DISLOCATION SUBSTRUCTURES OF INTERSTITIAL-FREE STEEL SUBJECTED TO LOW CYCLE FATIGUE AT VARIOUS STRAIN AMPLITUDE

The aim of this wok is to study the fatigue properties in relation to the microstructures of interstitial-free (IF) steel, a representative of body center cubic (BCC) materials, subjected to low cycle periodic stress. In order to compare the grain size effect on dislocation development, we used samples of two distinctive grain sizes, 70 mu m and 210 mu m, prepared by different thermo-mechanical processes. We found (I) there was an initial softening when the stain amplitude was lower than 1.6% and the microstructure of the fatigued samples are cellular independent of the grain sizes and the amplitude of strain. (2) A re-entrant hardening following an initial softening stage took place for strain amplitude range from 0.2% to 0.4% in the samples of 70 mu m grains and 0.4% to 0.8% for those of 210 pm grains. Small cellular structures formed within a larger uncondensed cells structure at an initial stage. (3) While the mechanical behaviors of fatigue for both grain sizes are similar, differences do exist and are determined by strain localization as well as the multiple slip systems which can be triggered into active operation