New thermomechanical treatment of high temperature aging nickel alloys with high dispersed γ — Phase

A new thermomechanical treatment that permits a significant increase in the time to failure, as well as the resistance in stress rupture tests of the hard aged nickel-based alloys has been proposed. It is shown that new treatment consisting of a small prestrain in creep regime and subsequent one- or two-step aging brought about the formation of a high-dispersed microstructure of gamma-prime phase particles from several nm to 50 nm. This microstructure is more stable towards coarsening upon subsequent annealing. Improvement of the mechanical and creep properties is explained by a stable microstructure of the alloy, produced by the thermomechanical treatment, a higher degree of order inside particles, and coexistence of the mechanisms of cutting and by-passing of particles by single and pair dislocations.     

A high potential material—zirconia

This is a review as well as an overview of zirconia. Although there are several methods of preparation of zirconia powder, we have described some of the methods of its preparation in this paper, and their application in ceramic bodies.     

让初中英语课“high”起来

如何让英语课“high”起来,做到寓教于乐、寓情于乐？以学生为主体在英语教学中创造一种生动活泼的氛围,激发学生潜能,生学生带着尚涨的热情愉快地学习和思考,在快乐的学习心境中主动求知求学,实现教与学良性循环,达到最佳教学效果。     

Phase boundary between Na–Si clathrates of structures I and II at high pressures and high temperatures

Understanding of the covalent clathrate formation is a crucial point for the design of new superhard materials with intrinsic coupling of superhardness and metallic conductivity. It has been found that silicon clathrates have the archetype structures, which can serve an existent model compounds for superhard clathrate frameworks Si–B, Si–C, B–C and C with intercalated atoms (e.g., alkali metals or even halogens) that can assure the metallic properties. Here we report our in situ and ex situ studies of high-pressure formation and stability of clathrates Na₈Si₄₆ (structure I) and Na_24+x Si₁₃₆ (structure II). Experiments have been performed using standard Paris–Edinburgh cells (opposite anvils) up to 6 GPa and 1500 K. We have established that chemical interactions in the Na–Si system and transition between two structures of clathrates occur at temperatures below silicon melting. The strong sensitivity of crystallization products to the sodium concentration has been observed. A tentative diagram of clathrate transformations has been proposed. At least up to ~6 GPa, Na_24+x Si₁₃₆ (structure II) is stable at lower temperatures as compared to Na₈Si₄₆ (structure I).     

Metal-supported high crystalline Bi₂Se₃ quintuple layers

Atomically flat thin films of Bi(2)Se(3) were grown on Au(111) metal substrate using molecular beam epitaxy. Hexagonal atomic structures and quintuple layer steps were observed at the surfaces of grown films using scanning tunneling microscopy. Multiple sharp peaks from (003) family layers were characterized by x-ray diffraction measurements. The atomic stoichiometry of Bi and Se was considered using x-ray photoemission spectroscopy. Moiré patterns were obtained at the surfaces of one quintuple layer films due to lattice mismatch between Bi(2)Se(3) and Au. Our experiments suggest that Au is a reasonable material for electrodes in Bi(2)Se(3) devices.     

Microstructure and mechanical properties of high purity nanostructured titanium processed by high pressure torsion at temperatures 300 and 77 K

Several structural states of nanostructured high purity Ti with average grain size down to 100 nm were achieved by high pressure torsion (HPT) at temperatures 300 and 77 K. As a result of HPT processing, changes of crystallographic texture, of grain and crystallite size, and of the dislocation density have been measured and analyzed. Mechanical properties of the nanostructured Ti were studied by uniaxial compression at temperatures 300, 77, and 4.2 K. The texture components indicate simple shear deformation arising from HPT. With subsequent compression, the yield strength appears to be governed by the grain size rather than by crystallite size, dislocation density, and/or impurity content. Considerable changes of texture were observed after low temperature compressive deformation indicating that twinning markedly contributes to plasticity.     

