IN-SITU OBSERVATION OF INITIATION AND PROPAGATION OF CLEAVAGE MICROCRACK IN TIAL

Nucleating and propagating of nanocrack formed in dislocation free zon e (DFZ) for the brittle TiAl alloy has been studied through in-situ te nsile test in TEM and analyzed using microfracture mechanics. The resu lts show that a lot of dislocations can be emitted from a crack tip wh en the applied stress intensity K-la is larger than the stress intensi ty for dislocation emission K-le = 1.4 MPa . m(1/2) and a dislocation free zone, which sometimes is a close zone, can form after reaching eq uilibrium. The DFZ is a elastic zone with large strain and then the st ress in the DFZ might equal to the cohesive strength sigma(th) because the crack tip is still sharp. When K-la is larger than the stress int ensity for nanocrack nucleation K-li = 2.4 MPa . m(1/2), the stress wi thin a certain range in the DFZ would equal to sigma(th) and then a na nocrack initiates in the DFZ or sometimes at the notch tip. The nanocr ack formed in the DFZ is stable and can propagate a small distance in cleavage mode through multiplication and movement of dislocation in th e plastic zone, during keeping constant displacement. Increasing K-la, can make the crack stably propagate continuously or discontinuously a nd it means that the stress intensify for crack propagation, K-lp, is larger than K-li. Therefore, K-le [K-li [K-lp [K-lC.