Nucleating and propagating of nanocrack in dislocation-free zone in brittle materials

Nudeating and propagating of nanocrack forming in dislocation-free zone (DFZ) for a brittle material, TiAl alloy, was studied through in situ tensile test in TEM and analyzed using micro-fracture mechanics. The results show that many of dislocations can be emitted from a crack tip when the applied stress intensity KIa is larger than the stress intensity for dislocation emission KIe = 1.4MPa·m1/2 and a DFZ is formed after reaching equilibrium. The stress in a certain site in the DFZ, which is an elastic zone and is thinned gradually through dislocation multiplication and motion in the plastic zone, may be equal to the cohesive strength, resulting in initiating of a nanocrack in the DFZ or sometimes at the notch tip. The nanocrack forming in the DFZ is stable and can propagate into a cleavage microcrack through multiplication and movement of dislocations in the plastic zone under constant displacement condition.