Fracture behavior of a B2 Ni-30Al-20Fe-0.05Zr intermetallic alloy in the temperature range 300 to 1300 K
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I.
THE
INTRODUCTION
stoichiometric Ni-50AI*
intermetallic
com-
*Unless otherwise stated, all compositions are given in atomic percent.
pound has several attractive properties in comparison to currently available commercial superalloys. These include a B2 crystal structure, excellent oxidation resistance, high thermal conductivity, a high melting point (1911 K), and a relatively low density (6 Mg m -j) so that NiAI is presently being considered as a candidate material to replace superalloys in aircraft engine applications. Despite this attractive combination of properties, polycrystalline NiA1 has little or no low-temperature ductility and poor high-temperature strength which presently restrict its use as a turbine blade material. As a result, there is a considerable amount of effort being made at present to improve these mechanical properties. Although the creep properties of NiA1 have been demonstrably improved by well-established techniques, such as solid solution alloying and precipitate, dispersion, particulate, and long fiber strengthening methods, Ill improving the low-temperature ductility of this alloy poses a considerable challenge for alloy development. As a result of several earlier investigations, it is now known that {110} (001) is generally the favored slip system in NiAI. t2-71 Thus, only three independent slip systems can operate in this alloy, tS~ which are insufficient to satisfy the von Mises criterion for generalized plasticity in a polycrystalline material. While the operation of additional slip systems, such as those involving (I 10) (I 11) slip on the ~110} planes, can potentially satisfy the yon Mises criterion and maintain grain boundary compatibility in a polycrystalline material, it is important S.V. RAJ, Materials Research Engineer, is with the Materials Division, MS 49-1, NASA Lewis Research Center, Cleveland, OH 44135. Manuscript submitted October 7, 1991. METALI.URGICAL TRANSACTIONS A
to note that this is only a necessary, but not a sufficient, condition for improving the ductility of many polycrystalline intermetallic alloys. This fact is borne out by two important observations. First, recent studies on Ni50AI single crystals tested along the (001) direction and favorably oriented to induce (111) slip showed little or no tensile ductility at temperatures below about 5 9 0 K . 19'1r~'111 Second, polycrystalline Fe-50Ai does not exhibit significant room-temperature ductility, although {110} (I 11) is the favored slip system in this alloy. I~21 Clearly, other factors must be satisfied to ensure that try
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