I.G. Sharma

Production of Al–Zr master alloy starting from ZrO2

Abstract A process involving in-situ reduction of zirconium oxide with excess aluminum in the presence of cryolite flux has been developed for the preparation of a Al–Zr master alloy. The findings of this process are reproduced on doubling the batch size, and pilot scale results indicated viability of the process for commercial scale exploitation. It has been found possible to produce Al–5%Zr alloy in batches of 20 kg with over 92% yield on holding the charge for 1 h in the temperature range of 1100–1200°C. The addition of the master alloy to pure aluminum to the extent of 0.3%Zr in Al, effectively reduced the grain size from 1100 to 163 μm and correspondingly improved the ultimate tensile strength (UTS) of Al. The present process is found to have an edge over the conventional process of direct alloying.

Preparation of carbon incorporated NbAl alloy and its subsequent conversion to pure niobium by electron beam melting

Abstract Aluminothermic reduction of niobium pentoxide coupled with either direct electron beam melt refining of thermit niobium or a combination of pyrovacuum treatment of thermit niobium followed by electron beam melt refining of niobium sponge are the most widely adopted processes for the production of high purity (greater than 99.9 wt.%) niobium metal. Prior to pyrovacuum treatment, the thermit niobium ingot is crushed to smaller size of the order of 6 to 8 mm. Owing to very high hardness of thermit ingot, the crushing has been found to be quite difficult and energy intensive. The effectiveness of electron beam melt refining of thermit niobium is directly dependent on residual aluminum and oxygen concentration in the feed, which are inversely related. In the present studies, it has been found possible to eliminate problems encountered during crushing of thermit niobium as well as the pyrovacuum step by incorporating a judicious amount of carbon in the thermit niobium. A modified charge composed of niobium pentoxide, aluminium reductant corresponding to 10 wt.% excess over the stoichiometric amount and carbon amounting to 1 wt.% of Nb2O5 when reacted, resulted in a product alloy of NbAlC with higher niobium recovery (equal to or greater than 99 wt.%) and low residual aluminium concentration (less than or equal to 2 wt.%). It has been observed that the presence of carbon has not affected the overall yield. Carbon in the form of NbC preferentially precipitated at the grain boundaries of thermit alloy and rendered it easily crushable. Crushed alloy product could be subjected to direct electron beam melt refining without undergoing a pyrovacuum treatment step. No operational problems have been encountered, and in fact residual carbon assisted deoxidation in the final stage of electron beam melt refining. The electron beam melted button exhibited a hardness value of 65–70 VPN and a total impurity concentration of less than 0.05 wt.%.

Aluminothermic preparation of Hf-Ta and Nb-10Hf-1Ti alloys and their characterization

Abstract The high temperature materials capable of withstanding high stress levels at elevated temperature together with excellent oxidation resistance have received considerable attention since the emergence of aerospace industries. Alloys such as Nb–10Hf–1Ti and Hf–20 to 30 wt.% Ta are being considered seriously for development to meet the growing demand for aerospace and other high temperature requirements. Currently the alloys are prepared by conventional melting route either by arc or induction melting techniques. However, use of high vacuum and high temperature expensive furnaces, high cost of the alloying constituents, probability of segregation of alloying components during melting are some of the major deterring factors in the production of these alloys through melting route. In the present investigation, the above problems have been eliminated to a great extent by adopting aluminothermic (thermit) smelting technique for the preparation of Nb–10Hf–1Ti and Hf–Ta alloys by direct co-reduction of the oxides of individual components. In the adopted process the heat generated from the chemical reactions, serves the function of melting and alloying. By maintaining 10% excess aluminium in the charge, specific heat in the range of 600–700 kcal kg −1 and varying CaO and CaF 2 from 10 to 15% of overall charge has enabled to achieve a yield figure of 95% for both the alloys with excellent quality of the alloy product. The alloys are further remelted and characterized with respect to composition, microstructure, thermal analysis, hardness and rolling.

A study on preparation of copper-niobium composite by aluminothermic reduction of mixed oxides

Cu–Nb composites have come into prominence because of their high strength and high electrical conductivity. A number of preparative techniques have received attention among the investigators due to difference in melting point, density and crystal structure of the constituents. In the present paper a simple, easy to scale up, non-furnace process involving co-reduction of mixed oxides (CuO, Nb2O5) with aluminium in the presence of slag fluidizer CaO has been attempted to prepare a Cu–2.5Nb (wt%) composite. A typical charge composition of about 58.2% CuO, 3.5% Nb2O5, 23.3% CaO and 15% Al (specific heat 715 kcal/kg) resulted in a Cu–Nb product with 83% yield of nearly targeted composition. As reduced composite was homogenized by arc melting. Remelted and consolidated composite was drawn into wires of <0.5 mm diameter to convert the dendritic niobium phase into the filamentary form. Property evaluation studies such as optical metallography, hardness, electrical resistivity measurements etc. have been carried out.

Thermal decomposition of ammonium polymolybdate in a fluidized bed reactor

Abstract Studies on thermal decomposition of ammonium polymolybdate were carried out in a fluidized bed reactor using air as a fluidizing medium. The ammonia vapour released during the decomposition process was passed directly into hydrochloric acid solution. The change in pH of the solution was monitored with respect to time and temperature and the degree α of decomposition was calculated at intermediate stages of the process. The kinetics of the overall decomposition process were found to fit well into a ‘power law’ model. The overall activation energy E a and the rate constant k were determined to be 16.3 kJ mol −1 and 1.24 × 10 −3 s −1 respectively. The value of the activation energy suggests the process to be surface controlled. The preparation of MoO 3 from ammonium polymolybdate in a fluidized bed reactor was found to be accomplished at a lower temperature and in a lesser time than that needed in a static bed reactor.

Preparation and characterization of iron aluminide based intermetallic alloy from titaniferous magnetite ore

Abstract Intermetallic alloys such as iron aluminide (Fe 3 Al) display an attractive combination of physical and mechanical properties. It is a potential candidate for application demanding high strength, superior oxidation, sulphidation and corrosion resistance. However, the utilization of this alloy is limited owing to problems of its preparation by conventional techniques and poor room temperature ductility. The present paper is, therefore, an attempt to prepare an Fe 3 Al-based alloy (iron aluminide alloy) of composition Fe–17.24Al–5.54Cr–1V–0.05C (wt%) by a non-conventional non-furnace process. The process essentially involves the use of cheap, indigenously available titaniferous magnetite ore containing oxides of iron and vanadium and oxide of chromium that are subsequently co-reduced by aluminium in the presence of excess aluminium and a requisite amount of carbon in a specially designed water-cooled copper reactor. The charge composition is judiciously adjusted to attain the alloy composition by utilizing the exothermicity of the overall reactions. The alloy is further characterized with respect to composition, phases and microstructure and properties such as fabricability, hardness, strength and oxidation resistance were studied to indicate its suitability for high temperature applications.