Mechanical Behavior of Monocrystalline Nial Using a Miniaturized Disk-Bend Test
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MECHANICAL BEHAVIOR OF MONOCRYSTALLINE NiAl USING A MINIATURIZED DISK-BEND TEST Ha K. DeMarco and Alan J. Ardell Department of Materials Science and Engineering, University of California, Los Angeles. California 90024-1595 ABSTRACT Miniaturized disk-bend tests were conducted on stoichiometric monocrystalline NiAl alloys of two different nominal purities. Disks 3 mm in diameter and -250 mm thick were prepared with faces oriented parallel to either (100) or (110) and tested in biaxial bending. The specimens exhibited some ductility, even in the "hard" (100) orientation, prior to catastrophic failure. The yield strength of the specimens was higher in the (100) orientation than in the (110) orientation, as expected, but the specimens in (110) orientation were considerably more ductile. The higher purity alloy was considerably more ductile in both orientations. The estimated CRSS of the samples (110) in orientation is -80 to 85 MPa. which is somewhat lower than reported values for deformation on the (001){010) slip system. For the most part, the fracture surfaces are similar in both alloys, with cleavage being the dominant mode of fracture. There is no visual evidence on the fracture surfaces that can account for the differences in ductility of the two alloys tested. INTRODUCTION The ordered intermetallic compound NiAl has stimulated considerable interest in the aerospace industry as a high-temperature structural material due to its low density and good oxidation resistance. However, the fabrication of NIAW into useful components is hampered by its lack of room-temperature ductility. It has recently been shown that the ductility of monocrystalline NiAl is enhanced by minor alloying additions (- 0.2 at. %) of Fe, Mo or Ga [I]. Micro-alloying of polycrystalline NiAl, however, does not produce the same enhancement of ductility [2]. This suggests that microalloying affects cleavage, but not the factors affecting crack propagation along grain boundaries. The mechanism responsible for the enhanced ductility of microalloyed single crystals has not been identified. In the present investigation a miniaturized disk-bend test (MDBT) is being used to study the changes in ductility of monocrystalline NiAl brought about by micro-alloying additions of Fe and other elements. The idea is to exploit the small size of the typical MDBT specimen (3 mm in diameter and about 250 jim in thickness) to prepare alloys of various compositions using the same starting material. The proposed method of alloy preparation involves deposition of controlled amounts of the desired alloying element onto disks of specific orientation, followed by annealing in vacuum at high temperatures (-1300 °C) and slow cooling to avoid complications associated with excess vacancy concentrations. Since mechanical testing using the MDBT apparatus has never been performed on single crystals of any material, let alone NiAl, it is essential to establish baseline data with which the results of future experiments can be compared. The results of our initial experiments are repor
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