ROLE PLASTIC DEFORMATION ON DUCTILE-BRITTLE FRACTURE TRANSITION

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In toughness of steels at the ductile-brittle fracture transition region, both the brittle and ductile natures are involved. The origin of the temperature dependence of toughness must be thus examined from both mechanisms. The ductile nature is represented by the resistance against the stable ductile crack propagation, i.e. R curve. By using steels with various microstructures and compositions, precise measurements of R curves were conducted. It was revealed that the temperature dependence of R curve is small and that factors such as grain size and toughness of ferrite matrix have almost no effects. However, carbon con-tent and particularly grain boundary carbides which act as obstacles of slip propagation were found to have remarkable effects.Accordingly, the origin of the temperature dependence of toughness is at the initiation stage of brittle fracture, while the processes leading to the initiation constitute the toughness value. By referring to the analyses by means of fracture mechanics calculation, it was shown that the initiation is controlled by the local tensile stress in the crack front area at a low toughness level. On the other hand, at the toughness level where the crack tip blunting is substantial and the stable crack advances, the brittle fracture initiation is controlled by local strain. It implies that the plastic deformation is the key process for the determination of toughness at the ductile-brittle transition region.Transmission electron microscopy of the area near the brittle fracture ini-tiation site revealed complicated dislocation cell configurations and formation of micro-voids at cell walls even at a very low toughness level. It suggests that deformation structure rather than the cracking of secondary particles induces the brittle fracture initiation.