X-Ray Investigation on the Mechanism of Fatigue Deformation and Fracture in Metals

Abstract

In their previous papers, the authors made some studies of annealed 6-4 brass, and particularly investigated its changes in residual stress, in half-value breadth and in micro-structure, during its fatigue process under constant and varying stress amplitudes, and found that its second phase showed some peculiar phenomena.One of these results showed the dependency of the changes in diffraction line widths on its crystal lattice plane in its α-phase. The authors tentatively interpreted this phenomenon as resulting from the heterogeneity of fatigue deformation as in the case of plastic deformation.Considering the character of fatigue fracture, however, it is questionable to ascrible the variation in the change of half-value breadth on diffraction crystal lattice planes only to the heterogeneity of plastic deformation.The authors examined therefore, during his X-ray investigations of fatigue deformation and fracture mechanism, first, the relationship between the tensile and the fatigue deformation, and secondly, fracture as dependent on the presence of a second phase.Heterogeneity in the tensile deformation had been commonly indicated as anisotropy of crystal, that is, orientation factor μ Accordingly, the authors attempted to make it clear whether this conception was applicable to a case of fatigue deformation or not. In case the application failed what sort of factors would be required?Heterogeneity as due to external forces had been investigated by Laue's reflection methods in single crystals, and the presence of asterism in the Laue spots on an individual crystal lattice plane showed anisotropy in tensile deformation. The reports, of which there had been a few, that no asterism was found in fatigue deformation process, requires reinvestigation, being a merely qualitative matter. The authors made inquiries, therefore, as to in what kind of changes in Laue spots the heterogeneity in the fatigue deformation appeared, and whether the dependency on the crystal lattice plane should have anything to do with asterism.The synopsis in this paper are as follows:(1) The changes in half-value breadth of 6-4 brass during the stretching process showed a variance in feature. The crystal plane dependency was seen in the β-phase but not in the α-phase on (420) and (331) planes. The changes in the half-value breadth of 7-3 brass and pure copper (α-phase only) were quite similar in general feature to the α-phase of 6-4 brass. From these experimental results it is evident that the crystal lattice plane dependency of the changes in diffraction line widths of the α-phase crystals was not affected by that of the β-phase crystals.(2) The interpretations of the dependency of the changes in the diffraction line widths on the crystal plane are based on the idea that each crystal in the polycrystalline aggregates is fit for the slip due to its orientation. Therefore, the authors adopted this idea to explain the changes in the half-value breadth. It is thought that the diffraction planes lying in parallel with the specimen surface occasioned by the experimental condition of X-rays considerably inclined up ward by some external force, and by reason of the crystals getting slips diverged broadly, while those planes of which the crystals are free from slips remained sharp.As for the plastic deformation of crystal, what is component of the shear stress τ in the slip plane along the slip line is all that is important. This shear stress is the result of the relation τ=μ×σN, where σN represents the normal stress and μ is the orientation factor. The latter is also indicated as μ=cosφ·cosα, where φ=the angle that the slip plane normal makes with the direction of the stress, and α=the angle that the slip direction makes with the direction of the stress.