Powder processing of ductile-phase-toughened NbNb₃Al in situ composites

Model high temperature Nb-Nb₃Al in-situ intermetallic matrix composites have been successfully fabricated using powder metallurgy and arc-melting techniques. Vacuum hot pressing of elemental Nb and Al powders, mixed in the ratio Nb-6wt.%Al and synthesized at 1200 °C, yeilded a dual-phase microstructure consisting of equiaxed islands of ductile Nb particles in a brittle Nb₃Al matrix. Subsequent thermal treatment at 1800 °C for 24 h resulted in an alternative composite microstructure with the Nb present in a lamellar morphology following a peritectic transformation, similar to structures obtained via arc-melting and thermal treatment procedures. The precipitation of ductile Nb phase following thermal aging was found to nucleate heterogenously at grain boundaries and to progress along specific growth directions, resulting in a uniform and fine distribution of filamentary Nb within the Nb₃Al matrix. Both equiaxed and lamellar Nb-Nb₃Al composite microstructures show improved fracture toughness of about 6 MPa m^1/2 compared with unreinforced Nb₃Al (about 1 MPa m^1/2), principally because of crack bridging and crack renucleation effects induced by the presence of the ductile Nb phase.