Microscopy and tensile behavior of melt-spun Ni-Al-Fe alloys
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I.
INTRODUCTION
FOR high-temperature
structural applications, such as those requiring nickel-base superalloys, the NiA1/3 (ordered bcc B2 structure) nickel aluminide may offer significant advantages in specific strength, use-temperature capability, and oxidation resistance. Similar incentives have already caused a great deal of study on the Ni3A1 3" (ordered fcc L12) nickel aluminide. The major barrier to structural use for either system has been inadequate ductility at ambient to moderate temperatures. In 3" aluminides, where the structure is capable of providing a sufficient number of operative slip systems, this problem has been attributed to intrinsic grain boundary embritdement resulting from poor cohesive bonding at the boundary.t~] Adjustments in composition and small additions of boron appear to have overcome the poor bonding and resulted in significant improvements in lowtemperature ductility. [2,3,4] The brittleness of/3 aluminides arises from inherent crystal structure characteristics. The NiA1 lattice suffers from a limited number of independent slip systems and does not satisfy the von Mises plasticity criterion tS~for homogeneous polycrystalline flow. At temperatures greater than about 500~ polycrystalline, near-stoichiometric NiA1 becomes ductile, presumably due to diffusion-aided deformation. I61 Plasticity at ambient temperature has been reported for single crystals of NiA1 when tested in compression, I6,7] polycrystalline rapidly solidified ribbons when tested in bending, I8~ and polycrystalline wrought NiA1 when tested in tension.~9] However, the strain to fracture for the poly-
S.C. HUANG, Staff Metallurgist, is with General Electric Corporate Research and Development Center, P.O. Box 8, Schenectady, NY 12301. R.D. FIELD and D.D. KRUEGER, Staff Engineers, are with General Electric Aircraft Engines, Mail Drop M89, 1 Neumann Way, Cincinnati, OH 45215. Manuscript submitted June 22, 1989. METALLURGICAL TRANSACTIONS A
crystalline materials was only about 2 pct, inadequate for most structural applications. Recent work tl~ on nickel aluminides has indicated that an attractive approach for improving the ductility of polycrystalline materials is to select Ni-rich Ni-A1-X ternary alloys that form dual-phase structures. The advantages of these aluminides should fall between those of NiA1 and Ni3A1, with the opportunity for more immediate structural applicability than NiA1. In that study, dualphase Ni-A1-Fe filaments processed by melt quenching exhibited as much as 17 pct strain to fracture at ambient temperature. This ductile behavior was not well understood and was attributed to the combination of a rapid solidification-induced fine grain size, a suppression of the development of order, and the suppression of grain boundary segregation. However, the effects of annealing were not studied. The study of annealed filaments would have more rigorously defined the role of the rapid solidification microstructures. Also, this study would pertain more directly to practical alloy evaluation, since most of the r
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