| In-situ tensile deformation tests of molybdenum were carried out in a transmission electron microscope at temperatures ranging from 120K to room temperature. At all test temperatures, brittle crack propagation was observed to precede plastic deformation. The characteristics of cracks, either ductile or brittle, was found to vary depending on crack geometry and loading condition. In some cases the coexistence of brittle and ductile crack was observed and in other instances crack propagation changed from brittle to ductile mode. Such phenomena could be successfully explained by the concept of local stress intensity factor for dislocation emission and brittle crack propagation. Dislocations generated at a crack tip are thought to shield the crack. The shielding of a crack by emitted dislocations would then increase the fracture toughness of materials. On the other hand, the anti-shielding of a crack by externally generated dislocations could occur, just like the shielding due to dislocations generated at the crack tip to reduce the toughness of materials. |
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