Dislocation Structures during Fatigue Deformation of Rolled Low Carbon Steel

Abstract

It is known that fatigue induced dislocations structures in iron and low carbon steel are cell structure at high stress amplitude, and band structure at low stress amplitude.The authors made fatigue testing of low carbon steel specimens, which had been rolled beforehand to give cell structures, at low stress amplitude, and examined the dislocation structures of these specimens, comparing with those of the annealed specimen by the transmission electron microscope.The results obtained are as follows:(1) Two types of subboundaries have been recognized in the fatigue process of the rolled specimen at low stress amplitude. The one has larger misorientation between the neighboring subgrains, and the other has almost no misorientation. The former is characteristic of the rolled specimen, and can be imagined to be made well defined by the recovery during the fatigue deformation. The latter is similar to the subboundary recognized in the annealed specimen for the low amplitude fatigue.(2) The cell size and dislocation density both in the annealed and rolled specimens become similar just before the fatigue failure. These results are similar to those observed in high stress fatigue.(3) The annihilation of dislocations occurs in the rolled specimens, perhaps by abnormally high vacancy concentration produced during the fatigue process.(4) The consumption of vacancy by the climb motion of the edge dislocation, or the annihilation of edge dislocation, seems to delay the generation of pores on the surface in the rolled specimen, and thus the fatigue strength of the rolled specimen seems to increase.