The Fatigue Deformation Mechanism of the Annealed and the Rolled Low Carbon Steels through the Observation of Dislocation Structures

Abstract

Recently transmission electron microscopy has come to be widely used as efficient means to study the dislocation microstructural changes during the fatigue of f.c.c. metals, but not so widely in the case of b.c.c. metals, because of difficulties in dealing with the thin foil technique in the latter instance. No examination has so far been made of the fatigue behaviors of cold worked b.c.c. metals, such as rolled low carbon steel, by means of transmission electron microscope, inspite of the important practical problems involved therein.
In this paper report is made of the direct observation of dislocation microstructure conducted by means of transmission electron microscope during the fatigue, with a view to clarifying the fatigue deformation mechanism of the annealed and the rolled low carbon steels, with discussion on the strengthening mechanism in the fatigue strength of the rolled material.
The results are summarized as follows:
(1) In the case of the annealed material, the initial stage in fatigue deformation process is characterized by increase in dislocation density, arrays of dislocation loops in parallel rows, dislocations in clusters and tilt boundaries composed of dislocation network. The second stage marks formation of substructure, and the final stage is reached by fragmentation of subgrains. This subgrain size is 2∼4μ in diameter.
(2) The rolled material contains, before it is subjected to fatigue test, various lattice defects which cause imperfect substructures in its crystal grains. These imperfect substructures can be divided into two main regions; one is the rough substructure region in which the difference in dislocation density between the sub-boundary and the inside of the subgrain is low, and the dislocation density comparatively low on the whole, and the other is the band structure region which is composed of lamellar distribution of highly tangled dislocation band. This band structure region presents almost no change during the fatigue. In that rough substructure region, there have been, with increase in the number of cycles, interaction of dislocations, annihilation of dislocation, and reversible movement of dislocations toward the positions of lower potential energy, and then there have been formed distinct substructures to a larger number than at the time of rolling process. This subgrain size is 2∼6μ in diameter.
(3) The fatigue life of the rolled material is prolonged longer than in the case of the annealed one. This phenomenon is due to the resistance to the mobility of dislocation under the cyclic loading of the small dislocation loops, dislocation network and these tangled dislocations in the rolled material. This fact shows that greater energy is required for the formation of favorable substructures for crack initiation in the rolled material than those in the annealed one.