Ti-5Al-2.5 Sn ELI合金の極低温高サイクル疲労におけるき裂の内部発生

Abstract

Internal crack initiation in high cycle fatigue at cryogenic temperatures has been investigated for Ti-5A1-2.5Sn ELI alloy. The fatigue crack initiation site changed from the sample surface to the sample interior at about 105 cycles. Neither defects nor foreign material was detected at the internal nucleation sites. However, the region of the nucleation site had a grain morphology and composition different from that of the bulk material. From observation it is concluded that internal cracks initiate in small a grain regions with a low Al content and an Fe enriched second phase at their grain boundaries. At maximum stress lower than three-quarters yield strength, where internal fatigue crack initiation occurred, dislocation pile-ups were primarily on prism planes and were blocked at the grain boundaries. The piled-up dislocations are believed to produce a high stress concentration in the small a grain regions. This stress concentration may provide a site for internal crack initiation. At higher maximum stress the dislocation pile-ups are more homogeneously distributed in the material and the dislocation arrays can pass through the grain boundaries. As a result, fatigue crack initiation must occur at the specimen surface.