On the tensile behaviors of a hard chromium coating plated on a steel substrate with periodic subsurface inclusions

The tensile behaviors of a hard chromium coating plated on a steel substrate with periodic laser pre-quenched regions have been investigated by experimental and theoretic analysis. In the experiment, three specimens are adopted to study the differences between homogeneous and periodic inhomogeneous substrates as well as between periodic inhomogeneous substrate of relatively softer and stiffer materials. The unique characteristics have been observed in the specimen of periodic inhomogeneous substrate under quasi-static tension loading. With the periodic laser pre-quenched regions being treated as periodic subsurface inclusions (PSI), the unique stress/strain pattern of the specimen is obtained by analytical modeling and FEM analysis, and the mechanisms accounting for the experimental results is preliminarily illustrated.

Characteristics of the interface of a laser-quenched steel substrate and chromium electroplate

The interface of a laser-discrete-quenched steel substrate and as-deposited chromium electroplate was investigated by ion beam etching, dissolving-substrate-away and using a Vickers microhardness tester, in an attempt to reveal the mechanism that the service life of the chromium-coated parts is increased by the duplex technique of laser pre-quenching plus chromium post-depositing. The laser quenching of the steel substrate can reduce the steep hardness gradient at the substrate/chromium interface and improve the load-bearing capacity of chromium electroplate. Moreover, the laser quenching prior to plating has an extremely great effect on the morphologies and microstructure of the substrate/chromium interface: there is a transient interlayer at the original substrate/chromium interface while there is not at the laser-quenchedzone/chromium interface; the near-substrate surface microstructure and morphologies of the free-standing chromium electrodeposits, whose substrate was dissolved away with nital 30% in volume, inherit the periodically gradient characteristics of the laser-discrete-quenched substrate surface. (c) 2006 Elsevier B.V. All rights reserved.

Degradation failure features of chromium-plated gun barrels with a laser-discrete-quenched substrate

The effect of substrate laser-discrete quenching on the degradation failure of chromium-plated gun barrels was metallurgically investigated. The results show that substrate laser-discrete quenching changes the failure patterns of chromium coatings during firing, and some periodic through-thickness cracks in the fired chromium coatings are justly located at original substrate zones between two adjacent laser-quenched tracks. Moreover, chromium coatings and the laser-quenched zones on the substrate are simultaneously degraded in microstructure and property during firing. Furthermore, the periodic structure of the laser-discrete-quenched steel (LDQS) substrate near the breech remains after firing, and the hardness of the fired laser-quenched zones is still higher than that of original substrates. The specific failure features were utilized to illustrate the mechanism of the extended service life of chromium-plated gun barrels with the LDQS substrate. (c) 2007 Elsevier B.V All rights reserved.

Undercooling and Rapid Solidification of Nb-Si Eutectic Alloys Studied by Long Drop Tube

Niobium-silicide alloys have great potential for high temperature turbine applications. The two-phase Nb/Nb5Si3 in situ composites exhibit a good balance in mechanical properties. Using the 52 in drop tube, the effect of undercooling and rapid solidification on the solidification process and micro-structural characterization of Nb-Si eutectic alloy was studied. The microstructures of the Nb-Si composites were investigated by optics microscope (OM), X-ray diffraction (XRD) and scanning electron microscope (SEM) equipped with X-ray energy dispersive spectrometry (EDS). Up to 480 K, deep undercooling of the Nb-Si eutectic samples was successfully obtained, which corresponds to 25% of the liquidus temperature. Contrasting to the conventional microstructure usually found in the Nb-Si eutectic alloy, the microstructure of the undercooled sample is divided into the fine and coarse regions. The most commonly observed microstructure is Nb+Nb5Si3, and the Nb3Si phase is not be found. The change of coarseness of microstructure is due to different cooling rates during and after recalescence. The large undercooling is sufficient to completely bypass the high temperature phase field.

