β型チタン合金Ti-15Mo-5Zr-3Alの疲労挙動に及ぼす組織の影響

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The fatigue behaviour has been studied under rotating bending using the materials with three different microstructures (β grain sizes) of a Ti-15Mo-5Zr-3Al alloy which were solution-treated at 735℃(STA735), 850℃(STA850) and 1000℃(STA1000) followed by aging at 500℃. It was found that STA735 showed the highest fatigue strength, and in the materials solution-treated above the β transus, STA850 and STA1000, the fatigue lives decreased with increasing solution temperature i.e. β grain size but the fatigue limit was almost the same in both materials. Fatigue cracks were initiated as a consequence of slip deformation in STA735, while at β grain boundaries in both STA850 and STA1000. In particular, STA850 exhibited an interesting initiation behaviour in which cracks were initiated at the specimen surface at high stress levels, but subsurface crack initiation was observed at low stress levels. The growth rate of small cracks was almost identical for all the materials, indicating no influence of solution temperature. Therefore, the observed difference in fatigue strength was attributed to the resistance to crack initiation of the microstructures studied.
1995-07-15