Ti-6Al-4V合金の疲労強度と微視的疲労き裂の発生

The fatigue strength in a high cycle region has been evaluated in this test using the representative titanium alloy, Ti-6Al-4V, under the rotating bending fatigue and the fractographic consideration is adopted to analyze the testing results. In addition, the fatigue crack initiation process has been investigated with the successive observation in the scanning electron microscope (SEM) equipment under out-of-plain bending fatigue.The main results obtained in this test are as follows:(1) Rotating bending fatigue tests in air; The fracture was often observed even after 107 cycles. When the specimen was broken by less repetitions than 1×106 cycles, the fracture mostly initiated from the outer surface with facet-like pattern at the origin. On the other hand, in the case of higher repetitions than 2×106 cycles, the fracture initiated from the sub-surface, where there were neither observed inclusions nor the facet-like fracture pattern.(2) Out-of-plain bending fatigue test in SEM; The ratio of crack initiation life of one grain length to total fatigue life is between 3 to 9% and the fatigue cracks are initiated by 90% from α grains and by the residual ratio from α/β boundaries. In addition, the slip bands are at first generated in α grains under the cyclic stress, then there appear cross-slip bands and finally those portions become micro cracks like cleavage-cracks.