Alloy Modeling and Experimental Correlation for Ductility Enhancement in NiAl
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ALLOY MODELING AND EXPERIMENTAL CORRELATION FOR DUCTILITY ENHANCEMENT IN NiAl R. DAROLIA*, D.F. LAHRMAN*, R.D. FIELD*, A.J. FREEMAN** *GE Aircraft Engines, 1Neumann Way, Cincinnati, Ohio 45215 "**Physics& Astronomy Department, Northwestern University, Evanston, Illinois 60201 ABSTRACT Single crystals of stoichiometric NiAl and NiAl+V alloys were tested in compression and tension from room temperature to 871"C to determine deformation behavior. The dislocations were predominately in the plastically deformed specimens. Attempts to ductilize NiAl by the addition of vanadium are described. The lowering of the anti-phase boundary energy by vanadium addition to NiAl, believed to promote the formation of dislocations, was predicted by the all electron self consistent total electron band structure calculations. The vanadium additions caused considerable solid solution strengthening in NiAl, rendering the ternary alloys more brittle than stoichiometric NiAI. INTRODUCTION Low density intermetallic alloys offer significant payoff to aircraft turbine engines by increasing the thrust to weight ratio. The predicted payoff for an intermetallic alloy such as NiAl with a density of 5.8 g/cc, compared to 8.3 glcc for state-of-the-art superalloys, is a rotor weight savings of up to 50%. A major problem in NiAl is its limited ductility which must be improved for aircraft engine service. To overcome the ductility problem, research is being carried out to experimentally assess the capability of alloy modeling based on first principles. The objective is to predict the role of ternary element additions on the electronic structure, phase stability, bonding characteristics, elastic properties, anti-phase boundaries and slip systems. An iterative refinement of the theoretical predictions is being made by experimental testing on single crystal NiA1 alloys. Alloying concepts based on first principles that can predict mechanical behavior of intermetallic compounds can serve as an extremely powerful tool for accelerating the development of useful intermetallic compounds. A major obstacle to ductility in NiAl is the slip direction resulting from the ordered B2 structure, yielding only three independent slip systems rather than the five required to satisfy Von Mises criterion and no active systems to respond to stress along . Lowering of the anti-phase boundary energy (APBE) could result in the formation of superdislocations, two 1/2 superpartials separated by an APB, as observed in other B2 systems such as CuZn [1,2]. Reduction of the formation energy of superdislocations would provide an ample number of slip systems and, provided the Peierls stress was low enough to give sufficient mobility, might well promote ductility in NiAl. All electron self consistent total energy band structure calculations for anti-phase boundaries in NiAl were conducted by using a supercell approach. APBE was determined by constructing supercells with and without APB's and comparing the resulting energies. Different atomic species were then substituted on the APB plane
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