Morphology and thermal characteristics of nano-sized Pb–Sn inclusions in Al

Nanoembedded aluminum alloys with bimetallic dispersoids of Sn and Pb of compositions Sn82–Pb18, Sn64–Pb36, and Sn54–Pb46 were synthesized by rapid solidification. The two phases, face-centered-cubic Pb and tetragonal Sn solid-solution, coexist in all the particles. The crystallographic relation between the two phases and the matrix depends upon the solidification pathways adopted by the particles. For Al–(Sn82–Pb18), we report a new orientation relation given by [011]Al//[010]Sn and (o11)A1//(101)Sn. Pb exhibits a cube-on-cube orientation with Al in few particles, while in others no orientation relationship could be observed. In contrast, Pb in Sn64–Pb36 and Sn54–Pb46 particles always exhibits cube-on-cube orientation with the matrix. Sn does not show any orientation relationship with Al or Pb in these cases. Differential scanning calorimetry studies revealed melting at eutectic temperature for all compositions, although solidification pathways are different. Attempts were made to correlate these with the melting and heterogeneous nucleation characteristics.

Quasicrystalline Coatings Through Laser Processing: A Study on Process Optimisation and Microstructure Evolution

Composite coatings containing quasicrystalline (QC) phases in Al-Cu-Fe alloys were prepared by laser cladding using a mixture of the elemental powders. Two substrates, namely pure aluminum and an Al-Si alloy were used. The clad layers were remelted at different scanning velocities to alter the growth conditions of different phases. The process parameters were optimized to produce quasicrystalline phases. The evolution of the microstructure in the coating layer was characterized by detailed microstructural investigation. The results indicate presence of quasicrystals in the aluminum substrate. However, only approximant phase could be observed in the substrate of Al-Si alloys. It is shown that there is a significant transport of Si atoms from the substrate to the clad layer during the cladding and remelting process. The hardness profiles of coatings on aluminum substrate indicate a very high hardness. The coating on Al-Si alloy, on the other hand, is ductile and soft. The fracture toughness of the hard coating on aluminum was obtained by nano-indentation technique. The K1C value was found to be 1.33 MPa m1/2 which is typical of brittle materials.

Comparison of CVD and MOCVD-grown Al2O3 coatings in the performance of cemented carbide cutting tool inserts

Low-pressure MOCVD, with tris(2,4 pentanedionato)aluminum(III) as the precursor, was used in the present investigation to coat alumina on to cemented carbide cutting tools. To evaluate the MOCVD process, the efficiency in cutting operations of MOCVD-coated tools was compared with that of tools coated using the industry-standard CVD process.Three multilayer cemented carbide cutting tool inserts, viz., TiN/TiC/WC, CVD-coated Al2O3 on TiN/TiC/WC, and MOCVD-coated Al2O3 on TiN/TiC/WC, were compared in the dry turning of mild steel. Turning tests were conducted for cutting speeds ranging from 14 to 47 m/min, for a depth of cut from 0.25 to 1 mm, at the constant feed rate of 0.2 mm/min. The axial, tangential, and radial forces were measured using a lathe tool dynamometer for different cutting parameters, and the machined work pieces were tested for surface roughness. The results indicate that, in most of the cases examined, the MOCVD-coated inserts produced a smoother surface finish, while requiring lower cutting forces, indicating that MOCVD produces the best-performing insert, followed by the CVD-coated one. The superior performance of MOCVD-alumina is attributed to the co-deposition of carbon with the oxide, due to the very nature of the precursor used, leading to enhanced mechanical properties for cutting applications in harsh environment.

Temperature deformation behaviour of a Mg-0.8Al alloy

There is considerable interest currently in developing magnesium based alloys as replacements for aluminum alloys in automobile applications, due to their high specific strength as compared to aluminum alloys. However, the poor formability of magnesium alloys has restricted their applications; superplasticity can be utilized to form components with complex shapes. In the present study, the compressive deformation characteristics of a Mg-0.8 wt% Al alloy with an initial grain size of 19 +/- 1.0 mum have been studied in the temperature range of 623-673 K and at strain rates ranging from 10(-7) to 10(-3) s(-1). The stress exponent was observed to decrease with a decrease in stress. The results are analyzed in terms of the existing theoretical models for high temperature deformation. Furthermore, the potential for superplasticity in this alloy is explored, based on the mechanical and microstructural characteristics of the alloy.

SOS films and interfaces: chemical aspects

The possible chemical reactions that take place during the growth of single crystal films of silicon on sapphire (SOS) are analyzed thermodynamically. The temperature for the growth of good quality epitaxial films is dependent on the extent of water vapor present in the carrier gas. The higher the water vapor content the higher the temperature needed to grow SOS films. Due to the interaction of silicon with sapphire at elevated temperatures, SOS films are doped with aluminum. The extent of doping is dependent on the conditions of film growth. The doping by aluminum from the substrate increases with increasing growth temperatures and decreasing growth rates. The equilibrium concentrations of aluminum at the silicon-sapphire interface are calculated as a function of deposition temperature, assuming that SiO2 or Al6Si2O13 are the products of reaction. It is most likely that the product could be a solid solutio n of Al2O3 in SiO2. The total amount of aluminum released due to the interaction between silicon and sapphire will account only for the formation of not more than one monolayer of reaction product unless the films are annealed long enough at elevated temperatures. This value is in good agreement with the recently reported observations employing high resolution transmission electron microscopy.

