PHASE-TRANSFORMATIONS IN AN IN-SITU NB-REINFORCED NB3AL INTERMETALLICCOMPOSITE

The development of two-phase Nb/Nb3Al in situ composite microstructure s by thermal treatment in a Nb-6wt.%Al alloy, processed through powder -metallurgy techniques, is examined in detail using transmission and s canning electron microscopy. Observations reveal that the precipitatio n of Nb3Al in a heavily dislocated Nb solid solution matrix initiates at grain boundaries and progresses along the [110] and [211] direction s in the matrix and Nb3Al precipitate, respectively; the precipitates eventually fuse into small, elongated grains with 1-10 mu m dimensions . The evolution of the in situ composite microstructure from the hot-p ressed equiaxed structure proceeds by a diffusion-controlled nucleatio n and growth transformation and not by a massive transformation. The l amellar microstructure of the alloy displays a five-fold increase in t oughness over unreinforced Nb3Al primarily due to crack bridging and p lastic deformation associated with the ductile Nb phase.