbF₃ crystals were investigated by x-ray powder diffraction (XRD), scanning electron microscopy (SEM) and fluorescence spectrophotometer. The results revealed that large-scale and uniform disks with a mean thickness of 20 nm can be easily synthesized, and the spindle-like and peanut-like TbF₃ crystals were assembled from nanodisks. The effects of synthesis parameters such as NaBF₄, reaction time and molar ratio of the reactants were systematically investigated. The room temperature PL properties of these different morphological TbF₃ microcrystals were measured.     

Synthesis of alpha-alumina nanoribbons: characterization and their morphological evolution

Single crystalline α-alumina nanoribbons were synthesized by reacting aluminum with silicon monoxide at high temperature. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), and high-resolution transmission electron microscope (HRTEM). The intense peaks of XRD pattern indicate that the prepared nanoribbons have a high degree of crystallinity. In the present work, silicon monoxide was used as an oxidant and precursor, which served to control the reaction rate. An interesting morphological evolution that nanoribbons resulted from nanosaws was described, and these novel nanosaws were also carefully characterized.     

Sintering and characterization of bismuth-oxide-containing zinc oxide varistors

Zinc oxide with different contents of Bi₂O₃ was prepared via a solid-state reaction to be used as varistors. Sintering was performed at 1200 and 1300 °C. Densification was achieved through liquid-phase sintering. A zincite phase, together with bismuth zinc oxide (Bi_7.65Zn_0.35O_11.83) and zinc bismuth oxide (Bi_25.33Zn_0.667O₄₀) phases, was formed at 1200 °C. A free Bi₂O₃ phase was still present at 1200 °C. Only the zincite phase was developed, and the other phases disappeared at 1300 °C. I–V characteristics show nonlinear behavior in all samples. The behavior was expected to be a result of the development of intergranular phases that crystallized from the Bi-rich liquid phase.     

国内外纳米ZnO研究和制备概况

概述了纳米ZnO的应用前景及国内外的研究现状,对纳米ZnO各种制备方法的基本原理、影响因素、产物粒径大小等进行了详细的分析讨论,同时提出了每种工艺的优缺点,并提出了研究方向。对其表征方法进行了介绍。     

Lubricious zinc oxide films: synthesis, characterization and tribological behaviour

Solid lubricants that are effective over an extreme range of operating temperatures are necessary for the development of new generation high-performance gas turbine engines with increased propulsion capability. While oxides have the potential to perform as high-temperature lubricants, they typically have high friction and create abrasive wear debris at low temperature. The objective of this work was to create oxides that have good tribological properties at room temperature through control of microstructure and stoichiometry. Zinc oxide films were grown by pulsed-laser deposition. The stoichiometry and microstructure of the films were controlled by adjusting substrate temperature and oxygen partial pressure during pulsed-laser deposition. Chemistry and microstructure were probed using SEM, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy. Friction coefficients and wear life were measured using a ball-on-flat tribometer. The degree of similarity of the coatings to bulk ZnO was RT, vac<RT, O2<300°C, vac<300°C, O2. Coatings with oxygen deficiency and nanoscale structure have low friction (i.e. μ<0.2) and long wear lives (i.e., greater than 106 cycles) at room temperature. As the chemistry and crystal structure of a coating approaches that of bulk ZnO, its tribological properties degrade and can become load/speed sensitive. An important result of this study is that oxides can be made to provide good tribological properties at room temperature. Thus, there is significant potential to produce low-friction, low-wear oxide coatings for wide-temperature range applications by controlling nanostructure and oxygen vacancies. This revised version was published online in November 2006 with corrections to the Cover Date.     

Mechanical characterization of microwave sintered zinc oxide

The mechanical characterization of microwave sintered zinc oxide disks is reported. The microwave sintering was done with a specially designed applicator placed in a domestic microwave oven operating at a frequency of 2.45 GHz to a maximum power output of 800 Watt. These samples with a wide variation of density and hence, of open pore volume percentage, were characterized in terms of its elastic modulus determination by ultrasonic time of flight measurement using a 15 MHz transducer. In addition, the load dependence of the microhardness was examined for the range of loads 0.1–20 N. Finally, the fracture toughness data (K _IC) was obtained using the indentation technique.     

