Approaches to enhance elevated temperature erosion resistance of Ni-base superalloys

Erosive wear is also known as impact wear. Several industrial components are degraded due to solid particle erosion at high temperatures. Solid particle erosion of metallic materials at high temperatures is influenced by the nature of interaction between erosion and oxidation. The main objective of the present work is to critically examine how strength and oxidation behaviour can be tailored to enhanced resistance to solid particle erosion of Ni-base superalloys at high temperatures. It is noted that alloys which form Al₂O₃ scale are likely to have good erosion resistance. This study examines methods of improving the erosion resistance by enhancing the elevated temperature mechanical properties. Methods for forming various scales and improving their adhesion characteristics are also elaborated.     

Microstructural characterization of the failures of thermal barrier coatings on Ni-base superalloys

Abstract

Typical thermal barrier coating (TBC) systems consist of a nickel-base superalloy substrate coated with a MCrAlY or diffusion aluminide bond coat, onto which is deposited a yttria-stabilized zirconia (YSZ) TBC. The bond coats are usually deposited via diffusion aluminizing processes or low pressure plasma spray processes (LPPS). The YSZ can be deposited by air plasma spraying (APS) or electron beam physical vapor deposition (EBPVD). A layer of thermally-grown oxide (TGO), which is usually alumina, forms between the bond coat and YSZ during TBC deposition and subsequent high-temperature exposure. The conventional wisdom is that APS coatings tend to fail in the YSZ and that EBPVD coatings tend to fail at the interface between the TGO and bond coat. However, current research has shown that the situation is much more complex and that the actual fracture path can be a function of the type of bond coat, the type of high-temperature exposure, and coating process parameters. This paper describes the results of a study of the failure of state-of-the-art EBPVD TBCs deposited on NiCoCrAlY and platinum-modified diffusion aluminide bond coats. The failure times and fracture morphology are described as a function of bond coat type. The failure times were found to be a strong function of temperature for both bond coats. The failure for NiCoCrAlY bond coats was found to initiate at defects in the coating, particularly at the TGO/YSZ interface, but the fracture propagated primarily along the TGO–bond coat interface. The failure times and morphologies for platinum-modified diffusion aluminide bond coats depended strongly on bond coat surface preparation. The mechanisms for failure of the two bond coats are described. Also, the effects of modifications to the bond coats and variations in processing parameters on these mechanisms are presented.     

Comparison of the quench rates attained in gas-atomized powders and melt-spun ribbons of Co- and Ni-base superalloys: influence on resulting microstructures

The cooling rates of gas-atomized Stellite 6 powders range from 10³ to 10⁶ K sec^–1 for 400 to 10m diameter powder particles, respectively. The microstructures vary from the equiaxed type in the coarser particles to dendritic and finally microcrystalline in the finer particles. In the melt-spun ribbons, the microstructure along a cross-section of the ribbon changes from microcrystalline at cooling rates higher than 10⁷ K sec^–1 to columnar dendritic at cooling rates of the order of 10⁶ K sec^–1.     

无限长铁路声屏障对运行列车噪声降噪模型研究

利用国家标准GB/T172 47.2— 1998中声屏障对点声源插入损失的计算公式 ,得到无限长声屏障对线声源的降噪模型 ,并针对一个特定问题在MATLAB软件中编制计算程序 ,计算结果和实测结果以及经典图表进行比较 ,证明模型是正确可行的。     

Improving resistance to dynamic embrittlement and intergranular oxidation of nickel based superalloys by grain boundary engineering type processing

Abstract

Polycrystalline nickel based superalloys are prone to grain boundary attack by atmospheric oxygen either in the form of time dependent intergranular cracking during dwell time within a low cycle fatigue loading spectrum, known as hold time cracking, or in the form of intercrystalline oxidation at higher temperatures. In the case of hold time cracking of IN718 it has been shown that the crack propagation velocity is determined by local microstructure and environmental conditions, reaching values up to 10 μm s^?1 under four-point bending conditions at 650°C in air. The governing mechanism for this kind of time dependent quasi-brittle intergranular failure has been recognised to be 'dynamic embrittlement', i.e. diffusion of the embrittling element into the elastic stress field ahead of the crack tip, followed by stepwise decohesion. In a very similar way to intercrystalline oxidation, this damage mechanism seems to depend on the local microstructure. Assuming that oxygen grain boundary diffusivity is particularly slow for special coincident site lattice (CSL) grain boundaries, bending and oxidation experiments were carried out using specimens that underwent successive steps of deformation and annealling, i.e. grain boundary engineering. It has been shown that an increase in the fraction of special CSL grain boundaries yields a higher resistance to both intercrystalline oxidation and hold time cracking by dynamic embrittlement.     

Surface resistance of type II superconductors: The surface barrier effect

The surface impedance of a type II superconductor plate occurring in the mixed state was calculated. In the case when a constant external magnetic field is perpendicular to the plate surface, the real part of the impedance (considered as a function of the magnetic field strength) exhibits a maximum, which is a manifestation of the dimensional effect. Behavior of the maximum was followed depending on the plate thickness and the alternating field frequency. On the passage to a planar geometry, an allowance for the Bean-Livingston barrier explains the formation of vortex-free regions near the plate surface, which leads to a decrease in the surface resistance and suppression of the size effect.     

