• D C Agrawal

      Articles written in Bulletin of Materials Science

    • Preparation and properties of iron-boron metallic glass ribbons melt spun in air

      D C Agrawal E A Chakachery

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      The preparation of metallic glass ribbons by melt spinning is a simple technique; however, a researcher often faces many problems when undertaking the preparation of such ribbons. The difficulties encountered are oxidation of the melt, choking of the nozzle by the solidified metal, formation of metal globules or ribbon fragments, oxidation of the ribbons, etc. These problems are particularly severe when working with high melting point alloys. This report describes certain experimental techniques used to overcome problems in preparation of metallic glass ribbons of iron-boron and other high melting point alloys. The ribbons thus produced have been characterized by transmission electron microscopy, mechanical tests and differential thermal analysis. The mechanical strength of the ribbon is comparable to the best recorded strength for ribbons of similar composition. Thedta data support the view that the formation of the phase Fe23B6 takes place upon crystallization of the eutectic alloy.

    • Interfacial mechanisms controlling phase formation in sol-gel derived lead zirconate titanate thin films

      S B Majumder V N Kulkarni Y N Mohapatra D C Agrawal

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      An RBS study has been carried out to study the relative extent of Pb diffusion in the substrate from PZT films. It is found that extensive diffusion of Pb occurs into the quartz substrate making the film severely deficient in Pb. No such diffusion occurs in the case of PZT films on sapphire and the concentration of Pb is near stoichiometric, except for a small loss due to volatilization. Excessive Pb deficiency, rather than any epitaxial effect, has earlier been proposed by us to be the crucial factor responsible for the existence of the pyrochlore phase in PZT thin films on substrates such as Si, glass, quartz etc. The present results confirm this. The effects of other process variables such as thickness and chemical composition (Zr/Ti ratio) of the film can also be understood in terms of the same phenomenon.

    • Enhancement in strength and toughness of Al2O3 due to dissolution of carbon

      S Bhattacharyya D C Agrawal U S Mehrotra S K Gupta

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      An improvement in fracture toughness and strength is observed in carbon atmosphere sintered alumina on reheating the sintered samples in air at 1200°C. SEM photographs show the presence of secondary precipitate particles at grain boundaries of the sintered samples, which disappear after reheating in air. Infrared spectroscopy indicates that the C-O bond intensity increases on reheating in air. An AES/XPS study reveals that carbon enters the alumina lattice on reheating. These results lead to the conclusion that the enhanced fracture toughness and strength are related to the dissolution of precipitates and consequent dissolution of carbon in alumina lattice.

    • Preparation of tetragonal zirconia powders by a solid state reaction: Kinetics, phases and morphology

      V V Mishra A K Garg D C Agrawal

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      Powders of tetragonal (t)ZrO2 have been prepared by a solid state reaction between sodium metazirconate and sodium metaphosphate. The reaction temperatures and times have been varied between 450 and 550°C and 5 and 75 h, respectively. Zirconia powder, mostly in thet andt′ phases, is obtained. The yield of ZrO2 powder increases monotonically with time at all reaction temperatures according to a phase boundary controlled kinetics. The fraction oft phase also increases with time at 450°C and 500°C but goes through a maximum at 550°C, the highest temperature employed. A maximum of 55% of the precursor monoclinic zirconia (used to prepare sodium meta zirconate) is converted tot phase at 500°C/75 h. The ZrO2 powder consists of crystallites of size 9–25 nm agglomerated into particles having average size between 2 and 4µm. The agglomerates have a breaking strength of 100 MPa. A hydrothermal treatment is found to break the agglomerates into smaller sizes. Grinding the powder in a mortar and pestle converts only 12% of thet phase into monoclinic, indicating that substantial fraction of the tetragonal phase is the non transformable varietyt′. Heating experiments also confirm this.

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