Microstructures and mechanical behavior of NiAl-Mo and NiAl-Mo-Ti two-phase alloys

Abstract

The phase relationship in the NiAI-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 NiAI-Mo two-phase alloys. Above room temperature, the NiAI-Mo alloys showed an increase in bend strength compared to monolithic NiAl, with reasonable strength retention up to ≈800 °C. The results demonstrated moderate improvements in toughness in the NiAI-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) rein-forcement and propagate preferentially along the (Mo)/NiAl interface. These attributes limit the potential for significant ductile-phase toughening in the NiAI-Mo system. An addition of 0.2 at. pct Ti resulted in a marked improvement in the room-temperature fracture toughness of NiAI-Mo. Fractography observations show some evidence for (Mo)/NiAl interface strengthening with the Ti addition.