Program Organizer: P.W. Keefe, Special Metals Corporation, New Hartford, NY 13413; M. Sohi, Allied Signal Engines, Phoenix, AZ 85072-2118
Monday, PM Room: B4
February 5, 1996 Location: Anaheim Convention Center
Session Chairperson: M. Sohi, Allied Signal Engines, Phoenix, AZ 85072-2118
PROCESSING, STRUCTURE AND PROPERTIES OF REACTION-INFILTRATED NiAl AND NiAl COMPOSITES:
T.A. Venkatesh, C. San Marchi, D.C. Dunand, A. Mortensen, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Room 4-117 Cambridge, MA 02139
Bulk NiAl is synthesized by infiltration of nickel preforms with molten aluminum and chemical reaction to form the intermetallic. Continuous fiber reinforced NiAl composites are fabricated with the same process by adding tungsten fibers to the nickel preform. Success of the process is found to be highly sensitive to the relative proportions of aluminum and nickel used. This issue is analyzed, and critical parameters are identified. Microstructures of the materials are characterized, particular attention being paid to macrosegregation, porosity, grain size and fiber-matrix interface structure and reaction. Mechanical properties of the material produced by this process are studied by compression creep testing and discussed for both bulk and reinforced NiAl.
ENGINEERING OF TOUGHENED NICKEL ALUMINIDE INTERMETALLICS: R. Ramasundaram, R. Bowman, Y. He, J. J. Lannutti, W. O. Soboyejo, Department of Materials Science and Engineering, the Ohio State University, 2041 College Road, Columbus, OH 43210-1179
A review of efforts to design and process toughen nickel aluminide composites is presented in this paper. This includes the use of micromechanics-based design methodologies to engineer the development of composites with improved fracture toughness and fatigue crack growth resistance. The paper will discuss the use of: ductile phase toughening with Mo particles and fibers; transformation toughening with partially stabilized zirconia (PSZ); hybrid/synergistic toughening with PSZ and Mo fibers/particles, and functionally graded architectures in the toughening of NiAl. The paper illustrates how the processing and applications technologies have guided the development of composites with improved damage tolerance. The damage tolerance issues that may hurt the potential aerospace applications of toughened nickel aluminide composites are also discussed.
FABRICATION OF STAINLESS STEEL TOUGHENED NiAl INTERMETALLIC COMPOUND MATRIX COMPOSITE: Shou-Yi Chang, Su-Jien Lin, Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan, China
The stainless steel fibers with a diameter of 150 um were wound and then nickel plated to a diameter of about 225 um. Alternate layers of the properly spaced Ni-coated stainless steel fibers and aluminum foils were stacked and hot pressed at 500[[ring]]C, 100 MPa in vacuum for 10 minutes to produce a S.S.f//Ni/Al composite. A following attractive process, hot pressing at 650[[ring]]C 150 MPa for a long period of time, 20 hours, allowed the nickel and aluminum to react to form the NiAl intermetallic compound matrix. Fewer pores were left if the hot pressing at 900[[ring]]C was followed. The variance of microstructures, which resulted from the different conditions of heat treatment and hot pressing, was analyzed by optical microscopy, scanning electron microscopy and X-ray diffraction. And the analyses revealed that some Ni-Al reaction phases existed in the composite.
MECHANICAL BEHAVIOR OF IN-SITU DIRECTIONAL SOLIDIFIED Cr(Mo)/NiAl COMPOSITE: S. M. Jeng, J.-M Yang, Department of Materials Science and Engineering, University of California, Los Angeles, CA 90024-1595; K. Bain, R. A.Amato, GE Aircraft Engines, Cincinnati, OH 45235
The NiAl-based composites reinforced with refractory metals in situ by directional solidification (DS) technique composites have been recognized as one of the most promising candidate materials for high temperature structural applications. The ductile reinforcing phase in the DS eutectics can assume a fibrous or lamellar morphology. The well-aligned structure can provide significant improvement in strength and toughness over the monolithic NiAl. The Cr(Mo)/NiAl eutectic composite has been successfully fabricated using an Edge-defined Film-fed Growth (EFG) technique. The mechanical behavior of the composites including tensile properties at room and elevated temperatures, fracture resistance and creep behavior were studied. The damage evolution and fracture characteristics at room and elevated temperatures were also investigated.
2:50 pm BREAK
NiAl SINGLE CRYSTALS WITH HIGH TENSILE DUCTILITY: Igor A. Bul, Vladimir I. Levit, Michael J. Kaufman, Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611
NiAl single crystals were grown by the Bridgman method from arc-melted rods. Some crystals were prestrained by compression at a temperature of 1273K. The dislocations b=<100> in the prestrained crystals were primarily located at low angle boundaries. Very low dislocation density was observed inside the subgrain volumes. Tensile tests were performed on both prestrained and undeformed NiAl single crystals oriented in  direction. The test temperatures were from 300K to 1273K, with a strain rate of about 10-4 sec-1. All the samples tested at RT obtained high tensile elongation (up to 13.6%). The prestrained samples showed slightly lower yield points and slightly higher elongation than undeformed ones at all temperatures. The factors related to ductility of NiAl single crystals are discussed: toichiometry, point defects and interstitials, dislocation density and mobility, sample design, method of machinery and quality of the sample surface preparation. This work is sponsored by the Air Force Office of Scientific Research (URI Grant F49620-93-0309) under the direction of Dr. Charles Ward.
AN ANALYSIS OF TENSILE DUCTILITY OF NiAl  ORIENTED SINGLE CRYSTALS: Jeffery S. Winton, Vladimir I. Levit, Michael J. Kaufman, Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611
The tensile properties of  oriented NiAl single crystals were investigated between RT and 700[[ring]]C. The tensile ductility exhibited three temperature regimes. At high temperatures of >500[[ring]]C, the ductility increases with temperature because of an increase in strain rate sensitivity, which results in less localized necking. At intermediate temperatures of 200[[ring]]C-500[[ring]]C, the ductility shows a plateau owing to a loss of strain hardening rate, which results in a reduction of maximum uniform strain. At low temperatures of RT-200[[ring]]C, fracture occurs after yielding, but before necking. The change of ductility of these three temperature regimes will be discussed in terms of strain hardening behavior, slip localization and necking. This work is supported by the Air Force Office of Scientific Research (URI Grant F49620-93-0309) under the direction of Dr. Charles Ward.
THE EFFECT OF ORIENTATION, TEMPERATURE, AND STRAIN RATE ON THE MECHANICAL PROPERTIES OF SINGLE CRYSTAL NiAl: Jeffery S. Winton, Vladimir I. Levit, Michael J. Kaufman, Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611
While the mechanical behavior of NiAl has been studied extensively, questions
concerning the fundamental deformation mechanisms and mechanical properties of
single crystal NiAl still remain. Little tensile testing has been performed,
and a review of the literature points out that there is relatively poor
agreement for such properties as yield strength and ductility, derived from
tensile testing. The tensile properties of single crystal NiAl were
investigated between RT and 1273K. The four crystallographic orientations
tested were , , , and . The single crystals tested
exhibited higher plasticity and a lower yield point at RT then previously
published data for all orientations tested. The dislocation mechanism at RT for
these tests will be discussed. Kinking was observed from RT to 773K in samples
slightly deflected from . Strain rate sensitivity was measured using
strain rate changes between 1x10-5, 1x10-4 and
1x10-3 and the results for all orientations will be presented. The
mechanical behavior corresponds to TEM observations of the dislocation density
and distribution. This work is supported by the Air Force Office of Scientific
Research (URI Grant F49620-93-0309) under the direction of Dr. Charles Ward.
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