Preparation of coatings with high adhesion strength and high corrosion resistance on sintered Nd–Fe–B magnets through electroless plating

Electroless Ni–P-based coatings have been deposited on sintered Nd–Fe–B magnets through applying ultrasonic irradiation and adjusting the [Cu²⁺]/[Ni²⁺] ratio in the solution. The effects of the ultrasonic power on the adhesion to the magnet substrate and the [Cu²⁺]/[Ni²⁺] ratio on the corrosion resistance of the coatings were investigated. It was found that the adhesion of the coating to the substrate could be greatly improved through applying ultrasonic irradiation. Maximum adhesion strength reached 56 MPa at 150 W. The results also showed that the addition of Cu²⁺ could improve the corrosion resistance of Ni–P-based coatings. When the [Cu²⁺]/[Ni²⁺] ratio was 0.02, the coating could be as long as 512 h free of corrosion in the neutral salt spray. The compact amorphous structure was responsible for the improved corrosion resistance of the coating.     

Structure–property relationships for high thermal conductivity carbon fibers

Previous work at Clemson University has shown that ribbon-shaped mesophase pitch-based carbon fibers graphitized at only 2400°C can develop thermal conductivities comparable with those of commercial round-shaped pitch-based carbon fibers graphitized at temperatures above 3000°C. The thermal and electronic transport properties (i.e. thermal conductivity and electrical resistivity) of ribbon-shaped carbon fibers produced at Clemson University are being studied. In addition, the structure of these fibers is being analyzed by electron microscopy and X-ray diffraction techniques. This paper will discuss the relationships between processing conditions, fiber structure and fiber properties.     

Failure analysis on high temperature superheater Inconel® 800 tube

This paper presents failure analysis on a super alloy Inconel^® 800 superheater tube in Kapar Power Station Malaysia. Visual inspection, microscopic examinations and creep analysis utilizing available related data are carried out to evaluate the failure mechanism and its root cause. The failed high temperature superheater (HTSH) tube was found snapped into two parts, heavily distorted shape and bent at several points. Microstructures of the failed tube showed that creep crack initiated at both external and internal surfaces of the tube and propagated as grain boundary creep cavities coalesced to form intergranular cracks. The severe geometry of tube causing steam flow starvation is identified to have caused increasing tube metal temperature resulting in overheating of the failed tube. Creep rupture is revealed as the cause of failure of the superheater tube.     

Steel and nickel alloys at high temperatures: Part D of ‘The requirements for and limitations of materials at high temperatures’

The range of steels considered includes corrosion resistant ferritic and austenitic steels and low alloy, martensitic 12% Cr and austenitic steels for higher strength applications. Nickel superalloys are discussed under the gas turbine applications for which they were largely developed. Nickel alloys for corrosion resistant applications are discussed and a short section on the refractory materials Mo, Nb, Tn and W is included.     

Microstructure and mechanical properties of biocompatible high density Ti–6Al–4V/W produced by high frequency induction heating sintering

High frequency induction heating sintering method is used for sintering of the metal and ceramics powder. This technique has been used to produce high density compacts, containing as small grains as possible of powders. The alloy of Ti–6Al–4V was modified by addition of 2.5, 5, and 10 wt.% tungsten through powder metallurgy. Ti–6Al–4V/W was prepared by high-energy mechanical milling. The use of the high frequency induction heating sintering technique allows sintering to nearly full density at comparatively low temperatures and short holding times, and therefore suppressing grain growth. Different process parameters such as sintering temperature, and applied pressure have been investigated. The obtained compacts are characterized with respect to their densities, grain morphologies and pore distributions as well as hardness. Ti–6Al–4V/W powder precursors have been successfully compacted and consolidated to densities exceeding 98.8%. The maximum compressive strengths were obtained at sintering temperature 1000 °C for the samples containing 5% W, and at 1100 °C for the samples with 10% W. Maximum hardness was obtained 45 HRC at 1100 °C for 10% W.     

