透過X線回折法による金属の疲労に関する研究(I)

抄録

As one of the X-ray studies of fatigued metals, the author adopted the transmission X-ray diffraction method in this study. Specimens of pure aluminum poly-crystals which were completely annealed were used, and they were fatigued by cyclic tension and compression strainings with a small fatigue test machine which was designed by the author. The test pieces were of the size of about 0.5mm in thickness, 2∼5mm in length and 6mm in width at the smallest section. For most of those fatigue tests, the strain amplitudes were considerably large, which could not be measured, then the fatigue lives were about 10⁴ cycles. The surface of the fatigued specimen was observed at times with an optical microscope, which could be directed and adjusted in three dimensional bearings. It was noticeable that most of the fatigue cracks occurred at grain boundaries.
In process of the fatigue test, the transmission X-ray diffractions of that specimen were exposed on a Laue camera. The X-ray source was of cobalt target, and the electron beam was 30kV and 8mA. A slit of 0.5mm in diameter was used for the X-ray beam. The asterisms of the diffraction patterns were measured statistically, and it was confirmed that the asterisms increased with increase in the number of strain cycles. It seemed that there were correlations between the asterisms and the fatigue damage, though no clear conclusion could be deduced.
The fatigued specimens were further thinned by electrolytically polishing until about 0.1mm in thickness. In the transmission X-ray diffraction of the thinned specimen, it was clarified that the diffracted spots were isolated to fine spots which were elongated; and at the same time, the diffraction rings by the characteristic X-ray, CoK_α, appeared. Since those diffractions were constructed of groups of many fine spots, it seems that the substructure or poly-crystallization developed in the grains of those specimens.
An experiment was also performed of a specimen which was subjected first to tension of over 10% strain. It was then subjected to cyclic straining of small amplitude in tension and compression. Comparative study was made of the X-ray diffraction patterns (a) as the specimen was cyclically strained and (b) as it was statically strained. It was clarified that the diffractions of the statically strained specimen were completely broadened, and these broadened diffractions decreased as the result of cyclic straining. This phenomenon implies that microplasticstrains by the static straining in the grain differ from those by the cyclic straining.