Ramana G. Reddy

Modeling of sulfide capacities of silicate melts

An examination of the solution chemistry of sulfides in silicates leads to a method for the calculation of the sulfide capacities of silicate melts using the Flory model for polymeric silica chains. Calculations for the CaO−SiO2, FeO−SiO2, and MgO−SiO2 binary systems at 1773 K and 1923 K are shown to be in good agreement with available experimental data and to be more reliable than calculations based on the empirical concept of basicity.

Thermal stability and corrosivity evaluations of ionic liquids as thermal energy storage media

Ionic liquids have been proposed as a new class of heat transfer/storage liquids for solar parabolic trough systems. In this research, thermal stability and corrosivity of various ionic liquids were investigated. TGA analysis showed that ionic liquids exhibit decomposition temperatures as high as 450°C. Among the ionic liquids investigated, [C 8 mim]PF 6 and [C 4 mim][Tf 2 N] were found to be the more thermally stable. The corrosivity of ionic liquids against 316 stainless steel and 1018 carbon steel was investigated by electrochemical techniques at room temperature. Corrosion rates less than 13 μm/yr were obtained, thus indicating outstanding resistance of the alloys to uniform corrosion in ionic liquids. Localized corrosion was observed using [C 4 mim]Cl, presumably due to the presence of deleterious Cl - ions.

Physical properties of selected brazing filler metals

Abstract A suitable selection of the filler metal is vital for producing satisfactory brazed joints. The wettability of brazing alloys with base metals depends on physical properties such as surface tension, density, melting point, and viscosity. Thermal conductivity and electrical resistivity are also important since the filler metal is frequently required to have similar values to those of the base metal. In the present paper, the physical properties of liquid alloys relevant to brazing have been evaluated. Six different filler metal systems were analysed, comprising alloys based on Ag, Al, Au, Cu, Ni, and Ti. Results show that the viscosity values for most binary brazing filler alloys are of the order of 2–8 mPa s, with Cu and Al alloys exhibiting the lowest viscosities. The surface tensions of brazing alloys vary from 800 to 1800 mN m-1, with the lowest surface tension values corresponding to the Ag and Al alloys. Thermal conductivity and electrical resistivity values fall in the range 10–200 W m-1 K-1 and 17–300 μΩ cm, respectively.

Thermodynamic properties of Ti–Al intermetallics

Abstract Thermodynamic properties of the titanium–aluminum system have been investigated over the entire composition range. A solid-state reversible galvanic cell employing a calcium fluoride (CaF 2 ) solid electrolyte and an Al–CaAlF 7 reference electrode was used in the temperature range 820–900 K for solid alloys containing 31.3, 47.9, 64.7, 73.6 and 99.0 atom percent aluminum. Experimental results of the activities of Al in the alloys showed a large negative deviation from ideal behavior. For instance the activity of Al in an alloy with 0.48 atomic fraction of Al was determined to be 2.98×10 −3 at 850 K.

Sulfide capacities of MnO−SiO2 slags

Sulfide capacities of binary MnO−SiO2 slags at 1773 and 1923 K were calculated thermodynamically. Only known data, such as the standard free energy of formation of MnO and MnS and activities of MnO in the melt, are used in making calculations based on fundamental concepts. Excellent agreement is found between our calculations and published experimental data. Correlations of sulfide capacities, based on optical basicity using Pauling electronegativities or empirically deduced optical basicities, differ from the experimental data in both magnitude and concentration dependence. Our method provides useful predictions of sulfide capacitiesa priori.

Novel applications of ionic liquids in materials processing

Ionic liquids are mixtures of organic and inorganic salts which are liquids at room temperature. Several potential applications of ionic liquids in the field of materials processing are electrowinning and electrodeposition of metals and alloys, electrolysis of active metals at low temperature, liquid-liquid extraction of metals. Results using 1-butyl-3-methylimidazolium chloride with AlCl3 at low temperatures yielded high purity aluminium deposits (>99.9% pure) and current efficiencies >98%. Titanium and aluminium were co-deposited with/without the addition of TiCl4 with up to 27 wt% Ti in the deposit with current efficiencies in the range of 78–85 %. Certain ionic liquids are potential replacements for thermal oils and molten salts as heat transfer fluids in solar energy applications due to high thermal stability, very low corrosivity and substantial sensible heat retentivity. The calculated storage densities for several chloride and fluoride ionic liquids are in the range of 160–210 MJ/m3. A 3-D mathematical model was developed to simulate the large scale electrowinning of aluminium. Since ionic liquids processing results in their low energy consumption, low pollutant emissions many more materials processing applications are expected in future.

