Both monotonic and cyclic deformation of a steel with three duplex microstructures the fine debris structure was the best in terms of strength and ductility while the network structure showed the wrost due to the early cleavage of ferrite phase. However, the network structure has the highest cyclic hardening potentiality because of the shell structure of martensite surrounding ferrite grains, and the fine debris has the lowest because of the good continuity of ferrite matrix. When cycled at rather high strain amplitudes, specimens of all the three microstructures hardened rapidly at first followed by a slight softening before the stable saturation stages, whereas the specimens hardened gently for a large number of cycles to saturate at relatively low strain amplitudes. It was concluded from the transmission electron microscopic observations that the cyclic softening was caused by the dislocation rearrangement into low energy dislocation structures(LEDS). Dislocation structures observed in cyclically deformed duplex microstructures at various strain amplitudes were found quite similar to those observed in fatigued f.c.c. metals. Especially, the dislocation parrallel wall structure produced by cyclic deformation at a plastic strain amplitude Δεp/2 of about 2.2 * 10~(-3) were similar to PSB structures. Such well arranged dislocation structures were believed to be responsible for the stable stress response as in f.c.c. metals. The similarities were considered to be due to the silicon solution strengthening and the effect of other alloy elements. Generally it was found that martensite/ferrite interfaces did not affect the dislocation structures in ferrite significantly, which indicate a uniform strain distribution around interfaces. Meanwhile, hetrogeneous distributions were also observed in differen ferrite regions as well as in some interfaces, which would become the main sources of fatigue crack initiation on the surfaces of specimens. Scanning electron microscopic observations on fatigued specimens' surfaces and fracture topographies demonstrated that the fatigue crack initiate mainly from the extrusion and intrusions in ferrite grains in the fine debris structure while mainly from the phase boundaries and ferrite grain boundaries in the other two microstructures. The S/N curves of the three duplex microstructures and the magnetic susceptibility of the fatigued specimens were also determined. The network structure showed the lowest fatigue limit and the block structure, which showed monotonic properties between network and fine debris structure, showed the best fatigue behavior. Because of the low resistance to fatigue crack propagation the fine debris structure did not show the best fatigue behavior despite the combination of best strength and ductility. Cyclic deformation induced high density dislocations caused decreases in alternating magnetic susceptibility. The higher the cyclic strain amplitude or the peak stress at which the specimens were fatigued, the greater the decrease in the magnetic susceptibility was. It is surprising that, a cyclic deformation at a Δεp/2 of about 2 to 3 * 10~(-3) to saturation caused a much smaller decrease in magnetic susceptibility in comparasion with cyclic deformation at a lower strain amplitude, which was explained in terms of the concepts of the formation of LEDS and internal stress. Deformation at even higher strain amplitude caused a further reduction of susceptibility because of the pinning effect of dislocations on the movement of magnetic domain walls.
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