MICROSTRUCTURES AND MECHANICAL-BEHAVIOR OF NIAL-MO AND NIAL-MO-TI 2-PHASE ALLOYS

The phase relationship in the NiAl-Mo system is characterized by a eutectic equilibrium between binary NiAl and the terminal (Mo) solid solution, thereby offering the potential for development of ductile-phase-toughened composites. A study was conducted to evaluate the effect of varying volume fraction of the (Mo) phase on the microstructure, bend strength, and ambient temperature fracture behavior of selected NiAl-Mo two-phase alloys. Above room temperature, the NiAl-Mo alloys showed an increase in bend strength compared to monolithic NiAl, with reasonable strength retention up to similar to 800 degrees C. The results demonstrated moderate improvements in toughness in the NiAl-Mo alloys in comparison to monolithic NiAl. A further enhancement in toughness was realized through hot working. Fractography studies showed evidence, for substantial decohesion between the (Mo) phase and the NiAl matrix, thereby suggesting the presence of a weak interface. This weak interface between the (Mo) phase and the NiAl matrix, in conjunction with modulus mismatch stresses, causes the crack to deflect from the (Mo) reinforcement and propagate preferentially along the (Mo)/NiAl interface. These attributes limit the potential for significant ductile-phase toughening in the NiAl-Mo system. An addition of 0.2 at. pct Ti resulted in a marked improvement in the room-temperature fracture toughness of NiAl-Mo. Fractography observations show some evidence for (Mo)/NiAl interface strengthening with the Ti addition.