The mechanism of PEO process on Al–Si alloys with the bulk primary silicon

This study focuses on mechanism of ceramic coating on Al-Si alloys with bulk primary Si using plasma electrolytic oxidation (PEO) technology. Al-Si alloys with 27-32% Si in weight were used as substrates. The morphologies, composition and microstructure of PEO coatings were investigated by scanning electron microscopy (SEM) with energy dispersive X-ray system (EDX). Results showed that the PEO process had four different stages. The effect of bulk Si is greatly on the morphology and composition of coatings at first three stages. Anodic oxide films formed on Al and Si phases, respectively. When the voltage exceeded 40 V, glow appeared and concentrated on the localized zone of interface of Al and Si phase. Al-Si-O compounds formed and covered on the dendrite Si phase surface, and the coating on bulk Si, which was silicon oxide, was rougher than that on other phase. If the treatment time was long enough, the coatings with uniform surface morphologies and elements distribution will be obtained but the microstructure of inner layer is looser due to the bulk Si.

Effect of chromium intermediate layer on properties of silver coatings

Coatings with layer structures of Ag/glass, Ag/Cr/glass, and Ag/Cr-Ag/Cr/glass deposited with magnetron sputtering are investigated. The results indicate that the performance for reflectance, hardness, adherence, and humidity durability of the silver coatings with Ag/Cr/glass and Ag/Cr-Ag/Cr/glass structures are better than pure silver film. In addition, the silver coatings with an Ag/Cr-Ag/glass structure present more advantages than that of the Ag/Cr/glass. Reasons are analyzed accordingly. (C) 2004 Society of Photo-Optical Instrumentation Engineers.

Effect of alumina and chromium interlayers on microstructures and optical properties of thin Ag films on glass substrates

Effects of alumina and chromium interlayers on the microstructure and optical properties of thin Ag films are investigated by using spectrophotometry, x-ray diffraction and AFM. The characteristics of Ag films in Ag/glass, Ag/Al2O3/glass and Ag/Cr/glass stacks are analysed. The results indicate that the insertion of an Al2O3 or Cr layer decreases the grains and influences the reflectance of Ag films. The reflectance of the Ag film can be increased by controlling the thickness of alumina interlayer. The stability of Ag films is improved and the adhesion of Ag films on glass substrates is enhanced by alumina as an interlayer.

Simultaneous determination of chromium(VI) and chromium(III) of bioavailable fraction in bottom mud by flow injection-atomic absorption spectroscopy

A rapid and sensitive method for separation and determination of Cr(VI) and Cr(III) in bottom mud of lake by flow injection on-line preconcentrtion system and GFAAS was developed. The available Cr(VI) and Cr(III) were extracted by HOAc or EDTA + NH4 NO3 and adsorbed simultaneously by an anion and a cation resin microclummn and then eluted simultaneously by 2 mol/L NH4 NO3 + 0.05 mol/L ascorbate and 2 mol/L H2SO4, respectively. The elution was performed for 50 s after adsorption for 2 min, and the efficiencies of elution were 85.4% - 94.8% and 96.7% - 106% for Cr(VI) and Cr(M) respectively. The detection limits of the method were 0.9 mu g/L and 2.7 mu g/L with relative standard deviations of 3.5% and 6.4% for the determination of Cr(VI) and Cr(III) in sample, respectively.

Elasticity, band structure, and piezoelectricity of BexZn1-xO alloys

Lattice constants, elasticity, band structure and piezoelectricity of hexagonal wideband gap BexZn1-xO ternary alloys are calculatedusing firstprinciples methods. The alloys' lattice constants obey Vegard's law well. As Be concentration increases, the bulk modulus and Young's modulus of the alloys increase, whereas the piezoelectricity decreases. We predict that BexZn1-xO/GaN/substrate (x = 0.022) multilayer structure can be suitable for high-frequency surface acoustic wave device applications. Our calculated results are in good agreement with experimental data and other theoretical calculations. (c) 2008 Elsevier B.V. All rights reserved.

Mg acceptor energy levels in AlxInyGa1-x-yN quaternary alloys: An approach to overcome the p-type doping bottleneck in nitrides

The Mg-Ga acceptor energy levels in GaN and random Al8In4Ga20N32 quaternary alloys are calculated using the first-principles band-structure method. We show that due to wave function localization, the MgGa acceptor energy level in the alloy is significantly lower than that of GaN, although the two materials have nearly identical band gaps. Our study demonstrates that forming AlxInyGa1-x-yN quaternary alloys can be a useful approach to lower acceptor ionization energy in the nitrides and thus provides an approach to overcome the p-type doping difficulty in the nitride system.

