Continuous and Discontinuous Yielding Behaviors in Ferrite-Cementite Steels

The continuous and discontinuous yielding behaviors in ferrite-cementite steels were complementarily investigated via nano- and macro-scale deformation examinations. The continuous yielding behavior was observed in heat-treated specimens with a lamellar or cementite-spheroidal structure even after strain-aging treatment. However, the discontinuous yielding behavior accompanied by the Luders elongation appeared in ferrite-cementite steel that was recrystallized after cold rolling. The results obtained by electron microscopy, synchrotron X-ray, and neutron diffractions indicate that the ferrite-cementite interface of the heat-treated specimen is semi-coherent with a high internal stress field, whereas that of the recrystallized one is incoherent with a low internal stress field. Moreover, coherency strain, which depends on the total area of the ferrite-cementite interface, and thermal strain, which is governed by temperature, are the two factors that influence peak broadening. The nanoindentation tests revealed that the critical loads are significantly lower near the semi-coherent interface than those near the incoherent interface and the ferrite grain boundary; this suggests that dislocations are easily emitted from the semi-coherent interface.