Completeness of beta-phase decomposition reaction in Cu-Al-Ag alloys

The completeness of beta-phase decomposition reaction in the Cu-11wt%Al-xwt%Ag alloys (x = 0, 1, 2, and 3) was studied using differential scanning calorimetry (DSC), X-ray diffractometry (XRD), and optical microscopy (OM). The results indicated that beta-phase transformations are highly dependent on cooling rate and on the presence of Ag. on slow cooling, the silver presence prevents the beta- and beta(1)-phase decomposition; thus, inducing the martensitic phase formation. After rapid cooling, a new thermal event is observed and the reverse martensitic transformation is shifted to lower temperatures.

Niobium to alcohol mol ratio control of the concurring esterification and etherification reactions promoted by NbCl5 and Al2O3 catalysts under microwave irradiation

Alcohols and acids can be switched to produce ethers or esters by varying the alcohol to catalyst mol ratio, in a new etherification and esterification method using NbCl5/Al2O3 catalyst under "solvent free" conditions and promoted by MW (microwave) irradiation. A "two sites" mechanism for the reaction is proposed, in an attempt to clarify the tendency of the catalyst to be dependent on the alcohol alone during the esterification process. (c) 2008 Elsevier B.V. All rights reserved.

Influence of the heat treatment in the electrochemical corrosion of Al-Zn-Mg alloys

The localized corrosion of Al-(5.03%)Zn-(1.67%)Mg-(0.23%)Cu alloys and high purity Al has been studied using electrochemical techniques, optical microscopy, SEM and EDX. The samples were previously submitted to different heat treatments in which coherent and incoherent MgZn 2 precipitates with different distribution and aggregation degree were produced. The influence of NaCl and Na 2SO 4, dissolved oxygen, immersion time and convection were studied. In NaCl solutions, pitting potentials for the alloys were more negative than for aluminium, indicating an increase in their susceptibility to localized corrosion. Moreover, annealed and cold-rolled alloys presented more negative pitting and repassivation potentials than those submitted to age hardening with direct or interrupted quenching. In annealed and cold-rolled samples, pit nucleation and propagation takes place in the zones where MgZn 2 is accumulated. In the case of the age-hardened alloys, a double pitting behaviour is observed, the first one in the magnesium and zinc enriched regions and the second in the matrix. While the cold water quenched alloy is susceptible to stress corrosion craking, the alloy submitted to the interrupted quenching process is less susceptible to intergranular attack. The sulphate ion shifts the pitting potential of aluminium and the alloys by chloride towards more positive values because it impedes local accumulations of the latter. © 1992 Chapman & Hall.

Niobium filtration of conventional and high-frequency x-ray generator beams for intraoral radiography. Effects on absorbed doses, image density and contrast, and photon spectra

We have studied the effects of niobium beam filtration on absorbed doses, on image density and contrast, and on photon spectra with conventional and high-frequency dental x-ray generators. Added niobium reduced entry and superficial absorbed doses in periapical radiography by 9% to 40% with film and digital image receptors, decreased the radiation necessary to produce a given image density on E-speed film and reduced image contrast on D- and E-speed films. As shown by increased half-value layers for aluminum, titanium, and copper and by pulse-height analyses of beam spectra, niobium increased average beam energy by 6% to 19%. Despite the benefits of adding niobium on patient dose reduction and on narrowing the beams' energy spectra, the beam can be overhardened. Adding niobium, therefore, strikes the best balance between radiation dose reduction and beam attenuation, with its risks of increased exposure times, motion blur, and diminished image contrast, when it is used at modest thicknesses (30 μm) and at lower kVp (70) settings. © 1995 Mosby-Year Book, Inc.

Phonons in binary BCC lithium-sodium alloys

The dispersion relations along the principal symmetry directions in BCC lithium-sodium alloys are calculated using second-order perturbation theory. The local modified Hoshino-Youngmodel potential was used for the lithium and the local Harrison model potential for sodium. The phonon density of states, the root mean square displacements and (Θ-T) curves are also calculated. In the absence of experimental data, just the theoretical predictions are presented here.

Thermal decomposition of the magnesium, zinc, lead and niobium chelates derived from 8-quinolinol

Solid M-Ox compounds, where M represents Mg(II), Zn(II), Pb(II) and NbO(III), and Ox is 8-quinolinol, have been prepared. Thermogravimetry, derivative thermogravimetry (TG, DTG), differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR) and infrared absorption spectra (IR) have been used to characterize and to study the thermal stability and thermal decomposition of these compounds. © 1997 Akadémiai Kiadó.

Raman scattering study of zincblende InxGa1-xN alloys

We report on first-order micro-Raman and resonant micro-Raman scattering measurements on c-InxGa1-xN (0 ≤ x ≤ 0.31) epitaxial layers. We have found that both, the transverse-optical (TO) and longitudinal-optical (LO) phonons of InxGa1-xN alloy exhibit a one-mode-type behavior. Their frequencies at Γ lie on straight lines connecting the corresponding values obtained for the c-GaN and c-InN binary compounds. Evidence for phase separation is shown in the sample with the alloy composition x = 0.31. The Raman spectra, with excitation energy close to 2.4 eV, show an enhanced additional peak, with frequency between the values found for the LO and TO phonon modes of the C-In0.31Ga0.69N epitaxial layer. We ascribed this peak to the LO phonon mode of a minority phase with In content of ≈0.80.

