Phase transformations in an in situ Nb-reinforced Nb₃Al intermetallic composite

The development of two-phase Nb/Nb₃Al in situ composite microstructures by thermal treatment in a Nb-6wt.%Al alloy, processed through powder-metallurgy techniques, is examined in detail using transmission and scanning electron microscopy. Observations reveal that the precipitation of Nb₃Al in a heavily dislocated Nb solid solution matrix initiates at grain boundaries and progresses along the <110> and <211> directions in the matrix and Nb₃A1 precipitate, respectively; the precipitates eventually fuse into small, elongated grains with 1-10 μm dimensions. The evolution of the in situ composite microstructure from the hot-pressed equiaxed structure proceeds by a diffusion-controlled nucleation and growth transformation and not by a massive transformation. The lamellar microstructure of the alloy displays a five-fold increase in toughness over unreinforced Nb₃Al primarily due to crack bridging and plastic deformation associated with the ductile Nb phase.