Micropyretic synthesis of NiAl containing Ti and B
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Micropyretic synthesis of NiAl containing Ti and B G.K. Dey and A. Arya Materials Science Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
J.A. Sekhar International Center for Micropyretics, University of Cincinnati, P.O. Box 210012, Cincinnati, Ohio 45221 (Received 12 June 1998; accepted 30 September 1999)
The effect of alloying additions of Ti and B on the process of micropyretic synthesis on NiAl and on the microstructure of the synthesized alloy was examined. It was observed that the combustibility of the quaternary alloy is good despite the presence of the alloying elements because of an additional combustion reaction between Ti and B. The microstructure of the quaternary alloy was found to consist primarily of the NiAl and Ti boride phases. The effect of preheating of the specimen prior to synthesis on the process of synthesis was also examined. It was observed that preheating not only can change the morphology of the phases but also influence the nature of the phases present in the alloy. The mechanism of the formation of the two phase microstructure during the synthesis from the elemental powders was established by stopping the combustion front and by carrying out a detailed microstructural characterization of regions around the stopped combustion front.
I. INTRODUCTION
NiAl has been the focus of several investigations because of its attractive properties.1 The lack of ductility has been one of the major drawbacks of this alloy. In a recent study, it has been demonstrated that this intermetallic compound can show significant ductility under certain conditions of impurity and imperfection content, heat treatment, and surface perfection.2 Observations of this nature are likely to enhance the interest in this alloy. Several alloying additions have been tried out with a view to improve the ductility of NiAl.1 Ti is known to impart ductility to some phases with B2 structure.1 Ti additions have been found to improve the fracture toughness of NiAl.3 Additions of Ti beyond a certain limit lead to the formation of the Ni2AlTi phase which can be used for precipitation hardening of NiAl.4 Additions of B have been found to be effective in improving the ductility of some of the aluminides.5 With this point in view, the changes in the properties of NiAl on B addition have been examined by George et al.6 Composites containing TiB2 in NiAl have also been examined in considerable detail.7,8 However, studies involving micropyretically synthesized NiAl containing Ti and B in amounts sufficient for the formation of TiB2 as a second phase during synthesis have not been carried out earlier. In this study, the micropyretic synthesis of NiAl with Ti and B additions was implemented and the effects of these two diluents on the process and on the microstructure were analyzed. Diluents have been found to influJ. Mater. Res., Vol. 15, No. 1, Jan 2000
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ence the combustibility of alloys during micropyretic synth
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