Oxidation of a ternary Ti3Al-Ta alloy

The isothermal oxidation behavior of a ternary Ti-25Al-18Ta intermetallic alloy has been investigated in pure oxygen over the temperature range of 850°C to 1100°C. The oxidation kinetics was found to follow a parabolic rate. Effective activation energy of 259 kJ/mol was deduced from the oxidation data. The oxidation products were a mixture of TiO 2 , the main component, Al 2 O 3 (alumina), and small amounts of tantalum oxide. The addition of Ta to Ti 3 Al alloy decreased the oxidation rate of the alloy. However, the oxidation scale was not compact and exhibited significant spallation especially at high temperatures.

In-situ multi-layer formation in the oxidation of Ti3Al-Nb

The effect of niobium on the oxidation of a Ti3Al alloy was studied in pure oxygen in the range of 850-1,100°C. The oxidation products for the Ti-30Al-2.7 Nb alloy were mainly TiO2 (rutile) mixed with A12O3 (alumina) and small amounts of niobium oxide. The oxidation resistance of Ti3Al was improved by the addition of niobium. An in-situ multiple-layer structure comprising a mixture of rutile and alumina formed on the oxide scale of the alloy at temperatures 1,000°C and above. The number of layers increased as the temperature increased but the individual layer thickness decreased.

Distribution of cobalt between liquid copper and copper silicate slag at 1523 K

The solubility of cobalt in Cu2O-SiO2 slags held in silica crucibles and equilibrated with copper-cobalt alloys has been investigated for conditions of varying amounts of cobalt in the alloy charged at a temperature of 1523 K. The solution of cobalt in slag from its alloy is explained by the equation: Co (in alloy) + Cu2O (in slag) = 2 Cu (in alloy) + CoO (in slag)The average distribution ratio, valid from 0.04 to 0.10 pct Co in the equilibrated metal, is: (Pct Co in slag)/(Pct Co in metal) = 400 ± 50 The results are explained with the help of SEM, X-ray diffraction and EDAX, which indicated that the phase assemblages encountered as charged cobalt is increased are: SiO2 + melt; Co2SiO4 + melt and CoO + Co2SiO2 + melt. In the last case, a coherent layer of cobalt silicate was found to have formed on the crucible surface. A tentative phase diagram of the ternary Cu2O-CoO-SiO2 system is presented. A change in the equation giving the activity coefficient of cobalt in Co-Cu alloys is suggested to bring calculated activity in agreement with the presence of CoO found experimentally as a separate phase.

Laser processing of titanium aluminides

Using a Nd-YAG laser, laser processing of a series of Ti-Al alloys including pure Ti and Ti-Al intermetallic compounds has been studied. Scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), and optical microscopy were used to determine the surface morphological, chemical, and compositional characteristics of the laser-processed samples. Analysis of results showed that cracks along grain boundaries caused by rapid heating and cooling of laser processing were the dominant characteristics of the surface morphologies of the laser-processed samples. The Al content in the Ti-Al alloys played a very important role in crack initiation and/or development. The more Al content in the samples, the more severe the cracks that developed after laser processing under the same conditions. The experiments were conducted at ambient conditions, resulting in surface oxidation layers being observed on the processed samples. The XPS results indicated that the oxidation layer consisted of adsorbed O2, Al2O3, TiO2, and TiO. In addition, Al enrichment was found in the oxide film of TiAl as well as in the oxidation layers formed on the surfaces of TiAl and Ti3Al intermetallics that were processed by the laser; this differs from the reported results for traditional oxidation of TiAl at elevated temperature.