TOUGHENING MECHANISMS IN DUCTILE NIOBIUM-REINFORCED NIOBIUM ALUMINIDE (NB/NB3AL) IN-SITU COMPOSITES

An in situ study has been performed in the scanning electron microscope (SEM) on a niobium ductile-phase-toughened niobium aluminide (Nb/Nb3Al) intermetallic composite to examine the crack-growth resistance-curve (R-curve) behavior over very small initial crack extensions, in particular over the first similar to 500 mu m of quasi-static crack growth, from a fatigue precrack. The rationale behind this work was to evaluate the role of toughening mechanisms, specifically from crack bridging, in the immediate vicinity of the crack tip and to define the size and nature of bridging zones. Although conventional test methods, where crack advance is monitored typically over dimensions of millimeters using compliance or similar techniques, do not show rising R-curve behavior in this material, in situ microscopic observations reveal that bridging zones resulting from both uncracked Nb3Al ligaments and intact Nb particles do exist, but primarily within similar to 300 to 400 mu m 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 similar to 1 to 6 MPa root (m) over bar, 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.