Oxidation behaviour and microstructural evolution of FeCrAl ODS alloys at high temperature

Abstract

The oxidation behaviour of a commercial oxide dispersion strengthened (ODS) FeCrAl alloy (MA 956) and of a dispersion-free FeCrAl alloy (APM) were studied during isothermal exposures in air between 1,100°C and 1,300°C. After short heat treatments, the oxide film developed on MA 956 was more protective than that developed on APM. Longer isothermal exposures were conducted on MA 956 in order to examine the long-term behaviour of the oxide layer and the microstructural evolutions of the alloy. Chemical analyses of the substrate for increasing treatment duration revealed a continuous decrease of the matrix aluminium content due to oxide growth. After 4 months at 1,300°C, aluminium content of the alloy decreased to a critical value below which a continuous and protective oxide film could no longer be formed. Longer exposure times were carried out in order to relate the different stages of the catastrophic breakaway oxidation and identify the parameters involved in this phenomenon. Other substrate evolutions like cavity growth and nature and morphology changes of the Y₂O₃ particles were observed and are discussed in relation with the oxidation behaviour.     

Friction-stir welding: microstructural characterization

 The microstructures of friction-stir welded aluminum alloys (1100 and 6061) are dominated by dynamic recrystallization. Friction-stir welding of 6061 aluminum to copper produced a complex, intercalated microstructure which was also dominated by dynamic recrystallization. The friction-stir welding process is characterized by extreme plastic deformation in the solid state; there is no associated melting. Received: 5 November 1997 / Accepted: 13 December 1997     

Single-photon emission and quantum characterization of zinc oxide defects

Room temperature single-photon emission and quantum characterization is reported for isolated defects in zinc oxide. The defects are observed in thin films of both in-house synthesized and commercial zinc oxide nanoparticles. Emission spectra in the red and infrared, second-order photon correlation functions, lifetime measurements, and photon count rates are presented. Both two- and three-state emitters are identified. Sub-band gap absorption and red emission suggest these defects are the zinc vacancy. These results identify a new source of single photons in a readily available wide band gap semiconductor material which has exceptional electrical, optical, and biocompatibility properties.     

Growth and characterization of electrosynthesised zinc oxide thin films

Zinc oxide (ZnO) films have been electrodeposited from an aqueous solution containing 0.1 M zinc nitrate as the electrolyte with pH around 5±0.1. The deposition was carried out by galvanostatic reduction with an applied cathodic current density in the range between 5 and 20 mA cm⁻². The influence of bath composition on the preparation of ZnO films is studied. The effects of zinc nitrate concentration and cathodic current density on the deposition rate of ZnO films were also studied. An optimum current density of 10 mA cm⁻² is identified for the growth of ZnO film with improved crystallinity and optical transmittance. The crystalline structure of the deposits studied by X-ray diffraction reveals the possibility of growing hexagonal ZnO films under suitable electrochemical conditions. The surface morphological studies by scanning electron micrographs revealed the presence of nodular appearance for films deposited at 800 °C bath temperatures.     

Growth of tin oxide rod-like and sheet-like structures

We have obtained the rod-like and sheet-like structures of tin oxide (SnO₂) by carrying out the thermal evaporation of solid Sn powders with varying the O₂ partial pressure. We have employed X-ray diffraction, scanning electron microscope, transmission electron microscope and photoluminescence (PL) spectroscopy to characterize the as-synthesized product. Higher O₂ partial pressure in the synthesis process mainly gave rise to wider sheet-like structures, whereas lower O₂ partial pressure facilitated the growth of rod-like structures. The product was single crystalline rutile structure. PL spectra exhibited the visible light emission.     

Ultra-high temperature Mo-Si-B alloys — Synthesis, microstructural and mechanical characterization

Molybdenum boron silicides containing between 18 and 31 vol.% α-Mo are prepared by reactive hot-pressing from Mo-Si-B powder mixtures. Their microstructures and mechanical properties are investigated. The multiphase alloys consist of Mo₃Si, Mo₅SiB₂ and discontinuous α-Mo. The results demonstrate that the volume fraction of α-Mo and B content exerts a significant effect on the flexural strength and fracture toughness of the multiphase alloys.     

