Influence of ferrite-martensite microstructural morphology on the low cycle fatigue of a dual-phase steel

A CMnSi dual-phase steel has been heat-treated to yield three different microstructures — a dispersion of martensite in fine-grained ferrite (Type I), a continuous network of martensite around ferrite grains (Type II) and martensite islands encapsulated in a ferrite matrix (Type III) — and low cycle fatigue tests have been conducted. All three conditions have been found to exhibit cyclic hardening. The lower hardening in the Type I microstructure and its cyclic softening at lower strains are attributed to a more uniform initial distribution of dislocations because of the dispersion of the martensite. The Type I microstructure displays the best fatigue performance and is shown to obey the Coffin-Manson law. The deviation of the other two microstructures from this law is shown to be due to a transition from a ductile mode of fracture at lower strains to a brittle type of fracture at higher strains.