Electrodeposition of amorphous Fe-Cr-P-Ni-C alloys. Morphological and structural characteristics

The effect of bath composition and electroplating conditions on structure, morphology and composition of amorphous Fe-Cr-P-Ni-C deposits on Cu substrate was investigated. The deposition efficiency of Fe-Ni-P-C alloy increased significantly with the addition of formic acid, but decreased with the addition of Cr to the plating bath. The increase of charge density activates the inclusion of Cr in the deposit. However, above a specific value of charge density, which depends on deposition current density, the Cr content in the deposit decreases. SEM analysis showed that the increase of Ni, Cr or charge deposition promotes susceptibility to microcracking.

Effect of thermal treatments on tensile strength of commercially cast pure titanium and Ti-6Al-4V alloys

Heating titanium structures is assumed to relieve tensions induced by the casting process as well as possibly optimizing some mechanical properties. The aim of this investigation was to evaluate the effect of thermal treatments on tensile strength of commercially pure titanium (CP Ti) and Ti-6Al-4V alloy. Thirty dumbbell rods, with diameters of 3.0 mm at the central segment and lengths of 42 mm, were cast for each metal using the Rematitan System. CP Ti and Ti-6Al-4V specimens were randomly divided into three groups of ten: a control group that received no thermal treatment and two test groups. One (T1) was heated at 750°C for 2 h and the other (T2) was annealed at 955°C for 1 h and aged at 620°C for 2 h. Tensile strength was measured with a universal testing machine (MTS model 810). Tensile strength means and standard deviations were statistically compared using a Kruskal-Wallis test at a α = 0.05 significance level. No statistically significant differences in tensile strength were observed among CP Ti groups. For the Ti-6Al-4V alloy, the control and T1 groups revealed statistically higher tensile strengths when compared to the T2 group, with no significant difference between the control and T1 groups. © 2005 Springer Science + Business Media, Inc.

Anelastic relaxation due to interstitial solutes diffusion in Nb and Nb-1 wt% Zr

Metals and alloys containing solute atoms dissolved interstitially often show anelastic behavior due to a process know as stress-induced ordering. The application of mechanical spectroscopy measurements to diffusion studies in body-centered cubic metals has been extensively used in the last decades. However the kind of preferential occupation of interstitial solutes in body-centered cubic metals is still controversial. The anelastic properties of the Nb and Nb-1 wt% Zr polycrystalline alloys were determined by internal friction and oscillation frequency measurements using a torsion pendulum inverted performed between 300K and 650K, operating in a frequency oscillation in the hertz bandwidth. The interstitial diffusion coefficients of oxygen and nitrogen in Nb and Nb-1 wt% Zr samples were determined at two distinct conditions: (a) for low concentration of oxygen and (b) for high concentration of oxygen.

Martensite aging kinetics in the Cu-10 wt.%Al and Cu-10 wt.%Al-10 wt.%Ag alloys

The martensite aging kinetics in the Cu-10 wt.%Al and Cu-10 wt.%Al-10 wt.%Ag alloys was studied using microhardness measurements, classical differential thermal analysis (DTA), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and in-situ high-temperature X-ray diffractometry (XRD). The results for the Cu-10%Al alloy indicated a process dominated by the martensite ordering assisted by migration of quenched-in vacancies and followed by the consumption of the α phase. For the Cu-10%Al-10%Ag alloy the dominant process is the consumption of the α phase associated with a decrease in the ordering degree of the martensitic phase. © 2007 Springer Science+Business Media, LLC.

Diffusion phenomena of the oxygen and nitrogen in niobium by mechanical spectroscopy

The short-range diffusion phenomenon (Snoek Effect) was investigated by mechanical spectroscopy measurements between 300 K and 650 K, in a polycrystalline niobium sample, containing oxygen and nitrogen, using a torsion pendulum. Experimental spectra of anelastic relaxation were obtained under three conditions: as-received sample; annealed sample and subsequently annealed in an oxygen atmosphere for three hours at 1170 K in partial pressure of 5°10 -5mbar. The experimental spectra obtained were decomposed in elementary Debye peaks and the anelastic relaxation processes were identified. With anelastic relaxation parameters and the lattice parameters, the interstitial diffusion coefficients of the oxygen and nitrogen in niobium were calculated for each kind of preferential occupation (octahedral and tetrahedral). The results were compared with the literature data, and confirmed that the best adjustment is for the preferential occupation octahedral model for low concentrations of interstitial solutes, but at higher concentration of oxygen were observed deviations of experimental data for the interstitial diffusion coefficients of oxygen in niobium when compared with the literature data, this could be related to the possible occurrence of a double occupation of interstitial sites in the niobium lattice by oxygen interstitials. © (2010) Trans Tech Publications.