Morphological modification and fracture toughness of the niobium aluminide eutectic

The processing of alloys of the niobium aluminide system has been investigated to determine their applicability to elevated temperature service. Several alloys have been examined including the eutectic of Nb₂Al and NbAl₃. This is a degenerate, irregular eutectic whose secondary phase morphology consists of a coarse network of slightly elongated NbAl₃ particles, which appear globular in cross-section. Control procedures have been developed for the directional solidification via levitation zone refining of this eutectic and of the stoichiometric NbAl₃ compound. Ternary additions of Ni and Ti alter the arc-melted morphology of the eutectic structure. The addition of titanium promotes a semi-lamellar structure around the periphery of the dendritic regions, but does not alter the eutectic regions of the microstructure. Nickel additions refine the structure to a very fine, rod-type morphology. However, the thermal instability of this structure prevents its growth as an "in-situ" composite during levitation zone refining. The fracture toughness of the binary and Ni-ternary eutectics were determined by a microhardness indentation cracking technique. No change in toughness resulted from the drastic morphology alteration . Elemental analysis, along with the current tentative ternary Nb-Al-Ni phase diagram, indicate that the Ni addition is taken up almost entirely by the Nb₂Al sigma phase. The morphological modification is most likely due to an alteration of a growth mechanism for this phase during solidification. Possible modes include a change in the triple point interfacial energy balance, and absorption of Ni on certain faceting planes in the sigma structure that inhibits growth in the high velocity directions.