Low-energy dislocations and ductility of ferritic steels

Abstract

Extended dislocation half-loops, of a 2〉111〈 Burgers vector, appear in ferrite of dual-phase type HSLA steels and welds as controlling to a large extent the progress of (micro) plastic deformation of the ferrite. The criss-cross configuration of such half-loops was not very sensitive to heat inputs, and on intersections of the half-loops a reaction was observed resulting in mobile a〉001〈 edge dislocations. A recently proposed mechanism (1990) explains how these a〈001〉 edge dislocations participate in nucleation of brittle cleavage fracture. During straining around and below the brittle/ductile transition temperature, generation of such mobile a〈001〉 edge dislocations is substantial, and embrittled HSLA steels usually contain numerous immobilized a〈001〉 edge dislocations which, interacting with the generated ones, nucleate the cleavage cracks.