Effect of purity on the tensile mechanical behavior of pure iron

Abstract

The current work systematically investigated the microstructures, mechanical properties, textures, dislocation density and dislocation substructures of 3N2, 4N2 and 5N2 pure irons (3N2, 4N2 and 5N2 pure irons represent that the content of Fe are 99.9264 wt%, 99.9925 wt% and 99.9992 wt%, respectively) using SEM, EBSD, XRD and TEM. The results indicated that the recrystallization temperature decreased with increasing purity, and higher purity accelerated grain growth. 4N2 and 5N2 pure irons both presented equiaxed ferrite grains, while the microstructure of 3N2 pure iron was consisted of irregular polygonal ferrites and equiaxed ferrite grains. Low-density LAGBs were observed in annealed pure irons. The fraction of LAGBs increased significantly after deformation, and the LAGBs density decreased as the purity increased. The dislocation substructures of 3N2 and 4N2 pure irons were dominated by sub-grain boundaries with high dislocation densities and dislocation tangles, whilst 5N2 pure iron primarily consisted of fewer sub-grain boundaries and random dislocations. Work hardening was the primary strengthening contribution of pure irons. As the purity decreased, the capacity of work hardening increased, which resulted from the effective obstacles to dislocations by higher fraction of LAGBs, sub-grain boundaries with high dislocation densities and dislocation tangles. Therefore, the improved mechanical properties may be attributed to the increase of work hardening ability with the decrease of purity. 3N2 pure iron presented discontinuous yielding behavior, while 4N2 and 5N2 pure irons exhibited continuous yielding behaviors. The continuous yielding was attributed to the minimal interstitial atoms rarely segregated on mobile dislocations to form Cottrell atmosphere.