Ti-5Al-2.5Sn 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

概要

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Internal crack initiation in high cycle fatigue at cryogenic temperatures has been investigated for Ti-5Al-2.5Sn ELI alloy. The fatigue crack initiation site changed from the sample surface to the sample interior at about 10^5 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 α 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 α 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.
社団法人日本鉄鋼協会の論文
1989-01-01