Dislocation structures in fatigued iron-carbon alloys

Pure iron (0.005% carbon), Swedish iron (0.031% carbon) and iron-carbon alloys containing 0.08 and 0.25% carbon were fatigued in plane bending and examined by transmission electron microscopy. At low stress dislocations in pure iron had a kinked or helical appearance. Over a range of stress amplitudes, the dislocation structures in Swedish iron and the iron-0.08% carbon alloy were similar to those reported for pure iron, i.e. clusters of heavily jogged dislocations and prismatic loops, with a cell structure developing at high stress. Restriction of dislocation motion by cementite precipitate particles and carbon segregation are suggested as responsible for the decrease in dislocation cluster formation in the normalized iron-0.25% carbon alloy. Quench-ageing of the iron-0.25% carbon alloy produced fine metastable carbide particles in the ferrite matrix. After cyclic stressing the metastable carbides are broken down and replaced by dense clusters of jogged dislocations and prismatic loops.