Synthesis of intermetallic NiAl and Ni 3 Al fine powders through organometallic precursors
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. INTRODUCTION
II. EXPERIMENTAL
Intermetallic NiAl and Ni3Al have potential as high-temperature engineering materials because of the high melting temperatures (1638 °C/NiAl and 1385 °C/ Ni3Al), high strength, and excellent corrosion resistance.1 3 However, poor ductility has limited the industrial applications of these materials.4-5 One technique to improve the ductility was the refinement of grain size with or without second-phase dispersants.6"9 The grain size refinement has other advantages, namely improvement of high-temperature strength10 and oxidation resistance.11 Small grain size is effectively attained by powder metallurgical procedures such as hot extrusion, hot pressing, hot isostatic pressing, and sintering using powders synthesized through atomizing,12 melt spinning,13 mechanical alloying,14 and others. Chemical synthesis is also an attractive technique for the production of homogeneous and fine powders. The homogeneity of chemical composition, which is a most important peculiarity of chemical synthesis, is useful to prevent the formation of unexpected phases with low melting temperatures (for example, Ni2Al3 and NiAl3) during synthesis. However, there are few studies on chemical synthesis of intermetallics, although it is widely applied for the production of fine ceramic powders.
A. Preparation of organometallic precursors
In the present paper, a new synthesizing technique of fine powders of NiAl and Ni3Al through organometallic precursors is proposed. Co-precipitation and thermal decomposition processes of organometallic precursors, reactions forming the intermetallics, and some properties of synthesized powders are discussed. Rapid co-precipitation presented in this paper results in homogeneous precursors. The benzene group contained in the precursors provides free carbon during the thermal decomposition to reduce Ni2+ and Al3+ ions to the metallic elements by carbothermal reduction in the following synthesizing reactions. 928 http://journals.cambridge.org
J. Mater. Res., Vol. 6, No. 5, May 1991 Downloaded: 13 Mar 2015
Stock solutions of NiCl2 (1 mol dm"3) and A1C13 (1 mol dm"3) were independently prepared by dissolving NiCl2 • 6H2O and A1C13 • 6H2O into distilled water and filtering through 0.1 fim membranes to remove solid impurities. The concentration of Ni2+ and Al3+ ions was determined by using ion-sensitive electrodes (Toa Electronics, Ltd., IM-40S). Precipitants were ammonium benzoate (2 mol dm"3 aqueous solution) and hydradinium monochloride (used as powder). A buffer solution of 1.2 mol dm"3 ammonium acetate and 1.2 mol dm"3 hydrochloric acid was used to adjust the pH to 3.5. All starting materials were high purity reagent grade chemicals (Wako Pure Chemical Industries, Ltd.). Appropriate volumes of stock solutions (Ni:Al = 3:1 or 1:1, totaling 200 cm3) were measured into a 1.0 dm"3 Pyrex container. The solution was heated up to 90 °C after adjusting pH = 3.5 by adding 200 cm3 of the buffer solution. Preheated precipitant solution (400 cm3) was added to the mixed solution during stirrin
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