Oxidation electronics: bond–band–barrier correlation and its applications

This report features the recent progress in understanding the behaviour of atoms and valence electrons involved in the process of oxidation, and some technological development driven by the new knowledge. It is initiated and verified that a chemical bond contracts spontaneously at a surface associated with magnitude rise of the bond energy due to the coordination imperfection and that an oxygen atom hybridizes its sp orbitals upon reacting with a solid surface. The former leads to the bond order–length–strength (BOLS) correlation for the physical aspect of a surface and a nano-solid and the latter to a bond–band–barrier (BBB) correlation for chemical reaction. In the process of oxidation, non-bonding lone pairs, anti-bonding dipoles and hydrogen-like bonds are involved, which add corresponding density-of-states (DOS) features to the valence band of the host. Bond forming also alters the sizes and valencies of the involved atoms and causes a collective dislocation of these atoms, which corrugate the morphology or the potential barrier of the surface. Based on the above premises, the oxidation of the low-index surfaces of transition metals Cu, Co, Ag and V, noble metals Rh, Ru, and Pd and non-metallic diamond has been consistently analyzed. Identities probed with various techniques, such as STM, LEED, XRD, STS, PES, TDS, EELS and Raman, have been systematically defined in terms of atomic valencies, bond geometry, valence DOS, bond strength and bond forming kinetics. It is understood that formation of the basic oxide tetrahedron, and consequently, the four discrete stages of bond forming kinetics and the oxygen-derived DOS features, are intrinsically common for all the analyzed systems though the patterns of observations may vary from situation to situation. What differs one oxide surface from another in observations are: (i) the site selectivity of the oxygen adsorbate, (ii) the order of the ionic bond formation and, (iii) the orientation of the tetrahedron at the host surfaces. The valencies of oxygen, the scale and geometrical orientation of the host lattice and the electronegativity of the host elements determine these specific differences extrinsically. Extending the premise of sp-orbital hybridization to the reactions of (C, N)–Ni(001) surfaces has led to a novel approach neutralizing the diamond–metal interfacial stress and hence strengthening the diamond–metal adhesion substantially. The BOLS correlation has provided consistent insight into the shape-and-size dependence of a number of properties for nano-solids. The BBB correlation has led to new findings in designing and fabricating materials for photoluminescence, electron emission and ultrahigh elasticity, etc.     

Binocular vision: moving closer to reality

Vision science is a truly interdisciplinary field of research, lying at the interface between psychology, computer science and neurobiology. It spans a broad range of research questions, from what visual neurons in the brain do, to the design and use of virtual reality technology. Here, I will take one well-researched area of human vision binocular depth perception, as an exemplar of the various ways in which the processes of human perception can be studied: from basic questions about the sensitivity of binocular vision to understanding how having two eyes helps us function in the natural environment.     

Glass fibers with clay nanocomposite coating: Improved barrier resistance in alkaline environment

Coatings made from neat vinyl ester and nanoclay reinforced vinyl ester composites are applied onto individual glass fibers as well as rovings to evaluate their barrier resistance against alkali and moisture attack. The fibers coated with clay nanocomposites present a significantly less damage caused by the diffusing alkali ions, giving rise to a much higher residual tensile strength after aging than the fibers without coating or those with a neat polymer coating. The static fatigue test performed on individual fibers verifies the advantage of using nanoclay composite to retard the corrosion process under the combined stress and alkaline environment. Similar beneficial effects of incorporating nanoclay on residual strength are identified for impregnated fiber bundles. The above observations confirm the excellent barrier characteristics of intercalated/exfoliated nanoclay in polymer that are applied in composite structures on both the microscopic and macroscopic scales.     

Natural bioceramics: from coral to bone and beyond

Recent advances in the production and use of natural bioceramics for applications in hard and soft tissue replacement are discussed. The synthesis of complex inorganic forms, which are based on natural structures that can mimic natural scaffold, upon which the cells are seeded, offers an exciting range of avenues for the construction of a new generation bone analogs for tissue engineering. The use of natural and synthetic calcium phosphate bioceramics as bone grafts in orthopaedics and dentistry is considered. Issues affecting the use of different materials in vivo are outlined, of particular importance are osteoconductivity (ability to support tissue ingrowth and bone formation) and osteogenicity (formation of bone from cells within a bone graft). A variety of other natural alternatives including sol–gel coated coralline apatite are evaluated, and other key success factors (strength, longevity and stability) are reviewed. Several treatments for improving performance are outlined, and speculation on future advances, including combination of traditional bioceramic implants with more recent advances in stem cell research is made.     

Computational approach to systems biology: from fraction to integration and beyond

Systems biology is an approach to understanding the workings of whole biological systems. The various methods used for systems analyses range from experimental to computational. In this paper, we describe basic concepts of systems biology, modeling challenges that arise from the massively parallel interaction among components in biological systems, and what lies beyond integration of modular knowledge.     

Rapid prototyping: from product development to medicine and beyond

This paper discusses the current status of layer-based manufacturing rapid prototyping (RP) technology and how it is currently being implemented as a tool for product development (PD). A discussion on RP for PD is given, focusing on the limitations of existing technology. The paper then goes on to discuss the specific application field of medicine, explaining how this application may influence changes in the technology. Future trends for RP development are then discussed with further consideration for software issues in future applications and how the technology is being accepted worldwide.     

Rapid prototyping: from product development to medicine and beyond

This paper discusses the current status of layer-based manufacturing rapid prototyping (RP) technology and how it is currently being implemented as a tool for product development (PD). A discussion on RP for PD is given, focusing on the limitations of existing technology. The paper then goes on to discuss the specific application field of medicine, explaining how this application may influence changes in the technology. Future trends for RP development are then discussed with further consideration for software issues in future applications and how the technology is being accepted worldwide.