Solubility and activity of oxygen in liquid nickel in equilibrium with -Al2O3 and NiO.(1+x)Al2O3

The limiting solubility of oxygen in liquid nickel in equilibrium withα-alumina and nickel aluminate has been measured by inert gas fusion analysis of suction samples in the temperature range 1730 to 1975 K. The corresponding oxygen potential has been monitored by a solid electrolyte cell consisting of calcia stabilized zirconia as the electrolyte and Mo + MoO2 as the reference electrode. The results can be summarized by the following equations: log(at. pct O) = \frac - 10,005T + 4.944 ( ±0.015)log(atpctO)=T−10005+4944(0015) % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn DmO2 /4.606RT = log P O2 1/2 = \frac - 13,550T + 4.411 ( ±0.009)O24606RT=logPO212=T−13550+4411(0009) From simultaneous measurements of the potential and concentration of oxygen in melts, not in thermodynamic equilibrium with alumina and aluminate phases, information on the composition dependence of the activity coefficient and the standard free energy of solution of oxygen is obtained. For the reaction, $\frac{1}{2} O_2 \to \underset{\raise0.3em\hbox{$Missing close brace ΔG o = -72,930 - 7.11T (±840) J gr.at.–1 = + 0.216 at. pct OlogfO=T−500+0216atpctO where the standard state for dissolved oxygen is that which makes the value of activity equal to the concentration (in at. pct) in the limit as concentration approaches zero. The oxygen solubility in liquid nickel in equilibrium with solid NiO, evaluated from thermodynamic data, is compared with information reported in the literature. Implications of the results to the deoxidation equilibria of aluminum in nickel are discussed.

Physical chemistry of the reduction of calcium oxide with aluminium in vacuum

The physical chemistry of "aluminothermic" reduction of calcium oxide in vacuum is analyzed. Basic thermodynamic data required for the analysis have been generated by a variety of experiments. These include activity measurements in liquid AI-Ca alloys and determination of the Gibbs energies of formation of calcium aluminates. These data have been correlated with phase relations in the Ca-AI-0 system at 1373 K. The various stages of reduction, the end products and the corresponding equilibrium partial pressures of calcium have been established from thermodynamic considerations. In principle, the recovery of calcium can be improved by reducing the pressure in the reactor. However,, the cost of a high vacuum system and the enhanced time for reduction needed to achieve higher yields makes such a practice uneconomic. Aluminum contamination of calcium also increases at low pressures. The best compromise is to carry the reduction up to the stage where 3CaO-Al,O, is formed as the product. This corresponds to an equilibrium calcium partial pressure of 31.3 Pa at 1373 K and 91.6 Pa at 1460 K. Calcium can be extracted at this pressure using mechanical pumps in approximately 8 to 15 hr, depending on the size and the fill ratio of the retort and porosity of the charge briquettes.

Production of niobium powder by electronically mediated reaction (EMR) using calcium as a reductant

Explored in this study is an electronically mediated reaction (EMR) route for the production of niobium powder using calcium as a reductant for niobium oxide (Nb2O5). Feed material, Nb2O5, and reductant calcium alloy containing aluminum and nickel were charged into electronically isolated locations in a molten salt (e.g. CaCl2) at 1173 K. The current flow through an external path between the feed and reductant locations was monitored. A current approximately 0.4 A was measured during the reaction in the external circuit connecting cathode and anode location. Niobium powder with low aluminum and nickel content was obtained although liquid Ca–Al–Ni alloy was used as the reductant. This clearly demonstrates that niobium metal powder can be produced by an electronically mediated reaction (EMR), without direct physical contact between feed (Nb2O5) and reductant (calcium). Mechanism of calciothermic reduction of Nb2O5 in the molten salt is discussed using an isothermal chemical potential diagram.

Synthesis, structural characterization and ferroelectric properties of Pb(0.76)Ca(0.24)TiO(3) nanotubes

A capillary-enforced template-based method has been applied to fabricate Pb(0.76)Ca(0.24)TiO(3) (PCT24) nanotubes via filling PCT24 precursor solution, prepared by modified sol-gel method, into nanochannels of anodic aluminum oxide templates. The morphology and structure of as-prepared PCT24 were examined by scanning electron microscopy, transmission electron microscopy (TEM) and X-ray diffraction techniques. The obtained PCT24 nanotubes with diameter of similar to 200 nm and wall thickness of similar to 20 nm exhibited a tetragonal perovskite structure. High resolution TEM (HRTEM) analysis confirmed that as-obtained PCT24 nanotubes made up of nanoparticles (5-8 nm) which were randomly aligned in the nanotubes. Formation of some solid crystalline PCT24 nanorods, Y-junctions and multi-branches were observed. Interconnections in the pores of template are responsible for the growth of Y-junctions and multi-branches. The possible formation mechanism of PCT24 nanotubes/nanorods was discussed. Ferroelectric hysteresis loops of PCT24 nanotube arrays were measured, showing a room temperature ferroelectric characteristic of as-prepared PCT24 nanotubes. (C) 2011 Elsevier B.V. All rights reserved.