Quantification of strain partitioning during low cycle fatigue of multi-phase steels containing a bainite matrix

Three multi-phase steels, all comprising 80% bainite and 20% ferrite have been studied under low cycle fatigue. Under monotonic loading the steels exhibit high strengths (>1 GPa) and reasonable ductilities of 10-20% elongation. Under cyclic loading the steels exhibited an initial cyclic hardening followed by a cyclic softening response. When compared to traditional dual-phase steels, the bainitic alloys showed an inferior fatigue lifetime, attributed to strain partitioning to the ferrite. The distribution of strain between the phases has been quantified using a surface micro-grid, and local strain values of 8% can be observed in the ferrite while the bainite strain is negligible. Finite element modelling has been used to quantify the dependence of strain partitioning on the relative strengths of the constituent phases, and this has revealed that the strength of the hard phase must be less than 3 times the strength of the soft phase to minimise stain partitioning.