Microstructural Size Effect on Strain-Hardening of As-Quenched Low-Alloyed Martensitic Steels

Quenched martensitic steels are known to show the characteristic feature of stress-strain relations, with extremely low elastic limits and very large work-hardening. The continuum composite approach is one way to express this characteristic feature of stress-strain curves. Although the overall stress-strain curves, as a function of alloy chemistries of steels, were well represented by this approach, the relationship between the macroscopic deformation behaviors and microstructural information is yet to be clarified. A high-spatial-resolution digital image correlation analysis was conducted to demonstrate the possible unit size corresponding to the microstructure. The continuum composite approach model was also modified to consider the size effect of the microstructure on the stress-strain curves of the as-quenched martensitic steels. Strain concentrations were observed at various boundaries, including lath boundaries, and the characteristic microstructural size was also predicted by the present model, which is smaller than the reported spacing between adjacent strain- concentrated regions.