Toughening mechanisms in ductile niobium-reinforced niobium aluminide (Nb/Nb3Al) in situ composites

An in situ study has been performed in the SEM on a niobium ductile phase-toughened niobium aluminide (Nb/Nb3Al) intermetallic composite to exanaine the crack-growth resistance-curve (R-curve) behavior over very small initial crack extensions, in particular over the first 500 or so microns of quasi-static crack growth, from a fatigue precrack. In situ microscopic observations reveal that bridging zones resulting from both uncracked Nb3Al ligaments and intact Nb particles exist primarily within 300 to 400 microns of the crack tip. Accordingly, rising R-curve behavior in the form of an increase in fracture resistance with crack growth is observed for crack extensions of this magnitude; there is very little increase in toughness for crack extensions beyond these dimensions. Ductile-phase toughening induced by the addition of Nb particles, which enhances the toughness of Nb3Al from about 1 to 6 MPa sq rt m, can thus be attributed to crack-tip shielding from nonplanar matrix and coplanar particle bridging effects over dimensions of a few hundred microns in the crack wake. (Author)