Micropyretic synthesis of tough NiAl alloys
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INTRODUCTION
INTERMETALLIC compounds have attracted considerable attention in recent times because of their vast application potential in the aerospace industry.[1] The enormous application potential of the intermetallics, especially aluminides, stems from their many attractive properties, such as relatively low densities, high oxidation, and corrosion resistance, combined with their ability to retain strength and stiffness at elevated temperatures.[2,3] Considerable amount of work has already been done on the aluminides in the TiAl and the Ni-Al systems.[4,5] Among the Ni aluminides, NiAl has emerged as a possible high-temperature structural material because of its low density, high melting temperature, good thermal conductivity, and excellent oxidation resistance. It has several advantages over superalloys and other intermetallics like Ni3Al.[6] As is the case with other aluminides, NiAl has certain drawbacks, the most important being the lack of ductility at ambient temperatures. Several attempts have been made to alleviate the problem of roomtemperature brittleness in NiAl through modification of slip systems, grain refinement, use of single crystals, and microalloying with boron.[7,8] Studies have shown that in addition to conventional casting and powder metallurgy fabrication techniques, micropyretic synthesis may also be used successfully to obtain net shapes in this alloy.[9,10] Micropyretic synthesis offers several advantages over other techniques of synthesis, the most important ones being the possibility of direct net shape fabrication and enormous saving in energy spent on fabrication and processing.[11] In the case of intermetallic compounds like NiAl, the examination of the effect of alloying additions, added with a view toward improving mechanical properties of the alloy, remains a study of paramount significance. In micropyretically synthesized alloys, the addition of a third alloying addition is likely to influence the micropyretic synthesis process and hence the properties of the alloy. In this study, the effect of minor alloying additions on the micro-
pyretic synthesis of NiAl has been examined. These alloying additions have been made with a view to improving the mechanical properties of the alloys. The influence of the following processing parameters on the process of synthesis of NiAl has been investigated in this article: particle size, diluents, and preheat. The effect of particle size and that of inactive diluents have been examined by Li et al.[12,13] The effect of preheat and the addition of Al on the process of synthesis of NiAl has been examined in other studies.[14,15] However, none of these studies have examined the effect of these process parameters on the microstructure. In this work, besides examining the effect of the process parameters on the microstructure, an attempt has been made to study the mechanism of the micropyretic synthesis of the multicomponent alloys by arresting the combustion front by squeezing the combusted specimen between two copper plates. A detailed micr
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