Processing of an α-2 aluminide alloy, Ti---24Al---11Nb

Compressive stress-strain curves have been generated over a range of temperatures (900-1100-degrees-C and strain rates (0.001-100 s-1) for two starting structures consisting of lath alpha2 and equiaxed alpha2 in a Ti-24Al-11Nb alloy. The data from these tests have been analysed in terms of a dynamic model for processing. The results define domains of strain rate and temperature in which dynamic recrystallization of alpha2 occurs for both starting structures. The rate controlling process for dynamic recrystallization is suggested to be cross-slip in the alpha2 phase. A region of processing instability has also been defined within which shear bands form in the lath structure. Recrystallization of the beta phase is shown to occur for different combinations of strain rate and temperature from those in which the alpha2 phase recrystallizes dynamically

Quasicrystals:Indian research accomplishments and imperatives

: Within two months of the first report on quasicrystals in PRL in November 1984, Indian research which had a 'premature discovery' in 1978 in this area got under way, In the past nine years these efforts have led to original discoveries relating to new types of quasicrystalline phases as well as extensive investigations involving tiling theory, hyperspace, positron annihilation and electrical properties, These researches have been multi-institutional and multi-disciplinary. Enlightened and generous funding was extended by DST from 1986 by recognizing it as a thrust area in basic research via SERC and US-India Funds. International recognition, subjective though it is, in the form of citation of Indian papers, invited lectures and reviews, books as well as the membership of International Advisory Committee has followed and is among the highest in the fields of condensed matter science covered at the Bangalore meeting, Future directions pertaining to the exploration of mechanical and electronic properties as well as structures beyond the quasicrystalline order will be pointed out.

Diffusion path during internal displacement reactions in multi-component oxides: Reaction between Fe and (Co,Mg)TiO3 solid solution at 1273 K

The reaction between Fe foil and a disc of ilmenite solid solution (Co-0.48 Ni-0.52) TiO3 was studied at 1273 K. At the metal/oxide interface, the displacement reaction, Fe + (Co,Mg)TiO3 = Co + (Fe,Mg)TiO3 occurs, resulting in an ilmenite solid solution containing three divalent cations. Ferrous ions diffuse into the oxide solid solution and cause the precipitation of Co-Fe alloy as discrete particles inside the oxide matrix. The morphology of the product layer was characterized by SEM. Only two phases, alloy and ilmenite, were detected in the reaction zone. This suggests that the local flux condition imposed by ilmenite stoichiometry (Co + Fe + Mg):Ti = 1:1] was satisfied during the reactive diffusion: (J(Co) + J(Fe) + J(Mg)) = J(Ti). The composition of the alloy and the oxide was determined using EPMA as a function of distance in the direction of diffusion. Although Mg does not participate in the displacement reaction, its composition in the ilmenite phase was found to be position dependent inside the reaction zone. The up-hill diffusion of inert Mg is caused by the development of chemical potential gradients as a result of displacement reaction. The evolution of composition gradients inside the reaction zone and the diffusion path in a ternary composition diagram of the system CoTiO3-FeTiO3-MgTiO3 are discussed. (C) 2010 Elsevier B.V. All rights reserved.

Substrate-dependent structure, microstructure, composition and properties of nanostructured TiN films

Titanium nitride films of a thickness of similar to 1.5 mu m were deposited on amorphous and crystalline substrates by DC reactive magnetron sputtering at ambient temperature with 100% nitrogen in the sputter gas. The growth of nanostructured, i.e. crystalline nano-grain sized, films at ambient temperature is demonstrated. The microstructure of the films grown on crystalline substrates reveals a larger grain size/crystallite size than that of the films deposited on amorphous substrates. Specular reflectance measurements on films deposited on different substrates indicate that the position of the Ti-N 2s band at 2.33 eV is substrate-dependent, indicating substrate-mediated stoichiometry. This clearly demonstrates that not only structure and microstructure, but also chemical composition of the films is substrate-influenced. The films deposited on amorphous substrates display lower hardness and modulus values than the films deposited on crystalline substrates, with the highest value of hardness being 19 GPa on a lanthanum aluminate substrate. (C) 2011 Elsevier Ltd. All rights reserved.

Influence of grain boundary sliding on diffusion in yttria doped zirconia

Grain boundary sliding during high temperature deformation can lead to stress concentrations and an enhancement of diffusion in mobile boundaries. Experiments were conducted on a fine grained 3 mol% yttria stabilized tetragonal zirconia, under conditions associated with superplastic flow involving grain boundary sliding. Tracer diffusion studies under creep conditions and without load indicate that there is no enhancement in either the lattice or grain boundary diffusivities. The experimental creep data are consistent with an interface controlled diffusion creep mechanism. (C) 2011 Elsevier Ltd. All rights reserved.

