High-temperature strength and damage evolution in short fiber reinforced aluminum alloys studied by miniature creep testing and synchrotron microtomography

The creep behavior of a squeeze-cast, short fiber reinforced Al metal matrix composite (MMC), consisting of an Al-11 wt.% Zn-0.2 wt.% Mg alloy reinforced with 15 vol.% Al₂O₃ Saffil® short fibers is investigated using miniature creep specimens. The small dimensions of the miniature creep specimens permit them to be machined from regions of an MMC block with different microstructures, thus allowing the effect of grain size and fiber texture on creep to be investigated on a more local level than is possible using conventional specimen geometries. The miniature creep specimens are subjected to uniaxial tensile stresses ranging from 3 to 40 MPa at temperatures between 573 and 623 K. It is shown that tests performed using the miniature creep specimen geometry are in good agreement with results previously obtained with standard creep specimens. Through interrupted creep experiments, it is observed that the creep back flow that occurs after unloading increases with increasing accumulated plastic strain. In the as-cast MMC, synchrotron microtomography reveals a fine distribution of pores whose spatial density increases with the spatial density of the fibers. The presence of fractured fibers in the crept MMC is also revealed. Some of the regions between fractured fiber fragments appear to be filled with matrix material, while others are voided.     

Room temperature aging behavior of Ti-Nb-Mo-based superelastic alloys

The effect of room temperature (RT) aging on the superelasticity of Ti-Nb-Mo-based superelastic alloys is investigated. The results show that annealing at relatively low temperatures such as 973 K after severe cold rolling results in poor resistance to the effect of RT aging. The transformation stress increases considerably due to the formation of an isothermal ω phase at RT. Addition of Sn is partially effective in suppressing the RT aging effect in the specimens annealed at 973 K. The RT aging effect is suppressed by increasing the annealing temperature, due to the annihilation of lattice defects or non-equilibrium vacancies introduced during cold rolling, which are responsible for accelerating the diffusion process, however, superelasticity is reduced by annealing at higher temperatures, due to a decrease in the critical stress for slip deformation (σ_CSS). The specimen annealed at 1173 K followed by aging at 773 K exhibits stable superelasticity with a high resistance to the effect of RT aging. Annealing at 1173 K causes the annihilation of lattice defects or non-equilibrium vacancies, while aging at 773 K induces precipitation of the α phase, which in turn causes an increase in σ_CSS, and further enhances the resistance to the RT aging effect by enriching the matrix with β-stabilizing elements.     

Metal Dusting of Alumina-Forming Creep-Resistant Austenitic Stainless Steels

Three developmental alumina-forming austenitic stainless steels were exposed to metal dusting conditions at 650?°C in a gas of 50%CO–49%H₂–1%H₂O (a _C: 36.7 and $ p_{{{\text{O}}_{2} }} $ : 2.83?×?10^?26?atm) under thermal cycling conditions. Metal wastage measurement showed initially slow kinetics followed by a fast weight loss. This observation is attributed to the formation of protective chromia/alumina oxide scales in the early stage of the reaction, followed by local oxide failure/spallation during cyclic reaction. Metal dusting initiated from these local defects, and pitting-type attack was observed after 131 cycles of reaction. After 352 cycles, severe dusting had developed, forming heavy and distinctive “tentacles” of superficial coke. This carbon deposit was composed of fine carbon filaments. Examination by TEM of the coke-metal reaction front showed direct surface metal disintegration, indicating that the dusting follows the classical mechanism for austenitic materials. Etching with aqua regia revealed a carburised zone formed in the alloy underneath the coke layer. Analysis by TEM of this zone revealed the formation of ultra-fine, needle-shaped chromium carbide precipitates within a chromium depleted austenite matrix.     

A Simple Expression for Predicting the Oxidation Limited Life of Thin Components Manufactured from FCC High Temperature Alloys

Chromia and alumina forming high temperature alloys suffer from breakaway oxidation if the concentration of the preferred scale forming element in the alloy decreases below the level required to sustain growth of the protective oxide scale. In thin components, the breakaway may occur even before oxide spallation starts to contribute to alloy depletion. In the present paper a simplified method is developed to predict the time to breakaway as a function of oxidation rate, initial concentration and diffusivity of the scale forming element in the alloy as well as component thickness. The first approach used is an approximation of the analytical solution previously derived by Whittle. The second method is based on a numerical solution and an exploration of the way in which the time to breakaway varies with the above mentioned parameters. Comparison with literature data reveals that for a number of applications good agreement between calculated and measured lifetimes can be achieved using both approaches. The lifetime equation derived using the numerical approach has the advantage that it allows prediction of breakaway oxidation in a larger range of experimental and alloy composition related parameters. It not only describes the behaviour of materials with a face centered cubic lattice but also includes the limiting case in which solute diffusion is fast compared to surface recession rate, as in, for example, the oxidation of ferritic alumina forming FeCrAl alloys at high temperatures.     