High temperature behavior of Ni-base weld metal: Part I. Ductility and microstructural characterization

The ductility dip cracking resistance of two Ni-base Filler metals, Filler Metal 52 (FM-52) (59%Ni–29%Cr–9%Fe–1%Mn–1%Al) and Filler Metal 82 (FM-82) (72%Ni–20%Cr–1%Fe–3%Mn–3%Nb) was evaluated using the strain-to-fracture (STF) test. The reinterpretation of previously reported STF test results for these filler metals, is discussed, based on detailed microstructural characterization performed using electron microscopy. In Part II of this paper, new insight into the creep-like, grain boundary (GB) sliding mechanism responsible for intergranular cracking in these filler metals is presented.     

Preparation and characterization of solid-state sintered aluminum-doped zinc oxide with different alumina contents

Aluminum-doped zinc oxide (AZO) ceramics with 0− 2 ·5 wt.% alumina (Al₂O₃) content were prepared using a solid-state reaction technique. It was found that AZO grains became finer in size and more irregular in shape than undoped ZnO as the Al₂O₃ content increased. Addition of Al₂O₃ dopant caused the formation of phase transformation stacking faults in ZnO grains. The second phase, ZnAl₂O₄ spinel, was observed at the grain boundaries and triple junctions, and inside the grains. In this study, a 3-inch circular Al₂O₃ (2 wt.%)-doped ZnO ceramic target sintered at 1500°C for 6 h has a relative density of 99·8% with a resistivity of 1·8 × 10^− 3 Ω-cm. The AZO film exhibits optical transparency of 90·3% in the visible region and shows an electrical resistivity of 2·5 × 10^− 3 Ω-cm.     

Preparation of nanocrystalline zinc carbonate and zinc oxide via solid-state reaction at room temperature

The precursor mixture was obtained by grinding ZnSO₄·7H₂O and NH₄HCO₃ with surfactant OP thoroughly at room temperature. Then soluble inorganic salts were removed by washing with water. After drying, the pure nanocrystalline ZnCO₃ was obtained. ZnO powder was obtained by one-step thermal decomposition of the ZnCO₃ at 400 °C for 1.5 h. Both the ZnCO₃ and the ZnO were characterized by TG/DTA, XRD and TEM. The particle size of the ZnCO₃ and the ZnO was found to be 50 nm and 20 nm, respectively. The smaller particle size of the ZnO suggests a fact that the crystals of the ZnCO₃ play a role in the self-crushing reaction.     

四针状ZnO晶须的掺杂及其表征

在平衡气量法制备四针状ZnO晶须过程中添加金属锡以研究其对晶须形貌的影响，采用高温固熔表面处理法制备了铝掺杂ZnO晶须。用X射线衍射分析方法和扫描电子显微镜分别对其结构和形貌进行了表征。结果发现，锡在反应过程中起催化剂的作用，添加10％～15％(质量分数)的单质锡能促进ZnO的C轴择优生长，更多锡的加入则抑制C轴生长。铝以置换固溶体和ZnAl2O4两种形式存在于ZnO晶体中。     

Microwave-assisted synthesis and characterization of flower shaped zinc oxide nanostructures

A microwave-assisted solution-phase approach has been applied for the synthesis of zinc oxide nanostructures. The synthesis procedure was carried out by using two reagents: hydrazine hydrate and ammonia. Flower shaped particles were obtained with hydrazine hydrate whereas mainly spherical agglomerated particles were observed with ammonia. The nanostructures were influenced by microwave irradiation time, reagent concentration and molar ratio of the precursors. High crystalline materials were found without the need of a post-synthesis treatment. The average crystalline size of ZnO nanostructures has been analyzed by X-ray Diffraction (XRD) pattern and estimated to be 18 nm. The presence of flower shaped zinc oxide with nanorods arranged has been confirmed from Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) micrographs. The samples were further analyzed by Fourier Transform InfraRed (FT-IR), Thermogravimetric Analysis (TGA) and photoluminescence spectroscopic techniques.