二相合金の疲労破損の機構に関するX線的研究

細束X線と電子顕微鏡による(α+β)黄銅の疲労現象の観察

抄録

In the field of mechanical engineering, the materials which commonly have been used are alloys that consist of many phases. It may be easily imagined that each phase of them shows the extremly complicated deformation under applicated stress. Thus, the external strain of the specimen is the mean value of the strain in each phase. In order understand the responses in each phase of the material to externally applied stress, the authors have adopted two phase alloys, (α+β) brass which is the simplest of them. Noticing the α-and β-phases, they have made study of (α+β) brass, and put particular emphasis on the change in the half-value of X-ray diffraction line during the fatigue process. To make clear the character of mechanism of crystal deformation in fatigue, the change in the diffraction line widths on a few lattice planes has been examined. Such examination has also been attempted during the plastic deformation process.
From those experiments results the following have been obtained.
(1) (310) and (321) diffraction line widths in the β-phase are greater than those of (400), (420) and (331) of the diffraction lines in the α-phase under the fatigue and stretching processes.
(2) In order to account for the crystal lattice plane dependency of the diffraction line widths in the α-phase, the authors have adopted an idea of the orientation factor which is calculated for each lattice plane. It is found in consequence that during the tensile deformation the crystal plane dependency of the changes is sufficiently accounted for by the values of the orientation factor, while during fatigue process it is accounted for by them and the effects of the crack initiation.
(3) The change in the residual stress of the α-phase due to stress repetitions show the same behaviour as those of plain carbon steel; the compressive residual stress appear in the first stage of fatigue and then decrease abruptly. On the othor hand, in the β-phase of the same material the tensile residual stress is generated in the first stage of fatigue, and in the second stage it decreases steeply. Thereafter no change occurs.
(4) In the observation of slip markings in the α-phase by means of optical microscope, it has been found that they are intensified gradually with increase in the number of stress repetition, while in the β-phase, no obvious slip phenomena has been observed.
Further, in pursuing those investigations, the authors have adopted back-reflection X-ray micro beam and electron microscopy techniques.
The information on fatigue mechanism supplied by the study of change in the half-value width as mentioned above gives us many suggestions on the change in micro-structure of materials under fatigue process. It is necessary to direct the light on the micromechanism because of such local nature of fatigue mechanism. The half-value breadth read from the diffraction lines taken by the ordinary beam is inevitably the mean value of radial breadth from many crystal grains and its variation includes various physical factors, although it has many advantages for detecting fatigue damages that have been observed in cyclically stressed materials.
In the present study, it is intended to apply the back-reflection X-ray micro beam technique to the observation of each crystal grain of material during fatigue process, in order to interpret the change in the half-value breadth under fatigue process of (α+β) brass.