The effect of fine precipitates on the initiation and propagation of micro cracks and the length of dislocation free zone (DFZ) in a low alloy Cr Mo V steel was studied by in situ TEM observation for the specimens both in embrittled and toughened conditions. The results show that in the embrittled specimen, dislocations emitted from the crack tip can easily slip by cutting through the precipitates and move away from the crack tip region, so that a long DFZ forms and the brittle fracture occurs via cleavage cracks initiating and propagating. The size of DFZ mainly depends upon the diameter and the interparticle distance of the precipitates in the matrix. In the toughened specimens, the size of DFZ extremely changes with the interparticle distance. Mobile dislocations are emitted from the crack tip and bypassing the particles. The interaction between the dislocations and the particles results in the dislocation pile up and tangle, leading to diminishing of the DFZ. The crack tip is easy to blunt or chang the propagating direction and the crack propagation shows a jagged path.
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