Elasticity, band-gap bowing, and polarization of AlxGa1-xN alloys

Elastic constants, the bulk modulus, Young's modulus, band-gap bowing coefficients, spontaneous and piezoelectric polarizations, and piezoelectric coefficients of hexagonal AlxGa1-xN ternary alloys are calculated using first-principles methods. The fully relaxed structures and the structures subjected to homogeneous biaxial and uniaxial tension are investigated. We show that the biaxial tension in the plane perpendicular to the c axis and the uniaxial tension along the c axis all reduce the bulk modulus, whereas they reduce and enhance Young's modulus, respectively. We find that the biaxial and uniaxial tension can enhance the bowing coefficients. We also find that the biaxial tension can enhance the total polarization, while the uniaxial tension will suppress the total polarization. (C) 2008 American Institute of Physics.

Epitaxial Growth of AlInGaN Quaternary Alloys by RF-MBE

AlInGaN quaternary alloys were successfully grown on sapphire substrate by radio-frequency plasma-excited molecular beam epitaxy (RF-MBE). AlInGaN quaternary alloys with different compositions were acquired by changing the Al cell's temperature. The streaky RHEED patterns were observed during AlInGaN quaternary alloys growth. Scanning Electron Microscope (SEM), Rutherford back-scattering spectrometry (RBS), X-Ray diffraction (XRD) and Cathodoluminescence (CL) were used to characterize the structural and optical properties of the AlInGaN alloys. The experimental results show that the AlInGaN quaternary alloys grow on the GaN buffer in the layer-by-layer growth mode. When the Al cell's temperature is 920 degrees C, the Al/In ratio in the AlInGaN quaternary alloys is about 4.7, and the AlInGaN can acquire better crystal and optical quality. The X-ray and CL full-width at half-maximum (FWHM) of the AlInGaN are 5arcmin and 25nm, respectively.

Band-gap bowing and p-type doping of (Zn, Mg, Be)O wide-gap semiconductor alloys: a first-principles study

Using a first-principles band-structure method and a special quasirandom structure (SQS) approach, we systematically calculate the band gap bowing parameters and p-type doping properties of (Zn, Mg, Be)O related random ternary and quaternary alloys. We show that the bowing parameters for ZnBeO and MgBeO alloys are large and dependent on composition. This is due to the size difference and chemical mismatch between Be and Zn(Mg) atoms. We also demonstrate that adding a small amount of Be into MgO reduces the band gap indicating that the bowing parameter is larger than the band-gap difference. We select an ideal N atom with lower p atomic energy level as dopant to perform p-type doping of ZnBeO and ZnMgBeO alloys. For N doped in ZnBeO alloy, we show that the acceptor transition energies become shallower as the number of the nearest neighbor Be atoms increases. This is thought to be because of the reduction of p-d repulsion. The N-O acceptor transition energies are deep in the ZnMgBeO quaternary alloy lattice-matched to GaN substrate due to the lower valence band maximum. These decrease slightly as there are more nearest neighbor Mg atoms surrounding the N dopant. The important natural valence band alignment between ZnO, MgO, BeO, ZnBeO, and ZnMgBeO quaternary alloy is also investigated.

p-type doping of GaInNAs quaternary alloys

Using the first-principles band-structure method, we investigate the p-type doping properties and band structural parameters of the random Ga1-xInxN1-yAsy quaternary alloys. We show that the Mg-Ga substitution is a better choice than ZnGa to realize the p-type doping because of the lower transition energy level and lower formation energy. The natural valence band alignment of GaAs and GaInNAs alloys is also calculated, and we find that the valence band maximum becomes higher with the increasing in composition. Therefore, we can tailor the band offset as desired which is helpful to confine the electrons effectively in optoelectronic devices. (C) 2008 Published by Elsevier B.V.