• T R Ramachandran

      Articles written in Bulletin of Materials Science

    • Analytical electron microscopy of aluminium alloys

      T R Ramachandran D C Houghton J D Embury

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      X-ray microanalysis and electron energy loss spectroscopy of thin foils constitute the important techniques of high resolution chemical analysis using the electron microscope. The technique of x-ray microanalysis is discussed in this paper with particular emphasis on the study of aluminium alloys using a dedicated scanning transmission electron microscope (stem).

      The principle of determining chemical composition from observed x-ray peak intensities including the absorption of x-rays and beam broadening in thin foils are considered. The accuracy of peak intensity measurement and detection limits in x-ray microanalysis are illustrated with reference to Al-Mn alloys. The Cliff-Lorimer (k) factors for manganese, iron and copper with respect to aluminium were obtained from standard samples. Identification of phases in 1100 and 1200 aluminium and 3008 (Al-Mn-Zr) alloy were carried out from measured intensities of x-ray peaks. The experimental results emphasize the value of developing techniques for extracting the particles from the aluminium matrix. The transition phases formed in Al-6%Zn-3%Mg and Al-4% Cu were investigated by micro-diffraction and x-ray microanalysis.

    • Physical metallurgy of aluminum-lithium alloys

      G J Kulkarni D Banerjee T R Ramachandran

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      The addition of lithium to aluminium reduces the density and increases the elastic modulus; precipitation of the metastableδ′(Al3Li) phase from supersaturated Al-Li solid solution leads to appreciable increase in strength. The enhanced values for specific modulus and specific strength favour the use of the Al-Li alloys as structural materials for aerospace applications. However the binary alloys suffer from problems of poor ductility and toughness associated with strain localisation (resulting from the ease with whichδ′ particles are sheared during deformation), the presence ofδ′-free zones near grain boundaries and the heterogeneous nucleation of the equilibriumδ phase on the grain boundaries. These problems have been overcome by the development of ternary and quaternary alloys containing copper and magnesium. A small amount (∼0·1%) of zirconium is added to these alloys to improve the recrystallisation characteristics. The properties of alloys developed for commercial exploitation are briefly discussed. An overview of the physical metallurgy of the Al-Li alloys is presented with emphasis on the following features: (i) phase equilibria and precipitation reactions in Al-Li, Al-Cu-Mg, Al-Cu-Li and Al-Mg-Li systems and extension of these results to Al-Li-Cu-Mg alloys, (ii) insoluble particles and their effect on precipitation in the alloys, (iii) microstructural studies on Al-2·3%Li-1·2%Cu-0·7%Mg-0·12%Zr alloy, (iv) lithium depletion during solution treatment, (v) coarsening ofδ′ particles and development of precipitate-free zones near grain boundaries and (vi) microanalysis of the lithium containing phases.

    • Recent trends in alumina and aluminium production technology

      H Mahadevan T R Ramachandran

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      For well over a century the Bayer process for the production of alumina from bauxite and the Hall-Heroult process involving the electrolysis of alumina dissolved in cryolite to produce aluminium have been commercially exploited. However, significant advances based on intense research and development activities in the last five decades have led to considerable improvements in operating parameters and productivity, efficient disposal and utilization of wastes. This review presents the developments that have taken place in technology, equipment and treatment of wastes in both the alumina and aluminium production processes.

      Technology and equipment developments in the field of alumina are concerned with usage of different qualities of bauxite, improvement in extraction efficiency of alumina from bauxite, obtaining better yield during precipitation, enhancing scale of operation and minimizing soda losses in the process. Improvements in electrolysis involve utilization of modified electrolytic baths, controlled feeding of alumina, better design of cell based on mathematical modelling of magnetic field and heat distribution and optimization of the performance of electrodes on the basis of good understanding of physical, mechanical and electrical properties. Information on the present situation in the Indian plants is also presented. Proper treatment and disposal of wastes generated in alumina and aluminium production are of great importance as they contain significant amounts of sodium hydroxide, fluorides, sulphur dioxide, oxides of carbon and cyanide. Some attempts made to utilize the waste material are briefly summarized.

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