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 eut ectic equilibrium between binary NiAl and the terminal (Mo) solid solu tion, 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-M o two-phase alloys. Above room temperature, the NiAl-Mo alloys showed an increase in bend strength compared to monolithic NiAl, with reasona ble strength retention up to similar to 800 degrees C. The results dem onstrated 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, fo r substantial decohesion between the (Mo) phase and the NiAl matrix, t hereby suggesting the presence of a weak interface. This weak interfac e between the (Mo) phase and the NiAl matrix, in conjunction with modu lus mismatch stresses, causes the crack to deflect from the (Mo) reinf orcement and propagate preferentially along the (Mo)/NiAl interface. T hese attributes limit the potential for significant ductile-phase toug hening in the NiAl-Mo system. An addition of 0.2 at. pct Ti resulted i n a marked improvement in the room-temperature fracture toughness of N iAl-Mo. Fractography observations show some evidence for (Mo)/NiAl int erface strengthening with the Ti addition.