Deformation of a NI-AL-FE Gamma/Beta Alloy
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DEFORMATION OF A NI-AL-FE GAMMA/BETA ALLOY R.D. FIELD*, D.D. KRUEGER*, AND S.C. HUANG** *GE Aircraft Engines, 1 Neumann Way, Cincinnati, OH, 45215 **GE Corporate Research and Development Laboratories, PO Box 8, Schenectady, NY, 12301 ABSTRACT Room temperature tensile tests were performed on annealed melt-spun ribbons of a 50Ni20Al-3OFe alloy. The ribbons were found to possess a duplex structure consisting of fcc y and B2 1 grains and exhibited tensile elongations in excess of 10% while still maintaining good strength. The tested specimens were found to contain high dislocation densities in both the y and p grains, with no indications of stress-induced martensite formation. Dislocation analysis revealed that the vast majority of dislocations in the p3have Burgers vectors; however, dislocations were also observed. Slip transfer was often facilitated by specific orientation relationships between the y and 13grains. BACKGROUND For high temperature structural applications, such as those requiring Ni-base superalloys, JINiAl may offer significant advantages in specific strength, use temperature capability, and oxidation resistance. Similar incentives have already encouraged a great deal of study on Ni 3Al. The major barrier to structural use for either system has been inadequate ductility at ambient to moderate temperatures. Unlike the Ni3Al system, which possesses a sufficient number of independent slip systems and can be rendered ductile by micro-alloying [1-31, the brittleness of NiAl arises from inherent crystal structure characteristics; that is, von Mises plasticity criterion for polycrystalline flow is not satisfied [4]. At temperatures greater than about 500°C, polycrystalline, near stoichiometric NiAl becomes ductile [5]. Plasticity at ambient temperature has been reported for single crystal NiAl, when tested in compression [5-7], and polycrystalline rapidly solidified ribbons when tested in bending and tension [8]. However, strain to fracture for the polycrystalline materials was only about 2%, still inadequate for most structural applications. Recent work on NiAI has indicated that an attractive approach for improving the ambient ductility of polycrystalline materials is to select Ni-rich Ni-AI-X ternary alloys that form dual phase structures. The advantages of these aluminides should fall between those of NiAl and Ni3 AI, with the opportunity for more immediate structural applicability than NiAl. In the study of Inoue et al. [9], dual phase Ni-Al-Fe alloys processed by melt quenching exhibited as much as 17% strain to fracture at ambient temperature. This ductile behavior was not well understood, and was attributed to the combination of fine grain size, a depression of the development of order, and the suppression of grain boundary segregation. In the present investigation, the structures, tensile properties, and deformation features in annealed ribbons of Ni-Al-Fe alloys with 20 at % Al and 20-40 at % Fe are being studied. This range in composition considers the Vy (disordered fcc) and 3-+i-' phase fields. I
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