Micropyretic synthesis studies of Ni-, Al-, Ti-, and Nb-containing alloys
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I. INTRODUCTION
A considerable amount of research has been carried out on NiAl because of its attractive properties.[1] However, the lack of ambient temperature ductility in this alloy has been one of the major drawbacks. In a recent study, it has been shown that single crystals of this alloy demonstrate ductility under certain conditions of specimen purity.[2] Observations of this nature give new impetus to the research being carried out on this intermetallic compound. Aluminides such as TiAl and NiAl possess a somewhat poor creep resistance at high temperatures and are therefore unable to meet the demands made by advanced propulsion systems, where good creep resistance at very high temperatures is of paramount significance.[3] As a result, it has been necessary to study other intermetallic compounds with higher melting points, which hold the promise of improved mechanical properties at high temperatures. Nb3Al is one such compound and is being studied in considerable detail.[4,5,6] Nb3Al occurs in the A 15 crystal structure, and though it has high-temperature strength, it is very brittle at ambient temperatures.[4] The brittleness of the alloy stems from the fact that it has a limited number of operating slip systems. Studies have shown that by alloying, it is possible to induce a crystal structure transition to a structure that will permit the operation of a greater number of slip systems, thus rendering the alloy ductile.[5] In some of the earlier studies on this alloy (involving meltspun ribbons), a high dislocation density has been observed after the structure changed from the A15 type to the B2 form.[6] It was, therefore, realized that the B2 structure in this alloy may indeed be ductile. In order to stabilize the B2 structure in this alloy, which otherwise has the A15 structure, alloying additions were attempted, and it was found that Ti may stabilize the B2 phase.[6] It is well known that Ticontaining B2 phases are ductile.[7] With this point in view, ternary Nb-Ti-Al alloys have been developed, and these G.K. DEY, Visiting Scientist, International Center for Micropyretics, and J.A. SEKHAR, Professor, Department of Materials Science and Engineering, and Director, International Center for Micropyretics, are with the University of Cincinnati, Cincinnati, OH 45221-0012. Manuscript submitted March 4, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
have been found to show considerable ductility at ambient temperatures.[4] Nb45Al15Ti40 is one such alloy. It is interesting to note that whereas the NiAl has the B2 structure and is extremely brittle at room temperature under most testing conditions, the alloy Nb45Al15Ti40 with a similar structure is ductile. The alloy Nb45Al15Ti40, unlike NiAl, however, displays poor oxidation resistance. There is a need consequently to search for a combination of intermetallic phases of the B2 type that exhibit both high ductility and oxidation resistance. Micropyretic synthesis (also sometimes called SHS or combustion synthesis) has emerged as a novel method of developing mate
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