The X-Ray Investigation of Microstructural Change Caused by Low-Cycle Fatigue in Low Carbon Steel

Abstract

The authors have applied X-ray diffraction techniques to the study of low-cycle fatigue in order to discuss its macroscopic behavior from the microscopic point of view. In a previous paper, it was reported that a good correlation was obtained between the particle size measured by the profile analysis of the X-ray diffraction peaks and the damage fraction in 0.16% C steel under low-cycle fatigue of constant strain amplitude for some range of the strain range. It is found that the nondestructive measurement of the remaining life might be possible using this correlation. However, the above finding was not successfully used for the discussion of the mechanism of deformation and fracture in low-cycle fatigue, because the publications dealing with the analytical method and the application of this X-ray technique gave little available information for relating the data obtained by the authors to the microstructural changes reported by various investigators.
In these circumstances, the authors applied the X-ray microbeam diffraction technique to a further study of the microstructural changes which occurred during the low-cycle fatigue. The behavior of the subgrain was observed by this technique, and was made to bear on the measurements of the X-ray profile analysis.
The following was obtained as the results of this investigation:
(1) Low-cycle fatigue before visible cracks occurred had two stages in its process, each of them having its own particular X-ray phenomenon. The first stage was the stage of subgrain formation indicated clearly by the changes in the subgrain size and the interaction function of dislocations. The subgrain size was reduced rapidly and was inclined to saturate. The interaction function, which was defined as the ratio of the dislocation density obtained from the value of microstrain to that from the particle size, was reduced considerably. On the other hand, the excess dislocation density at the subgrain walls increased remarkably in the second stage, which was termed the stage of the subgrain development. The number of cycles at which the first stage was completed was around 270 in the case of the strain amplitude of 2.0%.
(2) The observation of surface patterns by electron microscopy showed a clear development of surface irregularities like extrusion and intrusion after the first stage was completed. This development may be related to that of the subgrain.
(3) The particle size had a linear relation with the subgrain size, and the interaction function indicated well the state of the subgrain. However, for the practical use of these measurements of X-ray profile analysis for closer examination of the substructure which appears in plastically deformed metals, it is necessary that further investigation will be made along this line.