Active Oxidation of Liquid Silicon: Experimental Investigation of Kinetics

Small scale laboratory experiments on the oxidation of liquid silicon have reproduced important features of the industrial refining of liquid silicon: active oxidation led to the formation of amorphous silica spheres as a reaction product. The boundary condition for active oxidation in terms of maximum oxygen partial pressure in the bulk gas was found to lie between 2·10^?3 and 5·10^?3?atm at T?=?1,500?°C. The active oxidation of liquid silicon had linear kinetics, and the rate was proportional to bulk oxygen partial pressure and the square root of the linear gas flow rate, consistent with viscous flow mass transfer theory. Classical theory for unconstrained flow over a flat plate led to mass transfer rates for SiO_(g) which were 2–3 times slower than observed. However, computational fluid dynamic modeling to take into account the effects of reactor tube walls on flow patterns yielded satisfactory agreement with measured volatilization rates.     

Effects of rapid thermal process on the junction properties of aluminum rear emitter solar cells

N-type silicon with aluminum emitters for rear junctions was studied; aluminum back surface fields were replaced with n-type silicon wafers. Aluminum rear emitters for n-type silicon solar cells were studied with various rapid thermal processing conditions. With fast ramping-up and fast cooling, an aluminum rear junction was formed uniformly with low emitter recombination current. The effects of junction quality on solar cell efficiency were investigated.     

Synthesis of high-purity silver colloids using a thermal decomposition method

A thermal decomposition method was employed to produce high purity Ag particles. Silver carbonate was dispersed in an H₂O solvent that was heated above 80°C, followed by rapidly injecting H₂O₂ into the solvent. Then, the silver carbonate was decomposed into nanosized Ag particles using the decomposition heat of hydrogen peroxide as follows: Ag₂CO₃→2Ag+CO₂+1/2O₂. The size of the synthesized Ag particles is approximately 100 nm. This method can be used to produce high-purity Ag colloids withuut impurities, unlike other methods.     

Effect of yttrium on the stability of aluminide-yttrium composite coatings in a cyclic high-temperature hot-corrosion environment

A composite coating of aluminide-yttrium has shown excellent corrosion resistance in a cyclic high-temperature hot-corrosion environment. To understand the effect of yttrium on the stability of the composite coating, the specimens were prepared with various coating parameters of Y thickness, sequence of post heat treatment and surface condition before Y-ion plating. Performance of the composite coating was evaluated by isothermal oxidation and cyclic high-temperature hot corrosion. Isothermal-oxidation-test results show that the Y in the composite coating helps to form a thick and dense Al₂O₃ scale which is ductile and resistant to thermal stress. The Y in Al₂O₃ may act as a donor which leads to an increase in concentration of interstitial oxygen and, thus, increases in oxidation rate. The presence of Y₂O₃ and (Y, Al) O-type compounds in grain boundaries of Al₂O₃ and boundaries between the Al₂O₃ and NiAl effectively prohibits the fast diffusion of oxidants (such as O and S) and Al along grain boundaries. Consequently, it may induce slow diffusion through the matrix, and thus the corrosion resistance of the composite coating under cyclic hot corrosion increases substantially.     

Vibration and noise characteristics of coaxial magnetic gear according to low-speed rotor structure

This paper presents the analysis of the vibration and noise characteristics of Coaxial magnetic gears (CMGs) according to the Lowspeed rotor (LSR) structure. Surface-mounted permanent magnet CMG (SPM_CMG) and Flux concentrated CMG (FC_CMG) are selected for the comparison models. The electromagnetic force distributions in the air-gap adjacent to the LSR are calculated to predict the deformed shape of the LSR. In addition, the electromagnetic forces acting on each pole of the LSR according to the rotation of the rotors and their frequencies are obtained to investigate the vibration and noise characteristics. The mode shapes of the LSR core and their resonant frequencies are obtained through modal analysis to consider the resonance of the analytical models. The vibration and noise analysis result indicates that SPM_CMG has less vibration and noise compared than the FC_CMG.