Characterisation of two Al–Fe based high temperature alloy powders

Abstract

Aluminium alloys containing additions of iron and cerium are among the alloys being developed as potential replacements for titanium based alloys for moderately high temperature applications. Development of these alloys is possible using rapid solidification technology, which results in a very fine distribution of dispersoids in the aluminium matrix. The microstructures of two rapidly solidified high temperature alloy powders of composition (wt-%) Al–6·7Fe–5·9Ce (alloy A) and Al–6·2Fe–5·9Ce–1·63Si (alloy B) have been characterised using transmission electron microscopy and the results are explained on the basis of some of the major solidification parameters, such as nucleation undercooling and recalescence. It was observed that most of the powder particles in the +10 to ?20 μm size range contained both microcellular and cellular regions, which could be explained in terms of an initial large undercooling followed by recalescence. The decomposition of the powder microstructure after exposing the powders to temperatures of 350, 420, and 500°C for 1 h was investigated using transmission electron microscopy. This work was complemented by phase identification studies using X-ray diffraction. The equilibrium precipitates Al₁₃Fe₄, Al₈Fe₂Si, and Al₃FeSi were detected in the powder microstructure of alloy B, whereas Al₁₃Fe₄ precipitates were detected in alloy A after high temperature exposure (500°C).

MST/1571     

NLR experience with high velocity burner rig testing 1979–1989

The NLH. has two high velocity burner rigs for oxidation and hot corrosion testing. A large, custom-built rig was commissioned in 1977 to enable actual components to be tested. A smaller rig for specimen testing was purchased in 1986. Experience with these rigs over the last decade is reviewed.     

Formation of non–equilibrium alloys by high pressure melt quenching

Melt quenching under high pressure can promote the formation of metastable materials. High pressure accelerates the amorphization of Cu₆₀Ti₄₀ and Cd₄₃Sb₅₇.For an alloy systems having volume expansion after solidification, the higher the applied pressure, the lower the melting point and the higher the amorphization temperature, which promotes the formationof metallic glass. High pressure also enhances nucleation and suppresses grain growth, so solidification under high pressure can refine the crystal grains to form nanocrystalline alloys, such as Ti₆₀Cu₄₀,Cu₇₀Si₃₀ and Pd₇₈Si₁₆Cu₆ alloys.     

A flexible route to high strength α-alumina and aluminate spheres

The formation of hollow alumina spheres is accomplished by coating polystyrene beads of 3 m and 50–80 m diameter with carboxylic acid functionalized alumina nanoparticles (alumoxanes) from aqueous solution 2–8 wt%. The resulting coated beads were heated to 220°C to calcine the alumoxane to porous amorphous alumina before washing with toluene to remove the polystyrene from inside the ceramic coating. The resulting hollow spheres were sintered at 1000°C to form -alumina. The -alumina spheres have been characterized, by SEM (scanning electron microscopy), BET, and hardness measurements, that show the hardness of the hollow alumina sphere (1900 ± 100 Kg.mm^–2) approaches that of corundum (ca. 2000 Kg.mm^–2). Multilayered bi-phasic spheres may be prepared by subsequent coating the -alumina spheres with a solution of a metal-doped alumoxane. After calcining, the mixed metal oxide phase (CaAl₁₂O₁₉, Er₆Al₁₀O₂₄, MgAl₂O₄, Al₂TiO₅, and Y₃Al₅O₁₂) forms outside of the alumina sphere resulting in a composite like ceramic bi-layer sphere. Pre-formed hollow alumina spheres were incorporated into a resin and ceramic thin film formed from a 1 wt% A-alumoxane aqueous solution. The hardness of the composites is compared to the matrix materials themselves.     

Graphite → diamond transition under high pressure: A kinetics approach

The graphite diamond transition in the diamond stability field can be either direct or solvent-assisted. The direct transition may proceed by spreading a puckered basal plane of graphite in the direction perpendicular to it. The kinetics of such a transition may be approximated by the growth of a two-dimensional nucleus. The threshold temperature of the transition appears to depend on the degree of perfection of the original graphite. Hence, the more perfect the graphite is, the lower temperature it will transform into diamond. The solvent-assisted transition normally proceeds by rapid nucleation followed by growth of these nuclei. The kinetic model for continuous nucleation may be applied to the early stage of such transition. The activation energies for the transition can then be calculated. It is found that these activation energies seem to vary inversely with the solubility of carbon in these solvents at ambient pressure. Hence, the higher the amount of carbon a solvent dissolves at ambient pressure, the more effective it can be as a catalyst for the graphite diamond transition under high pressure.