Kinetic evolution studies of silver nanoparticles in a bio-based green synthesis process

Silver nanoparticles are being extensively studied due to their widespread applications and unique properties. In the present study, the growth kinetics of silver nanoparticles as synthesized on reduction of silver nitrate solution by aqueous extract of Azadirachta indica leaves was investigated. The formation of silver nanoparticles was preliminarily monitored by measuring the absorption maxima at different time intervals after adding the reducing agent to the silver salt solution (0.5, 1, 1.5, 2, 2.5, 3, 3.5 and 4 h). At different time points characterization studies were conducted using X-ray diffraction studies, FT-IR techniques, zeta potential studies and transmission electron microscopy. The total available silver in the reaction medium was determined at different durations using ICP-OES. The changes in reduction potential in the medium were also monitored using potentiometric analysis. The results confirm a definite change in the medium pertaining to formation of the stable nanoparticles after 2 h, and a significant increase in the agglomeration tendency after 4 h of interaction. The growth kinetic data of the nanoparticles till 3.5 h was found to fit the LSW model confirming diffusion limited growth. (C) 2011 Elsevier B.V. All rights reserved.

Stability of copper tolerance in Thiobacillus ferrooxidans

A strain of Thiobacillus ferrooxidans MAL-4-1 was adapted to grow at higher concentrations of copper by repeated subculturing in the presence of increasing levels of added cupric ions in 9K medium. The strains adapted to copper were found to be more efficient in bioleaching of copper from concentrates. When copper tolerant strains were back cultured repeatedly in 9K medium without cupric ions, the initially developed metal tolerance was observed to be lost. This indicates that the copper tolerance developed is stress-dependent and not a permanent trait of the adapted strain.

Theoretical estimation of the effect of minor elements on the solubility of oxygen in silicon melt

The effect of fourteen minor elements (Al, As, B, Bi, C, Ga, Ge, In, N, P, Pb, S, Sb and Sn) on the solubility of oxygen in silicon melt has been estimated using a recently developed theoretical equation, with only fundamental physical parameters such as hard sphere diameter, atomic volume and molar heat of solution at infinite dilution as inputs. The results are expressed in the form of interaction parameters. Although only limited experimental data are available for comparison, the theoretical approach appears to predict the correct sign, but underestimates the magnitude of the interaction between oxygen and alloying elements. The present theoretical approach is useful in making qualitative predications on the effect of minor elements on the solubility of oxygen in silicon melt, when direct measurements are not available.

A critical assessment of the standard molar gibbs free energy of formation of NiWO4

Three independent studies have been reported on the free energy of formation of NiWO4. Results of these measurements are analyzed by the �third-law� method, using thermal functions for NiWO4 derived from both low and high temperature heat capacity measurements. Values for the standard molar enthalpy of formation of NiWO4 at 298·15 K obtained from �third-law� analysis are compared with direct calorimetric determinations. Only one set of free energy measurements is found to be compatible with calorimetric enthalpies of formation. The selected value for ?f H m 0 (NiWO4, cr, 298·15 K) is the average of the three calorimetric measurements, using both high temperature solution and combustion techniques, and the compatible free energy determination. A new set of evaluated data for NiWO4 is presented.

The structure of amorphous platinum disulfide as studied by anomalous x-ray-scattering

Anomalous X-ray scattering (AXS) has been applied to study the structure of amorphous platinum disulfide, Pt1-xS2, prepared by the precipitation process. The local atomic arrangement in amorphous Pt1-xS2 was determined by the least-squares variational method so as to reproduce the experimental differential interference function at the Pt L(III) absorption edge by the AXS method as well as the ordinary interference function by MoK alpha. The structural unit in amorphous Pt1-xS2 is found to be a PtS6 octahedron, similar to that in crystalline PtS2. These octahedra share both their corners and edges, while only edge-sharing linkages occur in crystalline PtS2.

Anomalous X-ray scattering study of local structures in the superionic conducting class (CuI)(0.3)(Cu2O)(0.35)(MoO3)(0.35)

The anomalous X-ray scattering (AXS) method using Cu and Mo K absorption edges has been employed for obtaining the local structural information of superionic conducting glass having the composition (CuI)(0.3)(Cu2O)(0.35)(MoO3)(0.35). The possible atomic arrangements in near-neighbor region of this glass were estimated by coupling the results with the least-squares analysis so as to reproduce two differential intensity profiles for Cu and Mo as well as the ordinary scattering profile. The coordination number of oxygen around Mo is found to be 6.1 at the distance of 0.187 nm. This implies that the MoO6 octahedral unit is a more probable structural entity in the glass rather than MoO4 tetrahedra which has been proposed based on infrared spectroscopy. The pre-peak shoulder observed at about 10 nm(-1) may be attributed to density fluctuation originating from the MoO6 octahedral units connected with the corner sharing linkage, in which the correlation length is about 0.8 nm. The value of the coordination number of I- around Cu+ is estimated as 4.3 at 0.261 nm, suggesting an arrangement similar to that in molten CuI.