Fracture Toughness and the Effects of Stress State on Fracture of Nickel Aluminides
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FRACTURE TOUGHNESS AND THE EFFECTS OF STRESS STATE ON FRACTURE OF NICKEL ALUMINIDES
JOHN J. LEWANDOWSKI, GARY M.MICHAL, IVAN LOCCI, AND JOSEPH D. RIGNEY
Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106
ABSTRACT The effects of stress state on the fracture behavior of Ni3AI, Ni3AI + B, and NiAl were determined using either notched, or fatigue-precracked bend bars tested to failure at room temperature, in addition to testing specimens in tension under superposed hydrostatic pressure. Although Ni3AI is observed to fail in a macroscopically brittle intergranular manner in tension tests conducted at room temperature, the fracture toughnesses (i.e. K IC presently obtained on Ni3AI exceeded 20 MPa4"I', and R-curve behavior was exhibited. In-situ monitoring of the fracture experiments was utilized to aid in interpreting the source(s) of the high toughness in Ni3AI, while SEM fractography was utilized to determine the operative fracture modes. The superposition of hydrostatic pressure during tensile testing of NiAI specimens was observed to produce increased ductility without changing the fracture mode. INTRODUCTION It is well documented that various nickel aluminide intermetallics possess low ductility and that the addition of small amounts of boron can significantly increase the room temperature tensile ductility of polycrystalline Ni3AI (1,2). It has also been established that single crystal Ni3AI may be very ductile (e.g. elongation to failure > 50%), while the ductility of NiAI single crystals depends on orientation(3). Although considerable work has focussed on the tensile ductility of these materials, a review of the literature indicates that there is little information available on their behavior under different stress states as well as their fracture toughness. The anticipated use of such materials in structural components where damage tolerance and resistance to catastrophic fracture are important thus necessitates investigations into the factors influencing the fracture toughness of this class of materials. Some of our recent work(4-6) has begun to address the issue of the effects of stress state on fracture of monolithic nickel aluminides, where both notched and fatigue precracked specimens have been utilized to determine the notch toughness and the fracture toughness of various intermetallics. Another means to systematically examine the effects of stress state on deformation and fracture is to conduct tensile tests under superposed hydrostatic pressure. The benefits of conducting such experiments are summarized elsewhere (7-11). Similar testing has been conducted on particulate reinforced composites where it has been demonstrated that the superposition of pressure may significantly increase the tensile ductility and flow stress, in addition to changing the fracture morphology. Part of the intent of the present work was to additionally document the effect(s) of superposed pressure on the deformation and fracture of a brittle intermetallic as part of a larg
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