Cyclic hardening behavior for interstitial-free steel

Strain-controlled fatigue experiments were employed to evaluate automotive-grade interstitial-free ferrite steels under R = 0. Hundreds of grains were examined by scanning electron microscope (SEM) under electron channeling contrast image technique of backscattered electron image mode (BEI/ECCI) for comprehensive comparison of micrographs with those taken under transmission electron microscope (TEM). It is clearly revealed that cyclic hardening was virtually unobvious and dislocation cell structures were very rare when Δε/2 was controlled to within 0.1%. When Δε/2 was increased to 0.2%, the general dislocation structure exhibited a predominately dislocation wall structure prior to the secondary cyclic hardening, after which the formation of dislocation cells were observed. At Δε/2 = 1.0%, following an initial rapid-hardening stage, the dislocation cell structure of low-angle misorientation formed in the early stage was gradually converted into high-angle misorientation as the cyclic strain continued to be imposed.