Microstructures and mechanical behavior of NiAl-Mo and NiAl-Mo-Ti two-phase alloys
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I.
INTRODUCTION
THE B2-ordered nickel aluminide NiA1 exhibits an attractive combination of properties, such as high melting temperature, high stiffness, and low density, thereby offering the potential for use as a high-temperature structural material. However, these characteristics are accompanied by a lack of ductility and inadequate fracture toughness at low temperatures. Although singlecrystalline NiA1 exhibits some plasticity, t'J polycrystalline NiAI shows little or no room-temperature tensile ductility t2-6~ and exhibits premature fracture. The room-temperature fracture toughness of coarse-grained NiAI is below 5 MPaV'-m, tTl and5 to 6 MPaV'-m for a 20/zm grain-size material, t8~ Intergranular fracture is the primary mode of failure in polycrystalline, stoichiometric NiAI. t3,41The inadequate toughness and damage tolerance of polycrystalline NiAI at low temperatures limits its potential, particularly in monolithic form, for aerospace applications. A composite systems approach, such as toughening by fiber reinforcement or by ductile-phase reinforcement, may provide the solution to the intrinsically poor damage tolerance of brittle materials. In the former approach, toughness is provided to the brittle matrix through mechanisms such as fiber pullout, crack branching, and crack deflection by interface debonding, as observed in ceramic composites.t9] On the other hand, the predominant toughening mechanism in ductile-phase-reinforced systems involves energy dissipation by plastic deformation P.R. SUBRAMANIAN, Senior Scientist, and M.G. MENDIRATI'A, Division Director, are with the Materials Research Division, UES, Inc., Dayton, OH 45432. D.B. MIRACLE, Materials Engineer, is with the Air Force Wright Laboratory, WL/MLLM, Wright-Patterson AFB, OH 45433. Manuscript submitted October 30, 1992. METALLURGICAL AND MATERIALS TRANSACTIONS A
of ductile ligaments bridging the crack surfaces behind the advancing crack tip. t~~ Examples of ductile-phase toughening include systems such as Co/WC, t"l A 1 / A I 2 0 3 , tl~ T i N b / T i A 1 , tl21 and Nb/NbsSi3 .tl3'141 The need for long-term composite stability at elevated temperatures requires that the reinforcing phase in these composite systems be in thermodynamic equilibrium with the matrix material. In this context, phase relationships showing equilibrium between the mafxix phase and a terminal refractory phase, such as in eutectics, can be exploited for obtaining an in situ composite. Such a two-phase system offers the best prospects for long-term thermodynamic stability. Examination of the literature t2-'5-2'1 showed a number of NiAI-X (X = Mo, Cr, W, Re, and V) quasi-binary eutectic systems, where NiAI is in equilibrium with a nominally ductile, refractory phase. Past studies t2,z~ on the mechanical behavior of directionaUy solidified (DS) NiA1-Cr and NiA1-Mo eutectics have focused on the role of the fine, fibrous (Cr) or (Mo) phases in improving the high-temperature strength and creep resistance of NiAI. However, the potential for toughening NiA1 by the refractory pha
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